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either pressurising or depressurising the building, and measuring the airflow required to maintain the test pressure (in the UK this pressure is 50 Pascals). Building Regulations require the level of air leakage to be no greater than 10m3
/m2 /hr (7m3 /m2 /hr in
Scotland), and in most cases achieving this presents little difficulty.
The impact of uncontrolled moist air movement can have a long-term detrimental effect on the durability and life of the building
Although Building Regulations provide a framework to exceed minimum airtightness levels (via dwelling emission rate DER in Standard Assessment Procedure SAP, and BER building emission rate in SBEM) substantial benefits can be realised in exceed- ing the minimum base requirement. For example, if we consider the ‘notional dwelling’ used within the SAP calculation, and vary the levels of thermal insulation (in terms of U-values) and air leakage, these benefits become clear. By varying the U-value from 0.15 to 0.05, with an air leakage rate of 7, the DER will drop by 6.7 per cent, but achieving this reduction in U-value will require almost three times the thickness of insulation. By contrast, retaining the 0.15 U-value, but dropping the air leakage rate from 7 to 1 will achieve a similar improvement in DER, but with little or no corresponding increase in thickness,
allowing a reduction in building footprint, or an increase in internal space, reducing build costs considerably.
Achieving airtightness by design
The two main ways to achieve airtightness in the building envelope are internally or externally. Traditional use of internal air barriers can be more complex and costly to install, due to the need to accommodate building services such as electrical, lighting, heating and drainage systems. An internal air barrier is only as good as its installation. If all the service penetrations are not adequately sealed, performance will be compromised. External air barrier systems allow for an almost penetration-free airtight layer, which can be installed faster and more robustly. This offers an effective but simple system comprising self-adhesive vapour permeable air barrier membrane, which provides effec- tive secondary weather protection while preventing trapped moisture and air leakage. Far simpler than internal options an external air barrier system will maintain the envelope’s integrity, with less building services and structural penetrations to be sealed, and less room for error.
SPECIFY AND INSTALL CAVITY BARRIERS Call 01304 842 555 or visit
www.envirograf.com
Envirograf Protecting the irreplaceable
WWW.ARCHITECTSDATAFILE.CO.UK
®
ADF FEBRUARY 2018
MAKE SURE YOU COMPLY WITH BUILDING REGULATIONS
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