the use of lime hemp composite products in the uk

8/11/2019 The Use of Lime Hemp Composite Products in the UK

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LEEDS METROPOLITAN UNIVERSITY

The Use of Lime Hemp

Composite Products in theUK

Academic and On-Site Perspectives

Sam Boys

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Contents1. Introduction .............................................................................................................................. 3

2. Literature Review ...................................................................................................................... 4

Mechanical Properties .................................................................................................................. 4

Thermal Properties ....................................................................................................................... 4

Hygrothermal Properties .............................................................................................................. 4

New Parameters ........................................................................................................................... 5

Acoustic Properties ....................................................................................................................... 5

Preparation Methods .................................................................................................................... 6

Curing Conditions & Drying Times ................................................................................................ 6

Whole Building Modelling ............................................................................................................. 7

Full Scale Experimental Test Building ............................................................................................ 7

Sequestration & Life Cycle Analysis .............................................................................................. 7

Related Studies ............................................................................................................................. 8

Interim Conclusions ...................................................................................................................... 8

3. Publications and Secondary Sources on Hempcrete ............................................................... 10

Supplier Literature ...................................................................................................................... 10

Books & Other Literature ............................................................................................................ 10

4. Existing Case Studies ............................................................................................................... 11

Haverhill, Surrey .......................................................................................................................... 11

Haverhill Thermographic Testing ................................................................................................ 12

County Down Eco House ............................................................................................................. 13

The Wales Institute for Sustainable Education ........................................................................... 13

5. Carleton School, Pontefract .................................................................................................... 14

Timing of HemcretePlacement ................................................................................................. 15

Method of Placement of Hemcrete........................................................................................... 15

Knock-on Effects of Extended Drying Times ................................................................................ 16

Internal Solutions ........................................................................................................................ 17

External Render .......................................................................................................................... 17

Ventilation Issues ........................................................................................................................ 18

6. Conclusions ............................................................................................................................. 19

Bibliography ........................................................................................................................................ 21

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The use of Lime Hemp Composite

products in the UK.

1.

Introduction

In response to environmental concerns a number of innovative new construction products have

recently become available for use. Amongst these is a product called hempcrete, also known by its

trade name Hemcrete. Hempcrete isa bio composite material made from hemp shiv and a lime

based binder(Lime Technology Ltd, 2011) which can be mixed and poured in-situ, much like in-situ

concrete, or supplied pre-cured as panels, cassettes or blocks. The leading supplier of this product in

the UK, Lime Technology Limited, herald the product as having a number of benefits:

Good thermal insulation

Excellent thermal inertia

Negative embodied carbon

Easy to use

Made from renewable/ abundant UK materials (Lime Technology Ltd, 2011)

Anecdotal evidence from sites using the product suggests that there are a number of problems with

its use in the UK climate, particularly on larger jobs where the in-situ method is used. This paper will

attempt to investigate the claims made for the material, and the problems encountered on the

aforementioned sites, and where possible offer some advice on the avoidance of future problems.

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2.

Literature Review

Mechanical Properties

Lime-hemp composites (LHC) have been used since the early 90s in France, and much of the early

academic work originates from Western Europe. Arnaud and Cerezo (Arnaud & Cerezo, 2001),

(Arnaud & Cerezo, 2002), studied various LHC mixtures, intended for different purposes: wall, floor,

roof. The different mixtures vary in their binder content, from 100kg to 530kg of binder per 110kg

of hemp. The mechanical properties were evaluated by means of compression testing. It is

concluded that LHC is characterized by its ductile behaviour in comparison with other building

materials, making it useful in preventing cracking where differential strains are likely, and making the

risk of sudden collapse very low (Arnaud & Cerezo, 2002). It was found that the lightweight mixtures

(intended for roof insulation) exhibited more ductile behaviour than that of the more dense floor

mix, which showed properties closer to that of the pure binder. It was also noted that the mixtures

become more brittle after the setting period of 9 months. Although the material was proven to have

a large range of maximum stress M

(between 0.4 and 1.2 MPa) and Youngs modulus E (40 to 90

MPa) it was noted that these values were not sufficient to use in loadbearing applications (Arnaud &

Cerezo, 2002). Dry thermal conductivity was also measured using an isothermal box and an attempt

made to produce a formula for the conductivity as a function of the density and binder content,

however, Evrard (2003) contended that the water content and humidity behaviour was not taken

into account in this model. Nervertheless, the results showed that the LHC had good insulating

properties ( between 0.06 and 0.11 Wm-1K-1for densities between 200 and 400 kg/m2) and the

theoretical model produced showed reasonable agreement with the measured results.

Thermal Properties

In 2002, BRE produced a report on the construction of the hemp houses at Haverhill in Suffolk

(Yates, 2002). As well as reporting in detail on the processes involved in the construction of this

development, the report also finds that the energy performance of the hemp homes is noticeably

better than the static state models predicted. It was found that the hemp houses consistently

maintained an internal temperature one or two degrees higher than in the brick houses for the same

amount of heat input (Yates, 2002). This effect is also noted by Evrard & De Herde (2005), when

using WUFI HAM (Heat, Air and Moisture) modelling software it was established that thermal

damping was taking place in a theoretical setting. Three theoretical cases were simulated; thermal

shock, thermal cycles and hygric shock. It was found that LHC took 9 months to achieve hygric

equilibrium (and therefore constant transfer conditions) following hygric shock, and therefore

concluded that standard steady state models (such as SAP) do not give an accurate picture ofthermal comfort and therefore energy usage (Evrard & De Herde, 2005).

