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airtight goodness or moisture problem?

Posted by Noctua (My Page) on
Sat, Jan 25, 14 at 13:39

Redoing the house we went with 3 inch exterior xps to be above dew point.

Inside in the basement I am looking at my rim joists.
Mayny places online say air tightness is key and under no circumstance should fiberous insulation be used.

I am concerned is I spray foam the rim joist I am makinging a structurally reducing mold sandwich.

My basement walls are 6 feet concrete then 2 feet then joists. The first 6 feet is 2 inch xps glued. The cement top has a 2x4 set into the outside edge (not treated/no vapor barrier/capilary break -1950s) I have not seen any rot along it anywhere.

From the out side I have 1 foot exposed concrete before the wall starts.

Is the whole area from cement to joist-top the rim joist or just the area the joists are in?

Any links to the/a proper method would be great


Follow-Up Postings:

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RE: airtight goodness or moisture problem?

Just the top portion would technically be the rim joists. But you could certainly spray foam the whole area between the top of the concrete and the underside of the first floor. It would provide the most effective insulation and air seal in a complicated hard-to-reach framing jungle.

However, I would use closed cell spray foam (ccspf) to provide the best seal and highest R Value.

See the document below from Building Science Corp. on using spray foam on rim joists. Note though that the exception for thermal protection may not apply in Canadian jurisdictions; I am in Ontario and have been required to protect sprayed rims.

Three inches of XPS (R15) is quite the retrofit. I take it this is behind new siding.

Spray foaming rim joistsPhoto: Building Science Corp.

Here is a link that might be useful: Spray Foam at Rim Joist

This post was edited by worthy on Sun, Jan 26, 14 at 0:53


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RE: airtight goodness or moisture problem?

I have been using tiger foam in the ceiling joists so I would use that in the rim too.

My big concern still being external foam (air barrier) and internal foam (air barrier) with wood (mold food) between.

The stack wall just adds extra flavor to the question.

ps is this the right forum, I coukd not find an insulating one. Could have done basements or this old house I guess.


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RE: airtight goodness or moisture problem?

might try hvac forum. There is a good energy guy there with many tips on how to air seal properly. I think his name is energyrater or something like that. Give it a try and be prepared for lots of advice!


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RE: airtight goodness or moisture problem?

I see your concern: 3.5" of XPS on the outside and a similar amount of spf on the inside of the above-grade portion of the basement. Since both foams in that thickness are relatively impermeable, what happens to the moisture?

But where is that moisture to come from if both applications sealed in the first place? Certainly not from condensation in the rim joist area. Not from air leaks from the exterior. (That still leaves rising damp.) If you're still concerned, use open cell spray foam on the interior. Open cell foam is not a vapour retarder. Also, be aware that in all cases you should still consider mechanical dehumidification of the basement to keep relative humidity to below 50% in the summer.


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RE: airtight goodness or moisture problem?

Where will the water come from indeed. I prefer to plan as if water will get in while doing all I can to keep it out.

I have a HRV that should handle the dehumifing, I think.

So far I am thinking of just spraying the rim joist with tiger and using roxul on the other areas... cutting around the strong backs will be a terrible job. Still reading online for other ideas.

The bottom portion where the pony wall conects to the cement seems like a problem area for air. Maybe I can get open cell for that or caulking.


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RE: airtight goodness or moisture problem?

I prefer to plan as if water will get in while doing all I can to keep it out.

SPF, Roxul above grade and foam boards are all unaffected by water or moisture. OCSPF is not a vapour retarder, so any moisture drive to the interior in summer will not trap moisture.


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RE: airtight goodness or moisture problem?

I know open cell will dry inwards... I just happen to have 200 board feet of closed cell and no open cell.
:D


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