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Old 07-02-2019, 11:15 PM   #1
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Insulating half-height basement foundation


I'm working on remodeling my 1984 basement. The house sits on a hill, so the basement foundation is half-height in the front, and full height in the back. The foundation is uninsulated (concrete touches dirt). I'm on the west coast of Canada (near Vancouver) so it doesn't really get cold (very rarely below freezing) but it's wet outside during winter.

The builder stuck some drywall in front of the concrete without insulation to provide a "finished" look. No insulation, just an air gap around 1". The plan is add a bit of insulation and fully cover the foundation with drywall. Attached is a photo of the foundation on the inside.

I've been watching a lot of videos and reading articles, but I'm struggling with the best insulation option for my project. I'm thinking 1-1.5" rigid foam box around the concrete (secured using plastic anchors) followed by 2x2 framing or strapping then mold-resistant drywall. Maybe even a bit more foam within the framing if I go with the 2x2. I like the character and function of a half-height ledge, but don't want the ledge to stick out too far, hence why I don't want to go with 2x4.

Do I need a vapor barrier before the drywall? How do I protect against mold between the concrete and the foam? I've read a lot of contradictory information online and could use some help.

Thanks
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Old 07-02-2019, 11:25 PM   #2
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Re: Insulating half-height basement foundation


Neither concrete or foam will support mold. It needs moisture, air and food. so all you have to do is keep moist dirty air from the basement from getting in the system.
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Old 07-03-2019, 12:56 AM   #3
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Re: Insulating half-height basement foundation


Right, that's what I thought as well until I saw this post: greenbuildingadvisor DOT com/question/why-is-mold-mildew-growing-behind-my-rigid-foam (can't post links apparently... replace DOT with a .)
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Old 07-03-2019, 02:36 AM   #4
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Re: Insulating half-height basement foundation


Mold is a spore that is in the air all the time if it lands on a food source and has moisture it will go. That food source can be many things dust in the air, that is deposited on walls when warm air come in contact with a cold surface.

You had far bigger chance of mold growing on the drywall that was glued to the cold concrete. All you have to do with insulation is make sure the cold from the concrete does not get to the surface where warm moist air is. Or don't let the warm moist air get passed the drywall and insulation to deposit dust and water on a cold surface.



If you were to buy a new house in BC today the exterior basement walls would be framed with 2x4s one inch away from the concrete with fire stopping on top, it would insulated with bats and covered with a barrier that seals around outlets and switches and anything else and then drywall. As long as the insulation is done right there is no problem .
So put up you foam and seal the seams and the connection to the floor add your strapping and put your foam between them and tape and seal that too. You will be fine. Just make there are no open spaces and you have taped all the places that air could get thru.
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Old 07-03-2019, 05:23 AM   #5
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Re: Insulating half-height basement foundation


Thanks for the info, fellow British Columbian. Out of curiosity, what's the purpose of keeping the 2x4 framing an inch from the concrete? Assuming there's poly between the wood and concrete, is there much risk in butting the wood up against the concrete?


I've created a mock-up for the foam concept. If I go with thin strapping, then I can glue it to the foam. Or screw it into the concrete. Either way it wouldn't leave much room for additional foam around the strapping, so I suppose it'd be best to use start with thicker foam next to the foundation (1.5-2"?).


If I do proper 2x2 framing in front of the foam, then maybe I could start with thinner foam (maybe 1"?), then use another inch or so between the framing?


What do you think? Am I on the right track for the foam thickness?
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Old 07-03-2019, 06:12 AM   #6
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Re: Insulating half-height basement foundation


The gap is because we mostly do concrete foundations and the inside is not smooth enough to build to close to it and you don't want wood to ever touch concrete.
The vapour barrier goes between the drywall and the studs on the warm side of the insulation in this climate. . Foam would be your vapour barrier.

On a half wall like you have we usually come 6" above the concrete so there is not shortage of insulation at the ledge.

There is a foam that is made for strapping, if you want you could use that and add another over that and use longer drywall screws to reach the strapping.



