What is thermal bypass, and why does it matter?

I am greatly indebted to the meticulous working and thinking of architect Mark Siddall (LEAP architecture) here. He’s been talking about thermal bypass for a while, so I asked him to walk me through the basics.

What the term ‘thermal bypass’ means is cold air (usually cold – though of course in summer it could be too hot instead) washing through building fabric, and undermining the thermal performance.

This means that in winter, cold air may be moving around in the walls and roof and taking heat from indoors, even if it doesn’t actually break through any air barriers. A classic example is when insulation is loosely bunged into a cavity, and warm air behind the inner leaf is drawn away and replaced with cold, when it should be held snug against the fabric.

My interview with Mark and some explanations and examples were written up for SIGA, and you can read them here.