Insulating a framed half-high wall in a basement
Hello.
We have just completed construction of an addition on our home in zone 5A. The addition has a walk out basement that does not have a heat or a/c duct located in the space. The walls were built with concrete block to above grade (floor to about 38″) and then the rest is a 2×6 studded wall (about 58″) The wall construction as of now is as follows from outside in. Brick veneer – 2″ vent space – OSB sheathing attached to 2×6 stud wall OSB is wrapped with Tyvek on the exterior.
For interior insulation I plan on adding 2″ of XEPS to the concrete block and furr with 1×3 strips. I also plan on adding 2″ to the framing so that the furring strips will be plumb vertically for sheet rock installation. My question lies in the wood stud cavities. Is it possible to do a flash and batt type insulation here with 1″ of XEPS in the studcavity (against the exterior OSB) and then fill the rest of the cavity with fiberglass batts or should I just batt these spaces? I don’t know exactly how mositure would move in this wall if the XEPS were in the cavity next to the OSB – would proper drying take place or would mositure be trapped between the XEPS and OSB?
Also, the floor of the new room (on first floor) is very cold. Is it okay to insulate those floor joists with fiberglass batt with a vapor barrier up towards the room? We plan on hanging 5/8″ sheet rock on the ceiling of the basement.
Thank you for your help.
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Replies
Martin,
Are you planning on heating the basement?
I'm not sure what you mean by "XEPS." If you mean extruded polystyrene, the usual abbreviation is XPS. If you install 2 inches of XPS against your concrete block wall, that will barely meet minimum code requirements. If you install 2 inches of EPS (expanded polystyrene), that won't be enough R-value to meet code requirements.
Later you refer to using "XEPS" for a flash-and-batt job. I'm not sure what you mean. Flash-and-batt jobs require closed-cell spray polyurethane foam, not rigid polystyrene. In your climate, I think that I would use at least 2 inches of closed-cell spray polyurethane foam for flash-and-batt -- not less.
If you decide to use flash-and-batt for the stud bays, you'll get better performance than you would with just fiberglass batts. Be sure that the spray-foam installer connects the top of the rigid foam that covers the concrete blocks with the rest of the spray foam (in other words, you'll need at least 2 inches of spray foam to cover the top of the concrete blocks and the mudsill, all the way back to the sheathing).
Thanks for the reply.
We do not plan on heating this part of the basement. There is flex duct going through the floor joists but for the supply vents in the addition room.
Sorry about my confusion of terms. Yes, on the concrete block we were trying to acheive R-10 via 2" of XPS; should we do more? Spray insulation is out of our budget right now; that is why I wondered if I could put XPS in the stud cavity (against the OSB) and then batt behind it up to the edge of the studs. I out some XPS in some of the stud bays (temporarily) with batt and it seems to be warmer then just the batt.
Also, could you advise about insulation of the floor joists?
Martin,
If you aren't planning to heat the basement, it may make sense to insulate the floor joists. It's up to you. In any case, make sure that the supply duct is insulated well.
Whether or not you insulate between the joists, it certainly makes sense to insulate your basement walls. If you can do a good job air sealing your basement and insulating the walls, you'll keep the floor above more comfortable.
I like to see people exceed minimum code requirements for insulation, so if you can afford it, it makes sense to insulate your below-grade walls with more than R-10 of insulation. Certainly, R-15 or R-20 is better.
If you want to insulate your stud cavities with a combination of rigid foam and fiberglass batts, you should read this article: Calculating the Minimum Thickness of Rigid Foam Sheathing.
In your climate zone, the minimum R-value of rigid foam installed in 2x6 walls is R-7.5. Each piece of rigid foam should be installed with attention to airtightness; that means that you need to seal the perimeter of each piece of foam with caulk, canned spray foam, or high-quality housewrap tape.
Don't forget to insulate the top of your concrete-block wall with a layer of horizontal rigid foam. Connect the rigid foam installed over the concrete block wall to the rigid foam installed between your studs. The rigid foam should be continuous, without gaps, and with all seams sealed so that the seams are airtight.
Martin, Put a pellet stove in the basement and use it during your coldest periods. That is the least expensive solution to warm floors and it will make your unuseable cellar useable. Two birds, one inexpensive stone.
Thank you for the replies. So my plan was to insulate the stud cavity the same manor that I did the rim joist. Rigid foam in tight against the OSB and Great Stuff Pro around the edges to create a tight enclosure along with tyvek tape to seal any seams in the board.
I should not worry then about moisture being trapped between the rigid foam and the OSB and causing rot?
Martin,
Q. "I should not worry then about moisture being trapped between the rigid foam and the OSB and causing rot?"
A. No -- as long as (a) the foam is thick enough, as explained in the article I linked to, and (b) you do a good job making the job airtight.