Hygrothermal Properties

Evrard, et al. (2005), in Dynamical interactions between heat and mass flows in Lime-Hemp

Concrete, went on to test LHCs transient behaviour further using WUFI software, with

consideration given to the mechanisms of heat transfer, particularly the interaction between

moisture content and heat flow. This study also simulated the drying of the LHC from its wet state as

would occur following its implementation in practice. It used an ambient temperature on both sides

of the wall of 20C and 80% RH (which may be unrealistically high for typical UK conditions), and it

was found that to achieve hygric equilibrium from a starting point of 400kg/m3

took around 2 years.It is, however, also noted that finishing materials such as lime render/ timber cladding (Evrard & De

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Herde, 2005) can in practice be installed after a few months, depending on climate conditions

(Evrard, et al., 2005), although said conditions are not specified. The study also tested LHC against

cellular concrete and mineral wool in transient conditions; both sudden cooling and sudden heating.

The results of Evrard & De Herde (2005) are confirmed in that the LHC case studies confirm that

thermal performances of LHC in transient conditions are rather high, and LHC is found to have

strong thermal and hygric inertia (Evrard, et al., 2005).

New Parameters

Evrard and De Herde (2010) studies further the hygrothermal performance of LHC, in this instance

comparing realistic assemblies (including cladding/ render and internal plaster) with assemblies

representing more traditional building methods such as masonry cavity wall, aerated concrete and

internally and externally insulated monolithic constructions. The assemblies were subjected to

variations in temperature and relative humidity to simulate the real environment. The authors find

that the introduction of new parameters helps to explain the effective indoor comfort. Phase shifting

(delay) of a sine variation in temperature,phsth (h) and damping of the amplitude of a sine variation

in temperature, dmpth (%) reduce the impact of daily fluctuations in outside temperatures and avoid

summer overheating. Moisture buffering, MBVsimul, affects the internal relative humidity and

contributes to thermal buffering through latent heat effects (Evrard, 2006), (Evrard, et al., 2005). The

phsthof two LHC assemblies is found to be 14 and 15 hours, meaning the internal surface reaches its

maximum temperature in the early morning if maximum external temperature is in the early

afternoon. The dmpthis 92% and 91% respectively, removing much of the daily internal temperature

variation. It is noted that these effects on indoor comfort go some way towards explaining the

discrepancies between predictions and measurements made by the BRE at Haverhill, Surrey (Yates,

2002). (Maalouf, et al., 2011) studied further the effect of moisture transfer on the thermal inertia of

LHC, comparing it with that of normal concrete and brick masonry, however realistic assemblies arenot used and these elements (brick and concrete) would normally be isolated from the internal

environment by a vapour control layer or similar. The object oriented program SPARK is used to

model the behaviour of the elements in real climatic conditions and, again, LHC compares favourably

with traditional materials. It was found that neglecting the effects of moisture transfer in the

simulation has a significant effect on the outcomes for LHC, especially as thickness increases

(Maalouf, et al., 2011).

Acoustic Properties

Arnaud and Cerezo (2001) concluded that the LHC thermal properties recorded far better than

those of conventional building materials, with coefficient of absorption better than 0.5 for all themixtures tested. However, at the BRE tests in Haverhill, Surrey it was found that the hemp homes

did not perform as well as the traditional homes in the acoustic tests, although they did meet the

requirements of the Building Regulations 1991 Part E, which were current at the time of testing

(Yates, 2002). This may well be due to the monolithic construction used with the hemp homes as

opposed to more complex cavity construction in the traditional homes. Further research into the

sound absorption properties of LHC (Gle, et al., 2011) characterizes different mixtures of LHC

according to their porosity, and then models the sound absorption of the mixtures. It is found that

the absorption can be adjusted significantly by control of the constituents; lime binders with small

shive particles being the most effective sound absorbers, although these changes will clearly affect

the mechanical and thermal properties as well.

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Preparation Methods

(Evrard, 2006) goes on to investigate the effect that the methods of preparation i.e. mixing speed,

tamping compression and water input, have on the final properties of the LHC, as well as a dynamic

sorption test using Nordtest (Rode, 2005) inspired methods. This is then compared with a WUFI

simulation. It is established that the strength of tamping has a large influence on the final properties

of the LHC. If the mixture is over compressed then the LHC will take longer to dry and carbonation of

the lime will occur much more slowly. Conversely, a very lightly tamped mixture dries much more

quickly and carbonation is more advanced at the end of the test period (Evrard, 2006)1. These results

are in agreement with those of Elfordy et al (2008), who described the influence of the projection

distance on the density of the product, and therefore the thermal conductivity and mechanical

properties. It was noted that both thermal conductivity and mechanical properties increase with the

mortar density (Elfordy, et al., 2008) and that final density is a function of the projection distance.

