insulated rammed earth for a cold...

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14TH CANAD IAN CONFERENCE ON BU I LD ING S C I ENCE AND T E CHNOLOGY

INSULATED RAMMED EARTH FOR A COLD CLIMATE

T. Wong and S. Cook

ABSTRACT

Rammed earth has worked for centuries in arid climates around the globe, moderating the intense heat of

the desert and keeping interiors cooler. Although rammed earth construction has been found in cooler

climates, including the UK and northern Europe, the low thermal resistance of the material has

historically mitigated against its use in very cold climates. The modern solution is to insulate the rammed

earth. The first insulated rammed earth house in Ontario was constructed in Castleton Ontario in 2011.

Insulated rammed earth maximizes both thermal mass and resistance. The Passive House concept, designed

originally for the Central European climate, utilizes a much higher design temperature and negates the

effects of thermal mass for winter but concedes its benefits in summer cooling. The moderating

effect of thermal mass will have greater influence in climates where temperatures fluctuate over a greater

range both daily and seasonally. The thermal mass effect increases energy efficiency year round, increases

comfort levels and reduces the amount of insulation required in comparison to a similar light weight

structure.

In a composite structure such as insulated rammed earth the thermal mass works in conjunction with the

insulation and improves the results dramatically. Previous studies have demonstrated the improved

performance of a high thermal mass wall assembly with the insulation in the centre of the wall. The

Castleton house utilizes inner and outer wythes of rammed earth with an interstitial layer of closed cell

insulation. The thermal mass of the exterior wythe buffers the temperature gradient across it, while the

interstitial insulation slows the transmission of the interior heat to the exterior. The interior wythe of

rammed earth also buffers the interior extremes, further reducing the temperature gradient across the

insulation and therefore the rate of heat flow from the interior toward the exterior. Combined, these

factors improve the performance of the wall in a non-linear fashion.

Historically, thermal mass is utilized as a heat sink for excess heat during the day, to be released in the

evening as the temperature drops. At the Castleton house, an interior two storey thick solid masonry faces

directly south. Air flow is encouraged around and over the wall. This is a modern interpretation of the

Trombe wall, utilizing the insulated exterior walls to protect it from the reverse thermosyphoning

traditionally associated with these walls. The large thermal mass allows for more flexibility when solar site

conditions are not optimal.

INTRODUCTION

Rammed earth has been in use from the earliest historical times as both a large scale construction method

and as an ongoing vernacular building system. It is incredibly durable, with hundreds of examples of

ancient buildings still in use over a wide range of climatic and seismic zones. Recent interest in rammed

earth has focussed on its use as a sustainable building system utilizing local, non-toxic materials with low

embodied energy. In 2012 builder Sylvia Cook of Aerecura Rammed Earth Builders and architect Terrell

Wong of Stone’s Throw Design Inc. joined forces to construct the first insulated rammed earth house in

Ontario. This 350m2 (3770ft2) house, situated in Castleton, Ontario, has withstood three full climate cycles

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with enviably low heating and cooling requirements. Based on observation, we believe that rammed earth

walls show thermal benefits in cold climates greater than earlier static models have indicated. Research is

required to test this theory using in situ temperature and moisture measurements as well as hot box testing

of insulated rammed earth wall sections with a goal of developing a usable dynamic model for this

material.

FIGURE 1: KARLSBORG FORTRESS IN SWEDEN, BUILT 1842

HISTORIC RAMMED EARTH

Rammed earth has been used worldwide for millennia, with extant examples such as the 6000 year old

Great Wall of China, extending for thousands of kilometres. Rammed earth has been used for large scale

construction, famously the Potala Palace in Lhasa and the Alhambra in Granada, as well as for vernacular

building such as China’s Hakku roundhouses, which have been continuously inhabited for over 500 years.

Although most commonly associated with hot and dry climates, rammed earth has been built in cold

climates. The Karlsborg fortress in Sweden is a significant example and in Ontario we have St. Thomas

church in Shanty Bay.

The simplicity of the technique, with damp local soil tamped into formwork, has permitted widespread use

wherever suitable soils are found. It was even promoted by the United States government during the

Depression:

“[Rammed earth]... from all reports is a method of construction with many advantages... and in somerespects is superior to frame and masonry.... No permanent building material is cheaper... and nowafter war has strained the economic resources of the world low-cost construction is demanded andthe merits of rammed earth are remembered.” (U.S. Department of Agriculture, 1937)

The adoption of stabilised rammed earth (SRE), incorporating Portland cement and other pozzolans, has

allowed for the use of a much wider range of soil conditions, thereby meeting structural requirements in

all climates. The addition of structural steel, especially in seismic zones and insulation in cold climates has

had rammed earth meet modern building codes. In recent decades rammed earth has been used extensively

in Australia and is becoming more common on the American west coast and in British Columbia.


