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Basement Insulation / Vapour Diffusion


A couple questions: Would there be any drying issues with insulating a greener foundation (Nov.2012) on two sides with foam? - Are we asking for trouble at the sill ?

We were planning to heavily insulate our (new house) basement with XPS on the exterior only, but may end up with additional foam in some areas of the basement interior as well, depending on how it's to be used / finished. The small mechanical room, for instance, could get EPS (type II, 2 1/2'') under plywood on the interior.

I know that the moisture in the concrete needs to go somewhere at sometime, and with the potential for such complete sealing around at the sill / rim joist area with spray foam, and possibly foam on two sides, can that (or future) moisture in the foundation still effectively dry, in or out ?

Also, as an aside, can anyone verify if longer lengths in IDP type foam fasteners are available ? -While we located one listed online (HILTI) http://www.hilti.com/holcom/page/module/product/prca_productdetail.jsf?&..., they informed us that the product is not available.
We are hoping to locate some IDP's long enough to secure two layers of reclaimed 3'' EPS panels (2'x4' w / shiplap) to the outside of our foundation in one shot. The longest ones we found were just 4 1/2'' overall.

Thank you,

Asked by jessie pratt
Posted Jan 21, 2013 12:13 PM ET


10 Answers

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Insulated concrete forms (ICFs) include rigid foam on both sides of the concrete, and these walls work fine.

Damp concrete is strong concrete. The longer you can keep concrete damp as it cures, the stronger it will be in the long run. So if ICFs or interior rigid foam help keep concrete damp, the result is stronger concrete.

I'm sorry to say that I can't help you locate the Hilti IDP fasteners you are looking for. Are there any GBA readers who can help?

Answered by Martin Holladay
Posted Jan 21, 2013 2:14 PM ET


Thanks Martin.

Answered by jessie pratt
Posted Jan 21, 2013 2:55 PM ET


Marc Rosenbaum used the Hilti IDPs on a recent retrofit to his (now former) Martha's Vinyard digs, mayhaps he has a distributor/retail type source.

They can also be had online direct from Hilti:


or from other online sources:


Answered by Dana Dorsett
Posted Jan 21, 2013 5:01 PM ET


Thanks Dana.
Yes, I read that piece by Marc Rosenbaum. Haven't checked eBay for IDPs yet.

If anyone has used longer foam fasteners . . . please pass along a source.


Answered by jessie pratt
Posted Jan 22, 2013 12:45 AM ET


I've just reread one of your pieces - (06 / 15 / 12) Joe Lstiburek Discusses Basement Insulation . . .

and I would like some expert clarification to my first question in this thread regarding where moisture can escape if a (green) foundation wall is foamed on both sides, and also likely spray-foamed at the sill / rim joist area.

Again, Is there a concern of running into problems (rot or mold) at the sill, and or (in our case) "RimBoard" ?
- which is awhem! - glorified OSB.

As in Dr.Joe's discussion, republished by you, Joe accepts that any moisture (or condensation) will...'experience some drying activity at the top of the foundation wall'.

GBA - Here's the news story: Lstiburek now says that the advice he gave in the Builder’s Guide was wrong. Lstiburek said, “I made a mistake. The insulation just needs to be warm enough to control condensation from the inside. The perm rating doesn’t matter. It’s OK for the concrete to be wet. The concrete doesn’t have to dry to the inside.”

Lstiburek's former recommendation to use vapor-permeable foam was due to a concern that imperfections in the foam installation might allow some warm interior air to contact the cold concrete, leading to condensation. However, experience has shown that most foundation walls, even those with small amounts of condensation, experience some drying activity at the top of the foundation wall.

Martin, Isn't it kind of like putting a piece of poly over a pile of wet wood on the ground (like the wet wood and TJI's and RimBoard above our foundation) and hoping to not discover a variety of different coloured molds when it's uncovered in late spring (or when the water is trying to escape from our wall assembly in the spring) ?

- when it warms up, heat and moisture is going to rise up into the sill / rimboard area if that's the easiest pathway ( i.e moisture and air contained on two sides by XPS, EPS and sprayfoam), no ?

To clarify, we were thinking of bringing the XPS (two layers of reclaimed 3'', 2'x4' panels) up to the top of our main floor deck level, which is sitting upon a foundation knee wall.


Answered by jessie pratt
Posted Jan 22, 2013 12:11 PM ET


First of all, at the top of your concrete foundation wall, you need a capillary break. Most builders use foam sill seal for this purpose. The capillary break will keep your rim joist dry, and prevent wicking.

It's perfectly OK to enclose concrete on all sides with polyethylene or rigid foam. Concrete is not wood. Concrete is used for bridge footings; it can sit underwater for decades. As I wrote earlier, damp concrete is strong concrete.

Answered by Martin Holladay
Posted Jan 22, 2013 12:22 PM ET



Yes, we've got the sill gasket in place.
Our concrete is certainly strong, though it's not our concern; we're somewhat worried about the pathway the water it contains will take when exiting the building. More specifically, are we asking that the moisture in our green foundation find its way through our OSB 'RimBoard' on it's way, eventually, to the atmosphere ?

About 8' of this RimBoard arrived on site with mold already on it - likely from sitting under a used lumber tarp / insufficient handling.

Again, I love strong concrete as much as the anyone. Not that big on molds.


Answered by jessie pratt
Posted Jan 22, 2013 1:26 PM ET


Again, the way to protect your rim joist is by careful installation of the capillary break.

It's also important to make sure that the rim joist stays dry; you can do that by installing rigid foam on the exterior of the rim joist, and keeping your basement air dry.

Answered by Martin Holladay
Posted Jan 22, 2013 1:36 PM ET
Edited Jan 22, 2013 2:17 PM ET.


The foamy sill gaskets are way better than nothing, but not nearly as good as metal flashing (yeah, I know, thermal bridging gone wild) or EPDM membrane.

If there isn't a capillary break between the footing and foundation wall there is still a rationale for keeping the interior foam semi-permeable- it's not ENTIRELY about wintertime moisture migration into the concrete from interior moisture drives (even if convected)- capillary wicking moves far more moisture than incidental air leakage, and allowing the concrete to dry toward the interior isn't a bad thing compared to allowing copious efflorescence on the above grade exterior (or even freeze/thaw spalling, should the moisture levels of the concrete reach that point.) This is obviously easier to deal with in new construction rather than retrofits.

Answered by Dana Dorsett
Posted Jan 23, 2013 4:24 PM ET


Thanks Dana and Martin.

Answered by jessie pratt
Posted Jan 23, 2013 8:16 PM ET

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