Insulating cantilever and rim joists

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Samster

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Hello,
I have been reading the information on insulating and there is a lot of good information out there.
I have 2x8 joists that extend about 2 feet past my foundation just over ground level on some sections of my house. Currently there is fiberglass insulation that is layed flat in the bay's with some room on top of it. It might be about 5 inched thick. I can feel cool air in that area. My questions are, what is a good way to insulate the bays? There is not much room to get in there. Also, what is a good way to insulate the joist boxes that are not cantilevered, just the rim joists. I live in southern Wisconsin. Thank you for the help.
 

hj

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You have to understand that insulation does NOT prevent things, like you floor, from getting cold. It only slows the process down, but unless something is done to add heat to help the insulation, it will eventually get as cold as the surrounding air.
 

Dana

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Hello,
I have been reading the information on insulating and there is a lot of good information out there.
I have 2x8 joists that extend about 2 feet past my foundation just over ground level on some sections of my house. Currently there is fiberglass insulation that is layed flat in the bay's with some room on top of it. It might be about 5 inched thick. I can feel cool air in that area. My questions are, what is a good way to insulate the bays? There is not much room to get in there. Also, what is a good way to insulate the joist boxes that are not cantilevered, just the rim joists. I live in southern Wisconsin. Thank you for the help.

If you can pop the sheathing off the bottom of the cantilever it's worth opening it up and air-sealing the framing to the subfloor, then stuffing full-depth high-density fiberglass or rock wool batts (R30) in there, or compress an unfaced R38 batt in there (easiser said than done, but probably cheaper.) Having the gap above the insulation is a "thermal bypass" path, for infiltration air to get in and chill the floor. The original insulation SHOULD have been snugged up the subfloor, not resting on the sheathing, since it's the cold sheathing that would potentially benefit from the ventilation gap for drying, not the sub-floor.

You don't absolutely need a vapor retarder for insulating the underside of a cantilever, but if you want to limit the mold/rot potential of the sheathing, staple 6-mil poly to the underside of the subfloor prior to air-sealing with foam, then seal the poly to the subfloor & joists with the foam.

If there isn't sufficient headroom room to re-build it from the bottom side, you can drill a 1.25" holes through the rim joist just below the rim joist and blow as much cellulose as you can stuff in there with a rental blower (you may have to use a reducing coupling taped to the end of the hose if the blower equipment doesn't come with a narrowing nozzle.) Cellulose is very air-retardent even at low density, and will reduce the air infiltration by 90% or more, while increasing the R-value to about R26-R27.

For the band joists in other areas you can cut'n'cobble 2" thick rigid EPS (~R8) cut with a 1/4-1/2" gap around the edges, lightly tacked in place, then fill the perimeter gaps with can-foam for a perfect air seal. Then put R15 rock wool trimmed for a friction fit to the interior side of that. With R8 foam & R15 rock wool you'll have minimal condensation at the foam/fiber boundary in your climate, and the vapor retardency of the foam will be sufficient to protect the band joist.

rimjoists.jpg


(^^Looks like they could have cut it a bit narrower before sealing to allow more fill with can-foam here- the air seal is less than perfect in places. The also used XPS, which could also be used, but it's more expensive, and lower vapor permenace, which limits the drying capacity toward the interior. In your climate EPS is a better choice, as long as it's at least R8, and not more than R12.)

If there is a ledge of concrete between the edge of the foundation sill in the interior, cutting in some 1.5" thick rigid foam and can-foam sealing it to the foundation sill AND concrete is advisable. Then stack a layer of rigid foam atop the foundation sill snugged up to the band-joist foam, and seal it in place, to protect the foundation sill from interior moisture drives in winter.

Alternatively, you could use 2" of closed cell spray polyurethane instead of cut'n'cobbled foam, which is likely to air seal better. You'll still want R15 rock wool on the interior side to fatten out the R-value, and as a thermal break against fire lighting off the foam.

If it's a full basement (or even a crawlspace) it's worth insulating the foundation walls too, but I won't detail that here unless asked.
 

Samster

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If you can pop the sheathing off the bottom of the cantilever it's worth opening it up and air-sealing the framing to the subfloor, then stuffing full-depth high-density fiberglass or rock wool batts (R30) in there, or compress an unfaced R38 batt in there (easiser said than done, but probably cheaper.) Having the gap above the insulation is a "thermal bypass" path, for infiltration air to get in and chill the floor. The original insulation SHOULD have been snugged up the subfloor, not resting on the sheathing, since it's the cold sheathing that would potentially benefit from the ventilation gap for drying, not the sub-floor.

