Need Advice on Insulation for Floor Above Crawlspace and Above Garage
So for the expansion off the existing house, there will be a non-ventilated, but insulated (with R-21 Insulation) crawlspace that will have a 6 mil polyethelene vapor barrier along the wall and floor. Any seams in the polyethelene will be sealed with caulk and/or liquid nails (still trying to figure out which holds up the longest). The polythene will be covered by stone.
So the question is, should I also insulate in between the floor joists of the 1st Floor? Or is the R-21 insulation in the crawlspace walls sufficient? Please note the Crawlspace walls are of a PWF design, so the insulation will it in between the 2×6 stud cavities with polyethelene on the inside and outside of the walls.
As for the space above the unheated but insulated garage with concrete floor, for the heated 2nd floor, should I install insulation in between the joists of the 2nd floor in that space? Or am I wasting money on insulation.
Thanks.
Ken L
Replies
If you are putting a room over the unheated garage you should insulate the space between the ceiling joists. Consider the garage the outside of the building. So would you insulate the outside facing ceiling or wall or roof? Yes, absolutely. For the garage ceiling insulate the underside of the ceiling with 1 or 2 inches of polyisocyanurate insulation board and tape all of the seams where the boards butt together. This becomes the air and moisture barrier. Then insulate the cavities between the joists, but you do not have to fill the space. Leave a few inches of space between the insulation and the subfloor. This is the same thing you should do for the floor facing the crawl space. The insulation board on the edges of the floor joists will prevent any moisture condensation on the edges. This condensation could lead to wood rot on the edges of the floor joists. For the new floor above the crawl space, put the aluminum face of the insulation board towards the floor of the crawl space.
You don't say what the R-21 insulation is on the permanent wood foundation is? If it is closed cell foam you will not need a plastic film on the wood portion. Closed cell foam is water tight. But you may want to put a capillary barrier between the wood wall and the underside of the perimeter joist to prevent any water wicking into the edge beam. This capillary barrier would go up the wall and over the top of the wood wall. The Building Science Institute has a paper about crawl spaces.
You are encouraged to put insulation under the plastic and gravel on the floor of the crawl space.
Knowing the weather zone you are in would also help.
I wanted to add that furring strips should be attached to the joists or rafters to develop a smooth planar surface for the sheet rock. That also provides a space to run any electrical wiring instead of drilling holes through the joists or rafters. So for the garage ceiling, attach the insulation board to the rafters, tape the joints and then attach the furring strips perpendicular to the rafters with screws. Then level the furring strips and attach the sheet rock to the furring strips. Furring strips should be at least #2 Southern Yellow Pine (SYP), 1X3 or 1X4. SYP is stronger and will hold the screws better than white wood or hemlock.
The unvented crawl space should be treated as conditioned space. Otherwise, you risk moisture buildup that can lead to mold and rot on the structural floor joists and subfloor.
Any discussion of insulation needs to start with the location and hvac zone, and should include moisture management as well as heat loss.
That information is covered in the attachment I gave him. All of the attachments are from the Building Science Institute and they cover insulating crawl spaces.
Agreed. He would be well-advised to read the attachments.
But for both the crawl space and the heated room/garage combination, it would be good to have a single vapor barrier layer, to avoid trapping moisture.
Several avenues to achieve reduced heat loss while mitigating moisture problems are discussed, combining them can lead to problems.
The specifics of how the garage is insulated and constructed would be of interest in the discussion.