Closed Cell Foam and Air Space
Hi all, my name is Chris. I have built a 22 x 2.5’ detached bricked garage with scissor trusses and a 3:1 pitched roof. There obviously isn’t a lot of space within the scissors and so I had planned on spraying 2-3 inches of spray foam underneath the roof sheathing. I also planned on putting mineral wool in the 2×6 wall studs. My question is, if I do 3 inches of closed cell under the roof sheathing I get R-18 or so. That is below code and where most of the heat is lost. Should I consider adding some bats above the drywall too? In that scenario there would be a couple of feet of air space between the bats and the 3 inches of spray foam.
Would I be further ahead just to do the 3 inches and forget the bats? I’m located in zone 5. Thanks for any input.
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You don't mention if the garage will be continuously heated or not. It makes a little bit of difference, mostly in how much it's worth insulating. Also check whether or not your local jurisdiction has code requirements for detached accessory buildings.
For a roof like that, the cheapest/best option is just to blow in a bunch of cellulose insulation on top of the drywall. You would also need to install roof vents (ridge and soffit is best) to vent the space above the insulation.
Thanks for your advice Pete. It will be heated and air conditioned with a heat pump throughout the year. I am using it for car storage and for a small carpentry workspace. I have already had it shingles with DaVinci roofscapes bellaforté plastic (slate looking) shingles and it is fully sealed airtight. I’d rather go with a hot roof application if feasible so I don’t have to create any venting.
I'm with Peter, the simplest is loose fill the attic space and vent. You don't need to touch the roof, standard gable vents work just well. Won't be fun to fill the low slope you have but much cheaper and higher R value overall.
If you must go with SPF, the simplest way to add extra R value is go with open cell spray foam over the 3" of closed cell. Even if you stay with just 3", make sure to cover the top chord of the truss with foam to reduce thermal bridging which significantly improves the assembly R value for not much extra cost.
你可以试着换出开放为b细胞泡沫atts, it will take some work to hold it in place. The batts need to be against the spray foam, they can't be on the attic floor. The simplest is probably perforated insulation hangers (aka insulation pins) adhered to the roof deck before the SPF goes on. You can get these pretty long. The batts get pushed on and held in place with washers.
That’s great advice. Thank you.
你觉得2”关闭细胞符合吗lowed by 5 inches of open cell would be a good assembly? R-32 ish in zone 5, 650 sq ft heated garage with 18,000 BTU heat pump. R-22 wall batts.
When it comes to heat loss, roof is just another surface. When you are using expensive insulation such as SPF, there is no ROI for going overboard. R30 is more than enough.
You do need slightly more cc SPF for condensation control though, 40% of your overall R value should be closed cell foam. Once the blowing agent diffuses, closed cell foam will eventually be between R5.5 to R6, aim for around 2.5" to be safe.