Replies

1. GBA Editor
   Martin Holladay | Apr 20, 2017 04:49pm | #1

   Jim,
   If he wants to install a DIY radiant barrier, it wouldn't cost much (and it would help).

   In Florida, however, it's hard to make your garage comfortable in the summer without air conditioning. It might be more realistic to just assume that garages are hot.

   -- Martin Holladay

2. 
   swazo | Apr 20, 2017 05:41pm | #2

   Thanks Martin, witch would be the most effective. Six inches of open cell foam on the inside surface of the roof or the radiant barrier?

   Jim

3. 
   user-2310254 | Apr 20, 2017 09:12pm | #3

   Jim,

   Is the garage radiating heat into the conditioned space? Is that the problem?

   FYI. You want to avoid installing open cell foam against the roof sheathing. There is a risk that the sheathing will stay damp.

4. 
   Anon3 | Apr 20, 2017 10:30pm | #4

   I was reading the roofer forums lately, and they came across roofs with foam insulation where basically the sheathings are just completely gone, only some mold dust remains, and the foam was what's left holding up the roof.

5. GBA Editor
   Martin Holladay | Apr 21, 2017 06:30am | #5

   Jim,
   Six inches of real insulation is certainly more effective at reducing the rate of heat flow than a radiant barrier. That said, it's true that the use of open-cell spray foam on the underside of roof sheathing is associated with moisture problems.

   Closed-cell spray foam is safer than open-cell spray foam in this location. Or you could install a conventional type of insulation like fiberglass, mineral wool, or cellulose. (If you choose one of these materials, the building code requires that there be a ventilation channel between the insulation and the roof sheathing.)

   Remember, though: Unless the space is being air conditioned, insulation can't really lower the temperature of the garage. All it can do is slow down the rate of heat flow through the roof assembly or wall assembly.

   -- Martin Holladay

6. 
   swazo | Apr 21, 2017 10:31am | #6

   Thanks guys,
   I asked him if he was trying to reduce the cooling load in the house and he said no, that's not his goal.

   I believe, and I think Martin implied the same, the spray foam is too expensive for the concern. It is interesting that you could have moisture problems considering the climate. I can see that a leak could trap water but I don't see how you could have condensation issues with this in Florida. I'll just accept that.

   I felt that ventilation was the answer. Provide a high point (in the ridge?) and let the relatively cool air come in the soffit vents and out the ridge vent. Doesn't this make sense?

   Thanks
   Jim

7. GBA Editor
   Martin Holladay | Apr 21, 2017 10:37am | #7

   Jim,
   Installing a ridge vent can't hurt. Is it likely to lower the garage air temperature much? I doubt it.

   -- Martin Holladay

8. Expert Member
   Dana Dorsett | Apr 21, 2017 12:05pm | #8

   Further venting the roof isn't going to do much, Tile roofs are inherently ventilated, and often installed on skip sheathing.

   Open cell foam on the underside of the roof deck may be problematic. Tell us more about the roof deck sheathing & tile stackup.

   Spraying the underside of the roof deck with a silvery low-E paint (not a scammy ceramic sphere additive or other "insulating paint"), AND adding radiant barrier on the underside of the rafters can make a measurable difference.

   Installing heat-rejecting window films on any windows (particularly windows that get direct sun) will help too. If garage has a lot of window area the heat gain from the windows could exceed the heat gain of a tile roof.