What area are you in?

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Old 07-03-2019, 07:20 AM   #7
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Re: Insulating half-height basement foundation


I read that link you provided and where I usually like GBA links I found that one to be confusing, both questions and answers. I do not like tolerating any form of mold, and mildew falls into that group. The best and probably only way to avoid mold is to eliminate the moisture, both from outside and inside (a cold wall can result in condensation). And moisture comes in two forms, liquid and vapor. Block the vapor and it accumulates to be a liquid.

The solution is not one home owners want to hear, your basement was never built to be finished living space. This link will help explain.

Bud
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Old 07-03-2019, 07:27 AM   #8
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Re: Insulating half-height basement foundation


Quote:
Originally Posted by Bud9051 View Post
I read that link you provided and where I usually like GBA links I found that one to be confusing, both questions and answers. I do not like tolerating any form of mold, and mildew falls into that group. The best and probably only way to avoid mold is to eliminate the moisture, both from outside and inside (a cold wall can result in condensation). And moisture comes in two forms, liquid and vapor. Block the vapor and it accumulates to be a liquid.

The solution is not one home owners want to hear, your basement was never built to be finished living space. This link will help explain.

Bud
Bud: A basement house in Vancouver built in 1985 was built to have living space, they often came new with one bedroom finished in the basement, all be it not up to today's standards but they were damp proofed and had drainage.
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Old 07-03-2019, 07:42 AM   #9
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Re: Insulating half-height basement foundation


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Originally Posted by Nealtw View Post
Bud: A basement house in Vancouver built in 1985 was built to have living space, they often came new with one bedroom finished in the basement, all be it not up to today's standards but they were damp proofed and had drainage.
Exactly !! Meaning they were half a$$ built to be living space. Even in 85 they knew how to make a dry basement but chose NOT to, to reduce cost of course. I'll guess 99% of new home owners look at basements with wide eyes thinking about all of that nice extra space, which it can be, but at an extreme cost and still not 100%.

We have to provide guidance to posters here but we need to be honest, there are no guarantees (unless you don't finish that basement) and if it fails it can be a disaster.

That GBA article says "live with it" I disagree and say "live without the finished basement" and I know what direction all home owners will go, but at least they have heard my concerns.

Bud
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Old 07-03-2019, 07:59 AM   #10
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Re: Insulating half-height basement foundation


Quote:
Originally Posted by Bud9051 View Post
Exactly !! Meaning they were half a$$ built to be living space. Even in 85 they knew how to make a dry basement but chose NOT to, to reduce cost of course. I'll guess 99% of new home owners look at basements with wide eyes thinking about all of that nice extra space, which it can be, but at an extreme cost and still not 100%.

We have to provide guidance to posters here but we need to be honest, there are no guarantees (unless you don't finish that basement) and if it fails it can be a disaster.

That GBA article says "live with it" I disagree and say "live without the finished basement" and I know what direction all home owners will go, but at least they have heard my concerns.

Bud
If He is anywhere close to Van living in a 3 million dollar house, you are not going to tell him not to finish the basement.
He is removing drywall that was glued to concrete and said nothing about a mold problem so It would be safe to assume it is good for finishing.
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Old 07-03-2019, 08:14 AM   #11
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Re: Insulating half-height basement foundation


Quote:
Originally Posted by Nealtw View Post
If He is anywhere close to Van living in a 3 million dollar house, you are not going to tell him not to finish the basement.
He is removing drywall that was glued to concrete and said nothing about a mold problem so It would be safe to assume it is good for finishing.

Sorry Neal but I just did. I know he will go ahead but perhaps my concerns will push his approach up a notch.

And "safe to assume ", down here we have learned that it is rarely safe to assume.

"a 3 million dollar house" must be your neighbor.

For the op, read that last link I provided, I read yours.

Bud
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Old 07-03-2019, 01:52 PM   #12
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Re: Insulating half-height basement foundation


Haha thanks guys. I'm in Victoria, so we're not quite at Vancouver levels of pricing, but who knows in 20 years, right?

