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airtight goodness or moisture problem?

10 years ago

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

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Comments (7)

worthy
10 years ago
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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Noctua
Original Author
10 years ago
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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The concrete pad may have cracks or expansion joints that are leaking, it sounds like an unusual construction in that someone went to all that trouble to build those piers. I am assuming the pad is open to the weather? Being late winter assuming you have snow, there will be a lot of freeze/thaw going on....the existence of the piers may be actually because of ground water, the underground parking in my old office building had its lowermost level on concrete piers and a plethora of sump pumps, because of ground water. The other factor to consider is that concrete is not really waterproof unless it is treated, or prestressed. The Romans knew how to make concrete waterproof but we haven't quite figured out how they did it..that's what my Engineer Dad used to tell me, anyway. Another possibility is that now you've enlarged that hole in the blockwork (I assume you checked whether it was safe to do structurally) - I assume the blockwork is in part of the heated part of the house? - you have effectively added that space to the conditioned part of the house. It could be that humidified air is entering the space and hitting the cold underside of the concrete slab, and creating condensation. It is not really desirable to have that moisture there, presumably it could run against the blockwork and cause dampness within the house. You are probably violating some codes attempting to use that space as you describe, too. You may need to get an engineer to look at that and determine where the moisture comes from and how to stop it. You may need to add a sump or something, you might have to apply insulation (spray foam perhaps) to the underside of the concrete. Ventilation is also something you may need, I doubt it's advisable for it to ventilate into the house. You may need some sort of "snorkel" type pipe to go up the side to the outside air, it would also equalise the temperature underneath so there won't be the condensation if that's what it is. Get an Engineer in there.... NOT a 'moisture control/basement flooding 'expert' although you may need them later. Meantime, 1) work out if the moisture is condensation dripping down 2) consider removing the plastic if it is making it worse, although I guess it's keeping the water out of the soil.
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Bottom section of the door itself. I can't remember at this point where else we saw signs of moisture problems as it's been months since we looked at it. We rather quickly ruled it out because of that and general lack of an anal retentive owner who took good care of the place. Obviously reconsidering it now though if price was low enough. I know a good GC in the area. I don't have all that much confidence in inspectors.
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cindywhitall
10 years ago
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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worthy
10 years ago
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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Noctua
Original Author
10 years ago
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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worthy
10 years ago
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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Noctua
Original Author
10 years ago
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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