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FIGURE 2: POTALA PALACE, HOME TO THE DALAI LAMA SINCE THE 7TH CENTURY

CONSTRUCTION METHOD

The key to the strength of rammed earth is a smooth gradation of particle sizes, where the largest particles

19mm (3/4in) are sufficient to impart strength but small enough to fit neatly into forms. Voids between

these particles are filled with successively smaller particles with the finest sizes yielding microscopic

spaces capable of taking up moisture, but impermeable to liquid water. Minimal addition of water, just

sufficient to permit thorough mixing, limits both the drying period and the potential for cracking.

Mechanical forces compact the mix to maximum density. The mix for the Castleton project, based on soils

from a local quarry, yielded16.8MPa (2437psi) based on cylinder crush tests.

Our formwork consists of reusable high-density overlay form-ply braced externally by LVL strongbacks

and walers. Insulation is placed in the centre of the form and the earth mix is placed and tamped in lifts.

Mixing and delivery are carried out with a combination of mechanical and hand shoveling. Ramming is

performed with pneumatic sand tampers and finished with hand tampers. The result is a relatively smooth

sandstone finish with noticeable surface variation and visible striations. The resultant wall is thus

interstitially insulated and airtight with thermal mass accessible both internally and externally.

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Our insulated walls consist of inner and outer wythes of 15cm (6in) of rammed earth sandwiching 15cm

(6in) of polyiso insulation. Rammed earth has generally low thermal resistance 0.8 to 1.0W/mK Fix,

2009) contributing only about U2.8W/m2K (R2), with the polyiso contributing the remaining thermal

resistance of the wall assembly. Two 7.6cm (3in) thicknesses of polyiso rated at U0.31W/m2K (R18.5),

each at a mean temperature of 24C (75°F) with a temperature differential of 28C (82°F), yields a total wall

value of U0.15W/m2K (R38). This value may be somewhat conservative in the context of a rammed earth

wall assembly as it takes aging into account: “The thermal resistance of polyisocyanurate decreases assome of the gasses in the pores from the manufacturing process diffuse out and are replaced with air.”(Building Science Corporation, 2013) This process is significantly minimised when encased in rammed

earth.

Structural integrity is ensured with a 0.6m (2ft) rebar schedule, which minimises cracking risk in each

wythe and connects the two wythes. Thermal bridging through the inter-wythe connectors, however, is a

possibility. Other materials have been proposed to alleviate this situation, including stainless steel or

fibreglass rebar, geogrid fabric, metal mesh, bamboo and uni-directional fibreglass.

FIGURE 3: TAMPING OF RAMMED

EARTH WALL (COOK)

FIGURE 4: TAMPING AROUND

CORNER INSULATION (COOK)


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FIGURE 5: FORMWORK AND FINISHED WALL SECTION – (WONG)

DESIGN STRATEGIES

For the Castleton house, passive design strategies such as orientation, materials choices, overhangs,

insulation, windows, and air tightness were emphasized over mechanical technology. The only

mechanicals are a 94% efficient, fully ducted ERV and a 60m2 (646ft2) main level radiant tube floor under

the centre section of the house. This is siphoned off the hot water tank, providing any additional space

heating requirements beyond passive solar.

Southern Orientation –Easterly Views

The easterly view and slope of the site dictated a less than optimal solar design. The north and south living

spaces have been shifted along the east-west axis, allowing south-facing windows to be installed in all three

main sections of the building. To maximize winter light on the interior, the center section of the building

extends above the roof and has many south facing windows which illuminate a 10m (33ft) wide by 7.3m

(24ft) high solid 0.45m (18in) thick internal rammed earth wall thermally broken from the exterior.

The north and west façades have minimal glazing; however, the number of windows on the east façade

directly result in the PHPP model showing an overheating issue. The model predicted that air

conditioning would be required 49% of the cooling season. However, since completion, the interior

temperature has never exceeded thermal comfort levels in the cooling season; no air conditioning has been

used.


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Summer Overhangs and Awnings

The overhangs on the south façade have been designed based on minimal summer solar gain and maximum

winter gain. Typical overhangs of 0.9m (3ft) extend over the rammed earth wall to deflect hot summer sun

and mass water from the rammed earth wall. The east deck completely shades the basement windows. A

retractable awning was installed over the eastern windows, providing even better comfort levels on very

hot days.