With regards to the dynamic sorption test, it was found that the LHC in practice took longer to reach

constant buffering than the WUFI model predicted, and it is postulated that this difference is due to

a retarded sorption effect (Evrard, 2006). The tests showed that buffering (of the) LHC wall mixturewas high, and on the Nordtest classification in Rode et al (2005) good to excellent.

Elfordy et al. (2008) also examines the competition between the hemp and the lime carbonation

process for water, and finds that adding just enough water to slake the lime just before the hose

outlet accelerates the setting kinetics and reduces drying times to less than one month. These

figures were obtained using a mould measuring 300mm x 600mm and filled to a depth of 200mm, to

simulate the production of LHC blocks, and as such, the drying time may not be accurate for a thicker

cast in-situ arrangement.

De Bruijn et al (2009) studies the mechanical properties of LHC containing fibres as well as shives, to

improve mechanical properties and simplify the production of the raw material for countries without

the facility to separate the shives from the fibres. It was found that the inclusion of fibres did not

create a mechanically stronger LHC, but samples were of a similar strength to contemporary LHC.

Curing Conditions & Drying Times

Arnaud and Gourlay (2012) studied the parameters which affect the curing process. Extreme

humidity conditions are studied, as well as the binder content and hemp particle size. It was shown

that extremes of humidity (30%, 75% and 98%) affect hydraulic lime binders detrimentally, however

air lime binders are only affected by high RH: The curing conditions can strongly affect the

mechanical behaviour of hemp concretes: high relative humidity conditions (75% and 98% RH) are

not really suitable for the binder setting (both hydraulic and air lime) and low humidity conditions

(30% RH) slows down very sharply the setting of hydraulic lime-based binders (Arnaud & Gourlay,

2012). The optimum curing conditions were found to be 20C and 50% RH. Considering that the

mean summer humidity (1971-2000) over most of the UK lies between 76% and 85% and in the

winter is above 80% (UK Climate Projections, 2009) and average temperatures are well below 20C,

this presents obvious concerns for curing times in the UK. This study also finds that higher binder

proportions in the mix create a higher strength product, with lower max strain, M, but increase the

curing time. Also, smaller hemp shive particle size distribution results in lighter concretes whose

1

Interestingly, it is also noted that the carbonation of the lime results in a gain in mass, as CaCO 3 is heavierthan Ca(OH)2. In theory this could mean an increase in mass of the LHC of 175% as carbonation takes place.

(Evrard, 2006)

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mechanical properties evolve more slowly due to reduction of macropore size but they finally have

higher modulus and compressive strength (Arnaud & Gourlay, 2012).

Colinart et al (2012) investigated further the drying times of LHC in relation to the manufacturing

process used (mixed and tamped or sprayed). It was found that the spraying process produces a

lighter weight, faster drying concrete than moulding or tamping. Furthermore, it was noted that

when using the spraying process, adding a slaked binder to the shives rather than adding water to

the shives and binder decreases the initial water content and therefore the drying time of the

resultant LHC (Colinart, et al., 2012). These results are in agreement with Elfordy et al. (2008) and

Evrard (2006), as mentioned above in Preparation methods. In relation to the mixing and tamping

process it is noted that the more the initial mixture is manipulated and tamped (or vibrated), the

more the density and drying time is. (Colinart, et al., 2012)

Whole Building Modelling

The transient hygrothermal behaviour of a whole building envelope of LHC is modelled by Tran-Le et

al (2010) using SPARK, a numerical modelling environment program. This research found that

accounting for moisture transfer and moisture production sources in HAM models can account for a

difference of 19.5% in energy consumption, and the use of LHC over cellular concrete can yield

savings of 45% in energy consumption.

Full Scale Experimental Test Building

Shea et al. (2012) constructed a full scale test building, known as the Hempod, and reported on the

thermal performance of the finished building. The calculated U-values were compared with a co-

heating test and found to give results within 3% of each other. It was noted that, as observed by

previous studies (Evrard & De Herde, 2010) (Yates, 2002), the steady state accounted for with U-

values and simulated by a co-heating test are rarely ever reached in reality, and a 300mm thick test

panel was constructed to investigate the energy transferred over 24 hours as a % of the steady state

prediction (Q24), and the time to steady state (ts-s). The study finds that the U-value calculations

agree broadly with the co-heating test, but the test panel takes in the order of 10 days to reach a

steady state. WUFI simulations of the same panel thickness predict a steady state in 3 days, without

taking into account the effect of relative humidity. This highlights the effect relative humidity has in

dynamic conditions as in (Evrard, et al., 2005), (Evrard & De Herde, 2010), (Maalouf, et al., 2011).

