Insulation for Rim Joist Sitting Directly on Hollow Concrete Block
Hello,
I have a 100 year old house in climate zone 6B. It costs a pretty penny to heat and am gradually trying to improve the efficiency of the house. I have already air sealed the attic and improved upon the insulation up there. The walls were insulated with cellulose at some point, but I know that some stud bays don’t have any insulation.
I want to insulate my rim joist using pieces of 2″ rigid foam with spray foam around the perimeter. Then add mineral wool for additional insulation. This seemed like a great plan, but upon more investigation I found that the concrete blocks that the rim joist is resting on (no sill plate) are hollow. There is a decent amount of air that is moving through the blocks.
I am concerned if I spray foam the blocks, then that might cause issues with moisture within the blocks. Has anyone run into a similar situation? I have asked around my neighborhood at what other homes have, but most homes have no insulation.
Thank you,
Evan
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Replies
You could probably jack those up without too much trouble, then slide in thin (1/32" to 1/16" or so) pieces of HDPE sheet to retrofit a capillary break. A capillary break will greatly reduce the risk of rot in that rim joist (and the ends of the joists) when insulating that area.
Bill
Thanks, Bill. That sounds like a good idea. Must admit though, I am hesitant to jack up the rim joists with the brick exterior.
Brick facades do not normally rely on the rim joist for support, the brick is usually resting on a ledge/step in the foundation wall. You should be able to lift the rim joist a LITTLE by using a short header close to the wall, spanning maybe 3-4 joists, and a pair of beefy bottle jacks. I usually use a stack of two squares of 2x10 (or thereabouts) orienter perpindicular to each other as a base for the bottle jacks when doing this.
Lift a section about 1/8" this way, then slide in the HDPE sheet. Lower that section, shift over to the next section, lift the next section about 1/8" being sure that the "next section" also lifts a bit of the end of the "last section" so that you can overlap the HDPE sheets between sections. 1/32" HDPE sheet is usually fine, 1/16" thick sheet is more durable if you have to jam it in the gap with more force to deal with crud that might be in the way.
HDPE is relatively rigid, very slippery, and cheap to buy. I try to buy black HDPE so that it has a bit of UV resistance and is less noticeable if exposed anywhere. Normally the material is white. Either color of HDPE will work as a capillary break. You just slide the sheets into the gap as you go along jacking up sections of wall. Get the sheet pre-cut into 2, 3, or 4 foot long pieces the same width as the depth of the top of your block wall. This way you know they're in all the way when the sheet is flush with the inside of the wall. Longer pieces install faster but require larger sections of wall to be jacked up each time. The thin material (especially 1/32") can be cut with scissors, so it's easy to cut down longer pieces if necassary. The exact thickness isn't critical, you just want it thick enough to be durable enough to slide into the gap OK during the installation process.
Bill
schardteven,
It's very hard to predict in these situations whether the insulation may cool the rim-joist and reduce the drying sufficiently to cause problems.
The two sure fire solutions are to either do as Bill suggests and add a capillary break, or insulate outside the rim-joist. If those are a step too far for you I would suggest insuating the block walls, but just air-sealing the rim-joists and leaving them un-insulated.
Thank you, Malcolm. I am interested in the capillary break but must plan that out more thoroughly. Unfortunately the outside is brick so insulation on the outside of the rimjoist is not possible.
Thank you for the idea to insulate the walls and only air seal the rim joist. That might be the path I am most comfortable with right now.
If that CMU is not grouted, you can get a pretty decent R value bump by filling it with something like peralite.
As for insulation, if soil level is bellow your joists by a couple of blocks and well graded to drain, water issues are less likely as is. Since you have no sill, Bill's suggestion of jacking and adding a plastic break is pretty simple and easy insurance. It is surprisingly easy to jack up a joist 1/8" if only wood construction above.
Thank you. I didn't consider filling with something insulative. I will have to clear out the old newspaper and inspect.
Everything above it framing wise is wood. But the exterior is brick, so I am nervous about that.
I've wondered about these kinds of retrofits -- very difficult to get every little crack. if the building is platform framed, western style, then the diagonal floor sheathing is going to have gaps that go won't get sealed with sprayfoam. air leakage will be through the diagonal sheathing gaps, then through the gaps in the strip flooring or behind the plaster grounds at the floor to baseboard intersections or at the electrical outlets. if you go ahead of the sprayer with a small straw nozzle then you might get more of the holes filled, but without stripping off exterior finishes, it's really hard to get a good air barrier as a retrofit. it's good to iterate with a thermal camera after the retrofits to see how the theory worked out with the physical install -- always variable with who is executing the work.
This house is actually balloon framing and the sheathing is some sort of gypsum product. You do have a point with old houses having many points for air infiltration no matter how much work is done to tighten them up.