Upload
others
View
5
Download
0
Embed Size (px)
Citation preview
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)
Managing Ground Gas Risks – NHBC
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)”
Managing Ground Gas Risks – NHBC
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
Application of BS8485 guidance - where NHBC ‘Traffic Light’ Guidance may be applicable
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
Application of BS8485 guidance - where NHBC ‘Traffic Light’ Guidance may be applicable
• 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.
Application of BS8485 guidance - where NHBC ‘Traffic Light’ Guidance may be applicable
• ‘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’.
Application of BS8485 guidance - where NHBC ‘Traffic Light’ Guidance may be applicable
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.
Application of BS8485 guidance - where NHBC ‘Traffic Light’ Guidance may be applicable
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.
Application of BS8485 guidance - where NHBC ‘Traffic Light’ Guidance may be applicable
• 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.
Ground Gas Protection - NHBC Current
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.
Ground Gas Protection - NHBC Current
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/
Update on NHBC ground gas guidance
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…..
Update on NHBC ground gas guidance
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….
Update on NHBC ground gas guidance
Some current issues
Poor installation….
ELQF - Update on NHBC ground gas
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.
ELQF - Update on NHBC ground gas
guidance
Thank you for listening!!!
Any questions?
01908 746645.
mailto:[email protected]