Ventilation

In the summer, night time ventilation is the key strategy to releasing heat from within the interior rammed

earth. The central feature wall stops 30cm (12in) below the roof, allowing natural ventilation to flow

unimpeded from the cooler north section of the house, exiting at the higher mezzanine windows. All of

the occupied floor space is within 5m (16ft) of an operable window.

FIGURE 6: SOUTH FACING IMAGE OF

HOUSE (COOK)

FIGURE 7: INTERIOR RAMMED EARTH

WALL (WONG)

THERMAL COMFORT

The main criteria for thermal comfort are consistent temperature and humidity levels, within reasonable

limits, based on the occupants’ ability to maintain adaptive behaviours such as wearing an appropriate

degree of clothing. Rammed earth enhances thermal comfort through air-tightness, hygroscopic properties

and radiant environment: “People who have lived in earth houses have stated a satisfactory standardrepeatedly, more comfortable than temperature measurements might lead us to believe.” (McHenry, 1989)


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Air Movement

In winter, the thermal comfort of a building is significantly enhanced by reducing air movement that

would arise from draughts and convection currents created by inconsistent temperatures within the

building envelope. As a monolithic material rammed earth is innately air tight and, with a continuous

insulation layer that wraps around corners and projects into openings, it provides a simple solution to

many common heat loss problems. The air tightness of the Castleton house 0.94AC/h @ 50Pa eliminates

draughts, and, in combination with highly insulated triple-glazed windows U0.829 (R6.85), air

temperature is kept consistent throughout the building, thus keeping air speeds extremely low.

Humidity

Higher relative humidity levels in winter also lead to increased thermal comfort. Based on physical

observation of the Castleton project over 2 years, it has been noted that indoor humidity levels have

remained in a tight range around 60% in winter, fluctuating no more than 2% when outside temperatures

drop below -15C (5°F) or with extended use of the ERV ventilation system. Normal occupant use,

including baths and showers, has no effect on this. In summer, with open windows providing natural

ventilation most of the time, levels have fluctuated by up to 10%, but have remained consistently lower

than ambient relative humidity. This phenomenon is attributed to the hygroscopic quality of rammed earth

as no other explanation, mechanical or otherwise, exists.

Radiant Environment

“Radiant exchange with mass surfaces is singularly the most efficient way of maintaining comfortcompared with any other technique as the body is more than twice as sensitive to radiant losses and gainsthan all other pathways combined (conduction, convection, respiration, evaporation) and more than fourtimes as sensitive than any other single pathway.”(Baggs, 2014) Rammed earth provides a stable mass

held at a consistent temperature through the winter as solar and interior energy gains balance the minimal

heat losses. The geometry of the house, with rammed earth walls in a direct sight line at all locations,

reduces asymmetrical radiant temperatures.

THERMAL MASS

The obvious benefit of rammed earth is the large amount of accessible thermal mass. This acts to store solar

gains and buffer temperature changes through the “thermal flywheel” effect. While these benefits have

been debated by Passive House adherents and others, it is our contention that the thermal mass of rammed

earth provides year-round thermal comfort, eliminates the need for summer air conditioning and reduces

heating demand in a cold climate.

Solar Gains

In winter, passive solar storage occurs as sunlight enters via the glazing and is absorbed directly and

indirectly by the rammed earth. The thermally decoupled interior wythe of the exterior rammed earth

walls and a large expanse of fully internal rammed earth contribute about 10MJ/K of thermal mass based

on:

1. total volume of interior rammed earth at 62m3 (2190ft3)

2. density of 1960kg.m3 (122lb/ft3) from sample tests of the mix used

3. heat capacity of 0.837kJ/kg K (199.9 BTU/lb·°F) as quoted in the literature.


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An approximate gain of 16MJ (15000Btu) could be expected on a typical sunny January day in southern

Ontario based on 12m2 (130ft2) of south glazing, illuminated for about 4 hours with solar angle of 25° and

an average of 150W/m2 (47Btu/hr.ft2) insolation. Without available thermal mass this energy gain would

cause significant overheating. Air temperatures in the house vary by a maximum of 3C (3.6°F) under these

conditions. A relatively high thermal conductivity of 1.0W/mK (0.60BTU/(hr·ft°F) facilitates the

movement of heat into the rammed earth.

Thermal Flywheel

The dynamic effect whereby heat gained by the walls during the day is released at night as ambient

temperatures drop contributes to the stability of indoor temperatures. The combination of interstitial

insulation and thermal mass in rammed earth increases the benefits of this thermal flywheel effect, almost

completely flattening out the daily thermal swings in temperature.

