Can I insulate the underside of my roof?
I haven’t tried to search this question because I don’t know how best to describe what I’m considering doing.
I have a 1980 single-story slab home in a warmer part of San Diego. It was built with trusses and a cedar shake roof. The previous owner replaced the cedar shake with regular-weight tiles (on top of OSB).
To improve energy efficiency and make the attic more useful, I would like to insulate the underside of the roof. There is, of course, already insulation in the attic on top of the ceiling drywall. I want to add it to the underside of the roof, so that when you are standing in the attic there is insulation both under your feet and above your head.
The temperature is very even here and vapor barriers aren’t much of a concern. The attic insulation has paper on its underside, but there is no vapor barrier installed. That is common here.
Is there any reason I can’t or shouldn’t add insulation to the underside of the roof?
Could weight be a concern? (The truss chords are 2×4’s, so I presume I’d use R13 fiberglass. I’d consider foam, but I was told it would weigh a lot more?)
Thanks!
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Replies
You want to spray the underside of the roof with foam, that will turn the attic into a conditioned space.
The FG can stay or go, it's not doing much now and will be doing nothing after you foam the roof.
Foam weighs too much? Who told you that?
Too much for what?
Joe H
Edited 11/2/2007 7:54 pm by JoeH
Well, the homes in my subdivision were built with cedar shake roofs (now illegal here in San Diego due to the fire hazard), which are very light.
Most of the houses now have tile roofs, but most are lightweight tile. I found out mine are regular weight. They were permitted and an engineer's report was done (I have it), but the roof is already supporting a good amount of weight.
I don't know how much foam would weigh. From what I've read, there are different weights of foam, which correspond to different R values. I'm guessing heavier foam has a greater R value than lighter foam.
So, when I say weight, I am only considering that. Would foam would weigh any more than an equivalent R-value of fiberglass bats?
I'd rather do foam. It would be more expensive, but much easier.
foam itIt is not appreciably heavier than the FG batts and much better for what you are trying to acomplish. if hte tiny percentage of increased weight of the foam will damage your roof or make things unsafe under it, you are already in danger the next time the wind blows
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Foam weighs next to nothing. You are only going to put it on a few inches thick.
If that little amount of additional loading is a concern to you, you best get an Engineer out to look at your roof structure.
Nothing else you can do to an existing trussed roof will do the job foam will no matter how much time you spend fiddling with it.
Joe H
If you could Nail 1/2" 4 X 8 sheets of thermax on the underside of the top cord of the roof trusses that may accomplish what you want to do. You are more concerned with the heating load causing your AC to run a lot. There are plywood and OSB sheets with alum foil on the underside that is suppose to reflect the heat back up and not let it penetrate the attic space. This is used in new construction only. There is also an aluminum mesh almost like window screen that can be nailed to the underside of the top cord of the roof truss that is suppose to accomplish the same thing. I have seen a number of articles on the internet about these types of installations. I think soffet vents and ridge vents are a fire issue in your area. It's not to have them but how they are installed is the critical issue. Properly installed vents with the addition of thermax on the bottom of the top cord of your trusses would be my best guess of your most effective way to keep the heat out. Good Luck Jay
Thanks for the responses.
I am concerned about the load on my A/C, of course. But I also would like to stablilize the temperature in the attic for things I put up there. Storage space is worth a lot here.
Due to the weight issue, I can't store anything heavy up there. But one section of the attic, in the middle of the "W" trusses, is over 6' tall and great for storing lightweight things.
And I also do a lot of tinkering around up there.
But it all leads back to the same goal: Making the attic cooler and more temperature stable. (I also intend to install an attic fan on one of the gable vents.)
I installed an attic fan at least 20 years ago. In fact I wore one out and had to replace it. I put it on a timer and took off the temperature sensor. That way I could turn the unit on before the attic was hot and cut it off if the noise bothered someone. Can't hear it anyway. It was one of the best things I did.
If the framing below your trusses allows. You may be able to add wood to the top of the bottom cord of the truss by scabbing plywood to the side of each truss. That way you could have adequate insulation and then put plywood or osb on top and have more usable area. A good carpenter could look at the structure and let you know if that is possible.
In any case sounds like you are heading in the right direction and picking the best time of the year to do the work. Jay
Icynene foam is extremely light weight. The manufacture claims that the when sprayed, the liquid expands at a rate of 100:1. Hence it is just tiny air cells. We had it sprayed onto 1/2" Styrofoam board, which made up the air channel for the soffit and ridge vent for our cathedral ceiling. We noticed a huge temperature difference from just the Styrofoam board, before the Icynene was applied.
kestrel