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ing the thermal insulation will contribute to lowering the building’s CO2 emission rate, but the implications of each approach can be substantially different. Air leakage is measured in m3
/m2 /hr - the
Identifying localised areas of reduced insulation or thermal bridging has become even more crucial
) over a given time period (hr). The measurement method commonly used is either pressurising or depressurising the building, and measuring the airflow required to maintain the test pressure (50 Pascals in the UK). The Building Regulations require the level of air leakage to be no greater than 10m3 (7m3
/m2 /m2 /hr in Scotland).
Although building regulations provide a framework to achieve minimum airtight- ness levels [via dwelling emission rate (DER) in Standard Assessment Procedure (SAP), and building emission rate (BER) in SBEM], the Code for Sustainable Homes (CSH) offers guidance on how to substan- tially exceed this. 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, the benefits from exceeding base requirements for airtightness 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 improve- ment in DER, but with little or no corresponding increase in thickness, allow- ing a reduction in building footprint, or an increase in internal space, while reducing build costs considerably.
Designing for airtightness
There are two main ways to achieve airtightness in the building envelope, inter- nally or externally. One way of thinking about this is ‘inside of the services zone’ or ‘outside of the services zone.’ 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 of the service penetrations are not adequately sealed, performance will be compromised.
A huge variety of ‘airtight’ accessories will be required when using an internal air
WWW.ARCHITECTSDATAFILE.CO.UK /hr
quantity of air moving through the building fabric (m3 (m2
), for a given building floor area
barrier system. These include airtight VCLs, pipe and cable gaskets, junction boxes, extractor fans, switch boxes, light fittings and sealing tapes. These accessories are generally more expensive compared to stan- dard non-airtight versions, and take more time, care and attention to install correctly. However, moving the air barrier system to the external side of the structural frame can allow for an almost penetration-free layer that can be installed faster and more robustly when comprising self-adhesive and mechanically-fixed vapour-permeable air barrier membranes and sealing tape. In essence, an external air barrier system prevents trapped moisture and air leakage and can be simpler to install than internal options, with less building services and structural penetrations to be sealed.
Best practice air barrier product solutions
The Anchorage at Dibden Purlieu is one of five buildings being built for Hampshire County Council to Passivhaus standard, which will help to achieve a significant reduction in energy bills for each property. The development required an airtightness level of less than 0.5. Wraptite-SA was applied externally to the timber frame panels in continuous pieces by chartered building company Raymond Brown Building creating a highly-insulated finished building, and achieving the required stan- dard. Initial air test results of 0.43 were achieved coming well below the 0.5 air permeability target. All the more impressive, since this was recorded even before the installation of the internal VCL Procheck 500, also provided by the A. Proctor Group, whose recommendation is that advice is sought when detailing Passivhaus to assess whether a VCL is required. This will be dependent on the building system, insula- tion used and the standard of workmanship.
The use of self-adhering vapour perme- able membranes makes a significant contribution to a building’s thermal performance by preventing lateral air move- ment. It also provides high vapour permeability, which allows any water vapour to escape the wall construction effi- ciently thereby avoiding any interstitial condensation problems.
Iain Fairnington is the technical director of the
A.Proctor Group
ADF FEBRUARY 2017
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