What type of foam on an exterior roof/cathedral ceiling?
Ok..I decided not to use my supply of 3″ x 16″ x 8′ Dow Blue XPS on the inside of my main ceiling only IF I can:
Use it on the exterior roof of the addition (single story, multi pitch 12×28) on the outside to insulate as an unvented unit? Two layers (R-30) on the top and R-30 fluffy on the inside. I’ve read and read the write up on here but I’m not sure if I can use this type of insulation as it’s specified for wall usage…but I can’t see any difference outside the fact it’s going to take a lot more tape. I’m also a bit confused on Vapor Barrier usage in the write up as the diagram says none but the text claims proper usage?
One of the reasons for this approach is I CAN’T BELIEVE how cheap cellulose is and on top of that they have a local rebate that will all but pay for the entire insulation job in the main part of the house.
thanks
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Replies
The Dow blue XPS is commonly used on roofs.
Geoffrey,
1. XPS can be used for roofs.
2. You don't have to tape both layers of XPS if your budget can't afford that much tape. You could just tape one layer of XPS -- or just tape the plywood or OSB roof sheathing. Whatever you do, stagger the seams on the two layers of foam.
3. I'm not sure what your vapor barrier question is. Two layers of 3-inch-thick XPS will definitely be a vapor barrier. Whatever you do, don't install any interior polyethylene.
Nice...thanks.
Do I put the self stick ice barrier on top of the new OSB that sits over the foam, on the first layer of OSB/Plywood on the roof or both? Anyone have a good place to purchase reasonably priced 8" screws?
And..Can I blow in Cellulose to a ceiling by using that insulation mesh and blowing it in through holes? Or would putting one sheet of drywall up and blowing it down work better? Thanks
G
Geoffrey,
I'm not sure why you want to use a peel-and-stick membrane on your roof. Is it an air barrier? If so, you don't have to worry about taping your plywood or XPS. Is it a water barrier? If so, it raises a question: are you worried that your roofing will leak?
Are you worried about ice dams? If so, you don't usually need peel-and-stick on your entire roof.
It's hard to give advice on where you should put the peel-and-stick unless you tell use why you want to install it.
Some suppliers of long screws include:
Best Materials — Dekfast 6-inch roofing screws and Dekfast 9-inch roofing screws
Wind-Lock (a source of long screws and plastic hold-down buttons)
FastenMaster (a source of screws up to 10 inches long)
Either method of installing cellulose can work, but if you've never done it before, you might want to hire an experienced contractor. More information here: How to Install Cellulose Insulation.
Martin,
Geoffrey may have seen a couple documents on the Building Science Corp website where Joe discusses using peel-and-stick membrane over an entire roof as an air barrier (and walls too). At least that's where I first saw that method mentioned.
Here's one, there have been others.
Joe's house covered in peel-and-stick membrane
You helped convince me to tape the seems in the sheathing with a quality tape (Siga, etc.) and not cover the entire structural roof deck with peel-and-stick membrane.
Thanks
Eric
Thanks Eric and Martin...I have seen many DIY articles where people recommend ice and water on the entire roof...so I just figured I had to. I'll be putting a metal roof on as it's an easy roof to use metal on by myself...thanks for all the info
This is a HUGE project and I couldn't do it without you out there in no face land...not to mention my loved ones..thanks again.
G
Ice and water shield should be used where ice dams are a concern and are required by code in Canada. Ultimately it comes down to how much snow will be on the roof. buildingscience.com has a couple of really good papers on the subject.
Any issues with metal roofs on a "Hot Roof"?
Q. "Any issues with metal roofs on a 'hot roof'?"
A. No, as long as you install purlins or plywood above the foam.
Purlins? That might make the roof not very walkable in any future service capacity no? I'm looking for ways to save money and the Metal Roof thing would actually be cheaper than shingles even after all the matching trim with the not having to buy a couple hundred dollars of ice/rain shield, which is not needed on a metal roof right? Thanks much..