Sealing the deal of airtightness

Iain Fairnington of the A.Proctor Group, discusses how the performance of the building envelope can be improved through airtight membranes.

The need for airtightness is driven by the global challenge to reduce carbon emissions and the drive to create more energy efficient buildings which waste less energy, reduce costs and protect both the building fabric and well being of its occupants.

The Government’s long-term carbon reduction strategy set out in the Climate Change Act means by 2050 the UK must achieve a reduction of 80 per cent against 1990 levels. While changes of government and the scrapping of the Zero Carbon Homes Policy have created some uncertainty, the need to follow through with these plans remains a fundamental part of the design and construction of the UK’s building programme.

Around 45 per cent of UK CO2 emissions come from the built environment, (27 per cent from domestic dwellings and 18 per cent from non-domestic), and space heating is accounted for much of this energy. The Building Regulations’ increasingly stringent performance criteria for building envelope has led to higher standards of insulation being specified for roofs, walls, windows and floors. However, identifying localised areas of reduced insulation or thermal bridging causing air leakage has become even more crucial.

Air leakage through cracks, gaps, holes and improperly sealed elements such as doors and windows can cause a significant reduction in the performance of thermally insulated envelopes, in some cases reducing their effectiveness by up to 70 per cent. Discrepancies between ‘as built’ and ‘as designed’ performance are largely attributable to uncontrolled air leakage, prompting architects and developers to increasingly turn to air barrier membranes as an essential part of the design process in achieving the most effective means of controlling and reducing air leaks.

A common misconception regarding airtightness is that well-sealed buildings mean uncomfortable, ‘stuffy’ indoor environments, which are in fact created by poor ventilation. Buildings with very low rates of air leakage require correspondingly higher levels of ventilation as part of a balanced design approach. It is a myth that increased ventilation hampers overall efficiency, because ventilation is controllable and can be accounted for within the design.

Guidance, legislation and compliance

The key guidance relevant to airtightness compliance is outlined in the Building Regulations Approved Document Part L1A Conservation of fuel and power in new dwellings and Part L2A Conservation of fuel and power in new buildings other than dwellings. However, it is also important to take a holistic approach when considering compliance with Building Regulations. Both reducing the rate of air leakage and increasing 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 quantity of air moving through the building fabric (m³), for a given building floor area (m²) 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/m²/hr (7m3/m2/hr in Scotland).

Although building regulations provide a framework to achieve minimum airtightness levels , the Code for Sustainable Homes (CSH) offers guidance on how to substantially 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 improvement in DER, but with little or no corresponding increase in thickness, allowing 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, internally 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 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 standard 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 standard. 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, insulation used and the standard of workmanship.

The use of self-adhering vapour permeable membranes makes a significant contribution to a building’s thermal performance by preventing lateral air movement. It also provides high vapour permeability, which allows any water vapour to escape the wall construction efficiently thereby avoiding any interstitial condensation problems.

Iain Fairnington is the technical director of the A.Proctor Group