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Old 11-30-2016, 12:14 AM   #1
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How is vapour barrier of basement wall typically tied in with rim joists?


I framed my basement years ago, and then moved on to other projects and never insulated it. Hoping to look into starting that this year, was going to save up and get it spray foamed but I'm thinking what I'll do is get the rim joists spray foamed, then use batts for the actual walls. So assuming I do that, what is the best way to tie the vapour barrier in with the rim joists? I don't imagine I can really tape it to the spray foam as it won't really stick that well and probably come off over the years. The joists will also be in the way making it harder to tie in properly.

The wall is built about 1cm away from the wall, so if I was to simply end the vapour barrier at the wall there is a huge gap where air can leak. Do I just tie it in to the top plate of the foundation? This is kind of tricky to do with the joists though.

Another option is I could cut strips of rigid foam and shove them between the top plate of foundation and use construction adhesive to glue and seal them in place.

Just wondering how this is typically done. Also will give me an idea if I should get it spray foamed first, or do the batts first.

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Old 11-30-2016, 07:00 AM   #2
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Re: How is vapour barrier of basement wall typically tied in with rim joists?


The recommendations for basement wall insulation don't exactly match where you are and where you are headed. Run this search "BSD 103 understanding basements" to access a good article. Sorry I can't post the link as they require a sign-up for the second visit but the information is good. But the search will get you in.

Although I don't like using a true vapor barrier some codes still require it, so check you local code department.

Also, all moisture issues need to be corrected before those walls go up. Covering an insulated wood wall with plastic is a sure way to run into mold issues.

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Old 11-30-2016, 01:05 PM   #3
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Re: How is vapour barrier of basement wall typically tied in with rim joists?


Vapour barrier is required here in Ontario.
I refused to use it so I had only one choice and that was to spray foam the walls as well.

Before you insulate, get a quote from them to do your walls as well as the rims. You may be surprised and it may only be a bit over your budget.
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Old 11-30-2016, 03:43 PM   #4
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Re: How is vapour barrier of basement wall typically tied in with rim joists?


Yeah I definitely want to use a vapour barrier and make it as tight as possible weather or not it's required, a leaky insulated wall may as well not be insulated. I originally was going to do full spray foam, I got a quote, I think it was around 3k to do the whole basement, and a bit over 1k to do the rim joists. So it's still an option but I'm leaning towards batts. Weeping tiles were recently redone so moisture won't be an issue.

Often people shove fibreglass in the rim joists, is that often the only thing that is actually done? I don't think I've ever seen it done properly come to think of it. A leak in vapour barrier pretty much negates the whole idea of having one, too.

My house is old so the existing vapour barrier is pretty poorly done, and not taped anywhere, and I suffer from it in winter, so if I'm going to do the basement I want to at least do that part right.
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Old 11-30-2016, 06:21 PM   #5
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Re: How is vapour barrier of basement wall typically tied in with rim joists?


Moisture can come through the foundation and condense on the vapor barrier.

Google "mold on basement insulation", hit images, and you may think the extra cost is worth it.

My spray foam is about 1/2 way down at post 575:

Total House Rebuild - Bungalow to Two Storey

No moisture is coming from the outside with this stuff installed.
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Old 11-30-2016, 06:58 PM   #6
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Re: How is vapour barrier of basement wall typically tied in with rim joists?


My vote would be spray foam as well. I'm not sure I understand your original post. If your wall has been framed, I assume the studs are in front of the foundation wall, along with a base a top plate, and the joist bays leading to the rim joist behind the stud wall. If so, you could stuff batts in the joist bays. If you want to use batts in the stud wall, the vapour barrier (req'd in Ont) would run from the top to the bottom plate and sealed around the perimeter with tube sealant.
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Old 11-30-2016, 09:05 PM   #7
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Re: How is vapour barrier of basement wall typically tied in with rim joists?


Problem if I only stop the vapour barrier at the top plate, then what about the space between the top plate and wall, the air can still leak through there. Also need to seal the rim joists themselves, batts arn't good enough to seal, so either have to cut vapour barrier and tape all around (kinda a pain) or spray foam. So will do spray foam for rim joists for sure. I guess I could ask them to spray it all the way to the top plate of my wall, then it will seal that area.

I might just go ahead and do spray foam for whole area but want to see what my options are for batts. It's been done many times so I'm sure there's a proper way of tying in vapour barrier for a proper seal.
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Old 12-01-2016, 02:56 PM   #8
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Re: How is vapour barrier of basement wall typically tied in with rim joists?


