09:38PM | 01/25/05
Member Since: 01/25/05
2 lifetime posts
I will be insulating my garage this weekend and have had a couple different people tell me different things. So i'm looking to clear up some things.

The garage is 40X22X12 with walls that are 2X6, and rafters are that are 2X4.

I'm going to use R19 (6 1/2" deep X 15" wide) on the walls, and then drywall over that - no biggy here.

The roof, however, is different. I thought i could just use insulation in between the rafters in the roof, much like the walls, but someone told me that the roof needs to be left alone (no insulation against it) so it is the same temp as outside. which to me seems odd, because owens corning has illustrations of using insulation in rafters on thier web site (the same way i would use them in my garage).

For Example!


I was planning on using R13 Foil insulation (about 1/8" thick) first inbetween the rafters in the roof, and then using R13 fiber insulation over that (remember the rafters are only 3 1/2" thick, so r19/21 is out of the question), and then finally using 2" thick R8 polyboard to cover everything up, giving me a nice white ceiling with a total R value of 34.

Now, will this work?


09:41PM | 01/25/05
Member Since: 01/25/05
2 lifetime posts


01:10PM | 01/26/05
Member Since: 11/27/04
174 lifetime posts
this is an attic space ,not for living or storing stuff??

insulate the ceiling with r40 blown in or 3 1/2" inch batt and then 5 1/2" inch batt over that.

insulating the top rafter space is used in hot climates as a way to control temperature in the attic space to reduce heat gain to the living space. attic still needs to be ventillated.

owens corning pic has to do with a livable attic space.


09:52AM | 01/27/05
Member Since: 01/28/01
171 lifetime posts
You need air flow to cool and dry the roof sheathing.

You need to buy thin foam troughs that preserve about 1 or 2" gap between the sheathing and the insulation. You can find them in the same aisle as the rolls of insulation.

Make sure you have eave and peak vents so the air can circulate.


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