So I am in the process of getting a 26 x 30 foot garage built (site cleared so far). Above the garage will be a single room built inside the trusses with two 8 foot dormers. This space will be heated and cooled all year and serve as a work area. Essentially this is a cathedral ceiling and I have read Martin Holladay's article on insulating it. http://www.greenbuildingadvisor.com/blogs/dept/musings/how-build-insulated-cathedral-ceiling The build site is located right in the middle of NC which puts it on the 3A / 4A climate zone and according to the chart in the article either R5 or R15 for foam. So in talking with the GC, I don't get the feeling he is really up on green building or air sealing. His response was we'll consult with the insulation company and see what they recommend. I'd kind of like to have a more definite plan than that. I understand foam is more expensive than batt but I don't have a handle on how much more and I'd like to have this space comfortable and am willing to put the money into to make it comfortable on the front end. I also understand that foam would act as an air sealer whereas batts would not. So what would the suggestions be from you guys as to how to get what I want and convey that to the GC and the insulation company. I am also thinking of a minisplit to heat and cool the space so any suggestions there would be nice too. I am thinking of LED lighting and if anyone has suggestions there I'd love to hear about that too. Not sure how to make the room bright without mounting fixtures on the slopey part of the ceiling. Thanks in advance for the help.

If the garage below is unheated, the floor is going to get cold, so you'll want to do a good job of insulating it. In central NC, it can get very hot, so you'll need good attic/roof insulation as well. A good insulation contractor should be up on proper air sealing. It really makes a big difference, as does providing a thermal break from the framing. Heat transfer through framing can reduce the insulation effectiveness by 25%.

We are in a similar climate, and use mini-splits for our small (1400 sq ft) ICF home. I don't know if it's just the units themselves, or the insulation and air tightness, but a single 12,000 BTU unit handles our HVAC easily. We have a separate 9K unit in the master suite just for individual control of the room. Since I designed and built the house myself, I keep accurate records for my own curiosity. A/C adds about $20 a month to the summer electric, and heat adds about $40 a month during the coldest part of winter. We have Mitsubishi HyperHeat units, and they are virtually silent, as well as very efficient. Our walls are 'technically' R-24 or so; the attic is R-49 cellulose. I say 'technically', as in our mild climate, I believe that the thermal mass of the ICF has a significant effect. Last year, we turned off the A/C in mid-September, and didn't turn the heat on until January 4th. By then, the house had cooled to 63 F., and the wife had had enough. I used an on-line Manual J calculator, and our entire house worked out to 9K cooling and 12K heating. If anything, the house has out-performed those numbers.

If the table says use minimum R-15 for your zone then do at least that.

Table R806.5 specifies the minimum R-value for the foam installed on top of the sheathing (not the R-value for the whole roof assembly) . The table calls for a minimum of:

R-5 foam for Climate Zones 1-3,
R-10 for Climate Zone 4C,
R-15 for Climate Zones 4A and 4B,
R-20 for Climate Zone 5,
R-25 for Climate Zone 6,
R-30 for Climate Zone 7, and
R-35 for Climate Zone 8.

Posted By patonbike on 22 Dec 2016 11:57 AM
If the table says use minimum R-15 for your zone then do at least that.

Table R806.5 specifies the minimum R-value for the foam installed on top of the sheathing (not the R-value for the whole roof assembly) . The table calls for a minimum of:

R-5 foam for Climate Zones 1-3,
R-10 for Climate Zone 4C,
R-15 for Climate Zones 4A and 4B,
R-20 for Climate Zone 5,
R-25 for Climate Zone 6,
R-30 for Climate Zone 7, and
R-35 for Climate Zone 8.

And mind you, those are at the IRC precribed total R values for those climate zones.

eg: R49 is code min for zones 4 and higher, so for say zone 5, the prescriptive R20 on the exterior of an unvented roof means it's deemed sufficient for up to R29 of cavity insulation or about a 40/60 ratio. If the cavity insulation increases, the exterior R requirements need to increase proportionally. With R38 between the rafters and only R20 on the roof the average wintertime temp at the roof deck is colder, and it takes on more moisture.

I'm wondering IF you do not heat the floors (no in floor radiant) but insulate and seal them well, will you have cold feet problems with unheated garage below?

The garage is just being finished and the upstairs in very comfy with a 1 head minisplit and open cell spray foam in the floor, walls and ceiling. The garage is partially underground and it stays warmish so far.

Great!

one other thing to note is that you have considered infiltration of critters. They can do a lot of damage to wiring. with a big garage door, its an invitation. keep a sharp eye as they like areas that are not constantly lived in. make sure there is no access through gaps in eaves, outside where wall meets cement foundation.