I have some questions regarding insulating the roof of a detached garage
It is fairly recent construction and currently not insulated. Truss roof with 2×6 top chords 24″ o.c. First floor will be used as a workshop and second floor for storage.
Planning on conditioning the space with a heat pump for heating and cooling. Located in upstate New York.
Would like to insulate the ceiling using a “cut and cobble” approach. Have done a good amount of research here and elsewhere and think I have a strategy that I’m comfortable with but would like to get some input as I haven’t seen my specific scenario discussed.
Building currently uninsulated but there are existing soffit vents and a ridge vent. What I’ve read indicates that the safest bet is to maintain the venting of the roof so that is what I plan to do. I have 5.5″ of rafter space – plan on using 1.5″ strips of rigid foam as spacers to maintain ventilation and add 4″ of rigid insulation to bring flush with inside of top chords. I have access to 4″ paper faced recycled polyiso which would be preferable as I would only have to cut and fit once to each bay. Planning to cut undersized and seal with spray foam. Can also source 2″ recycled XPS and install 2 layers if that is preferable. Would like to add an additional layer of rigid foam to inside of top chords so I can tape seams, improve air barrier, create thermal break. One complication – skylights in 2 of the bays so continuous venting from soffit to ridge not possible in these bays.
Any wisdom appreciated. Thanks!
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First time posting here. I guess I should have put the bulk of the information in the "more info" field. Oops!
[Editor's note: The missing text has been restored to the question, where it belongs.]
Theodore,
Do you have raised heel trusses by any chance? Is the garage detached?
Not raised heel unfortunately standard attic truss. Detached garage.
Theodore,
The skylights complicate things. You'll end up with blocked ventilation channels and possible ice dams. It's hard to predict whether the skylights are a small problem or a big problem. You can cross your fingers -- or just remove the skylights and replace them with OSB or plywood.
Note that there is a good chance that the roof trusses you describe aren't designed for storage. That probably means that you will have to retrofit 2x6 or 2x8 joists if you intend to use the second floor.
Thank you for the info. If I were to keep the skylights would it be better to apply the rigid foam directly to the sheathing in those bays only (or spray foam just those bays)? Would this create problems if the rest of the bays were vented?
Theodore,
If you intend to keep the skylights, I guess I would recommend (of the options you list) that you install closed-cell spray foam on the underside of the roof sheathing in any rafter bay blocked by a skylight.