My garage has a vaulted ceiling on 2x6 joists. The main roof of the building is a bit above it and the vaulted ceiling is not covered with any sheeting. I could feasibly put 2x8 batts of insulation in the cavities and then sheetrock it up. The insulation would stick out 2" or so above the joists. Would this achieve me any better R valve? or should I just use 2x6 insulation and call it a day. Thanks in advance.

  • Usually more insulation is better if you have the space. Your use of the garage will factor in the cost benefit of more insulation. Would assume the walls are already insulated or will be also. – crip659 Jun 22 at 17:07
  • You'll probably be ok, but you can reduce the R value of the insulation when you compress it too much. At least with fiberglass insulation. If you leave it sticking out I suppose that's fine but how are you fastening it between the joists? Is there a reason you want to use 2x8? Do you already have some and just want to save money? – Phaelax z Jun 22 at 17:11
  • I will be spending a lot of time in there as it's becoming my music room/man cave. The walls will insulated with 2x4 batts of mineral wool. The ceiling is open so thicker batts will not be compressed. The batts will just be friction fit between the joists and then sheetrock put under them. I was thinking the 2x8 insulation might up the r valve and the sound isolation over the 2x6 batts, that's why I was thinking of going that route. – Scotty McGregor Jun 22 at 17:42
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    Back to your original question, 8" batts provide more insulation that 6" batts, assuming the 8" batts are not compressed. So I would go with the 8" bats and just leave the insulation stick above the joists. – SteveSh Jun 22 at 18:39
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    Note that with any type of insulation, you're going to be losing heat through the joists, unless they are insulated in some fashion along their top edge. – SteveSh Jun 22 at 18:40

Compressing insulation designed for 2x8 into a 2x6 cavity will yield a negligible R-value difference.

You can buy R-21 insulation for 2x6 cavities but if you buy R-30C then it will only be worth R-22 once compressed.

Insulation is a delicate balance of air space and fiberglass. Air is a terrible conductor of heat.

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