Sequestration & Life Cycle Analysis

The sequestration of carbon is key to the claims made for LHC in respect of its negative embodied

carbon (Lime Technology Ltd, 2011) status. Sequestration is the process of the locking up ofcarbon in the LHC, both during the growing of the hemp, through photosynthesis and during the

carbonation of the lime binder. This concept is described in some detail in Hemp Lime Construction:

A Guide to Building with Hemp Lime Composites (Bevan & Wooley, 2008). The figure often quoted

for LHC of 110 kg/m2(Lime Technology Ltd, 2011) is explored in a little more detail, but there has

been little academic work done in this area to date. It is unclear whether this figure applies to the

pre-fabricated systems available which presumably use climactic controls and forced ventilation in

their manufacture, or just to the cast in-situ mode of delivery. Additionally, anecdotal evidence from

sites using cast in-situ LHC have shown that in UK conditions forced ventilation/ heating is required

in the drying stage, and no account seems to be made for this in any of the calculations. A French

Life cycle analysis (LCA) is cited (Bevan & Wooley, 2008), but the excerpt is from an unofficial

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translation by The Guardian. It is noted also that the sequestration is only considered to be

temporary, as the carbon stored in the hemp is likely to be released into the atmosphere again at

the end of its life, either through combustion or bio-degradation (Bevan & Wooley, 2008). Care must

be taken to ensure that the carbon is released back into the atmosphere as CO2rather than CH4, as

would be the case if allowed to rot in landfill.

A theoretical life cycle analysis of the CO2 impact of a LHC building against a traditional base case

(Rai, et al., 2011) takes into account operating CO2and embodied CO2. Ecotect software is used to

model a 25 year life cycle for a distribution warehouse. It is shown that against the steel clad base

case, LHC cladding can produce a CO2 saving of 279.4%. This figure takes into account the

sequestration benefit of the Hemp used in LHC manufacture. If sequestration is not taken into

account, the LHC is considerably worse than the steel cladding (138.5% increase in CO2), mainly due

to the presence of lime and cement in the binder. This model also compares LHC to timber cladding,

and it is noted that the sequestration benefit of LHC is higher than that of timber as considerably

more hemp by weight is used in the LHC cladding. Again, this highlights the importance of the

sequestration of CO2in attaining the carbon negative status claimed by the manufacturers, and the

need for viable recycling options if this claim is to be upheld beyond the life of the building in

question.

Related Studies

The delay in the drying of LHC is seen as one of the main drawbacks of its use, particularly in the UK.

Work has been done (Khazma, et al., 2012) on similar lignocellulosic composite materials (flax shive/

cement based) to improve the reliability of setting and hydration. The report finds that a pectin/

polyethylenimin mixture coating on the shives before addition of the binder reduces competition

between the porous shives and the binder for water, resulting in faster setting times, improved

hydration, improved mechanical properties and significant reduction in dimensional variation

(Khazma, et al., 2012). Similar coating methods could be investigated to improve the drying times

and properties of LHC, although minimizing the hydrophilic properties of the hemp shives may have

a detrimental effect on the moisture transfer and buffering properties mentioned earlier.

Another area of possible improvement which has been investigated is the substitution of a binder

with less CO2impact than the current lime based one. Clay binder has been investigated, (Busbridge

& Rhydwen, 2010) and the thermal properties established are similar to those quoted by Lhoist for

LHC (Lhoist UK Ltd, 2012). However, clay is found to be considerably more hygroscopic than lime,

and have poorer vapour permeability, and these parameters could again affect the moisture

buffering and thermal inertia of the finished product. It was also observed that mould grew on the

hemp-clay blocks during drying.

Other vegetable aggregates have also been studied (Nozahic, et al., 2012), finding that sunflower

stems share many of the same physical properties as hemp shives, however this research was

carried out on much lower aggregate contents than commonly used in LHC (binder/ aggregate ratio

by mass of 18 as opposed to around 1.65 in conventional LHC), and as such results of mechanical

properties are not comparable.

Interim Conclusions

It is clear from existing academic work that LHC provides good thermal insulation, and that excellent

thermal inertia values can be demonstrated. It has been shown in several studies that steady state

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models such as those behind U-values are not sufficient to give a clear picture of energy usage in

buildings constructed of LHC. This is due to the thermal inertia exhibited by LHC the time taken to

reach a steady state (ts-s) being a key parameter. Key to this is the moisture buffering capability of

LHC, and without taking relative humidity into account simulations showed much shorter ts-s. This

thermal inertia results in a damping of the effects of external conditions, meaning that the peak

internal temperature is both considerably lower than peak external temperature, and is also reached

in practice in the middle of the night, when external temperatures have usually dropped again. This

results in increased thermal comfort for the occupants, and a lower demand on heating and

therefore energy usage. This means that U-values, which are calculated on the assumption that a

steady state is reached instantaneously, do not give an accurate picture of the actual energy usage

of the building in use and the true picture is, in fact, much more favourable.

The claim of negative embodied carbon relies heavily on the sequestration argument, and while

carbon is undoubtedly locked up in the hemp as it is grown, it is yet to be shown that this carbon can

be managed at the end of the life of a building in a way that does not release it back into the

atmosphere. Incineration would release the carbon as CO2, which would mean that the

sequestration had only been temporary. Significantly, if put to landfill, the carbon could be released

as CH4, which is far worse as a greenhouse gas than CO 2. Currently the design life of most housing is

around 60 years, and commercial buildings considerably less. In the timescales involved in global

warming and climate change these periods are blinks of an eye. Viable recycling options must be

developed if LHC is to be considered truly carbon negative.

