Why air seal both sides of the insulation?

I am often asked why to air seal both sides of wall insulation. If the internal plasterboard is the designated air control layer, why tape and seal the wall wrap?

Answer: Air sealing just one side of the enclosure does not provide an optimised condition due to a phenomenon called “Thermal Bypass.”

The following explanation from Energy Vanguard explains it nicely.

Thermal bypass describes heat loss that gets around intended thermal insulation, including by wind washing, air infiltration, and convective loops.  Studies have shown that such conditions can dramatically reduce the effectiveness of installed thermal insulation.   Thermal bypass can encourage condensation, wetting and damage. This is true not only for fibrous insulation but rigid foam insulation too. The one sure approach to avoid the problems associated with thermal bypass is to install a continuous and robust air tight layer both inboard and outboard of the thermal insulation.

Our standard construction in Australia means our air control layer is normally the internal plasterboard line. We also normally do a terrible job of taping & sealing our wall wrap. We can do a much better job with our wall wrap, and we can also improve our internal lining. Some attention to detail to the things that we already have and install on site will make a big difference. Its’s the difference between erecting a building and assembling a building.  To erect means just put it up. To assemble infers that thought and precision is involved.

Thermal  bypass: the impact of natural and forced convection upon building performance.


Diagrams by Mark Siddall