You don't absolutely need a vapor retarder for insulating the underside of a cantilever, but if you want to limit the mold/rot potential of the sheathing, staple 6-mil poly to the underside of the subfloor prior to air-sealing with foam, then seal the poly to the subfloor & joists with the foam.

If there isn't sufficient headroom room to re-build it from the bottom side, you can drill a 1.25" holes through the rim joist just below the rim joist and blow as much cellulose as you can stuff in there with a rental blower (you may have to use a reducing coupling taped to the end of the hose if the blower equipment doesn't come with a narrowing nozzle.) Cellulose is very air-retardent even at low density, and will reduce the air infiltration by 90% or more, while increasing the R-value to about R26-R27.

For the band joists in other areas you can cut'n'cobble 2" thick rigid EPS (~R8) cut with a 1/4-1/2" gap around the edges, lightly tacked in place, then fill the perimeter gaps with can-foam for a perfect air seal. Then put R15 rock wool trimmed for a friction fit to the interior side of that. With R8 foam & R15 rock wool you'll have minimal condensation at the foam/fiber boundary in your climate, and the vapor retardency of the foam will be sufficient to protect the band joist.

rimjoists.jpg


(^^Looks like they could have cut it a bit narrower before sealing to allow more fill with can-foam here- the air seal is less than perfect in places. The also used XPS, which could also be used, but it's more expensive, and lower vapor permenace, which limits the drying capacity toward the interior. In your climate EPS is a better choice, as long as it's at least R8, and not more than R12.)

If there is a ledge of concrete between the edge of the foundation sill in the interior, cutting in some 1.5" thick rigid foam and can-foam sealing it to the foundation sill AND concrete is advisable. Then stack a layer of rigid foam atop the foundation sill snugged up to the band-joist foam, and seal it in place, to protect the foundation sill from interior moisture drives in winter.

Alternatively, you could use 2" of closed cell spray polyurethane instead of cut'n'cobbled foam, which is likely to air seal better. You'll still want R15 rock wool on the interior side to fatten out the R-value, and as a thermal break against fire lighting off the foam.

If it's a full basement (or even a crawlspace) it's worth insulating the foundation walls too, but I won't detail that here unless asked.
If you can pop the sheathing off the bottom of the cantilever it's worth opening it up and air-sealing the framing to the subfloor, then stuffing full-depth high-density fiberglass or rock wool batts (R30) in there, or compress an unfaced R38 batt in there (easiser said than done, but probably cheaper.) Having the gap above the insulation is a "thermal bypass" path, for infiltration air to get in and chill the floor. The original insulation SHOULD have been snugged up the subfloor, not resting on the sheathing, since it's the cold sheathing that would potentially benefit from the ventilation gap for drying, not the sub-floor.

You don't absolutely need a vapor retarder for insulating the underside of a cantilever, but if you want to limit the mold/rot potential of the sheathing, staple 6-mil poly to the underside of the subfloor prior to air-sealing with foam, then seal the poly to the subfloor & joists with the foam.

If there isn't sufficient headroom room to re-build it from the bottom side, you can drill a 1.25" holes through the rim joist just below the rim joist and blow as much cellulose as you can stuff in there with a rental blower (you may have to use a reducing coupling taped to the end of the hose if the blower equipment doesn't come with a narrowing nozzle.) Cellulose is very air-retardent even at low density, and will reduce the air infiltration by 90% or more, while increasing the R-value to about R26-R27.

For the band joists in other areas you can cut'n'cobble 2" thick rigid EPS (~R8) cut with a 1/4-1/2" gap around the edges, lightly tacked in place, then fill the perimeter gaps with can-foam for a perfect air seal. Then put R15 rock wool trimmed for a friction fit to the interior side of that. With R8 foam & R15 rock wool you'll have minimal condensation at the foam/fiber boundary in your climate, and the vapor retardency of the foam will be sufficient to protect the band joist.

rimjoists.jpg


(^^Looks like they could have cut it a bit narrower before sealing to allow more fill with can-foam here- the air seal is less than perfect in places. The also used XPS, which could also be used, but it's more expensive, and lower vapor permenace, which limits the drying capacity toward the interior. In your climate EPS is a better choice, as long as it's at least R8, and not more than R12.)

If there is a ledge of concrete between the edge of the foundation sill in the interior, cutting in some 1.5" thick rigid foam and can-foam sealing it to the foundation sill AND concrete is advisable. Then stack a layer of rigid foam atop the foundation sill snugged up to the band-joist foam, and seal it in place, to protect the foundation sill from interior moisture drives in winter.