In summer, although shading strategies reduce direct solar gain to the interior, some increase in

temperature does occur, with consequent heat absorption by the rammed earth: “One of the mainadvantages of rammed earth walls is their ability to regulate indoor air temperature due to the ‘thermalmass’ effect. Where passive cooling is required, peak indoor temperatures can be reduced when the excessheat gains are absorbed by the building fabric.” (Hall & Allinson, 2008)

This effect has been studied in Australia where rammed earth is a common building material and the

climate largely requires cooling:

“Empirical (in situ) measurements of temperature and heat flux were taken on the walls of anexisting rammed earth office building in New South Wales, Australia during the summer... duringoccupied hours. During this time the earth walls performed well. External walls were found totransmit comparatively little heat to the office and the internal walls absorbed heat during thistime.” (Taylor & Luther, 2004)

The Thermal Mass Debate

Wolfgang Feist, the architect of the Passive House concept, questions the value of thermal mass in

Passive Houses: “After all, the influence of the thermal storage capability of the external walls isextremely small (less than 0.5.)” (Feist, 2000) The argument has been expanded by other Passive House

proponents:

Thermal mass needs fluctuating heat inside the building to buffer a surplus for later, to allow thethermal mass to ‘store’ the energy for later release when the temperature inside the buildingdrops... The capacity of already warmed-up thermal mass to further absorb heat is reduced....Buffering is furthermore not required in a Certified Passive House as the high insulation level andgreatly reduced ventilation heat loss will leave only a tiny gap between demand and supply ofwarmth on very cold days anyway. (Passive House Institute New Zealand, 2014)

However, local climate will have a significant influence on the efficacy of thermal mass. It is

understandable that Feist sees minimal impact in Germany where the daily temperature swings and

amount of solar radiation are lower and temperatures are less extreme, while a very cold day in New

Zealand is -2C (28°F). In southern Ontario solar energy can considerably reduce the amount of purchased

energy, with an accompanying need for the storage and release benefits of thermal mass. The large


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amounts of thermal mass available from rammed earth mean that extra heat storage is always available,

even with smaller diurnal temperature changes found in highly insulated buildings. Conventional passive

house wisdom that colder temperatures merely require more insulation reaches a practical limit, at which

point the benefit of thermal mass becomes measurable.

The climate influence is clearly seen in the comprehensive article, “Mapping Thermal Mass Benefit”

(Ghattas et al, 2013) which looks at the entire United States and determines that there is an energy benefit

ranging from 1% in cold climates such as Alaska to 29% in mild marine climates, with an overall average

of 3-4% for the heating season. The main benefits are seen in the cooling season where thermal mass

provides both energy savings and increased thermal comfort.

Even in cold climates with lower solar gains, studies have demonstrated thermal mass comfort and energy

benefits. In a study of Lärkträdet, a Swedish Passive house, the effect on interior comfort compared to an

equivalently insulated light wood frame building makes a strong case for thermal mass. The building

construction for the exterior walls consists of 150mm (6in) concrete, 250mm (9.8in) EPS, 70mm (2.8in)

concrete. This is similar to the insulated rammed earth section which also has its insulation in the middle.

There is an energy reduction of 7.4% over that of the equivalent framed model. The study also clearly

shows a reduction in predicted overheating, a common complaint about Passive House designs.

(Andersson et al., 2012 )

A common argument against the use of thermal mass for heating climates is the minimal benefit of

“excessive, expensive thermal mass.” (Canadian Passive House Institute, 2014) However, rammed earth

provides abundant thermal mass without any increase in construction cost and at a significant reduction in

greenhouse gas emissions. In combination with interstitial insulation rammed earth provides year-round

thermal comfort, eliminating the need for summer air conditioning and drastically reducing heating

demand.

FUTURE RESEARCH

The Castleton house gives anecdotal evidence of the efficacy of rammed earth in providing thermal

comfort, but it may be more important as an opportunity to investigate the dynamic thermal properties of

the material. The authors feel it would be valuable to study the performance of rammed earth in a

temperate climate using data from the house as well as rammed earth samples in a laboratory setting,

recording thermal variations in both time and space.

The “time lag” in temperature changes is commonly referred to in discussion of thermal mass. A change

in ambient temperature leads to a slow change in temperature of the mass, generally peaking well after the

ambient temperature has swung in the other direction: “Transmission losses through the walls are not

dependent merely on the steady-state value of the material plus insulation, but are a time-dependent value.”

