Replies

1. 
   Chaubenee | Nov 23, 2015 11:23pm | #1

   I am pretty sure that you are going to hear that 2" of closed cell in that application in that zone is not enough and that you need at least three inches. How about cellulose? Or open cell, both with a vapor retarder and carefully sealed Sheetrock on the ceiling.???

2. GBA Editor
   Martin Holladay | Nov 24, 2015 05:06am | #2

   Steve,
   You can probably get away with breaking the rules, for two reasons: (1) Your garage won't be generating much moisture, and (2) Your garage won't be heated for many days each year.

   That said, it's better to follow the rules -- especially because the next owner of your house may convert your attached garage to a bedroom.

   What about doing it right? Among the options you haven't mentioned:

   1. Building a vented, insulated roof (one that includes soffit vents, a ridge vent, and an air channel connecting them).

   2. Installing a flat (horizontal) drywall ceiling, with cellulose insulation above the ceiling.

   For more information on the right way to insulate a sloped roof assembly, see How to Build an Insulated Cathedral Ceiling.

3. 
   Reid Baldwin | Nov 24, 2015 07:02am | #3

   How long will you be working in your garage at a time? When you first turn the heater on, most of the heat will go to heating up the stuff in the garage. The insulation level determines your energy usage at steady state. If you turn the heat on Friday night in anticipation of working out there all weekend, it will be steady state most of that time. If you decide after dinner to spend a couple hours working in the garage, the differences in insulation level won't make much difference.

4. 
   Steven G | Nov 24, 2015 09:52am | #4

   I wish I had the funds to rebuild the roof or spray foam the whole thing but it's just not possible. The roof is a very low pitch, almost flat so I don't think building a flat ceiling and cove ting is possible.

   Since it will only be heated when I decide to work in there, can I get away with roxul up to the sheathing with a vapor barrier over it? Or should I just shoot it with 2" of foam and call it a day?

5. GBA Editor
   Martin Holladay | Nov 24, 2015 10:05am | #5

   Steve,
   The correct answer is that you need a ventilation gap under the roof sheathing if you intend to install mineral wool between the rafters. Can you install soffit vents and a ridge vent?

6. GBA Editor
   Martin Holladay | Nov 24, 2015 10:07am | #6

   Steve,
   If you want an unvented roof assembly, then one of your proposals -- to just install 2 inches of closed-cell foam and call it a day -- is a relatively safe approach from a moisture perspective, but not so good from a fire-safety perspective. Any spray foam needs to be protected on the interior with a layer of gypsum wallboard.

7. 
   Steven G | Nov 24, 2015 02:15pm | #7

   Would I hurt anything by installing r15 roxul on top of the r14 foam? I read that to do this correctly, I need an r20 of foam with an r49 ceiling but was curious if I could get away with just the r14 foam since I will be at r29 ceiling total

8. GBA Editor
   Martin Holladay | Nov 25, 2015 04:37am | #8

   Steve,
   As my article ("How to Build an Insulated Cathedral Ceiling") notes, the type of flash-and-batt approach you have chosen for your unvented roof assembly requires that the spray foam layer have a minimum R-value of R-20 in Climate Zone 5. If the foam layer has a lower R-value, the roof assembly is susceptible to moisture accumulation.

   That rule applies to occupied homes. When it comes to a garage that is only occupied intermittently, you don't necessarily need to follow the rules, as long as (a) your local code enforcement official agrees, and (b) as long as you are proceeding with open eyes, aware of the potential risk.

   The risk increases (a) if you start heating the garage for longer and longer periods, and (b) if any moisture-generating activities occur in the garage.

   You still haven't explained why you are wedded to an unvented roof assembly instead of a vented roof assembly.

9. 
   Steven G | Nov 25, 2015 08:46am | #9

   the structure is a very low sloped roof with 2x8 rafters and rolled asphalt. From what I read, this will not work vented since so much of the cavity will need to be open.