Insulating "Rim Joists", But No Rim Joists
#1
01-09-17, 09:39 PM
Insulating "Rim Joists", But No Rim Joists
Hi,
Not sure how often this comes up, but my house has no rim joists. It's also not balloon frame construction. The first floor's joists sit on the bottom sill below, which sits on the cinder block foundation, and the wall's bottom plate sits on the floor joists. It's still standing 85 years later, so I'm not going to add one.
But it creates some air sealing and insulating questions.
Normally, from what I've read at least, the first step in insulating rim joists is to caulk the bay to air seal the four edges. Since I'm just looking at extremely leaky horizontal siding, rather than a Rim Joist, should I still attempt to air seal this? My thought was that if water ever gets behind this wall I'd be blocking the draining route. i.e. the same reason you don't caulk a wall from the outside. So my plan is as follows:
1) shove fiberglass down the cinder blocks and foam seal the blocks closed as well as the space b/t the cinder blocs and the bottom sill.
2) caulk at least the inside side of the wall bottom plates, but leave it clear where the wall's bottom plate hits the outside wall.
3) cut rigid foam for each chamber and spray foam it in place.
Is this what you think is appropriate in this situation? Or, do you think I should air seal the horizontal planks to the sill and plate, etc. before doing step 3)?
I'm attaching a photo of one of the bays.
Thanks for your help.
Not sure how often this comes up, but my house has no rim joists. It's also not balloon frame construction. The first floor's joists sit on the bottom sill below, which sits on the cinder block foundation, and the wall's bottom plate sits on the floor joists. It's still standing 85 years later, so I'm not going to add one.
But it creates some air sealing and insulating questions. Normally, from what I've read at least, the first step in insulating rim joists is to caulk the bay to air seal the four edges. Since I'm just looking at extremely leaky horizontal siding, rather than a Rim Joist, should I still attempt to air seal this? My thought was that if water ever gets behind this wall I'd be blocking the draining route. i.e. the same reason you don't caulk a wall from the outside. So my plan is as follows:
1) shove fiberglass down the cinder blocks and foam seal the blocks closed as well as the space b/t the cinder blocs and the bottom sill.
2) caulk at least the inside side of the wall bottom plates, but leave it clear where the wall's bottom plate hits the outside wall.
3) cut rigid foam for each chamber and spray foam it in place.
Is this what you think is appropriate in this situation? Or, do you think I should air seal the horizontal planks to the sill and plate, etc. before doing step 3)?
I'm attaching a photo of one of the bays.
Thanks for your help.
Last edited by Gunguy45; 01-09-17 at 11:30 PM. Reason: Changed title...though calling your rim joists fat and lazy would be insulting.
#2
01-09-17, 09:47 PM
Member
I'd still stick the 2" thick foam in and seal up any gaps with expanding foam from the inside.
#4
01-10-17, 03:48 AM
I did mine last week (after living here 17 years 
) and used Roxul. Waterproof, vermin proof, mold proof, and fire proof with a rating of R15 per 3 1/2". I could really tell the difference. No need to "stuff" it, as it is fairly rigid and will friction fit the opening. "Stuffing" fiberglas reduces its R value.
) and used Roxul. Waterproof, vermin proof, mold proof, and fire proof with a rating of R15 per 3 1/2". I could really tell the difference. No need to "stuff" it, as it is fairly rigid and will friction fit the opening. "Stuffing" fiberglas reduces its R value.
#5
01-10-17, 04:19 PM
I think I'm going to seal the area underneath the bottom sill and evaluate how bad the air leakage is in the bay. I'm thinking it's coming through the horizontal exterior wall boards as well as from below, but it's hard tell. When you say Roxul do you mean the board that they make or the batts? Unfortunately two things, the board is hard to find around here, at least at the orange or blue stores, and I don't think the board has a vapor barrier. But, if after sealing under the bottom sill the air infiltration is not as bad as I currently imagine it to be, then maybe the batts would work.
I was looking into the Roxul board for the reason you state, vermin. I already have a lot of Polyiso to use, but I bought it before realizing 1) the r-value in cold weather falls below that of other rigid foams and 2) the supposed potential carpenter ant issues of all rigid foams. I would buy the Roxul board in a heartbeat if it were available because of #2. Nonetheless, it's not a ton of rigid foam being installed here and can be monitored, so I'm not too concerned.
I was looking into the Roxul board for the reason you state, vermin. I already have a lot of Polyiso to use, but I bought it before realizing 1) the r-value in cold weather falls below that of other rigid foams and 2) the supposed potential carpenter ant issues of all rigid foams. I would buy the Roxul board in a heartbeat if it were available because of #2. Nonetheless, it's not a ton of rigid foam being installed here and can be monitored, so I'm not too concerned.
#6
01-10-17, 05:04 PM
Roxul is batt insulation made of rock wool. I think you may be referring to old tarboard used last century. Roxul Safe 'n' Sound 3 in. x 15-1/4 in. x 47 in. Soundproofing Stone Wool Insulation (12-Roll)-RXSS31525 - The Home Depot
#7
01-11-17, 10:03 AM
The product I was thinking about that's made by Roxul is referenced in this article:
GreenBuildingAdvisor.com | If Ants Like Rigid Foam, Should We Stop Using It?
It's called Roxul ComfortBoard. Here's a link to the company's website on this type of board.
ROXUL COMFORTBOARDâ„¢ 80 | Mineral Wool Insulation Board
GreenBuildingAdvisor.com | If Ants Like Rigid Foam, Should We Stop Using It?
It's called Roxul ComfortBoard. Here's a link to the company's website on this type of board.
ROXUL COMFORTBOARDâ„¢ 80 | Mineral Wool Insulation Board
#8
01-11-17, 03:33 PM
It's close to the same thing I referred to, but your link is for commercial products. The Safe 'n Sound is available at most home centers. Specifications are almost identical.
#9
01-11-17, 04:45 PM
One of the advantages of the rigid foam board is that it can be easily sealed into those cavities closing off all leaks behind it. Then the insulation value keeps the inside surface warm enough so there will be no condensation.
Your polyiso should be just fine to use as a bit of r-value loss is only a percentage of the total.
Bud
Your polyiso should be just fine to use as a bit of r-value loss is only a percentage of the total.
Bud
