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Old 02-26-2016, 10:17 PM   #16
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Originally Posted by charles3526 View Post
Hello i am preparing to insulate my rim joists. I wanted to use spray foam and fill the entire joist that hangs over the outside of the house. I hear and seen people just use 2" foam board and close it off when it meets the bsmt walls.------ correct, seal the basement air/moisture from the cantilevered cavities. Is that the same as spray foam? I know it would be cheaper then it but it is more efficient with insulation and also keeping bugs out from comming in via the outside ply being shot to sh*t. Thats my next questiom can i also install a 2inch foam board on the outside of the rimjoist and spray around it to seal up any gaps?---------- if you add foamboard on the floor sheathing, you don't need rim joist insulation, but it may help control air/moisture infiltration through the rim. My joist hang over about two ft over the foundation so thats what im trying to clear. --------- you NEED to air seal with plywood to stop the critters. Any help would bw great thanks in advance.

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Old 02-27-2016, 03:53 PM   #17
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Upon further reading, Post #8, 11, and 13 are wrong in that you don't want any basement air/moisture getting to the cantilever cavity. Basic air sealing/insulation, many links available- here are two; Fig.7; http://ccetompkins.org/resources/sealing-air-leaks

https://basc.pnnl.gov/resource-guide...ilevered-floor

Post 15; "Growing old is mandatory, growing up is optional." I've added/changed my signature as recently a member private messaged (PM) me and said he was glad I would help him with his insulation planning because he was fearful of posting his question publicly in that he would be ridiculed.

If we are to take the time/effort/money on a project, may as well try to do it right the first time.

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Old 02-27-2016, 05:18 PM   #18
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Hi Gary,
Although I do agree that moisture concerns need to be considered, if the op does a good job of insulating and air sealing each bay, bottom, ends, and sides where exposed, then leaving the upper area of those cavities open to basement air should not be a problem.

Now, the plywood should be repaired if it is bad, but once air sealed and with 4" of rigid those cavities will not be cold. As for the basement being cold, the op is already improving this area, not sure if he intends to do the rest, but once the entire rim joist is air sealed and insulated that basement will be much warmer. Also assuming there is a furnace or boiler down there. Charles hasn't answered that as yet.

Bottom line is, does the op want warmer floor above. If yes, then leave the gap between the floor above and the new rigid insulation and improve the temperatures down there.

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Old 02-27-2016, 07:08 PM   #19
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So which is it? You changed your answer in an attempt to make it look like I was wrong..

Insulating the floor of the cantilever will keep the floors warmer and more comfortable as well as consistent moisture levels with the interior.

By closing them off, they will be heat and moisture synced with the ground. This is the same reason we try to cover cantilevers from the exterior. Putting foam across the bottom doesn't address the thermal bridging of the framing, but that will still be warmed by the roomside air.

Short of cladding the exterior in foam, I would insulate the bottoms as stated previously. It will keep the floor warmer and if insulated sufficiently, both moisture and heat loss are mitigated.

As far as the ridicule inference goes, I have never, nor would I ever ridicule a poster for any question. As a matter reference, I have vehemently defended posters before and chastised a regular poster that was less than friendly.

Not sure what your reference to post 15 is for. The only reference I see was the "internet police". Are you the Internet police?
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Old 02-28-2016, 02:45 PM   #20
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"then leaving the upper area of those cavities open to basement air should not be a problem." ----------- leaving any opening at the top is not "air sealed" as per Energy Codes; " Floors (including above-garage and cantilevered floors)
Insulation shall be installed to maintain permanent contact with underside
of subfloor decking.
The air barrier shall be installed at any exposed edge of insulation."
From; https://www.energycodes.gov/sites/de...ntial_BECU.pdf

The first link I gave said an air barrier:

  1. Install a rigid air barrier or other supporting blocking to separate the cantilever from the conditioned space.*-------- per code it is not part of the conditioned space due to exposed on the underside (to ambient temps) rather than in basement-- requires solid blocking or foam board over the separating wall space.
  2. Seal all seams, gaps, and holes of the air barrier with caulk or foam. ----- all gaps would include any opening at top.
  3. Install insulation without misalignments, compressions, gaps, or voids and align it with the sub-floor, the rigid air barrier (A), and the exterior face of the cavity.----- so it is aligned on three (plus ends) sides which INCLUDES the over-wall blocking/FB.
  4. Once insulated, enclose the cavity with a rigid air barrier material.From; https://basc.pnnl.gov/resource-guide...ilevered-floor
These are Energy Codes, readers should at least know moving air robs insulation of R-value, especially fiberglass (or other fibrous and even SPF) insulation, eg.in a wall cavity- similar to floor cavity, just exposed on bottom rather than side; http://www.greenbuildingadvisor.com/...formance-walls



All non-code literature I've read (of 25 recent ones) except one (Canadian contractor), required/suggested air sealing over the wall between conditioned/non-conditioned spaces. Gary
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Old 02-28-2016, 04:30 PM   #21
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Since when is "air sealing" what would be referred to as creating a barrier between conditioned spaces? The point of insulated the bottom, back, and sides of the cantilever is, in effect, moving it into the conditioned space.

The top of the cantilever is, as I understand, the conditioned floor of the kitchen or main floor room...is it not.

The open area of the cantilever is communicating with the conditioned space at that point and hence the reason that I am recommending it stay open to keep it warm.

The outer edges and corners of the cantilever should be caulked and sealed prior to that application of rigid foam board to the bottom, back, and end bay-sides.

You didn't answer the question about why you changed your answer I see.

When will have to agree to disagree here if the situation is as I understand it.
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Old 03-14-2016, 07:45 AM   #22
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Re: Rim joists question?


Hello thank yous for all ur answers. Im looking cut off any out bugs and air from comming in idk where else they woulf be comming in from. But besides the hang overa of the house. I also want to cut off the air comming from them caise all there is thats blocking it is some old ass ply wood its not rotted but u can tell its old. Im basivally looking to keep iut the crickets. So i was thinking of filling the cavity of the rimjoists with either spray foam,2" foam board or just stuffing them with batt insulation
And then covering each up with a piece of wood or foam board to its even with the interior cement wall.

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