Spay foam on the inside of poured foundation to avoid mold?
I have a question regarding insulating a new construction poured foundation in zone 6B, close to 7A. Twenty miles north of Minneapolis.
Originally I was planning on either a T-Mass or ICF foundation. Both are somewhat more expensive and have a lower R value than what I am now proposing. The current plan is to insulate the outside with 2″ foam sheets. This will satisfy code. Then on the inside I would spray 2″ of foam and insulate a stud wall with either Rockwool or fiberglass. This wall would be around R-30.
I’m using spay foam on the poured concrete to eliminate any air movement between the conditioned space and the cool concrete to avoid any possibility of mold.
So the question is will spay foam keep me from having mold behind the insulation?
One more question about the foundation. I have been told that Minnesota code requires 5000 psi concrete and this will not need a moisture barrier. Does this eliminate the need to have a moisture barrier?
Thanks in advance,
Jim
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Jim,
If you install 2-inch-thick rigid foam on the exterior side of your concrete wall, you've gone a long way toward reducing the chance of condensation or mold. (The exterior rigid foam helps keep the concrete warm during the winter.)
If you want to install interior spray foam on the wall, you can -- and the interior spray foam will certainly further reduce the chance of condensation or mold.
You'll end up with a fairly complicated basement wall, however. You might find it easier to install all of your foam insulation on just one side of the concrete -- either the exterior or the interior.
I'm not sure what you mean by a "moisture barrier." It's always a good idea to include asphaltic dampproofing on the exterior side of a concrete basement wall. You might even consider the use of a waterproofing system that includes exterior dimple mat.
For more information on these issues, see How to Insulate a Basement Wall.
-- Martin Holladay
Martin, sorry I realize I used the wrong term. I meant capillary break. I will need to damp proof the outside of the wall.
I think MN code requires R 10 on the outside of the foundation. I feel that much more than 2" on the outside makes the transition from above grade wall to foundation difficult. Either way, it needs to be covered but if it is too thick it will need to be flashed out. (more?) You are right about using the insulation on the inside. I try to limit my use of spray foam but the last thing I want is mold. The interior stud wall will give me a finished wall which will need to be done at sometime anyway.
Jim,
If you want to limit your use of spray foam, you should install rigid foam rather than spray foam on the interior side of the basement wall. The technique is described in the article I linked to (How to Insulate a Basement Wall).
If you are interested in lowering your costs and being environmentally responsible, you can purchase reclaimed (recycled) rigid foam for about 1/3 the price of new rigid foam.
When it comes to capillary breaks, a minimum requirement is to install a capillary break (usually closed-cell foam sill seal) between the top of the basement wall and the mudsill. In addition, it's usually a good idea to install a capillary break between the footing and the basement wall. For more information on this topic, see Capillary Breaks Above Footings.
-- Martin Holladay
Thanks Martin, I have been watching for used rigid foam. I found some on Craig's list but the seller didn't respond. I can't help but wonder where this used foam comes from. Are there sources other than Craig's list that anyone knows of in the Minneapolis area.
I'll read the links provided.
And thanks for the capillary break info. That's what I suspected.
Jim
http://www.insulationdepot.com/ (877) 311-0175
I needed to talk to them so asked if they had anything close to Minneapolis. They have 1 1/2" Iso in Iowa City for under $11 per sheet. It's on Craiglist for Iowa City too. Another ad had 2 1/2 thick Iso in Quincy IL for $16
Even virgin stock 2" rigid polyiso at box store pricing is half the cost of closed cell spray polyurethane foam. (Menards currently has it on sale for $27/sheet for a 4'x8' x 2", which is 42 cents per board-foot, but even at full-retail it's under 50 cents per board-foot.)
Since it has R10 between the polyiso and "the great outdoors" and only R15 between the polyiso and warm interior it doesn't need to be derated for climate. You can assume it will perform near it's labeled R-value in this stackup. The mean temp through the polyiso will be about 40F at the mean January outdoor temp of ~13-15F in your location. Most 2" polyiso would be delivering ~R11 under those conditions:
https://buildingscience.com/sites/default/files/styles/panopoly_image_original/public/bscinfo-502_figure_02r_web.jpg?itok=wB_w82ml.
There is the additional cost of labor when using rigid foam vs. spray polyurethane but it's a fairly easy DIY when trapped to the foundation by a studwall.
Reclaimed foam can sometimes be sourced from commercial roofing companies.
These guys have some 2.5" EPS (which would also perform at about R11 at a mean temp of 40F through the foam) for $8/sheet, which is fair:
https://minneapolis.craigslist.org/hnp/mat/6055549574.html
Thanks- You guys are such a valuable resource!!
I do plan on doing the insulation myself.