Where should I put a vapour retarder?
I am going to add exterior insulation to an old timberframe building. The interior walls will be wood paneling attached on the outside of the beams. For the next layer, I am thinking to wrap with vapour barrier. Six to eight inches of rigid rockwool insulation would go up next, then strapping for the exterior sheath, board and batten. I have read that the optimum scenario is to have one-third of the insulation inside the vapour retarder and two-thirds outside. Would it make sense to put up one layer of insulation before wrapping with vapour barrier, and then two (or three) more layers outside? Thank you.
Where are you located? I see the word ‘vapour’ with a U in it which is indicative of Canada, in which case it is a likely yes, the vapour barrier is best at the 2/rds point if possible, but where exactly in Canada are you?
What you propose seems to make sense from a physics point of view as it sounds like you could put your VB at the 2/3rds point, but was this also intended to be your air barrier? And if so, you would need a method for attaching it so it won’t be perforated.
And another question, are you planning to have air conditioning as well? Here is our page that covers this issue-
Don’t install air conditioning in a home with an interior vapor barrier
If you aren’t planning to add air conditioning it may be a lot of effort to mitigate a very minimal risk, so the big question really is about whether or not you will have A/C, and where you live.
No air conditioning. North shore of lake Ontario.
It seems normal, practical, to put the vapour barrier on the outside of the interior wall, then add on the insulation. On the other hand it seems theoretical, or technical, to put on one layer of insulation, then the vapour barrier, and then the rest of the insulation. I am happy to go with theory over practice, just looking to guage a nod one way or the other.
Or to gauge, as the case may be.
Without AC an interior vapour barrier isn't really a problem, and frankly even with AC if you're fairly moderate about it. Meaning, don't be afraid to pop in a mini -split heat pump at some point to take the edge off a heat wave, the problem is more when people keep their homes at sub-arctic temperatures for an entire summer. So if it were me I'd be leaning towards the easiest, and that may well be keeping it on the interior, but....
If you have it sandwiched between two rigid sheets that could work well also, as long as you have a plan for how to hang it since there wouldn't be any solid surface to attach it. I think you're good either way Jeffrey, seems like you have a good understanding of sensible construction.