(Hall & Allinson, 2008)

The winter performance of the 45cm (18in) wall assembly relies on three distinct temperature gradients:

1. The gradient across the insulation layer has the most direct effect on the performance of the

insulation. In winter this is at its maximum when both surfaces are at ambient temperature,

improving when either of these temperatures are moderated.

2. The gradient across the inner wythe occurs as the interior surface is in contact with the warm

interior as well as receiving the winter solar gain most directly. The surface in contact with the


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insulation will generally be at a lower temperature due to the time lag as heat slowly diffuses

through the rammed earth. This has the effect of decreasing the temperature gradient across the

insulation layer, thus improving insulation performance.

3. Exterior walls warm up during the day due to higher temperatures and solar gain, losing heat at

night but maintaining higher than ambient temperatures due to stored solar gain and high heat

capacity. Higher than ambient temperatures in the outer wythe also decrease the temperature

gradient across the insulation layer. The outer wythe may also create a micro “heat island”

effect. In the winter with maximum insolation, this is beneficial. In summer the effect is reduced

due to shading, with shaded portions of the wall able to absorb excess solar gains by conduction

without exceeding the total heat capacity of the wall.

Tracking temperature changes across the width of the walls in different directions and throughout the

seasons to confirm the above hypotheses could be easily done with an array of sensors through the walls.

The ability to stabilise temperatures is also enhanced by a higher moisture content in the rammed earth:

“It appears that as moisture-content increases the peak–peak amplitude is suppressed, due to higher heat

capacity and bulk density, whilst the time lag is largely unaffected.” (Hall & Allinson, 2008) Thus

tracking humidity changes in the walls relative to ambient will be another important research component.

To complement in situ measurements it will be beneficial to conduct standard hot box tests on basic

rammed earth wall sections, enabling the determination of the benefits of varying insulation types and

depths of insulation or rammed earth layers. The ultimate goal will be to document the thermal benefits of

rammed earth in a cold climate and develop modelling tools for use by designers to encourage the

adoption of this construction system by the larger building industry. Rammed earth has a long history and,

in the view of the authors, a most sustainable future.

REFERENCES

Andersson, L., Engström, A. and Lindström, G. 2012. Energy-Efficient Passive House using thermalmass to achieve high thermal comfort. Federation of European Heating, Ventilation and Air-conditioningAssociations. Available from: http://www.rehva.eu/fileadmin/hvac-dictio/01-2012/energy-efficient-passive-house-using-thermal-mass-to-achieve-high-thermal-comfort_rj1201.pdf [accessed 14 January2014]Baggs, D. 2014. Thermal Mass & its Role in Building Comfort and Energy Efficiency [online] Availablefrom: http://www.ecospecifier.com.au/knowledge-green/technical-guides/technical-guide-4-thermal-mass-its-role-in-building-comfort-and-energy-efficiency.aspx [accessed 20 January 2014]Building Science Corporation http://www.buildingscience.com/documents/information-sheets/info-502-temperature-dependent-r-valueCanadian Passive House Institute, 2014. Passive Solar Design in North America. Available from:[accessed 20 January 2014]Feist, W. 2000. Is thermal storage more important than thermal protection? Passivhaus Institut,DarmstadtGhattas, R., Ulm, F-J. and Ledwith, A. 2013. Mapping Thermal Mass Benefit. Available from:cshub.mit.edu http://cshub.mit.edu/sites/default/files/documents/ThermalMassBenefit_v10_13_0920.pdf[accessed 10 January 2014]


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Hall, M. and Allinson, D. 2008. Assessing the moisture-content-dependent parameters of stabilised earthmaterials using the cyclic-response admittance method. Institute of Sustainable Energy Technology,School of the Built Environment, University of Nottingham, UKMcHenry, P.G. 1989. Adobe and Rammed Earth Buildings: Design and Construction. University ofArizona Press, Tucson, Ariz. p154Passive House Institute New Zealand, 2014. What is the role of Thermal Mass in a Certified PassiveHouse? Available from: [accessed 20 January 2014]Taylor, P. and Luther, M.B. 2001, Evaluating rammed earth walls: a case study. Solar Energy, Volume76, Issues 1–3, January–March 2004, Pages 79–84 Solar World CongressU.S. Department of Agriculture, 1937. Rammed Earth Walls for Buildings. Farmer’s Bulletin No. 1500

Figures

1. Karlsborg Fortress in Sweden, built 1842 Karlsborg-Fortress/ West Sweden Tourist Board

2. Potala Palace, home to the Dalai Lama since the 7th century


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