elqf - update on nhbc ground gas guidance gavin allsopp...

ELQF - Update on NHBC ground gas

guidance

Gavin Allsopp

National House Building Council

Update on NHBC ground gas guidance

Contents

• Who are NHBC?

• NHBC perspective:

• Managing ground risk

• Traffic Light System

• BS8485:2015 and NHBC guidance.

• NHBC current expectations

• Some current issues

Who is NHBC?

The National House-Building Council

- established 1936

Non-profit distributing company

Unique “stakeholder” company

structure

10 year Warranty introduced in 1968

– 80% market share

Approved Inspector licence granted in

1985 (England and Wales) – Approx

65%

Who is NHBC?

Land Quality - Managing Ground Conditions

Warranty - Chapter 4.1 provides framework &

expectations for managing ground

conditions

Building Control – Where NHBC acting as the Building

Control Body, we also assess

contamination aspects under Part

C of the Building Regulations

Managing Ground Gas Risks – NHBC

Perspective

Technical guidance - Guidance on evaluation of

development proposals on sites where methane

& carbon dioxide are present, 2007)

Technical Updates – advice on good practice

updates or changes to technical advice is

communicated by regular technical bulletins;

(these are now linked to the NHBC Standards)


Perspective

What NHBC Require – ‘the fundamental requirements

remain unchanged’

‘all sites are properly assessed and investigated for potential contamination hazards“

“sites are properly remediated where necessary or appropriate, and

design precautions are taken’

“documentation & verification provided to confirm that site is suitable

for the proposed use(s)”


Perspective

Managing Ground Gas Risks – NHBC Perspective

Statutory Requirements – Construction to comply with relevant Building

Regulations applicable at time of commencement of works.

Last amendment to Building Regulations in England was 2013

Demonstrating Compliance - Approved Documents / British Standards used

to demonstrate compliance with Building Regulations. But alternative

methods can always be adopted but……..

Note of Caution… If alternate approaches adopted, developer needs to

demonstrate solution /design compliant (equivalent or better) with Building

Regulations applicable at the time.

Managing Ground Gas Risks – NHBC traffic

light system

Most people will be aware of:

Managing Ground Gas Risks – NHBC traffic

light system

But how many people have read (and understood)

appendix F – Derivations of Gas Screening Values

used with traffic Light.

• Critical if you are using traffic light system that

you understand;

• How they were derived? Assumptions?

• What sites/construction types they are applicable to?

• Restrictions?

Application of BS8485 guidance - where NHBC ‘Traffic Light’ Guidance may be applicable

NHBC published

classification System in

2007– referred to as ‘Traffic Lights’. It is a modelled

approach

• Only applicable to housing

development (not apartments)

• Assumes 150mm ventilated

void

• Assumes 1 air change (sub

slab ) per day


Not applicable to:

• High Rise.

• Rafts, cast insitu suspended slabs, ground

bearing slabs….

• If using void formers, geocomposite void formers,

granular blankets for sub slab ventilation.

• Also need to consider if applicable to terraces or

larger detached dwellings – Modelled on 64m2


• Defendable approach –

Clauses 6.2 & 6.2.2 BS8485.

• Characteristic Situation:

• An empirical approach

based on relationship

between gas data &

judgement/experience to

yield practicable and

reliable outcomes.


• ‘Traffic light’ is a:

• Modelled approach.

• Greater emphasis on

ventilation element.

• Models potential for gases

to accumulate beneath (or

within) specific building type

using specific assumptions.

• Derives the GSVs & ‘Traffic

Light Classifications’.


What is Gas Screening Value?

Sub floor void

considered for

CH4 & CO2

Small room considered

for CO2 calculations

Small room

ventilation rateC02

LeakSub floor void

ventilation rate

Low-Rise Housing

Development

• For Amber 1 not

plausible for gas

hazard to be present.

• Subject to

development = model

assumptions.

• GSV for Amber 1

based on target gas

not being > critical

concentrations.


What is Gas Screening Value?

