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Chasester 12-15-2009 01:43 AM

Rigid foam Install in Basement

This is sort of close to the post a couple below me.

I have a basement that had moisture problems for sure. My dad and I ripped the interior walls off of the concrete blocks and the blocks were indeed wet. Not water running down but not dry either. We dried them out, applied moisture proof paint to them and put up the studs again (this time with pressure treated wood).

Plans are to dig a trench in the spring and configure for proper draining outside the walls.

Now I'm wanting to insulate what we've done and saw the manufacturers recommend rigid foam (I live in zone 4) And the Gov't says R value for basement wall should be 11. I got the Dow SuperTuff 2" which has the R value of 12.

My plan is to cut the foam inbetween the studs.

Question being. Should I allow an inch or so between the back of the board and the concrete blocks (ergo flush it with the studs) or should I put it directly on the concrete.

or should I go an entirely different route like spray foam?


Bob Mariani 12-15-2009 07:06 AM

spray foam is by far the best route. If using solid foam you want to install it directly on the masonry wall sealing all joints completely. Then build the stud wall 1" away to provide air barrier to allow any moisture to escape. The studs are not insulation and if you install only between the studs you will still have a fairly large heat loss.

Piedmont 12-15-2009 09:32 AM

You don't actually need that 1" space, the only reason it's done is because foundation walls are notoriouslly uneven or out of square (some are even stone) it's very hard to put a stud wall flat against the foundation. Also if water penetrates it should not hit the stud wall. The bad thing, that air space increases the chances of condensation pooling and you have to seal the wall every 10' horizontally and vertically (no space for fire blocking/drafting) and bumping it out makes it pretty hard.

I hate to be the bearer of bad news but the foam you got is polyisocyanurate, it doesn't allow drying to the interior and foundation walls need to dry. Fortunately you can install it on the top halfs of your foundation walls (fasten each sheet sideways on the top) then switch over to unfaced XPS foam (make sure it has no plastic facing and that's eXtruded PolyStyrene often pink or baby blue but can be other colors). The reason it can be installed on the top halfs of your foundation is because typically the tops of your foundation are exposed to the exterior so the top can dry to the exterior if you use polyisocynanurate. However the lower half can't, so you have to use XPS foam (unfaced) to allow drying to the interior. Polyisocyanurate on top and XPS on the bottom is a good setup because 70-80% of your heat loss in your foundation walls is out the top portion where you'll be using the most R-Value. That's a good idea even with sealing. It'll save you a whole lot of $ as polyisocyanurate is crazy expensive too. I would probably go 1" - 1.5" XPS foam on the lower half, and use expanding foam or silicone to seal the joints. You don't want to have any holes/penetrations if you can help it. The exceptions are walls with a lot of exposure like walkout basement walls. Walkouts can be fully insulated with polyisocyanurate and the whole thing dry to the exterior.

Don't worry too much about the R-Value. You have to double the R-Value to save 50% of the remaining heat lost. That's to say
R1 lets 50% escape
R2 lets 25% escape
R4 lets 12.5% escape
R8 lets 6.3% escape
R16 lets 3.1% escape
R32 lets 1.6% escape
R64 lets 0.8% escape

As you can see your $ is better spent elsewhere than worrying about the difference between say... R10 and R12 unless a building inspector is actually involved. Although likely the best possible scenario (polyiso on top and XPS on bottom) bang for buck it isn't. This is one of those situations it's expensive to add a fraction of insulation and often the $ is better spent on other things or elsewhere. You may want to return all the polyisocyanurate and do 2" XPS on top and 1.5" on the bottom and call it a day. That will give you the most bang for buck, the 2" polyiso (R12) will save you 0.8% more than 2" XPS (R10) of your heat loss through the walls.

Bob Mariani 12-15-2009 12:17 PM


Don't worry too much about the R-Value.
When you finish walls in a basement you are required by code to conform to building codes. At a minimum this wall needs to have R13 value of insulation. So I guess you are saying if no inspection do it less than code. When any good job or any good pro knows that code is a bare minimum and we try to exceed those standards.

