gypsum sheathing and aluminum faced ply
Everywhere I look these days, I’m seeing that yellow or silver-coloured Georgia Pacific fibreglass-faced gypsum board used as sheathing. So I cruised over to the Georgia Pacific website and apparently, this stuff meets code requirements as a structural sheathing. It even comes with a 90 day weather-resistance guarantee! (I’ve seen a lot of it up for more than 90 days before it’s covered- wonder what you’re supposed to do if you go beyond 90 days- rip it down and start again?!)
Personally, I used 1/2″ ply for my place- 7/16″ OSB seemed too chintzy. Am I just nuts?
They’re also offering an aluminum foil-faced plywood material, touting it as a radiant barrier to reduce the migration of solar heating from the shingles into the house if installed as a roof sheathing material with the shiny side DOWN. Yeah, I know there’s an air gap, but isn’t the “heat mirror” facing the wrong way in this case for it to work?
Any comments on either of these materials?
Replies
bump
Not an expert, but I think the point of the shiny side down is supposed to keep your house heat in, not solar heat out.
That's not what Georgia Pacific are claiming on their website- they're claiming a reduction in heat gain from solar heating of the shingles.
Interesting. My assumption would be then that if you were to peel the foil facing off (or whatever it is made of) it would be reflective on both sides. Still it seems that you would be allowing the sheathing to heat up.
Your right, intuition would be to put the foil side up...
mm,What Paul said in post 6 is correct. The foil rejects radiant transmission in and out. Until the shingles can be nailed to the air 1/2" above the decking, the shiny side has to go down.Bill
Edited 10/14/2005 7:00 pm by BillBrennen
The other reason for the shiney side to be down is so dust doesn't accumulate - creating a new exterior face for the heat to transfer out of through conduction (which aluminum IS good at).Rebuilding my home in Cypress, CA
Also a CRX fanatic!
Correct. My last post was hurried, so I omitted it.Bill
OK. I see that it isn't as simple as merely reflecting any infrared that the sheathing or shingles are emitting. If the reflective layer is buried between layers of conductive material it will simply heat up and pass the heat by conduction, eliminating the benefit of the radiant barrier. So what you're saying is that as long as the reflective layer has air on one side or the other, the radiant barrier will work.
So: any comments on the "exterior drywall" sheathing? I'm seeing huge quantities of it used up here these days. It just surprises me that nobody here even mentions it. Is it widely used in the US too?
The radient barrier needs the air space. It's actually not a mirror - it's more like a deadener. Aluminumn foil is very poor at radiating heat back out into the air... heat gets to it and stops, it does not pass through.
Rebuilding my home in Cypress, CA
Also a CRX fanatic!