Tackling local issues for airtightness compliance

As Building Regulations have imposed ever-tougher energy performance criteria on the building envelope, the significance of localised areas of reduced insulation or thermal bridging leading to air leakage has become even more crucial. A. Proctor Group reports.

The energy consumed by buildings accounts for a significant proportion of the UK’s total energy consumption. 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 therefore Building Regulations relating to energy performance continue to play a major role in helping to achieve our targets for improvement.

Controlling air leakage

Air leakage through cracks, gaps, holes and improperly sealed elements such as doors and windows can cause a significant reduction in the performance of even thermally insulated envelopes. As thermal insulation requirements increase, industry consensus suggests that discrepancies between ‘as built’ and ‘as designed’ performance can be largely attributable to uncontrolled air leakage. Architects are increasingly turning to air barrier membranes as an essential part of the design process for achieving the most effective means of controlling and reducing air leaks.

A misconception when it comes to airtightness is that well-sealed buildings mean uncomfortable, ‘stuffy’ indoor environments; this is largely an effect of poor ventilation rather than airtightness. Buildings with very low rates of air leakage require correspondingly higher levels of ventilation as part of a balanced, holistic design approach. A common misunderstanding is that this increased ventilation will undermine efforts to reduce air leakage and hamper overall efficiency. It’s important to bear in mind that ventilation is controllable, and therefore can be accounted for within the overall design, whereas uncontrolled air leakage is not.

Managing air flow

Unmanaged or uncontrolled air flow will act as a carrier for moist air, drawing it from outside to in, or pulling it from inside to out, into walls, ceilings, and roofs. The impact of uncontrolled moist air movement can have a long-term detrimental effect on the durability and life of the building.

In terms of the energy efficiency of a building, uncontrolled air flow will almost certainly have a major impact. Initial heat load calculations for heating and cooling equipment will usually make an allowance for a level of natural infiltration or uncontrolled air flow. The higher the infiltration rate, the lower the energy efficiency of the building. Efficiency levels can be affected by both natural and mechanical air movements. The forces of wind and stack effects will lead to a level of air filtration and subsequent efficiency loss. Sealing the shell of the building and any un-designed holes can reduce the impact of wind and stack effects.

Building Regulations compliance

Key guidance related 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. A typical approach might aim at reducing the rate of air leakage and increasing the thermal insulation, both of which will contribute to lowering the building’s CO2 emission rate, however, 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 (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.

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 exceeding 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 effective 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.