The claim that LHC is easy to use is also not as straight forward as it may seem. The process itself is

undoubtedly simple, but several trades are likely to be involved in the use of in-situ production, and

there is little margin for error in the placing and tamping operations. Obtaining a homogenous

product can be difficult, and over tamping will lead to poorer insulation properties and increased

drying times. In the UK drying times are of primary importance on most sites, and high relative

humidity and low temperature have been shown to seriously inhibit the drying and curing processes.

Little academic research has been done into drying times in realistic UK climatic conditions, and this

is a possible area for further research. As such, careful consideration should be given to when the

placing of LHC is scheduled in a build, and the length of time drying is expected to take. Also, training

of staff/ subcontractors will be of paramount importance in obtaining a consistent end product.

LHC is made from renewable/ abundant materials, but reducing the amount of transport involved

in the production process could reduce embodied energy even further. It is also unclear how much

CO2 is released from the earth during the farming of hemp, and this is something that may need to

be factored in to the embodied CO2 of LHC.

Overall, it is clear from academic work that LHC can provide comfortable buildings with low carbon

footprints, through low energy usage and low carbon materials. The construction of such buildings in

the UK must be carefully considered to allow for prevailing climatic conditions, and this is an area

where more research might be useful. Drying times in prejudicial conditions have not been widely

studied, and there are some possibilities to assist this. Also more work could be done to find viable

recycling options for the product at the end of its life. The claims made by the manufacturers are

broadly fair, but are open to some qualification and debate.

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3.

Publications and Secondary Sources on Hempcrete

Supplier Literature

In addition to the academic work reviewed above there are a number of texts and publications

available giving advice on the use of Hempcrete and related products. The UK supplier websites such

as Lime Technology Ltd and Lhoist UK Ltd provide installation instructions and various guides to

frequently asked questions etc. However, as one might expect in a suppliers literature, there is very

little information given on the potential problems and limitations associated with the product.

The Lime Technology Ltd Designers Information Pack (Lime Technology Ltd, 2011) provides design

related advice, including standard detail drawings and guides to U-values, air permeability and

general specifications. This publication also includes guidance notes from consultants AKS Ward on

domestic scale installations, highlighting the primary differences between standard timber framing

and that used in conjunction with Hempcrete.

Lime Technologys Installers Information Pack (Lime Technology Ltd, 2011) includes further practicaladvice on aspects of installation, such as mixing, storage, casting, drying and rendering, in addition

to relevant data sheets and safety advice. It is interesting to note that the drying times of 6-8 weeks

stated are based on an average temperature of 20-14C respectively, which as noted above, is

unlikely to be achieved under UK conditions.

Books & Other Literature

Mentioned briefly above, Hemp Lime Construction (Bevan & Wooley, 2008) provides

comprehensive guidance on building with LHC, and should be a must read for anyone involved in

projects using this material. The book is the result of a DEFRA funded study which was commissioned

by the National Non-Food Crops Centre, and as such, takes a generally positive bias, however theadvice given is thorough and practical. The publication cites many of the references mentioned

above, and addresses the scientific and environmental arguments as well as the practical aspects of

construction. Running to over 100 pages, much of the current experience of using the material is

discussed, and many tips and tricks are divulged to the reader.

BRE Information Paper IP14/11 (Sutton, et al., 2011) gives a summary of the key points to consider in

the use of LHC, including some of the limitations. It also contains example sections, and a useful

construction sequence for a standard wall section. The BRE guide quotes a drying time of 4-8 weeks

for the hemp-lime, but does highlight the fact that the material is more suited to the warmer

months.

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4.

Existing Case Studies

There are a number of short case studies available on the various supplier websites for the

components of LHC systems which give basic details of various projects that have used LHC (Lime

Technology Ltd, 2011). In addition to these details there are some more in-depth, independent case

studies of in-situ LHC builds.

Haverhill, Surrey

The most important case study to date is that of the hemp houses built at Haverhill in Surrey in 2002

(Yates, 2002). As noted above, this case study provided valuable data on the thermal and acoustic

properties. In addition to the empirical data mentioned above, the study also tells the story of the

build in detail, and gives a frank account of the practical problems experienced on-site. As such, this

is another recommended read for anyone who is to be involved in using LHC in its in-situ form.

The study compared the construction and subsequent performance of two hempcrete homes with

that of two (otherwise) similar traditional cavity wall homes on a new-build development, and was

carried out by the Building Research Establishment on behalf of the client, The Suffolk Housing

Society. The BRE studied the project to investigate:

Relative structural, thermal, acoustic, permeability and durability qualities;

Reduction in waste generated on site;

Environmental impact;

Construction costs.(Yates, 2002)

The main findings of these investigations were that the hemp homes stood up well against the

traditional homes in terms of structure and durability, thermal performance and waste, with the

structure and durability being described as at least equal to that of the traditional homes, andthermally heating fuel consumed by the hemp homes is no greater than that used in the

traditionally constructed houses (Yates, 2002).

