Insulating the inside of concrete basement walls?

[Knots]

Hi - I've lurked here plenty (and learned a lot), but now that I'm building a house around a wood stove I need to ask some questions.

The question: The builder seemed reluctant to put the foam board insulation on the outside of the foundation and instead wants to put 3" Thermax foam board on the inside. Does anyone have experience with this? I worry about mold/condensation between the foam and the concrete.

House/stove details: 1400 sq ft ranch with walk-out front. House will have 2x8 walls with blow-in icynene insulation. Lotsa insulation in the attic. Chimney roughly in the center of the house with the stairwell nearby. Jotul F55 Carrabassett for a stove.

I'm not even close to starting the house yet and I bought the stove already!

 

[begreen]

Hi and welcome. Sounds like you are correct. If the foundation is raw, I'd like it to be exterior insulated. But it would be good to understand his concerns to see if they are valid and addressable. It could just be he is more familiar with one method and wants to provide you a product he knows how to do well.

What is being done to eliminate thermal bridging on the studs? Will this be a staggered 2x4 wall equaling 8" thick or will there be an insulation wrap on the building? If yes, I think the stove may be overkill. You might want to rethink it to something that can run cleaner at lower output like a small Woodstock cat stove. Or is 1400 sq ft just the main floor and with basement is it 2800 sq ft?

Get the heatload calcs done on the house soon. You might be surprised how little heat it takes to heat a small, super-insulated home.

 

[Knots]

Thanks. Yeah - I just have visions of weird stuff happening at that interface, especially in the summer.

 

[begreen]

A good source of information on these kind of questions is www.buildingscience.com

http://www.buildingscience.com/search?SearchableText=basement+insulation

 

[Knots]

Doh! My first thread and I put it in the wrong forum Thanks for moving it.

I will dig through that site this weekend.

 

[begreen]

LOL, common mistake. Tell us more about the house. Can you answer some of my second part questions about the construction and total heated area size?

 

[Knots]

The first floor will be 1400 sq ft. The stove will be roughly in the middle of the unfinished basement. It will also be heating the 26x28 two-bay under garage. And, when needed I will be sending some of the heat to the 28x28 garage hanging off the end.

The thermal bridging will be mitigated by 3/4" of foam between the foil-backed drywall and the 2x8 studs. Lemme see if I can find a pic.

 

[Knots]

 

[save$]

My brother had foam insulation put under the cement floor and on all the exterior of the cement walls. It was coated and sealed. There is an apartment in the basement and never cold or damp. Very easy to heat.

 

[Knots]

My brother had foam insulation put under the cement floor and on all the exterior of the cement walls. It was coated and sealed. There is an apartment in the basement and never cold or damp. Very easy to heat.

Nice. When you say "coated and sealed" do you mean that the foam was glued to the concrete or something more than that?

 

[begreen]

For sure keep us posted on how it works out. Don't insulate the basement ceiling and consider a couple key returns to the basement near the outside walls. Cooler air will sink down these returns to replace the warm air heading up the stair case. If you can make that stairwell open and with an oversized door at the top, go for it.

 

[Coog]

A good source of information on these kind of questions is www.buildingscience.com

http://www.buildingscience.com/search?SearchableText=basement+insulation

I second building science. When I am not reading posts here at hearth.com I actually hold a day job and have consulted with them professionally. Very knowledgable but very expensive. They have a lot of free resources as well. May want to tap into that.

I gotta ask,why 2 x 8 walls? I built my house 6 years ago and my exterior walls are 2 x 6. I ran calcs comparing 2 x 4 walls with 2 x 6 walls and figured there would be a 20 year payback at higher energy costs. I have never compared 2 x 8 walls but I gotta wonder if there is a whole lot to gain. Just a thought.

 

[velvetfoot]

I put 4" of foam board on the inside walls of my basement and I think it's drier in summer. Thinking about some foam on the floor, with something or other on top.

 

[save$]

Nice. When you say "coated and sealed" do you mean that the foam was glued to the concrete or something more than that?

Yes it was coated with some sort of black stuff below grade and something gray stuff above grade. Foam left unprotected from daylight will degrade. I think the foam is a couple in. thick and tongue and groove. My parents lived in that apartment for 30 years without problems. The house was built into the side of a hill so the top level is street level then you drive down around back and apartment is walk out ground level. The attached garage was also set up te same way so there were cars parked upstairs and down. Both living areas are mostly heated with two pellet stoves.

 

[woodgeek]

Lots of folks have done foam on the inside. The trick is not having wood members in contact with the cold/wet concrete. The Thermax is fire rated such that it doesn't need to be drywalled over, which is presumably why he chose it. I wouldn't want bare foil/foam walls in my basement....they will look crappy fast. E.g. one of your kid's little friends will visit and punch it full of fist holes for fun.

I think I would want a vapor permeable board (no foil liners) like EPS or XPS so it could 'dry inward'. But then you need to drywall over for the fire code. If you ever want to finish the basement walls (e.g. yourself after construction to save $$) give a thought to how you will hang wall boards. I guess you could tapcon wood furring to the face of the foam board and hang the wall on that.

I blather. Lots of options on the inside and pretty conventional, neither concrete nor foam nor tapcons molder.

Just don't forget to do under the slab. at least a couple inches.

