Foam against the interior of the foundation wall is the recommended approach by Building Science Corp. and the US Department of Energy.

Not because it prevents moisture from entering the basement. (I hope your builder didn't actually say that.) But because by keeping the basement wall warm, it prevents condensation from forming on the interior concrete. Plus, for any bulk moisture (water) that may leak in and moisture drive toward the interior, it is unaffected while letting the moisture move towards the interior in a controlled manner.

A builder can use expanded polystyrene (EPS) or extruded (XPS) or closed cell spray foam (ccspf). Foam boards take much longer to apply to the wall and don't seal as tightly compared to spray but the material costs less. It's also possible to "flash" one-two inches of spf, followed by fibrous insulation.

Considering how hard it is to cover the walls properly with foam boards, the price quoted sounds reasonable. But I would only use ccspf, not open cell spray foam (ocspf).

Worthy, are you sure BSCs recommendation isnt for existing homes?

With new construction its usually easier to put it to the exterior which will keep the walls much warmer. It also does more to block moisture but certainly wouldnt rely on it as the only means. I think spray foaming closed cell on exterior basement walls is an excellent application.

The physics are the same.ÃÂ

Exterior insulation will do the same job. But all the problems with detailing the rim area--not to mention creating a home and road for termites--make interior insulation the first choice. Of course, most homebuilders build to a budget with Code minimum fibrous insulation and a vapour barrier, a combination often called the "diaper".

Here is a link that might be useful: Building Science Corp. on Basements

This post was edited by worthy on Fri, Dec 6, 13 at 10:01

Worthy, it sounds like foaming the interior of the foundation wall is the way. I think he said the foam would be an inch thick and then would put fibrous insulation next. That "diaper" doesn't sound good. Should I just make sure he doesn't put plastic or any other vapour barrier on the inside of the fibrous insulation?

Ha! Diaper.. Thanks for that link Worthy. Its a great one for sure.

The physics are certainly not the same though. From a strictly building science point of view, insulation is best on the exterior which keeps the dew points high (walls warm) and reduces thermal bridging. It also brings the thermal mass inside the conditioned space which definitely boosts energy performance.

From the link; "Exterior insulation best location from physics perspective, has practical problems with protection, thermal bridging and insects."

Iam surprised they are taking this newer stance of recommending insulation to Interior. Their main issue is the most surprising to me right now..

Protection: Exterior basement insulation is not exposed very long during the building process and not particularly vulnerable once its up in my opinion. Once its backfilled, what sort of damage will it incur? What am I missing here?

Thermal bridging: Ive always thought thermal bridging is usually less and was very surprised to see that one researcher found the brickledge location to be a major issue. If no brickledge, no issue as I see it. Even with a brickledge, a first course of foamglass would be ideal but realize the logistics and possible price increase is questionable.

Insects: Tough to argue with this one down south. There are insect resistant foams available. Best practice leaves a gap before wood framing which is a fairly challenging detail to get right in the field and of course this leaves a significant thermal bridge if not accounted for from the Interior.

This is why I love pre-cast concrete panels for basements because I dont have a choice in the matter, they only come with Insulation on the interior. Still, if pre-cast panels werent available and there was no brickledge I would probably spray foam to the exterior if the price was right and treat for termites which is required anyway. While I dont think the inspection strip would account for nearly the heat loss of a brickledge its probably significant enough to follow BSCs advice if building in termite country which is ever expanding these days.

Ha! Diaper.. Thanks for that link Worthy. Its a great one for sure.

The physics are certainly not the same though. From a strictly building science point of view, insulation is best on the exterior which keeps the dew points high (walls warm) and reduces thermal bridging. It also brings the thermal mass inside the conditioned space which definitely boosts energy performance.

From the link; "Exterior insulation best location from physics perspective, has practical problems with protection, thermal bridging and insects."

Iam surprised they are taking this newer stance of recommending insulation to Interior. Their main issue is the most surprising to me right now..

Protection: Exterior basement insulation is not exposed very long during the building process and not particularly vulnerable once its up in my opinion. Once its backfilled, what sort of damage will it incur? What am I missing here?

Thermal bridging: Ive always thought thermal bridging is usually less and was very surprised to see that one researcher found the brickledge location to be a major issue. If no brickledge, no issue as I see it. Even with a brickledge, a first course of foamglass would be ideal but realize the logistics and possible price increase is questionable.

Insects: Tough to argue with this one down south. There are insect resistant foams available. Best practice leaves a gap before wood framing which is a fairly challenging detail to get right in the field and of course this leaves a significant thermal bridge if not accounted for from the Interior.

This is another reason I love pre-cast concrete panels for basements because I dont have a choice in the matter, they only come with Insulation on the interior. Still, if pre-cast panels werent available and there was no brickledge I would probably spray foam to the exterior if the price was right and treat for termites which is required anyway. While I dont think the inspection strip would account for nearly the heat loss of a brickledge its probably significant enough to follow BSCs advice if building in termite country which is ever expanding these days.

Bridging is not an issue in a concrete wall. It is in above-grade framed walls, which is why I sheathe in XPS unless the architect has speced otherwise.

Most of the US is subject to termite problems. Ditto southern Ontario.

Dr. Lstiburek, who started his career as a builder, admits to having tried a number of ways to solve the rim problem, none of them satisfactorily.

Fiberglass in the rim joist area is a problematic choice. See this thread from theJournal of Light Construction forums, particularly the response of building expert Martin Holladay.

Termite Map of US

This post was edited by worthy on Fri, Dec 6, 13 at 19:13

Foam plastic used on the interior of the house will need a thermal barrier of at least 1/2" drywall.

OP - location means almost everything in a discussion like this.

I have a 4 year old house with fiberglass alone and no musty smell. The basement waterproofing needs to be done right, the ground needs to be graded right. Foam isn't necessary for that issue alone.

In a cold climate, you should absolutely insulate from the outside with rigid foam. If you are in a warmer area, this would be overkill.

Again location matters a lot.