Building Envelope  »  Air tightness

Airtight envelopes are essential for buildings with active space conditioning systems. A tightly constructed building can be achieved through careful planning during the design stage and careful construction techniques. In addition unwanted air leaks can also cause uncomfortable drafts. Another side effect of air leakage in heated buildings can be condensation within the fabric. When warm damp air escapes through the cooler building structure it will condensate. The moisture can lead to poor performance of the insulation and causes fabric deterioration and further damage to building construction. In hot and humid climates on the other hand, air leakage can be a significant source of indoor humidity affecting thermal comfort levels as well leading to moisture build-up and the formation of mold. Care should be taken with all construction and installations penetrating the building envelope as these can lead to air leak if not carefully sealed.

Chief sources of air leakage:

• Cracks in wall (masonry)
• Window and door joinery
• Flue and fireplace
• Ducts for heating, ventilation and air conditioning
• Wall to floor connection
• Wall to ceiling connection
• Penetrations in external wall for service components

Air leakage is usually measured using a blower door test. Occasionally thermal imagery, smoke deflection tests also can be used to identify the sources of air leaks and drafts, however, it cannot be accurately measured unless a blower door test is performed.

Solutions to air leakage

For advanced buildings like the Passive House and the Zero and Plus Energy buildings, it is crucial to provide, and sometimes even prove, solid detailing of all physical junctions in the structure. In such buildings uncontrolled air leakage can contribute the largest single part of the thermal losses. It must be noted that in extremely efficient buildings mechanical ventilation (ideally with a heat recovery system) should be included to guarantee a hygienic air exchange. Air tightness is however also a must to obtain ultra low energy buildings. It is recommended those air tightness tests be carried out on all buildings. This both during the building stage, necessary to find links and eliminate these when they are accessible, and when the building is finished to confirm these results.

A minimum supply of fresh air must be guaranteed in every case, in order to provide good indoor air quality, though. Recommendations are usually around 1 air change per hour; this means that the volume of air inside the building must be replaced by fresh air once an hour. This will avoid the negative effect of the ‘sick building syndrome’ where occupants react to the poor indoor environment. A continuous impermeable barrier, combined with other measures such as weather-stripping, can reduce rates of air leakage by a factor of five to ten compared to current standard practice in most regions of North America, Europe and the cold-climate regions of Asia (Harvey, 2006). It is recommended for LEB Buildings to have of air tightness at 50 pascal (n50) of 1.0 ACH or less and for ULEB and PZEB Buildings the air tightness should be at least 0.6 ACH or less.

Air locks can also be used to reduce energy loss by preventing heat and cooling losses through uncontrolled air leakage. Entrance rooms can serve well as air locks. Both the interior and exterior doors should not be open at the same time to avoid cross ventilation through the air lock.