The report notes that the predicted thermal performance, calculated from the U values and SAP,

was significantly lower than that of the traditional homes, but, as mentioned above, in practice the

hemp homes maintained a marginally higher internal temperature for the same fuel input. The

testing was carried out in the first three months of occupation, and Yates points out that the

occupants will have been un-familiar with the way the hemp homes perform thermally, and that

there is room for improvement by adjusting the behaviour of the tenants. A training day focussing

on overall thermostatic control and thermostatic radiator valves (Yates, 2002) is suggested, with

subsequent re-testing, but the results of this are not reported.

As noted above, the hemp homes did not perform as well as the traditional homes in the acoustic

tests, however they did still pass the building regulations that were current at the time.

In terms of permeability, the BRE undertook 96 hour spray tests of samples of the LHC, and it was

found that water penetration was limited to an average of 50-70mm, making penetration of the

external envelope (200mm) unlikely. Thus both forms of construction were found to give complete

protection from water penetration. However, the hemp homes generated less condensation (Yates,

2002).

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The amount of waste produced was also found to be similar, however it was noted that the volume

of excavation from the hemp homes was significantly lower (Yates, 2002).

The construction costs of the hemp houses was estimated to be significantly more than the

traditional homes, at 526/m2 against 478/m

2, however, it was noted that significant relative

savings were made on the second hemp home as opposed to the first, thought to be due to the

contractor becoming more familiar with the processes involved. The report highlights a number of

causes of inefficiency thought to have hindered the progress of the build in general. Some of these

are site management issues particular to this project specifically, however, some relate to the LHC

material or processes required to install it. The need for a prefabricated shuttering system is

mentioned as a problem, and this is something which successor projects have remedied. The drying

times of the LHC are also mentioned as a major issue.

Haverhill Thermographic Testing

Following on from the above study, the BRE commissioned a thermographic inspection of the

Haverhill project in March 2003 (Yong, 2003). The aim of this survey was to produce thermal images

of the front and rear aspects of the two kinds of construction and investigate any significant areas of

heat loss through the external envelopes.

The internal temperature of both buildings was measured to be the same. The data obtained shows

that the external temperature of the traditional home was 4-6C higher than that of the hemp

home. Given that the internal temperature was similar the conclusion drawn is that the external

envelope of the hemp homes retains more heat than that of the traditional cavity wall buildings.

Figure 1: Thermographic images from Haverhill (Yong, 2003)

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County Down Eco House

This case study (Bevan & Wooley, 2009), carried out by Bevan & Wooley, authors of the

aforementioned Hemp Lime Construction (Bevan & Wooley, 2008) detailed the construction of an

80m2 single storey eco house in County Down, Northern Ireland. It was a qualitative look at the

experience of building with LHC, and described the pros and cons of the building process. It was

noted again that the shuttering process slowed down the progress of the work, as did the drying

time associated with the LHC, which was placed in October. The paper also included a brief resume

of the thermal properties of LHC.

The Wales Institute for Sustainable Education

Harris et al. (2009) described the construction of the WISE building. The building utilizes materials

with as low an embodied energy as possible (Harris, et al., 2009). LHC was used in the non-

loadbearing walls and floor slab, in conjunction with other sustainable building methods such as

rammed earth walling and limecrete footings. It noted that the cast and tamped form of application

was more successful as it led lo less waste than spraying.

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5.

Carleton School, Pontefract

In 2011 works commenced on the Carleton School site in Pontefract, West Yorkshire. The main

contractor was BAM Construct, operating on a design and build basis, with an architect being

novated from the client team. The build involved a substantial extension to the existing school

buildings, some of which had been fire damaged, and had to be removed.

In-situ Tradical Hemcrete was specified by the clients architects for much of the external envelope

of the new buildings. However, as the contract was design and build, design liability fell with the

main contractor. Construction commenced in March 2011, with the placement of Hemcrete

beginning in August 2011. The in-situ Hemcrete was spray-applied into a frame with permanent

shuttering on the inner face, and a temporary proprietary shuttering system on the outer face. The

sprayed mixture was then tamped into place.

In May 2012 Leeds Metropolitan University was given access to the site for the purposes of

investigating the use of LHC in the project. Site Manager, Matthew Garnett gave his perspectives of

the build to date and provided a tour of the project site.

At the time of the interview with Mr Garnett the drying of the LHC was not complete, meaning that

plastering, rendering and associated works were being delayed. This was clearly the source of

considerable concern for the site team, given that they had been advised by the supplier that the

drying time would be in the order of 4-6 weeks, and it was now in excess of 9 months since

placement began. External sheeting had been erected on scaffolding to protect the drying LHC from

the elements, and the scaffold was causing some inconvenience and considerable extra cost. Forced

ventilation and heating had also been installed in the building in an attempt to expedite the drying

process.

Figure 2: Covered Hemcrete at Carleton School

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Yates notes that: Many builders apply too much force when tamping because they think that the

material needs to be firm. In fact, it should remain relatively open and not too dense, particularly in

the core, as this slows the drying process.(Yates, 2002, p. 34).

