Top plate vapour barrier - confused

Mark Harvey

I understand the concept of the vapour barrier and even why it is installed on the top plate between rooms, ... it simply creates a good ceiling seal. What I don't quite understand is that I have seen the plastic between the two top plates and on top of the top top plates ... under the trusses. The latter makes sense, but why would the plastic be between the plates? It still folds upward to the ceiling and , I think, the ceiling plastic is sealed and stapled over the wall plastic, correct?
Thanks.

jklingel

The vb is apparently still a Canadian code thing, and its function is to prevent vapor from getting to cold wood. Ideally, it should encase your house in a continuous layer; hermetically sealed, as it were. It is an awkward piece of doggy droppings to install sometimes, as choices have to be made, as you noted. I don't think it matters where it goes (of your two options) as long as you don't have a break in it where moisture can hit cold wood. Where two pieces meet, staple them down nice and snug, then tape them w/ tape made JUST FOR POLYETHYLENE. It won't hurt a bit to use something like Tremco acoustical sealant between each piece where they join before you tape, either. NOW, whether or not you need one, or whether or not it is even helpful, is a matter of debate. And, if you use a vb, do your best to air seal at any and all penetrations thereof. Air conditioning can cause problems w/ vb's, too.

woodworkbykirk

its done this way because most framing crews dont install vapour barrior on teh entire ceiling before back framing a house. they install strips on the top plate and on exterior walls where partitions will tie in so the insulators will have a continous vapour barrier when they do their thing. they can put up smaller sections of vapour barrier that are cut to the size of the room they are working in

jasin

Its the insulation people or general who are suppose to do the vapor barrier.