Where should I put a vapor barrier / vapor retarder?


Where should I put a vapour retarder?

Jeffrey Connell Published: March 14, 2020, 4:25 p.m.Last updated: Sept. 9, 2020, 5:03 p.m.

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.

Responses (4)

Jeffrey Connell March 16, 2020, 11:21 p.m. Reply

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.

Jeffrey Connell March 16, 2020, 11:23 p.m. Reply

Or to gauge, as the case may be.

Mike Reynolds
Mike Reynolds March 17, 2020, 12:01 p.m. Reply

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.