Figure 4: Differences in mix densities.

Bevan and Wooley suggest the use of spray application to overcome the problem of over tamping:

Spraying the material ensures a consistency in application as placing by hand and over-tamping can

lead to denser mixes.(Bevan & Wooley, 2008, p. 41). In this case both spraying and tamping were

used and it is possible that the tamping could have negated the benefit of the spraying process,

causing a denser product and contributing to the delays in drying time.

Knock-on Effects of Extended Drying Times

The delays caused by the drying of the Hemcrete had a number of effects on the scheduling of

follow on trades. As alluded to above, the render finish to the exterior of the building could not be

applied until the %RH of the Hemcrete had fallen below 15% for a depth of 40-50mm (Lime

Technology Ltd, 2011). The requirement to keep the exposed Hemcrete reasonably dry and

maintain air flow meant that scaffold had to be kept on hire for longer than expected. Matt Garnet

said: Externallywe have a lot of scaffold up, and the majority of that scaffold is simply there to

protect the Hemcrete. So, we have paid extended hire costs from September until now [the

following May].

The extended drying time was also delaying the application of lime plaster internally, meaning that

M&E installations, suspended ceilings and flooring operations were being delayed. The contractor

had pressed ahead with the plastering of one room as a test to see what would happen if the plaster

was applied before the recommended %RH was achieved. Site Manager Matt Garnett said of the

lime plaster: It took a few days of work to this one room for it to cure. We had to keep coming back

in to try and polish it off and the finish in there isnt very good because of that. It is not clear how

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much the %RH of the Hemcrete contributed to the difficulty in curing the plaster and thus obtaining

a good finish. It is possible that there may be a workmanship issue compounding the problem, as

lime plaster is not often used in modern buildings. It should also be noted that although Lime

Technology advises that a two coat wet-on-wet application of British Gypsum Durafinish can be

used internally (Lime Technology Ltd, 2011), British Gypsum were not prepared to warrant its use,

and were unable to provide an alternative from their range. Matt Garnett explained: We had a

meeting with British Gypsum to see if we could get one of their products that we could actually

plaster on here, but their products are not breathable, and they would not warrant anything going on

there.

Internal Solutions

In order to expedite the follow-on trades the contractor had installed a plastered bulkhead margin

where suspended ceilings abutted the Hemcrete walls to allow the installation of ceiling grids, and

where dado trunking and other services were to be fixed to the Hemcrete a plywood pattress had

been installed which could subsequently be plastered up to. In this way they had been able to

remove some of the criticality from the curing of the Hemcrete. Matt Garnett said of the

programme: Internally, because we have put this plasterboard margin in, and timbers for the dado

trunking, it has taken that part of it off the critical path. The only thing that it is preventing us from

doing is I dont want to start floor finishes in those areas and then have to go back in and lime-

plaster a room with a vinyl floor in place.

The contractors had also installed large fans inside the building to force air-flow over the drying

product, and Matt Garnett said this seemed to have helped significantly. In addition, the under floor

heating had recently been commissioned, and was now being left on in an effort to assist drying

which was also said to have produced a rapid improvement in drying. There was a suspicion though,

that the use of these measures would not have been accounted for in the carbon footprint

calculations of the designers, and the Lime Technology data for Hemcrete.

One advantage of the delay in rendering was that it allowed the contractor to take advantage of

more up to date advice regarding the placement of reinforcing mesh in the base coat. Mr Garnett

explained: Recently Lime Technologies have also said we need to introduce a layer of mesh into the

first coat of render as they have had experience of the product crackingso if we had rendered [the

building]last year, potentially we would have been looking at a problem with cracking.

External Render

In November of 2012 the site was re-visited as the project had reached practical completion, and the

school had opened for the new term. The external render had been applied to all but the north face

of the school, which was still below the threshold for rendering. This was thought to be due to the

northern aspect receiving less direct sunlight than the other faces. This face had been base-coated,

but was awaiting its finishing coat.

There was some concern over the render finish on the faces that had been top-coated, with the

finish being slightly irregular in appearance. It was thought that the differences in surface colour and

texture were due to the finishing being done at different stages of dryness of the top-coat.

The contractor was also concerned about the hardness of the render, and there was evidence ofsome streaking and discolouration to the window frames, glazing and the brick plinth on which the

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Hemcrete walls were sat. This streaking was thought to be due to the lime render dissolving in

rainwater and then running down the face and being deposited as lime scale. The window frames

were powder coated aluminium, in a dark blue colour, and it was noted that the depositions could

cause permanent damage to the frames and the glazing if it was not cleaned off regularly. The brick

plinth below the Hemcrete was in charcoal grey in colour, and the depositions were quite unsightly

and had to be removed with proprietary brick cleaner on a regular basis. The site team were

optimistic that this deposition would decrease as the render cured over a period of months. At the

time of writing the site team were still periodically cleaning the windows and frames with a

proprietary lime scale remover.

