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Best insulation to use for basement exterior walls (cement)

5 years ago

last modified: 5 years ago

Hi,

I am looking to get my basement finished soon and am looking for the best type of insulation to use. I thought I read somewhere about spray foam and/or foam board being the best? I had a contractor tell me they use R13 with the built in vapor “barrier”.

i just want to understand what to look for so I am educated about it.

Thanks!

Featured Answer

mainenell

5 years ago

I paid to have my basement spray foamed. It more than paid for itself the first year.

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Comments (11)

My3dogs ME zone 5A
5 years ago
last modified: 5 years ago
I Googled your question and came up with lots of sites with info -HERE
Bri Bosh
5 years ago
Spray foam. Subgrade cement walls are like a sponge. You’ll never have to worry about musty moldy insulation with spray foam.
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Technically 1" of XPS versus 2" of XPS would be about half the cost. So, is that worth the savings ultimately? I will not have in floor radiant heat for the basement. When you say slab I keep thinking you mean a slab built house, to reiterate, this is a basement and local code *here at least* does not require ANY rigid foam for the basement slab. Not sure if it even is required on the foundation walls. I guess this zoning ordinance is behind on code updates. -shrug- I see myself as one in a few that will be considering rigid foam for any basement application in new construction here. Is there any benefit to using XPS over the EPS for the foundation WALLS? I know I can get some EPS cheaper per R-Value but it will be thicker. I was thinking over 2" of EPS foam would be troublesome to fasten and then cover. Any recommendations on what to cover the foam with before backfill? Or should I only worry about covering the exposed foam on the walkout basement portion? I intend on using a waterproofing prior to installing rigid foam to the foundation walls.
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The building industry for some time now in basements has been engaged in a haphazard "cross your fingers" approach to trying to insulate without creating mold/moisture/etc problems by using vapor barriers, liquid membranes, sealants, etc. It turns out it was mostly for naught. The research shows the best insulation configuration is to just simply let the dang vapor go where it wants. No plastic, no vapor barrier, nothing is best when it comes to the insulation, even in cold climates. Rock wool is actually a good product for basements, I'd leave it, but I'd peel that vapor barrier off. Then I'd get some mildew resistant sheet rock(the green stuff) and drywall over it. The green sheet rock is not in any way moisture or mold invincible, but it is slightly better than normal sheet rock. Make sure to keep the sheet rock up off your slab and 1/2" or so away from that corner you talked about. Also, in the corner, if that is an untreated stud, I'd pull it and put a new ground contact rated treated stud. They "technically" are not approved for inside use but you don't want untreated wood up against your foundation. You can seal between the new stud and the foundation with caned foam and put a piece of synthetic trim like PVC or whatever you want in to cover the gap between the drywall and stone.
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Pinebaron
5 years ago
last modified: 5 years ago
When I personally finished our 1600sf basement in our last house in NJ, I first applied concrete sealer to all the walls, follwed by 1" foam board insulation glued to the walls using a suitable spray glue before framing the walls. Once I finished framing I had someone install R38 insulation between framing studs since I did not want to breath the stuff. I was extremely happy with final results.
robin0919
5 years ago
Spray foam. Does your area require vapor barrier?
seosmp
Original Author
5 years ago
Thank you all for the responses! I didn't get any notification of replies, so glad I decided to check today!

The latest information I've been tracking is here: https://www.homeconstructionimprovement.com/how-to-insulate-basement-walls/ - using 2" XPS foam. So that's what I was leaning towards.

I did see spray foam is the best but pretty costly. Perhaps I should just get an estimate or 2 to see what it would be. I know there is a company right around the corner from my house as I see the truck there.

I'm not sure about the vapor barrier but will try to find out.

Thanks!
worthy
5 years ago
last modified: 5 years ago
See Building Science Corp. here.

As long as you have at least 1 inch of XPS (extruded polystyrene) tightly sealed with the appropriate tape against the concrete wall, you will be fine in all but a very cold climate. Then you can add fibrous insulation to bring it up to at least the Code minimum in your climate.
Source: Building Science Corp.

Closed cell spray foam (ccspf) will work well too, but is much more expensive. But again, you can use a layer of ccspf followed by fibrous insulation.

A separate vapour barrier is not necessary, but still may be required by a municipality that is not up on the latest in building science.

As per the schematic, be sure the rim joists are similarly insulated, not just stuffed with fibreglass.
seosmp
Original Author
5 years ago
Thank you both!

mainenell, did they frame first and then spray? Do you recall the approximate cost / square feet? I have 2 companies coming to give estimates next Mon / Tues.
robin0919
5 years ago
You should have gone with ICF . The BEST!!!! IMO....you should have done your homework. See what Worthy said.
whaas_5a
5 years ago
ICF is extremely expensive in some areas, like mine. Rigid exterior and interior foam along with spray foam in the sills is a very efficient and cost effective option. So ICF isn't necessarily the best in all markets. My basement in the winter stays stable at 55 degrees as an unconditioned space right now without insulated interior walls. Even when it hits -20 outside.
mainenell
5 years ago
It was sprayed directly onto the interior walls. This is not, and has no potential to be, a finished basement. It is a rock foundation with a very thin cement floor. Maybe an inch. I wish I had sprayed to the basement floor.

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