Sealing Band Joists Above Basement Foundation Walls
The reason to air seal the band joist (the horizontal bottom-most piece of wood that sits on your home's foundation) is because the masonry foundation expands and contracts at a different rate than the wood does. The crack between them leaks air all around the building and is where up to 20% of the air infiltration takes place in a house.
Clean off the top of the foundation and around the band joist, so there isn’t loose dirt. You can use a vacuum or rags or a broom.
Electrical Wires. You can make your own decision about what to do about wires that run along the band joist. You can bury them under the insulation, or try to move them out of the way. Just make sure no volunteer is trying to pry up a wire with a sharp screwdriver or something. You don’t want them cutting the wire with the screwdriver and getting electrocuted.
Pipes. If there are any pipes that might get over 200 degrees, don’t get any insulation or foam within 2”’ of it.
Measure each stud bay. Use a steak knife or saw to cut the rigid insulation to be 1/2” short on each side and on the top and bottom. If this is an old house, each bay might be different.
Nail the insulation block in the joist bay center of every stud bay, so there is that 1/2" on all sides.
As the last task, spray Great Stuff spray foam around the insulation block, so the cured foam works both as glue to hold the insulation in and to seal the gaps around. Spraying at the end means you minimize everyone's exposure to the foam, which is not great to breathe for extended periods of time and is cumulative. Keep the bulkhead door and windows open while you work to air out the fumes and/or wear a respirator.
Safety. When working with the Great Stuff, make sure the volunteers wear protective eyewear at all times, because if the foam dripped down and got in someone’s eyes, it would be disastrous. They also have to wear gloves. If it gets on clothing or hair, don’t touch it until it dries, because it just smears everywhere. After it’s dry, you can cut it off or sand it easily. If it gets on the hands, you can use acetone to get it off; some are sensitive to this solvent.
The spray foam bottles need to be shaken for a while before they are sprayed. Whenever possible, they should be held upside down when sprayed to help the foam get out of them. Otherwise all the expeller gas can be used up and the foam is still in the bottle. (See Spray Foam Safety and Application.)
Band joists and balloon framing ("balloon framing" means the floor plates don't go all the way to the exterior walls. They stop short, leaving an open space between the outside edge of the ceiling in the basement and the exterior wall. Basically, standing beside the exterior wall of the basement, you can reach up into a stud bay, around the edge of the ceiling and wave your hand around inside the wall of the floor above. If your arm were elastic, you could extend your hand straight up through the inside of the wall right up into the attic.)
If a building you're working on has balloon framing, any fire in the basement could rise quickly up this space to take over the whole building. In order to minimize this risk, anywhere where there is balloon framing, it is a good idea to first stuff rock wool into the opening above each stud bay as a fire block. You aren't increasing the risk of fire by installing the foam, but the balloon framing is just dangerous in general and it's a good idea to do what you can to solve it before blocking any such solution off with the foam.
Installing Rock Wool Safely: Rock wool is made from rock slag and won't catch on fire. It has nasty stuff in it. Have everyone working with it wear respirators, long sleeves, gloves, and goggles. Stuff inside it can become airborne, so you want to treat it gently. Leave the doors and windows open while you work. Cut the rockwool with a saw to fit inside the space between the basement ceiling and the exterior wall inside each joist bay. This stops the fire from going higher. Press it in firmly.
Once the rock wool is in place, you can install the rigid insulation blocks in place in each stud bay.
It is best to have those cutting the rock wool and rigid insulation work outside to minimize exposure.
You can see some pictures of what this looks like at:http://www.heetma.com/docs/how_to_airseal.pdf. There is a part about 2/3 through that shows what you need to do and how it should look in the end.