Ventilation Issues

On the second visit to the site the team were experiencing some issues with smells emanating from

some of the rooms with Hemcrete external walls. On investigation by a representative from Lime

Technology the smells were attributed to dampness caused by a lack of ventilation in the affected

rooms. It was suggested that the buildings ventilation control strategy be amended in the short

term to ensure that ventilation continued to occur at times when the school was not in use, in order

to assist the drying of residual moisture in the Hemcrete. In addition, there were some rooms

which had no ventilation at all. It was suggested that a small fan unit with heat recovery was

installed in these rooms to introduce some air flow. Although the school had opened the rooms

affected by dampness were not in use, pending the resolution of the problem. To date the

ventilation issue has been raised periodically by the school, as recently as week ending Friday 8th

March 2013, and the smells continue to linger in a few rooms. The site team is optimistic that warm

weather in the summer months will put an end to this problem.

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6.

Conclusions

It is clear from the literature review above that the claims made for LHC can be substantiated with

academic work. However, there has been relatively little research carried out into the drying times

of LHC, and the effects of environmental factors on them, and research into realistic UK weather

conditions is a possible area for further study.

It seems that the biggest problem faced by sites using in-situ LHC on a commercial scale is the

balancing of programme constraints with the drying times of the product. This was mentioned by

Yates in reference to the Haverhill case study (Yates, 2002) as a major cause of inefficiency on the

build, the County Down Eco House (Bevan & Wooley, 2009) also experienced delays due to October

placement and it was also a major problem on the Carleton School build. It was hoped that more

empirical data could be collected from the Carleton School project in terms of %RH data for the

Hemcrete, in order to facilitate analysis compared to the known weather conditions. However,

although this data was apparently collected, the site team were unable to locate it at a later date

and the authors late involvement in the project made its collection impossible. The common thread

running through these case studies is the delay caused by placement of the LHC late in the season,

resulting in lengthy delays in drying the product. This suggests that the product is not suitable for UK

use in its in-situ form in the colder months on projects which are time constrained.

The Carleton School project suffered from a lack of good advice and research into the product

before the project started. Even without considering the academic work reviewed above, there was

ample information available in the form of the Haverhill case study (Yates, 2002) and the guide

Hemp Lime Construction (Bevan & Wooley, 2008). Reviewing these publications could have

prevented most of the issues that arose on the project and in view of the contractors liability under

the design and build contract it is surprising that they relied so heavily on the advice of the supplier.A little independent research could have easily avoided:

Late placement resulting in excessive drying times;

Dense finished product caused by over-tamping the mixture;

Excessive hire fees for external scaffolding/ internal ventilation;

Delays to rendering/ external works;

Extra expenditure on work-arounds such as dry-lined bulkheads and plywood mounts for

dado trunking.

That said, it seems that the supplier failed to effectively manage the expectations of the contractor,

and could have warned them more explicitly about the problems of placing the product so late in the

year. The supplier would have been well placed to advise the use of pre-cast cassettes or panels if

the build had to be scheduled during the winter months. This application could potentially have

retained all the environmental benefits of LHC without the uncertainty of the drying process. In fact,

Lime Technology now advise that the in-situ application of Hemcrete is suitable:

For use where the Tradical Hemcrete will be installed between the

beginning of March and the end of July.

For use where the programme is relaxed (e.g. self-build projects).

For use where a programme over-run does not have severe penalties.(Lime Technology Ltd, 2012)

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And conversely, for off-site application:

For use where the construction takes place all year round.

For use where programme certainty is critical.

For use where time on-site needs to be minimised. (Lime Technology Ltd,

2012)

Clearly the construction of a school by a main contractor would fall in the latter group, and as such,

the pre-cured method of construction may have been more suitable. The above advice has been

added to the supplier website since the Carleton project commenced, so it is possible that the

supplier was not aware of the impact of poor weather conditions.

The ventilation issue and the associated dampness was, it would seem, a combination of a design

issue (in the case of the rooms with no ventilation at all), and a systems control issue. In this case the

problem was easily overcome, but did involve additional expenditure in the form of extra fan units

and additional ventilation. This could probably have been avoided by better research at the designstage and better advice from the supplier.

The issue of the deposition of lime scale on the brick plinth and fenestration is one that has not been

raised by other case studies, and it is possible that the carbonation of the lime in the render will

harden it significantly over a number of months. There is currently only one approved system of

render for Hemcrete, but further research could investigate the possibility of using an existing

breathable non-lime based system, such as the organic resin/ silicone based K-Rend (Kilwaughter

Chemical Company Ltd, 2012). This could potentially eliminate the problem of extended curing times

and associated deposition, while still maintaining a breathable, self-coloured coating.

Future projects wishing to use LHC in its in-situ form should consider carefully whether their

programme will allow its placement in the warmer months of the year, and whether delays are likely

to push the drying period into the colder months. If delays could affect the placement then an off-

site system should be seriously considered in place of the in-situ method. Any prospective developer

should ensure that their design, planning and site teams are well educated in the use and constraints

of the product well in advance of the design and planning stages. Earlier involvement of the supplier

with the project design team could help to insure that the appropriate format of the product is used,

and ensure that the clients requirements are fully understood at the outset.

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