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Old 02-06-2013, 10:34 AM   #16
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fiberglass, okay in walls but not rim joist?


Can't go wrong there.... MN tested f.g. with paper-facing vapor retarder on rims: http://www.buildingfoundation.umn.ed...default.htm#@D

Their conclusion:
"The data thus clearly demonstrates that it is not prudent from a condensation management perspective to fill the rim joists with batt insulation, regardless of whether they are covered with a vapor retarder or not. Free circulation of interior drying air in the rim joist cavity, particularly during the summer, would inhibit condensation as well as prevent moisture accumulation. Exterior insulation is effective in controlling winter rim joist condensation but tends to exacerbate summer condensation conditions, particularly in the presence of an interior vapor retarder. With the absence of interior batt insulation and an interior vapor retarder, the negative consequences of exterior rim joist insulation are mitigated. However, especially in retrofit situations, the amount of exterior insulation that can be installed (if any) may not be sufficient to provide adequate thermal insulation (at least R-10). Thus in these cases, the alternative is to install rigid insulation (such as foil-faced polyisocyanurate) flush against the interior side of the sheathing. This still allows air drying of the rim joist cavity but reduces the potential for interior summer condensation by decreasing the condensation surface to the bare minimum of the interstices of the insulation/ sheathing interface. In the winter, the insulation is protected from interior-sourced water vapor by the foil-facing. " From: http://www.buildingfoundation.umn.ed...oistphenom.htm

A stud cavity, x4" or x6" deep is a lot easier to air-seal (drywall) than a 25' -30' x 16" or 24" joist cavity, open to interior air. Vapor diffusion through the rim joist is also an additional problem; the low R-value of wood (R-1.25 per inch) with only R--2, the dew-point on the inside face is very close to the outdoor temperature, unlike pictured; with fiberglass insulation enclosed by the drywall (air barrier) and at R- 3.7 per inch: http://www.buildingscience.com/docum...nsulation/view

Gary

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Old 02-06-2013, 11:52 AM   #17
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fiberglass, okay in walls but not rim joist?


All the responses are appreciated, but I'm a little confused with some of them. Here's what I've gathered about rim joist insulation...


no insulation = bad
fiberglass (faced or unfaced) = bad
seal cracks/corners/joints with GS + fiberglass = acceptable
1" XPS with GS + fiberglass = good
2" XPS with GS = very good
2 part spray foam = best

Where am I going wrong?
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Old 02-06-2013, 06:31 PM   #18
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fiberglass, okay in walls but not rim joist?


here is the crucks of it all R value period bottom line you want R value. air infultration is combatted with using Good stuff cuts the air flow the zone your in requires a R-30 for crawl space 2" of XPS is not going to give you that you need the R-30 equivlant you can put r30 batts in there after the xps and GS and you got it covered. what a lot of people want to advoid is vermin making a nest in there. that is why a lot of people suggest using foam. as long as you meet the R-30 the inspector could careless what type of insulation you used. I would if it were me hire a person to spray foam in and call it good. because how many times doe you really go in to your crawl space? I would foam it and for get it.
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Old 02-06-2013, 10:46 PM   #19
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fiberglass, okay in walls but not rim joist?


"seal cracks/corners/joints with GS + fiberglass = acceptable"-------------------- no,

the foamboard warms the cavity side/wood rim to prevent condensation (and air-seal), fill with fiberglass after the foamboard. Post 16, last link. Fiberglass does nothing to warm the surface of the wood rim joist to prevent condensation of basement air there. Rigid f.b.'s quality of "insulating" raises the temperature so there is no dew-point or condensation; http://www.buildingscience.com/docum...apor-retarders Air (infiltrating/exfiltrating) can go right through f.g. and leave its moisture on the cold rim to rot.

Gary
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Old 02-07-2013, 06:03 AM   #20
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fiberglass, okay in walls but not rim joist?


as Gary said, it is not ONLY the air leakage that you want to address there, but the warming of the interior surface of the rim joist itself to prevent condensation (or sealing it from interior moisture). That is why a coating of foam or XPS board DIRECTLY sealed against the wood is best.... Think of it like your windows: Those thermal shades do a nice job of keeping your room warmer, yet every winter morning you wake up to windows covered in water or frost because you have just isolated them from the warm interior air, but not necessarily from the moisture. FG batting acts the same as the blinds in this situation. The difference is, you don't want to seal over your windows with XPS board or foam, but here you can .... Is that making sense?
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Old 02-07-2013, 08:41 AM   #21
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fiberglass, okay in walls but not rim joist?


Yes it does make sense. My only question is with the statement below in this link http://www.buildingscience.com/docum...endations/#Map versus the most common advice I hear.

"No interior vapor control required on the interior side of framed walls in climate zones 1, 2, 3, 4a, or 4b. In hot, humid climates, a Class I or II vapor control layer on the interior of the framing can, and often does, cause premature building enclosure failure due to inward moisture drive condensation (see RR-9302: Humidity Control in the Humid South). BSC recommends avoiding Class I or II vapor control layer on the interior in these zones, or any material that acts inadvertently like a Class I or II vapor control layer such as reflective foil insulations, vinyl wall coverings, glass mirrors and epoxy paints."

I am in zone 4a. So, if I follow that recommendation to avoid class 1 or 2 vapor barriers, then doesn't that mean I would be better to use 1" XPS (1.1 perms - class 3 barrier) instead of anything thicker? (2" is the most common recommendation I hear, but it is a class 2 barrier - .5 perms)

It sounds to me like 1" xps + GS + unfaced FG is the best option for those in zones 1, 2, 3, 4a, or 4b. Does that sound right?

Thanks for all the replies.

Last edited by 88bomber; 02-07-2013 at 09:01 AM. Reason: typo
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Old 02-07-2013, 12:40 PM   #22
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fiberglass, okay in walls but not rim joist?


The problem I have with foaming in the ENTIRE cavity is that it is now hidden from inspection. It could be rotting or infested and you wouldn't know. Foaming up the whole thing is quicker and easier than just caulking or foaming the edges to air-seal and installing foam board, but I'd be worried that something is happening under the wood and it would be much harder to check.
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Old 02-09-2013, 07:38 PM   #23
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fiberglass, okay in walls but not rim joist?


" BSC recommends avoiding Class I or II vapor control layer on the interior"---------interior, next to drywall, not toward the exterior.With 1", you will be safe at 35% RH and less as per Post #9.

Gary

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Clothes taking longer to dry?
Clean the dryer screen in HOT water if using fabric softener sheets.
They leave a residue that impedes air-flow, costing you money.
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