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I'm in the midst of re-doing our garage. The space will be used as a nice woodshop/mancave. I'm looking for some suggestions in terms of insulating.

The garage is 12x20, and has about a 27 degree pitch in the roof with 2x6s as rafters. I live in Vermont so it's recommended to have around R50 insulation in the ceiling. If I retain the vaulted ceiling I'll need to furr out the 2x6's to achieve the recommended R value. From googling around people are pretty divided on furring (with a 2x4) vs. sistering a 2x10 to the existing rafter. I'm leaning towards furring because with sistering i'm adding 1.5" to every rafter which is less insulation value up there. But perhaps this is a non-issue?

If I do end up furring are there any sort of recommended techniques to do this? I plan on hanging 1" thick pine boards on the rafters and want to ensure the furring holds up with the added weight.

Forgive the myriad of questions, but any help/guidance is much appreciated!

  • Can you use insulation with a higher R value (lower U-value) instead so you don't have to modify the rafters? – ChrisF Jan 30 '17 at 15:51
  • @ChrisF I'm not aware of anything higher than R19 that would fit in in 5.5" space. – jrmack Jan 30 '17 at 16:05
  • What's the W/m.K value of R19 insulation? – ChrisF Jan 30 '17 at 16:16
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    This is the reason scissor trusses were invented. Unless you're ok with an unvented "hot roof", I'd look at floating 2x4 or 2x6 ceiling joists, suspended at the wall and ridge, and blowing in cellulose. It's going to depend on how much headroom you have, though. – isherwood Jan 30 '17 at 16:29
  • I would go with isherwood's suggestion. I'm doing the same thing, and in my area I'm keeping the heat out more than keeping heat in, but it is the same basic problem. My workshop space is half of a 3 car garage, and I plan to leave the doors operational, so in additio9n to the ceiling insulation, I'm also tasked with improving the garage door's seal to the wall, as well as insulating the door. My ceiling is already in, blown insulation is next, but with the west facing garage door, I need to address the heat coming in from the door before the insulation in the ceiling will do much good. – Bigtexun Jan 30 '17 at 17:36
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You cannot completely fill the attic space like you can a wall cavity. Remember, you'll need at least 1" air space above the insulation for "attic ventilation".

If you choose to use the 2x6 roof joists, you'll only be able to "fill" the joist space up about 4.5". So, your choices are 3 1/2" thick batt insulation for an R-value of about R-11.9 or rigid insulation board for a R-20 to R-30, depending on the type of board.

If you choose sistering 2x10's, then they have a net depth of 9 1/4". Therefore, leaving about 1" for venting, the net depth of insulation should be about 8" or so. That would give you R-19 to R-21 with batts or R-30 to R-60, depending on the type of board. (By the way, the R-60 is with polyisosanurate boards, which is very expensive.) Also, you're correct in assuming there is a significant R-value reduction because of the 3" of solid wood joist space.

If you furr down a 2x4 "ceiling" joist system, you could create a deeper attic space and use blown-in insulation, which is much cheaper. However, you'll need to verify that the 2x6 roof joists can support such a load. (Note: If you have an average load of about 25 lbs. per sq. Ft. Live load and 15 lbs. per sq. ft. dead load, then my calculations indicate 2x6 joists at 24" oc will only span about 8' to 10', depending on the grade of lumber. If the span is further, you could add 2x8 or 2x10 ceiling joists and support the existing 2x6 roof joists "up".

Also, you indicate the slope is about 27 degrees, which is slightly steeper than 6:12 (26.5 degrees). Blown-in insulation will tend to settle down to the walls, become ineffective and block the vent space on that steep of slope. Consider lowering the new 2x10 ceiling joists down to 2:12 pitch (about 9 degrees).

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'recommended to have around R50 insulation' that's a good recommendation but not a requirement if I understand correct, 6" foam won't get you R-50 but will get you well insulated and air sealed, if your already ridge vented you're gonna have more loss than insulation will compensate for anyways

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