rim joist confusion/question

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Old 02-16-17, 10:17 PM
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Rim joist confusion/question

Hello all,

Im a little confused about rim joist insulation as my house is a little older. When looking at other houses where the foundation goes up to the rim joist it makes sense to insulate with ridged foam and seal with great stuff as opposed to sticking fiberglass in the joist.

However, my house is built like this: Foundation, sill plate, 4 feet wood wall with studs , rim joist, and then subfloor.

Does it make sense to place ridged foam in the rim joist in this situation before i put fiber bats in the rest of the joist? To me it doesn't as I am reading that the reason to put foam there is to prevent condensation in the rim joist. My walls seem to be built like any other wall in the house that is located above the basement. The joist are high above the foundation unlike in other pictures you see when looking at reasons to insulate rim joist with rigid foam.

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Old 02-17-17, 12:47 AM
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If this structure is above grade then you are correct it is no different than any other exterior wall.
 
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Old 02-17-17, 02:44 AM
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Unless you have plans on covering the walls and rim, you won't be able to use exposed XPF. I would, likewise, treat it all like an exposed wall and insulate the walls and rim at the same time with the same material. I would recommend Roxul over Fiberglas, but both are good insulators, with Roxul being r15 for a 3 1/2" cavity and Fiberglas being R13 at best. Roxul is vermin proof, waterproof, fireproof, moldproof as well.
 
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Old 02-17-17, 03:40 AM
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I agree with Marq1 and Chandler.

Your 4' wall, the walls upstairs, and the rim joist all have the same issues, a cold exterior surface that needs to be protected from warm moist air from the inside. So a key component of those areas is a well sealed air barrier and that is why you often see rigid foam used as it blocks air as well as insulating.

In your application, air seal wall to foundation, add insulation, and cover with drywall sealed at all edges and penetrations like electrical boxes.

As for any exposed concrete it needs to be addressed in some manner as concrete is very close to an open window in terms of insulation value.

Bud
 
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Old 02-17-17, 04:38 AM
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I disagree that the same material should be used everywhere... the rim should always be insulated with foam as the insulation there will not be covered with an air barrier. (Warm air between floor joists can penetrate the fiberglass or Roxul otherwise and frost.) I agree that the rest of the walls can be insulated like any other exterior wall, provided you cover the wall with drywall which is your air barrier.
 
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Old 02-17-17, 07:04 AM
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@ Marq1 Every rim joist insulation recommendation I have seen are for joist that are above grade. So I don't think its accurate to say if its above grade no need. My point is that most joist in the reconditions are are an inch or two above grade on the foundation and mine is 4 feet.

http://www.energymastershome.com/wp-...reenshot_1.png


@ chandler I can use polyISO exposed. While i don't see any condition issues in the joist thats the whole point of the question. Generally your not suppose to use the same material (fiber) in the rim joist as the rest of the wall because of condensation build up in rim joist. So are you saying these are more like normal walls upstairs because of the distance from foundation and I can do same material up and down?

@ AMBud9051 the only exposed concreted is 2 foot foundation. How do people check for cracks and the condition of it if its covered? Would you cover it in my pic?

@ Xsleeper If thats the case then the entire wall needs foam as it is a exposed wall facing the outside. Where the wood framing starts above the sill plate is where the outside wall has contact with the outside.


And for everyone does it matter that its a stucco house? verse siding? Does the stucco act as air barrier ? And how do people check for termites if you drywall the framing in my pic?

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Last edited by Stormlight; 02-17-17 at 08:21 AM.
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Old 02-17-17, 07:51 AM
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Even a slab on grade home will address the exposed edge of the concrete. If you are going to the eddort to insulate the 4' wall area it would be important to insulate that concrete we are looking at.

As I posted, it doesn't matter where this assembly is located, crawlspace, or third floor, the issue is keeping the warm air away from a cold surface. Kraft faced insulation doesn't do that. Thus drywall all the way to the bottom of the floor above sealed at all edges, or you can go the rigid between the joists. Any basement or crawlspace can use fiberglass insulation in the rim joist IF they air seal it to keep moisture laden air out. People are using rigid as a convenient way to resolve the exposure to air and achieve a good r-value.

