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Garage roof insulation

NutmegCT

Great Pumpkin
Bronze
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My small garage (new for the MG) gets full summer sun. Roof temp gets up to around 150 between 10am and around 6pm; this heats up inside the entire garage so the air (and MG) gets well over 100 degrees. Walls actually stay at ambient temp (80-90).

I want to add insulation on the side of the roof that gets the full sun.

Garage isn't heated or cooled.

Just rolls of R-19 pink fiberglas stapled up between the rafters? Radiant barrier material instead of rolled insulation? Is there a better alternative?

Thanks.
Tom M.
 
How about an electric roof vent with a thermal set point.
 
Thanks Elliot. I may do that also; easy to do in my garage. But I'm interested in keeping the temp from going so high in the first place. The underside of that roof gets really hot. I've just heard about "radiant barrier", so am wondering about that also.

Tom M.
 
Tom, I just bought a roll of Radiant barrier. It’s still in the box. I got it on Amazon; there was a $10 coupon on Amazon.
 
Tom, I just bought a roll of Radiant barrier. It’s still in the box. I got it on Amazon; there was a $10 coupon on Amazon.

Thanks. I guess my question is really which is better for my specific use. The fiberglass insulation roll, or the radiant barrier. I can get my head around the idea of insulation, keeping heat from the roof from moving inward. Can't quite follow the idea of radiant barrier, especially as the recommendations are for the reflective surface to be facing toward the room area - not the roof.

Details, details.
Tom M.
 
I understand Tom. I’m putting in an attic; my expectations are low.
 
Is it a truss attic or stick built. Stick built may be easier to add the radiant barrier on the underside of the roof.

David
 
Thanks David. It is "stick built". I don't see a problem adding insulation or radiant barrier up there between the rafters; roof peak is only ten feet from floor, and I'm pretty handy with a staple gun. Big frustration is trying to find reliable reviews and comments about the radiant systems. Many of the negative comments I find are similar to this: "I put it behind my refrigerator, and it doesn't do anything." or "Here in Minnesota it didn't change the temp in my attic during snow storms." They're commenting on barriers in installations the barrier was never intended for.

I have heard from a few folks who say they have the radiant barrier in their attic, but they don't know if it helps anything; it was installed by a contractor who also blew insulation between the ceiling joists. As I just want to cut down the heat radiating from the underside of the roofing on a hot sunny day, it seems the radiant barrier is a good idea. But good ideas on paper dont' always pan out in the field.

This gives me an idea (always dangerous ...). I'll tack up a strip of aluminum foil between two rafters (leaving air space between barrier and roofing). Then compare temp of the underside of the roofing in the adjacent rafter section, to the temp of the underside of the foil where it's installed. Might give some indication of effectiveness.

Thanks.
Tom M.
 
I don't know what type of roofing material you have, but it matters as once the heat is collected, it transfers somewhere. I have a metal roof. It would collect the sun like yours and transfer it inside. I finally had 2 inches of closed cell foam sprayed on top of the metal roof. Sealed with elastomeric sealant and now the inside of the workshop is 10 degrees cooler than outside. An amazing difference. I do have the apply more elastomeric every 4 years to keep the foam from the sun.

Jerry
 
Thanks Jerry. Roofing is GAF asphalt shingles and "tar paper" nailed into roofing board. Nothing below the boards except the rafters.

Interesting your use of foam sprayed on top of the metal roof. Sounds like it serves the same purpose as a shade tree, keeping sun off the metal surface itself.

Just out of curiosity, did you consider putting something on the underside of the roof to prevent heat transfer?

Thanks.
Tom M.
 
This is interesting. I just put about two feet of household aluminum foil up, across the bottom faces of two rafters. Shiny side toward the roofing.

After two hours in the sun, bottom surface of the foil is 85F; bottom surface of the roofing (next section of rafters) is 120F.

There may be something to this "radiant barrier" method after all.

Wonder if I should just put up more aluminum foil on part of the area, instead of the "real" radiant barrier material, and see how it goes for the next month or so.
 
Thank you sir!

That's the first article I've seen giving so many details. Strange that it says the shiny side of the foil should be facing down. Wouldn't it seem more effective if the shiny side faced the hot surface (the underside of the roof)? That's where the heat is radiating from.

Thanks.
Tom M.
 
Since the foil will be right against the roof it will pick up heat through conduction and/or convection anyway. Having the shiny side down will re-radiate less of the heat into the room.

Over time, dust will build up on the upper surface. With the dust, it won't be shiny anymore. The dust will absorb radiation and transmit the heat into the foil through conduction. And again, having the shiny side down will re-radiate less of the heat into the room.

If you had a vacuum gap between the foil and the roof then yes, it could make sense to have the shiny side facing the roof.
 
Thanks again.

"If you had a vacuum gap between the foil and the roof then yes, it could make sense to have the shiny side facing the roof."

There is an air (not vacuum) gap between foil and roof - it's the depth of the 2x4 rafters.

Just so I'm understanding, are you saying the shiny side should only face the roofing if there's a *vacuum* in between?

Thanks.
Tom M.
 
Camilla's wedding hat might make an effective insulation:

105cd7c0b432a713f57e03c4b61c6766
 
RE: Metal roof. I had 2 inches of styrofoam in the form of 4 x 8 sheets on the inside of the roof. But once the heat is through the metal, the rafters became a radiator. So the foam on top made the heat not come through. The insulation on the inside may help when I heat the building but not to keep the heat out.

Jerry
 
Heat will travel three ways, conduction (solid touching solid), convection (through fluid movement and contact) and radiation (in this case, long wave Infrared light).

Shiny metal will reflect incoming radiation, causing less heating of the shiny object from external radiation. And it will also reduce the transmission of outgoing radiation from a hot, shiny object to other things.

In your attic, some heat will transmit to the foil by radiation from the underside of the roof. So one might assume that shiny side toward the roof it better. But the foil is still in contact with the rafters so it will heat up through conduction. And the air between the rafters will heat the foil through convection. So the foil is going to get hot anyway. Since it's hot anyway, having the shiny side down will transmit less into the room.

If you had a vacuum between the roof and the foil there would be no conduction or convection and heat would only arrive through radiation. So reflecting it back would prevent heat buildup. That's why spacecraft solar shields are shiny on the outside.

KSC-05pd2407a~medium.jpg
 
Tom,
I bought a staple hammer to tack up my radiant barrier material. I do have a staple gun but I figured my hand would be numb from all the "squeezes" of the trigger. Just swing the staple hammer to contact the wood and it's done.
076174373790.jpg
 
Thanks for the details on the reflectivity issue.

Elliot - could you show a couple photos of the barrier in place? I'm interested in what material you used, and how it's installed.

Thanks.
Tom M.
 
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