Sub floor void

considered for

CH4 & CO2

Small room considered

for CO2 calculations

Small room

ventilation rateC02

LeakSub floor void

ventilation rate

Low-Rise Housing

Development

• For CH4 sub slab

equib concentration

must not exceed 1%.

• For Co2 the limiting

conc within small

internal space must

not exceed 0.25%.

• Based on the

modelled approach

these cannot be

exceeded for amber 1.


• Therefore for Amber 1 sites:

• NHBC would not normally insist on 3 party

validation of gas protection measures.

• May do if we have concerns over gas

monitoring data, installer or if proposed

construction does not meet assumptions of the

model.

For Amber 2 sites more reliance on the

membrane & verification is required.

Ground Gas Protection - NHBC Current

Expectations

Gas

Regime

Minimum Gas Protection Expectations Verification/Information Requirements

Green • N/A – But need to comply with BR211

Radon requirements

N/A

Amber 1 • Ventilation - subfloor venting to achieve

at least 1 air exchange per day

(minimum 150mm void height;

1500mm2/m air vent opening or

500mm2/ m2 floor area spaced at not

more than 2m centres on at least two

opposing sides)

• Membrane – must be suitable for use.

• Membrane installation/design - to

achieve complete integrity across entire

building footprint. Penetrations and joints

sealed.

• Construction drawings -showing

position of membrane, sealing details

and ventilation points to be provided

• Membrane specification – technical

data sheet(s) for gas membrane

(including gas permeability data) to be

provided.

• Installation – photographic evidence

of installed membrane may be

requested.


Expectations

Gas

Regime

Minimum Gas Protection Expectations Verification/Information Requirements

Amber 2 • Ventilation –subfloor venting to achieve at least 1 air exchange per day

(minimum 150mm void height; 1500mm2/m

air vent opening or 500mm2/ m2 floor area

spaced at not more than 2m centres on at

least two opposing sides)

• Membrane specification must be suitable for

purpose (criterion detailed in BS8485:2015

clause 7.2.4)

• Membrane installation/design to achieve

complete integrity across entire building

footprint. Penetrations and joints sealed.

• Installation – Installers must be experienced

and appropriately trained and/or qualified.

• Construction drawings showing position

of membrane, sealing details and

ventilation points to be provided.

• Membrane specification – Technical data

sheet(s) for gas membrane (including

gas permeability) to be provided

• Installation – Third party verification

report(s). Supporting evidence to be

included (i.e. photographic evidence

/certificates of conformity, observations

relating to sealing, location of ventilators

and standards of installation etc.)

• Integrity testing – May be requested. The

testing plan to be agreed in advance.

Red Standard residential housing is not normally acceptable without further ground gas risk assessment and/or possible remedial mitigation measures to reduce/remove the source of the

ground gases.


Expectations

More Information:

Technical Extra Article April 2016 – Issue 20.

http://www.nhbc.co.uk/Builders/ProductsandServices

/TechZone/NHBCStandards/TechnicalExtra/

http://www.nhbc.co.uk/Builders/ProductsandServices/TechZone/NHBCStandards/TechnicalExtra/


Some current issues

• Gas protection measures not being designed.

Recommendations made but who is the designer?

• Gas membranes – Struggling to meet Criteria in table 7

BS8485. Amendment to be produced.

• Membrane – should be dependent on the gas risk.

What permeability does your membrane need to protect

the end users?

• Pressure relief should be installed as a minimum…..


Some current issues

• Verification Plans (CIRIA 735) – not be received before

construction starts (or even completes on some

projects).

• Verification of installation – murky world where sister

companies are signing off installers work!

• Poor design, installation and verification still

happening….


Some current issues

Poor installation….


guidance

Summary

• Our gas protection requirements remains unchanged –

characterise the gas risk and design appropriately to

mitigate the risk.

• BS8485:2015 – NHBC traffic light system still

applicable/relevant.

• For Amber 1 our verification requirements differ to

BS8485/C735.

• Gas protection design/installation/verification

improving.


guidance

Thank you for listening!!!

Any questions?

[email protected]

01908 746645.
mailto:[email protected]