Insulating Cathedral Ceiling with 2×6 Rafters
I’m turning my 600ish sf, 2 car detached garage into a woodworking shop. I want to be able to use it through the winter (Colorado, Zone 5) and am planning on installing a garage heater (17,00BTU). I could use this forum’s collective genius to decide on the best way to insulate the ceiling.
Current situation: I’ve framed 2×4 walls in front of existing CMU walls, and insulated with kraft faced r-15, covered with drywall. The gable roof uses 2×6 rafters spaced 24′ oc. There is no existing ceiling – you look up and see rafters and roofing nails. The rafters meet the exterior walls at about 7′ high.
Question: what’s the most cost efficient and effective (given medium skilled homeowner abilities) way of insulating the ceiling/roof? I’m assuming step one is to install soffit and ridge vents as none exist now. Then install baffles for proper ventilation. Then what? I’ve come up with these ideas and am at a total loss of which to do.
1) Fir out the 2×6 rafters to make a deeper cavity for r-30 faced batts. Cover with drywall. Cost – around $1500. Will be weird where they hit the rafters at 7′ off the ground.
2) Install faced r-19 batts made for 2x6s, cover with drywall. Cost – around $950
3) Install unfaced r-19 batts made for 2x6s, then 2″ r-13 foil faced foam board, then cover with drywall. Cost around $2100
4) Install cross members at about 8 or 9′ high, create osb ‘ceiling’, then faced r-38. There will be about 2 linear feet of space between the ‘ceiling’ and where the rafters hit the framed wall. Would use doubled up foam board (gets to r-26) and drywall there. Cost – around $1300,
Help! Which one can I get “away with” and/or is good enough so that the electric heater can make the garage comfortable without needing 4 hours to heat it up. I’m leaning towards number 2 or 4 but am happy to be convinced otherwise.
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I would do #4 but use blown-in insulation and drywall, just like a house attic. Are you going to heat the space all the time? If not, electric heat may not provide the big surge of heat needed to go from cold to workable in a reasonable time inspite of the insulation.
+1 on option 4. Use a layer of faced polyiso under the rafters plus the new framed ceiling with taped seams as your air and vapor control layer, batts (faced or unfaced, doesn't matter) above that. If you go with 1" polyiso, you can even nail up T&G as your finished ceiling using 2.5" nails straight through the foam. The short section of unvented 2x6 rafters bays don't matter all that much as moisture can readily diffuse through the fluffy insulation into the attic space.
The new attic space above still needs to be vented, this can be a combination or mushroom vents, gable vents and ridge vents. Since there should be no major moisture source in the shop (no cooking or showers), venting is not as critical but I would still at least aim for code min net free area.
Hey, did you end up completing this project? I think I have a similar setup/ project and wanted to hear how your design went.