Bud, I realize I'll never going to get a perfect living space in the basement, but the idea is to get it a bit better than what it is now. I lived in the basement for over a year while doing renos upstairs and never noticed any problems with water, mildew, must, etc. A tad bit cold at times, sure, but mostly tolerable. The foundation and inside of drywall look clean so far, but the area is heated and the concrete is essentially exposed, so it has been able to dry easily. Good article, by the way.

Neal, that's pretty neat they have foam with grooves for strapping. I'll have to see if they have that in town. I've attached a crude cross section. Are you implying 6" rigid foam insulation on the top of the ledge? I'm guessing I would use spray foam between the rigid foam and the bottom plate, and the rigid foam and concrete floor?

Btw, found a great great document for climate zone 4 (attached). Has some real nice illustrations (pdf didn't show up... link to pdf here: https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=4&ved=2ahUK Ewjbv__ps5njAhUD-lQKHaIPDFkQFjADegQIAhAC&url=https%3A%2F%2Fwww2.gov .bc.ca%2Fassets%2Fgov%2Ffarming-natural-resources-and-industry%2Fconstruction-industry%2Fbuilding-codes-and-standards%2Fguides%2Fclimatezone4.pdf&usg=AOvVaw2d sm8-qiIG36xxZHocrlv5)
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Old 07-03-2019, 02:03 PM   #13
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Re: Insulating half-height basement foundation


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Originally Posted by turbine View Post
Haha thanks guys. I'm in Victoria, so we're not quite at Vancouver levels of pricing, but who knows in 20 years, right?

Bud, I realize I'll never going to get a perfect living space in the basement, but the idea is to get it a bit better than what it is now. I lived in the basement for over a year while doing renos upstairs and never noticed any problems with water, mildew, must, etc. A tad bit cold at times, sure, but mostly tolerable. The foundation and inside of drywall look clean so far, but the area is heated and the concrete is essentially exposed, so it has been able to dry easily. Good article, by the way.

Neal, that's pretty neat they have foam with grooves for strapping. I'll have to see if they have that in town. I've attached a crude cross section. Are you implying 6" rigid foam insulation on the top of the ledge? I'm guessing I would use spray foam between the rigid foam and the bottom plate, and the rigid foam and concrete floor?

Btw, found a great great document for climate zone 4 (attached). Has some real nice illustrations (pdf didn't show up... link to pdf here: https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=4&ved=2ahUK Ewjbv__ps5njAhUD-lQKHaIPDFkQFjADegQIAhAC&url=https%3A%2F%2Fwww2.gov .bc.ca%2Fassets%2Fgov%2Ffarming-natural-resources-and-industry%2Fconstruction-industry%2Fbuilding-codes-and-standards%2Fguides%2Fclimatezone4.pdf&usg=AOvVaw2d sm8-qiIG36xxZHocrlv5)
Yes something like that.
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Old 07-04-2019, 02:58 AM   #14
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Re: Insulating half-height basement foundation


Went to my local hardware store today. They don't have any foam with in-built channels for strapping, so I'm thinking I'll just use some of the regular 2" interlocking pink stuff followed by 1x3" strapping on the outside.

What's your thoughts on screwing the strapping into the concrete to hold the foam in place? Or would it be better to attach the foam with plastic anchors then glue the strapping in place? I don't think gluing the foam to the concrete will work well. The foundation has a lot of bumps and ridges.
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Old 07-04-2019, 03:31 AM   #15
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Re: Insulating half-height basement foundation


I would stand up 2 sheets of foam, tape the joint and the screw a 2x3 strapping over the joint with the blue tap con screws then add a sheet tape and and add strapping. Instead of screws you can just pin it in.

Drill a 3/16 hole with the rotary hammer insert two pieces of nylon string from the weed wacker and drive a 3" nail in the hole.


Then you could come back and fit another 3/4" foam between the strapping or you could pre cut the 3/4" foam to fit so you could put that in as you go and get nice tight joints on everything.
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Last edited by Nealtw; 07-04-2019 at 03:41 AM.
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