Bud
 
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Old 02-17-17, 08:33 AM
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@ Xsleeper If thats the case then the entire wall needs foam as it is a exposed wall facing the outside. Where the wood framing starts above the sill plate is where the outside wall has contact with the outside.
No, youre not getting it. The wall studs are just like any other exterior wall... once you put batts between the studs and cover it with drywall (air barrier) then paint it (class iii vapor retarder) there wont be as great of a potential for condensation.

The rim joist should be treated differently because it will not be directly capped with drywall (air barrier) or painted (vapor retarder). Putting fiberglass against the rim joist with no air barrier or vapor retarder allows air leakage through the floor joists. Air pressure on one side of the house (windward side) turns the joist bay into a conduit for cold air. As that cold air exits the house (Leeward side) it takes the moist warm air out where it forms frost behind the insulation on the opposite side.

All rim joists should be insulated with at least 2" of foam and air sealed because no one puts an air barrier over fiberglass between joists. This applies to your situation just as it would to the rim joists that are between floors on a 2 story home... or to a single story home with a rim joist that sits on a sill plate that rests right on the foundation. There is no difference.

As pointed out, any exterior wall that is cement will be cold and would benefit from insulation as well. But if foam is used, it must be covered with an approved fire rated barrier.
 
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Old 02-17-17, 10:16 AM
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Ah perfect explanation. Thank you so much! I think part of my confusion was that fact that we may not insulate between the wall studs and finish the area. Our goal was just to insulate between the joist space under the sub floor

Only questions left are if you were to cover the foundation to the floor notice how it juts out before it gets to the sill plate. (Then tapers in) That would be a truck load of insulation in that gap. Is there some way to box out the foundation concrete?

How are most people when they cover all of this checking for foundation issues or termites? Notice the effervescence in our foundation. Im a little leary of covering that as I like to measure how much is happing. (just bought a month ago)
 
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Old 02-18-17, 05:39 AM
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The potential for condensation in your case is minimized due to the fact that you have solid wood framing for a rim board and as sheathing. Solid wood is much more forgiving than osb or plywood in theses cases when it comes to the formation of condensation. The harder, compressed and chemically adhered construction of the osb make it much more prone to condensation formation. Solid wood can cycle vapor migration into and out of the fiber without issue unless a tremendous overload occurs or certain more dense woods are used.

I'm somewhat curious as to whether you even have a true separate rim joist. Are you sure your sheathing material is not in fact acting as the rim in your case? Not that it makes a difference from an insulation viewpoint but just an observation.

Insulation can be either the foam or a fiber material. The rock wool is certainly more dense and will limit air movement better than conventional fiberglass. You can also purchase R-15 high density fiberglass batts.

In any case, I would seal the gaps between sheathing boards with a foam sealant and carefully review all other framing intersects for air leakage. Since this area is at the bottom of the structure, air would typically be drawn inward from the exterior due to the "stack effect". The more control you have of incoming air, the more comfortable and stable the indoor air will be. Less movement will keep a more consistent temperature gradient within the dwelling. Of course, you should also carefully review the ceiling plane at the highest level to seal air leakage paths at light fixtures, mechanical openings, soffits and wall top plates that show within the attic space.
 
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Old 02-18-17, 02:29 PM
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"I'm somewhat curious as to whether you even have a true separate rim joist. Are you sure your sheathing material is not in fact acting as the rim in your case? Not that it makes a difference from an insulation"

Great point, the entire wall is one big rim joist. Now it makes sense.

With that said I if the goal is to insulate the floor above the basement then i still need to insulate these walls as they are acting as rim joist for the floor above.
 
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Old 02-19-17, 03:50 PM
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If you are insulating the floor then the space below that does not necessarily have to have the walls insulated unless there is some degree of heating delivered to the space.

The actual rim area is the space between the bottom of the floor sheathing and the top of the plate that forms the wall that the floor joists rest on. That area should be insulated to the same level that any exterior wall above is insulated. That insulation should have a suitable air barrier/vapor retarder applied to the inner surface. The floor insulation should then be installed between the floor joists and supported to insure that it will not fall out. Any vapor retarder on that insulation should likely be to the upside and put in contact with the back of the subfloor material.
 
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Old 02-19-17, 04:07 PM
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Let me add a wrinkle, half of this basement is a crawl space and half is concrete floor. Its vented.

I think the absolute best way to handle this is to seal the vents, vapor barrier the dirt on the crawl half and then only insulate the walls to treat the entire basement as its another level in the house.

However, until that happens does it even make sense to throw fiberglass in the rim joist below the sib floor with the vented air running through it?
 
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