Building envelope assessment

The walls, roof, windows and foundation, assessed as one system.

The envelope is everything that separates conditioned space from the outdoors, and it fails as a system rather than as a list of parts. A wall with excellent insulation and a discontinuous air barrier performs like a wall with neither.

An envelope assessment looks at four control layers in order of importance: water, air, vapour and thermal. That order surprises people — insulation comes last, because an assembly that is wet or leaking will not perform however much insulation is in it, and may rot.

For older houses the interesting question is usually whether the assembly can be improved without changing how it dries. Insulating a solid masonry wall on the inside, for example, makes the masonry colder and wetter; done without understanding that, it is how a beautiful century house gets destroyed politely over ten years.

We produce an assembly-by-assembly assessment with effective R-values rather than nominal ones, the thermal bridges that matter, and a sequence for improving it that does not create a moisture problem on the way.

For homeowners — the rest of it