Piedmont 12-15-2009 01:35 PM

Yup, you caught me. There are some things about code that are great and wonderful, and others that aren't and todays code for basement insulation is horrible. They make it very hard to do properly AND prevent mold/mildew. The most important thing about basements is moisture control, not insulating. What good is having R13 or R38 basement wall that's got moisture build-up and moldy? That wall will certainly exceed code... you'll likely be ripping it out later for moisture issues. Code mostly treats basements like any other floor, when it shouldn't. I'd rather have R10 basement walls that are dry and let my foundation breath/dry preventing moisture issues than R38 that's wet and causes me to throw everything out in my basement and I mean everything. By code, he can install R15 batts in the stud wall and call it a day... he only needs R11 that is much better than code. It sure won't insulate or perform like R10 foam, sure sounds like he will have to rip it later too from water & mildew issues, and when the fiberglass gets saturated with water it won't be R15 anymore, but that wall exceeds code and what I have found professionals often do because it's cheap and easy and meets code. Code requires a barrier for any insulation whose perm is under 1. Put up a stud wall, insulate in between the studs (with fiberglass, mineral wool, or cellulose), put a vapor barrier up, and then drywall over that. Frequently done, meets code, but the #1 design to create a studwall that will look like a science experiment gone bad. You might as well sealed your studwall & insulation in a well sealed plastic bag and throw water into it on occasion there ain't nothing good that can come of it but it meets code and code is a minimum. Where in code does it explain fiberglass should not be installed in a basement or when it should? That the bottoms of basement walls can't dry to the exterior so need to dry to the interior and preventing that has known to cause issues (so bottoms of basement walls should not be covered with polyiso but the tops are fine)? Or that wall paper is a barrier so should be banned from being installed in a basement... any pro will install wallpaper in my basement if I ask and I'll likely be ripping it all out later because of moisture issues that will ensue. Likewise basement floors also should dry to the interior and if you put vinyl/laminate over a plywood subfloor you've just created a barrier and your plywood will be soaking but every pro I know will happily install vinyl over my plywood subfloor in my basement without mentioning that.

He can install the 2" of polyisocyanurate making R12 for the whole thing and that will meet code (the dead air space adds R1 = R13) ... and risk moisture problems not letting the bottoms of the foundation dry. But, this option meets code.

Or 3" of XPS foam, but that is too thick he's now in the situation he's putting more emphasis on insulating vs. the risk of moisture problems (3"-4" is actually okay in parts of Alaska).

If you're that worried use 2" XPS foam, you are putting a slight emphasis on insulating vs. moisture control and the dead air space adds R1 to the deal (you have to seal it for fire drafting reasons anyway) and every building inspector I know calculates dead air space as R1 = R11 which is what you said. If you need R13 like Bob said, I think your best option is to insulate the wall with 1.5" XPS with all seams & holes taped (just a pinhole can let in up to a cup of water/season) and fill in the stud walls with unfaced mineral wool. Unfaced because you have to let the foundation breath, and the foam will move the condensation point inside it. Not something I prefer over just foam. I hate fiberglass/mineral wool in any foundation stud wall but the most important thing is XPS against the foundation (or polyiso on top and XPS on the lower) and then insulate the stud wall how you see fit just no vapor barriers, any moisture vapor that comes through the foundation you gotta let it into the living space. Make sure you use a dehumidifier.

Gary in WA 12-15-2009 02:29 PM

I would go with Building Science, 1" or less of EXP (1.1 –Pink or 1.5 Blue- vapor semi-permeable) unfaced and batts in the wall, rather than 2” EXP (.55 or 0.75- semi-impermeable): “The best foams to use have a perm rating of greater than 1 perm for the thickness used. This means limiting extruded polystyrene insulation to less than 1-inch thickness for walls (more than 1 inch thick and they do not breathe sufficiently) and making sure that the rigid insulation is not faced with polypropylene skins or foil facings.” From: I didn't learn about basement foam until I came to this forum, Keep up the good work you two!

"This is one of those situations it's expensive to add a fraction of insulation and often the $ is better spent on other things or elsewhere." ---- That's for sure:

Be safe, Gary

MJW 12-15-2009 06:27 PM

Use Thermax on the walls and rebuild the studs on inside of that. Glue to wall and tape seams. 1 1/2 gives you almost R10, plus the block, you should be fine. Everything has an R value.

Where are you at that requires R13 in basements?

Chasester 12-16-2009 02:52 AM

Wow, thanks for the reply everyone. The studs are already up so I'll need to do something inbetween them. My concern now is spray foam will just give a nice safe haven for mold.

I'll have to do a lot of research for it.

sweaty 12-16-2009 08:07 AM

I would not finish or insulate anything until you have completely solved the moisture problem. You don't want a moldy basement.

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