Bridge to Nowhere

I've written about attention to detail regarding insulation and draft-proofing in previous posts. Today's topic is a lesser-known factor in building performance but no less important. Thermal bridging. And attention to detail is key here as well.


This diagram does a great job of illustrating the cumulative effect of thermal bridging. Since wooden (or steel) framing has a much lower r-value than insulation, heat can flow (escape) through the studs and dwangs more quickly. In the example above, the framing makes up 1/4 of the total wall space where there is zero insulation. Since heat tends to rise, the problem of thermal bridging is even worse for ceiling insulation.


When insulation is installed between the joists, every joist becomes a thermal bridge, and the overall r-value of the ceiling is calculated as:

(r-value of insulation x area) + (r-value of joists x area)
----------------------------------------------------------------
total area

In other words, the r-value above your head is not simply the r-value as shown on the bag of insulation. If you have thermal bridging, then your r-value is lower.

We are dealing to this by using a suggestion from Ian Mayes, the Eco-Design Adviser for Hamilton City Council. (I reckon this is about the best use of rate-payers money there ever was or will be.) Ian suggests laying the batts perpendicular to the joists and covering them completely.


Not very exciting, but that's what it looks like. But we did run into some extra challenges around the extra framing we had to add to support the solar hot water on the roof.


But we dealt to that in the same way.


Even a small layer of insulation will break the thermal bridge enough to save energy. This is what we've also done in the bathroom behind our home-made medicine cabinet (future post). You can see the black building paper directly in contact with the exterior cladding. Brrr.

So instead of pushing the cabinet all the way back, we're installing a thin layer of insulation...


...and pulling the cabinet out about 25 mm (1 inch).


Please note that a frame will have to go around the medicine cabinet to hide the gap with the plasterboard. This will further disguise the fact that it hangs out into the room.

Voila! Eco, thrifty, attractive.





Peace in the Middle East (but keep the oil price high), Estwing