Insulation for exposed basement ceiling
Hi all,
I am building a “pretty good house” in central KY, zone 4A, mixed humid. R30 walls and R60 ceilings. The house will have a full basement with r10 rigid foam on the interior of the walls, running from the floor all the way up until it hits the insulation on the interior of the above-grade walls.
I asked the builder to design a tornado shelter in the basement, and he created doozy. I’ve affectionately named it my bomb shelter.
The only issue is that he created this space below our back porch, outside the main 4 walls of the basement. There is a doorway that provides access from the basement to the shelter.
What amount of insulation is needed for the ceiling in this area? The ceiling will be a solid concrete slab, which will be the floor of the back porch. The porch will be covered, but it will be obviously open to the elements.
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Replies
Clay,
There is one potential problem with this type of room, and it has nothing to do with insulation. It has to do with roofing.
Concrete slabs aren't waterproof. Depending on how much wind-driven rain and wind-driven snow ends up on your porch deck, this concrete slab can leak. For a tornado shelter, you may not care too much if the roof drips. But it's worth mentioning.
If you want to install roofing over your concrete slab -- for example, rubber membrane roofing -- you can. You will need flashing where the roofing meets the house. Once this roofing is installed, you can install some type of tile flooring on top of the roofing if you want.
If your basement walls are insulated on the interior with rigid foam, and if you install an exterior door to access your tornado shelter, then the tornado shelter will be outside of your home's thermal envelope, and there will be no need for insulation of that room.
I assume that most tornadoes occur during the summer, so that you don't care too much if the room is heated and cooled.
Or, I guess I could seal and insulate that doorway really good and then the heat loss would be minimized.
Thanks for the tip on the waterproofing. I hadn't thought about that. I had contemplated just leaving it as a concrete slab, but I could always brick it, and I assume the membrane could go under the brick. ETA: Plus, it will face the northwest, with a wall to the west, which should protect it from most of the weather.
However, the shelter will be inside the thermal envelope because a doorway connects the shelter to the basement. True, tornado season is during the warmer months, so comfort is not an issue while it may be in use, but I am concerned about heat loss from the basement to the shelter and up through the ceiling.
Clay,
If your basement wall insulation is installed on the interior side of the wall, it's going to be very hard to bring the tornado room into your conditioned envelope. I assume that the tornado room is separated from the basement by a poured concrete wall, with a 36-inch-wide door in the wall. Is that correct?
If so, there will be massive thermal bridging through the concrete walls of the tornado room (where these walls meet the poured concrete walls of the basement) if you try to include the tornado room in your conditioned space. It will be a lot easier -- and you will avoid the thermal bridging problem -- if you just leave the tornado room outside of your conditioned envelope.
So the next question is how would the best way to seal that door? Just attach the rigid insulation over the door (obviously cutting the foam so you could still open the door)? I'd have to leave a gap near the hinges so the door can swing, but it would be better than a door with no insulation, right?
Clay,
You go to your local lumberyard and you order an insulated steel door. (An ordinary entry door.)
P.S. Since this is a safe room (designed as a tornado shelter), make sure that the insulated door you choose meets requirements for safe rooms. Special hinges and latches may be required.
Awesome. Thanks!
ETA: Thanks, I'll look into safe room doors.
I also have a project with a conditioned basement and concrete deck (terrace) on top. the flashing points you make are well-taken. But the R-value of the ceiling is the question. If the walls only need to be R-10, why does the ceiling deck have to be 49?
The risk if you skimp in the roof insulation is that the basement ceiling will get below the dew point of the basement air. Should that happens water will collect on the ceiling and drip to the floor, so it could literally rain in your basement.
Walta
I've included electric radiant heat in the slab to keep snow from piling up. Would that help the condensation issue in the conditioned space below?