Does using T & G wood plank instead of drywall affect where we put the vapor & air barrier?

Sue Colbourne Nov. 9, 2019Updated: Aug. 26, 2020

We want to use fir T and G for our interior walls instead of drywall (new build with plywood exterior sheathing and Tyvek - which will eventually have cedar shingles). What's the best thing to use for air and vapour barriers and where should we put them in the wall assembly so they work best? 

Responses (3)

Sue Colbourne 3 years ago

Hi Mike

Thanks so much for all this information. We live in southern Vancouver Island. We definitely want to avoid using 6 mil poly anywhere in our home. We are wondering if vapour barrier paint would be acceptable to the local building inspector (just trying to do research before we approach him) but would want a paint that is not toxic. We will also be doing T and G for the ceiling and ultimately it will be painted to better show off the ridge beam and half scissor trusses but we read in one of your links that vapour barrier paint is more applicable for walls so not sure what to do for the ceiling (roof is metal). Any other thoughts you have are greatly appreciated.

Mike Reynolds 3 years ago

6 mil poly barriers are actually not as a big a problem with ceilings, the vapor drive isn't quite as problematic as with walls. In the Edelweiss Demo House we used vapour barrier paint on the walls but we used poly on the ceiling. As for your building inspector, that could go either way, so if you want to try vapour barrier paint (or vapour retarder primer) check with an inspector ahead of time. As for the toxicity, We used Benjamin Moore vapor barrier paint which was low VOC but not Zero VOC unfortunatetly. 

The honest answer is that from a scientific point of view, painted wood will actually act as a vapour barrier, meaning vapour will move through it so slowly that it would qualify as a vapour barrier, but it is highly unlikely youll find a building inspector to accept that for new home construction. And I say that with the caveat that you should not even consider that if you don't have a proper air barrier in some other form. Be sure to read this page about the difference between Air Barriers and Vapour Barriers as they are often combined in one product but perform completely different tasks.