You need to bring the VB over the top of your stud wall and into the rim board area. Staple it in place and the spray foam will spray right over it sealing everything off.


Insulate the walls first.

If you're gonna use insulation, at least use Roxul as it doesn't absorb moisture like fiberglass.
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Old 12-01-2016, 07:28 PM   #9
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Re: How is vapour barrier of basement wall typically tied in with rim joists?


Yeah plan is to use Roxul. Since the joists sit on top of the wall, what is best way to deal with that, I guess I can just kinda shove it all around and staple it. I'd probably use acoustic sealant for the seams, then tape it to the wood too for good measure. The foam would then cover that part. Or if I go that far I may even end up just vapour barriering the rim joists too... But foam is probably best bet.
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Old 12-01-2016, 09:05 PM   #10
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Re: How is vapour barrier of basement wall typically tied in with rim joists?


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Originally Posted by Red Squirrel View Post
Problem if I only stop the vapour barrier at the top plate, then what about the space between the top plate and wall, the air can still leak through there.
Ya, sorry, brain gap until I saw your diagram. Another option could be cutting rigid foam for the joist pockets and sealing around the perimeters with canned foam.
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Old 12-26-2016, 03:50 AM   #11
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Re: How is vapour barrier of basement wall typically tied in with rim joists?


Been thinking this over more as I want to try to do it all myself to save money and probably start on that this year. Easy route is to get the rim joists spray foamed and ask them to also extend a layer of foam all the way to the top plate of wall. That will create a continuous seal and then I can just vapour barrier to the top plate. Trying to bring vapour barrier into each individual cavity from the wall would prove to be tricky.


Another option that's cheaper and more DIY I've thought of is I could do the classic rigid foam, but also add a piece that is horizontal and extends to the top plate like in my attached pic. I could then seal them in with caulking or acoustic sealant. Maybe even fire caulk? As long as it's something that will seal decently and last. Think this would work ok? I'm leaning towards that route. Should probably do that before any electrical that way I can drill through the foam and top plate and pass wires in, vs trying to cut foam around existing wires. I'd then caulk the holes in. Then for the regular wall I just need to bring vapour barrier up to the side of top plate as normal and have continuous tape, but then also small sections to tie it in to the foam between each joist section.

I understand this is probably above code, I think most people just foam the end part of the joist and call it a day, but may as well try to make sure I get a very air tight seal as best as I can. Rest of house is terrible but at least I can do this right if I'm going to do it.

As a side note I plan to install a wood stove at some point, for the wall what is the best way to prep for the exit pipe? how is that general area typically vapour barriered/instulated? I imagine you can't have plastic tapped right to the pipe or it will melt.
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Old 12-26-2016, 06:22 AM   #12
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Re: How is vapour barrier of basement wall typically tied in with rim joists?


Use acoustic sealant. It doesn't dry up and crack.
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Old 12-27-2016, 02:21 PM   #13
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Re: How is vapour barrier of basement wall typically tied in with rim joists?


I think you may be missing the difference between air sealing and vapor barrier. You are correct in that you want to stop the movement of air (air sealing) but the vapor barrier is not what does that. Generally speaking, unless required by code, below ground walls should not have a vapor barrier but yes, they should be air sealed.
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Old 12-27-2016, 07:09 PM   #14
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Re: How is vapour barrier of basement wall typically tied in with rim joists?


I thought the whole point of vapour barrier was to air seal? Without one you get air leaks and your insulation is just acting as an air filter. Cinder block is not really air tight so definitely want to vapour barrier the wall too. This is about creating a continuous sealed envelope.

Though my pic may have been missleading, the light blue line represents the "cold side" that needs to be air sealed, the darker blue is the vapour barrier, and then the pink is the foam. So between the foam and wall vapour barrier it creates a continuous seal. I will want to caulk or foam the bottom plate of the wall too or air can leak through there.
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Old 12-28-2016, 10:39 AM   #15
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Re: How is vapour barrier of basement wall typically tied in with rim joists?


Quote:
I thought the whole point of vapour barrier was to air seal?
Then it would be called an air barrier. Yes, vapor barrier and air barrier are different things. Common on a concrete basement wall is rigid foam glued to the walls and then the seams taped. Done well, this can be an air barrier. It is not, however, a vapor barrier, as the foam is considered a vapor retarder and lets the wall dry to the inside instead of trapping the moisture in the wall.

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