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Vapour barrier suggestions needed

Posted: Wed Jul 17, 2019 9:13 am
by veedubb
Hello everyone. I just joined this form and this is my first post and I'm looking for suggestions and/or comments.

I'm no stranger to home construction and renovations but I'm stuck in deciding how to vapour wrap above the garage.

I intend to insulate the living room floor which is above garage (currently no insulation whatsoever). The floor is very cold in the winters and even laying carpet doesn't help much on cold days. The floor joists are 2 x 8, 16" OC construction and I intend to use Roxul (Rockwool) R24 batts for insulation as well as sound deadening.

My question is how to apply the vapour barrier on the warm side just beneath the hardwood floor. My sub-floor consists of diagonal planks with narrow strip hardwood which is typical of '60/'70 construction methods.

Should I:

A) apply a continuous sheet up from the bottom of the joist to the bottom of the planks, back down to the bottom of the adjacent joist...and so on?


B) cut strips of vapour barrier plus a a few of inches wider to cover under the subfloor and overlap a couple of inches at the top of the joists?

I fully intend to use Acoustic Sealant and staples to in attempt to get a proper air seal. It's cheap and each tube goes a long way.

The A) method is the best way to get a good air seal, but I think that would prevent the batts from being tight between the joists and the batts would eventually slide down along the plastic vapour barrier. I've done this in the past upon request on someone else' job and I'm not too fond of it for the reasons listed.

I'm leaning towards method B) even though it may be a little more work. I'll be applying Acoustic Sealant along the lengths of the joists where I'll be stapling.

I hope I'm making this clear of what I intend to do and if this makes sense to you.

I can't think of any deficiencies or drawbacks other than possibly introducing some air leakage somewhere, though it should be small and insignificant. I figure the same can be said if it was a continuous sheet.

I know what people are going to say -> spray foam. It has crossed my mind but 1) it's close to three times the price, 2) I'm only doing a small 12' by 11' area and 3) I'm looking for sound deadening which is an area that spray foam does not perform as well in. I'm also aware of the carbon monoxide aspect of exhaust fumes leaking into the living room but we've never had any issues since moving here in 1986 up until now, and there's just a huge air cavity currently.

Is there anything else I should consider? Or am I just 'barking up with wrong tree'?


Re: Vapout barrier suggestions needed.

Posted: Wed Jul 17, 2019 11:59 am
by veedubb
After some digging around, an alternative would be to cut rigid foam boards instead of the vapour barrier and can spray foam around the perimeter to seal it in place. That would add R-value as well as contribute to a tighter air seal.

While reading through some material, I've come across a controversy on whether to install the batt insulation in constant contact with the subfloor above (as per code?!?) or leave an airspace between the batt insulation and the subfloor for warmer floors as in this article: ... erformance

I have to say that this last article is intriguing and makes sense. Thoughts?

Re: Vapour barrier suggestions needed

Posted: Wed Jul 17, 2019 10:06 pm
by Aaron
You have the subfloor but not the finished floor? Pity you didn't stretch 6 mil poly across the joists beforehand. Not sure poly should be on the inside side of a subfloor; you may be better off with the 2" foam with the spray foam. Check with your jurisdiction to see if that alone is suitable vapor barrier, it often is if it's 2" thick.

Re: Vapour barrier suggestions needed

Posted: Thu Jul 18, 2019 7:25 am
by Shannon
If your garage is heated you may be ok to install the poly on underside of joists? Poly Must go on warm side of framing but in that case both would be warm??
Check your local code office to see what they require

Re: Vapour barrier suggestions needed

Posted: Fri Jul 26, 2019 9:45 am
by veedubb
Aaron, the floor is original and hasn't been replaced so there's no way to install poly across the joists. It has crossed my mind to rip up the whole main floor and lay down 3/4" plywood and re-floor but that's a lot of work since my flooring is in good shape. This is why I've come to ask these questions without disturbing the main floor.

It looks like I'll be using the rigid foam and spray foam. Though I may be using 1.5" rigid instead of 2".

Re: Vapour barrier suggestions needed

Posted: Fri Jul 26, 2019 9:49 am
by veedubb
Shannon, my garage is not heated but the garage itself rarely drops below freezing in the winter in Southern Ontario. Maybe it's because there's only two exterior walls and it's half underground along one exterior wall, while the other exterior wall contains the garage door.

Re: Vapour barrier suggestions needed

Posted: Fri Jul 26, 2019 5:35 pm
by Shannon
sounds good

Re: Vapour barrier suggestions needed

Posted: Mon Sep 16, 2019 8:01 am
by veedubb
I just finished the ceiling this past weekend. I took the time to cut, place and foam both the 1" styrofoam filmed sheets and R22 Roxul, though it was very technical and time consuming due to the cross bracing, heat pipes and other obstructions. I left approximately a 1" to 1.5" air gap between the styrofoam and the insulation as per the information from the linked article I posted initially.

I also installed a ceiling light, electrical outlet and control cables for a garage door opener "while I'm in there".

I'm hoping that this does the trick but the real test would be when it gets cold and the furnace comes on.

Re: Vapour barrier suggestions needed

Posted: Mon Sep 16, 2019 5:51 pm
by Shannon
Cross your fingers! Lol