Alternatively, you could use 2" of closed cell spray polyurethane instead of cut'n'cobbled foam, which is likely to air seal better. You'll still want R15 rock wool on the interior side to fatten out the R-value, and as a thermal break against fire lighting off the foam.

If it's a full basement (or even a crawlspace) it's worth insulating the foundation walls too, but I won't detail that here unless asked.

Thank you for your reply.
The sheathing is close to the ground and would be very hard to remove. A friend of mine suggested that I spray foam the area. That is what he had done. They sprayed the top, sides and bottom. I can access the bay's from the inside so could I dense pack those areas from the inside? I like the ideas you gave for the band joists. Not too difficult.
 

Dana

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If you can access from the inside, that's the PERFECT place to dense-pack it from.

I suppose this means there is no air-barrier in the joist bay where it crosses the foundation sill? If yes, you can staple in landscaping fabric mesh (tight 2" spacing on the staples) , then make a 3" slit to be able to insert the 2.5" diameter blowing hose to about 6" from the outer end of the bay. If there's only 2" of space between the fiberglass and subfloor you can still get it in there if you run the blower air-only while inserting it. Use a towel or something around the hose at the slit mesh to avoid major blow-back in your face, but then blow cellulose until the blower stalls (you'll hear the blower start to wind up), then pull the hose back 6" or so until it stalls again. You'll often have to do some push-pull motions on the hose to get the insulation flowing again after each stall. With a 1-stage blower you'll be able to get close to 3lbs density this way- the mesh will firm not like a mattress (and not a down pillow) to a finger-push.

When you're done blowing, cut'n'cobble in rigid foam over the mesh, can-foam sealed around the edges as a permanent air-barrier/vapor retarder. One inch foil faced polyiso would be appropriate here. If you use XPS it would have to be at least 2" thick to be sufficiently vapor retardent. Unfaced EPS would have to be at least 4" thick to be sufficiently vapor retardent.
 

Samster

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If you can access from the inside, that's the PERFECT place to dense-pack it from.

I suppose this means there is no air-barrier in the joist bay where it crosses the foundation sill? If yes, you can staple in landscaping fabric mesh (tight 2" spacing on the staples) , then make a 3" slit to be able to insert the 2.5" diameter blowing hose to about 6" from the outer end of the bay. If there's only 2" of space between the fiberglass and subfloor you can still get it in there if you run the blower air-only while inserting it. Use a towel or something around the hose at the slit mesh to avoid major blow-back in your face, but then blow cellulose until the blower stalls (you'll hear the blower start to wind up), then pull the hose back 6" or so until it stalls again. You'll often have to do some push-pull motions on the hose to get the insulation flowing again after each stall. With a 1-stage blower you'll be able to get close to 3lbs density this way- the mesh will firm not like a mattress (and not a down pillow) to a finger-push.

When you're done blowing, cut'n'cobble in rigid foam over the mesh, can-foam sealed around the edges as a permanent air-barrier/vapor retarder. One inch foil faced polyiso would be appropriate here. If you use XPS it would have to be at least 2" thick to be sufficiently vapor retardent. Unfaced EPS would have to be at least 4" thick to be sufficiently vapor retardent.
Thank you for the tips! The landscape fabric is a nice touch. The only thing in the bay is plywood on the bottom and it looks to be tight to the bottom of the joists. I can send a picture if needed. Is there a certain type to use? Because I am unable to really get a good moisture barrier, will the dense pack hold moisture? I believe it is critter resistant too.
Thank you again.
 

Dana

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"Is there a certain type to use?"

What it the subject category here?

Cellulose can buffer quite a bit of moisture without losing function or damage. Air tightness toward the interior is more important than vapor-retardency, since orders of magnitude more moisture can be transported by air than by vapor diffusion, even through fairly vapor-open materials such as unpainted drywall. A square inch of air leakage into a wall assembly from the interior moves much more moisture into the wall than 200-300 square feet of unpainted wallboard does by vapor diffusion alone:

water-vapor-research-building-science-diffusion-through-drywall.png



water-vapor-research-building-science-infiltration-through-drywall.png



Since the bottom of the cantilever is probably painted half-inch plywood, it's vapor permeance is going to be about 1-2 perms, but since it won't be rain or dew-wetted, it'll provide considerable drying capacity even in winter. The vapor permeance of polyurethane finished 3/4" hardwood over 3/4" plywood subfloor is about 0.5 perms. If the air-barrier in the joist bays between the basement & cantilevered section can be brought down to under 2 perms (say, with 2" of tightly sealed foam-board, any type), interior moisture drives will not have much affect on the amount of moisture in the cellulose or the bottom-side plywood on the cantilever.
 
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