 

[peakbagger]

I dont recomend foam on the outside as carpenter ants love to nest in it. If its outside any exposed foam is covered with a stucco coating but that doesnt stop the ants. Once they get in the foam, they will tunnel just under the surface and inevitably they will find a route into the house. I have 1" of foam on the outside that runs from ground line to the footings on all foru walls and they havent gotten into it but on the two sides I ran up the base of the siding, I wish I had never done it. I ended up cutting a 2" wide gap on top below the siding and after a few years of hitting it with pesticides, I got the buggers out of the foam. One the other two walls which are exposed concrete above ground line, I glued foam boards from the sill down 4 feet on the inside. There is still a thermal bridge via the concrete but it did make a big difference. Spray foam wasnt availlable in my area when I did it but I defintiely would have the interior sprayed next time.

 

[velvetfoot]

I guess you could tapcon wood furring to the face of the foam board and hang the wall on that.

That's what I did, only I routed a groove so it would be flush and then put another 2" layer over that. The drywall, attached by screws to the furring strips, is holding well. Whether it was cheaper than building a wall offset from the concrete and spraying, I don't know. You can run into my drywall and bounce off though.

 

[shoot-straight]

i would do a really good job with water proofing the outside with a membtrane, make sure i have good drainage as well. then stud the off the wall at least and inch and then sprayfoam behind.

 

[woodgeek]

That's what I did, only I routed a groove so it would be flush and then put another 2" layer over that. The drywall, attached by screws to the furring strips, is holding well. Whether it was cheaper than building a wall offset from the concrete and spraying, I don't know. You can run into my drywall and bounce off though.

Me too. They make 1.5" XPS that's precut: http://www.homedepot.com/p/t/100320...&productId=100320293&R=100320293#.UUHbmxnzI7A

I opted a little different (on a small area). I put up 1" continuous and taped XPS held in place by 1"x1" furring (over) with tapcons, and put a second layer of 1" XPS in the spaces between the furring. And then drywall into the furring. On a larger area, I would use the precut stuff, but I wanted 2" for some reason.

FYI, XPS uses a HCFC blowing agent so it has a large global warming footprint....they are working on replacement blowers, but for now XPS is not green.

 

[5 Furnace]

Hi,

I am an architect and a Certified Passive House Consultant. You are absolutely right to install insulation on the exterior of the foundation walls, under the ENTIRE slab, and ideally across the top of the footing before the block wall is laid up. You will find most builders are not into this, because this is way out of the normal for them. But you will find lots of helpful reference for such details at the Building Science Corporation's website: (broken link removed to http://www.buildingscience.com/index_html)

Getting the insulation on the outside is a no-brainer on new construction- the reason everyone insulates on the inside is that either they are doing so to existing basements, or they don't know any better. Once you backfill it is much more difficult to go back and do it over again, so get it right the first time. For those reasons get the best insulation, best vapor barrier, and best waterproofing you can find, and have them installed meticulously. Also check that sub-slab ventilation is not needed in your area- this would mostly concern radon or VOCs in the ground- such as from soil contamination.

The reason why you want to insulate any usable space is that the dew point happens around 54 degrees with air heated and humidified to room temperature and active use. That is also right about the average soil temperature- so it should be no surprise to anyone that basements are wet.

Insulation UNDER the block wall? Yes- Foamglas by Dow Corning is a load-bearing insulation board that you can use either under the block/on top of the footing, or under the spread footing. Good luck convincing your building inspector and your concrete guy- but point out to them that it has a greater bearing capacity than the soil you are bearing on.

Cheers!

 

[Swedishchef]

I agree with the above comments. Insulating the outside is the best option. However when I built 4 years ago I was not aware of this... (I learned lots since then).

Instead what I ended up doing in spraying 1.5 inches of closed cell, high density spray foam on the concrete on the inside and then placing R14 roxul between the studs that butt up against the insulation. I even sprayed in the rim joists to create a very air-tight seal.

The spray foam creates enough of a thermal barrier to prevent condensation on the inside (same concept of a cooler in the summer...how often do you see condensation on the outside when there's ice on the inside??). Works great for me.

Andrew

 

[velvetfoot]

FYI, XPS uses a HCFC blowing agent so it has a large global warming footprint....they are working on replacement blowers, but for now XPS is not green.

I'm not sure that's true anymore. I recall some printing on the later panels I installed about a new formulation and I copied this from the Owens Corning website:
• Zero ozone depletion potential
with 70% less global warming
potential than our previous
formula.

 

[Knots]

I gotta ask,why 2 x 8 walls? I built my house 6 years ago and my exterior walls are 2 x 6. I ran calcs comparing 2 x 4 walls with 2 x 6 walls and figured there would be a 20 year payback at higher energy costs. I have never compared 2 x 8 walls but I gotta wonder if there is a whole lot to gain. Just a thought.

My best guess is that I will be in this house for at least 25 years. I think it will pay in that amount of time.


Wow! Lots of good info here! It looks like I will be doing a lot of reading this weekend!

 

[woodgeek]

My best guess is that I will be in this house for at least 25 years. I think it will pay in that amount of time.

Why not use a proven high eff design and not guess? My concern is that 2x8 framing still gives disappointing assembly R-values due to thermal bridging if you don't put continuous foam on the outside. If you do put continuous foam on the outside, you could drop to 2x6 and put the money you saved toward making the foam on the outside thicker. IOW, 2x6 with FG or cellulose + foam sheets on the outside will be cheaper than 2x8 with cavity foam and perform much better.

A lot of people have done this optimization....the reason 2x8 with cavity foam is not done is because there are better, cheaper designs.

 

[Knots]

In the picture above it shows the continuous foam on the inside that allows the icynene to be between the drywall and the studs.