What are my choices in my roof remodel
Hello, I am now much farther along in the remodel of my 1965 home. We removed the old roof shingles, decking, and rafters. We rebuilt with new rafters from the walls up. Then I sheathed the roof with 5/8 inch Zip Sheathing. Now I plan on adding two layers of 2 inch rigid polyiso foam board. I live in Zone 3a. My question or hope for advice is, must I add another layer of 7/16 or 1/2 inch polyiso on top of my 4 inches of polyiso, then 1×4 lathes or strapping, with a metal roof on top? Or could I skip the second layer of Zip Sheathing or plywood. and install maybe a layer of roof underlayment or bubble wrap, then 1×4’s and metal. Would it cause any problems or concerns with skipping the second layer of Zip Sheathing? All suggestions and advice would be appreciated.
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
Depends on your metal roof, so check with the manufacturer.
Some are rated to be installed directly over insulation with clips, some need a wood deck and some can be installed over strapping. Usually the cheapest is strapping.
All metal roof manufacturers require an underlayment. No matter what, skip the bubble wrap, it is a waste of money.
Thank you sir for your reply. If indeed the metal manufacturer allows to roof over the polyiso foam board. What type of wrb would you suggest putting over the rigid foam board? I am installing this myself, Will the foam board need that weather barrier put over it as soon as it is installed, to protect it from the rain? I realize that I have a WRB with my layer of Zip Sheathing installed and taped. I was just trying to save the added expense of installing a second layer of Zip if possible. If the metal manufacturer allows me to put the 1x4 strapping directly over the rigid foam board, then in your opinion those layers won't cause any problems with the efficiency or integrity of my roof system? I plan on removing all of the insulation in my attic ceiling. Then I plan on putting R30 fiberglass batts between my rafters against the bottom side of my Zip Sheathing. Again, Thank You for your advice.
I generally go for a permeable underlayment since it is a stock item at my local roof suppliers, pretty much any synthetic one would work.
As long as you have the underlayment over the foam, whether the panels go directly over the foam or over strapping, the roof will work just as well. The ZIP underneath becomes your main air barrier.
In your warmer climate, with 4" of insulation above, you can get away with any amount of insulation underneath, the R30 is no problems at all.
One item with foam over-roof to watch is that you protect the edges of the foam. Critters like to get any there, so make sure that no foam edges exposed.
Again, Thank you so much for your helpful advice. I just talked to the metal manufacturer and he said as long as the 1x4 strapping is secured to the rafters with long screws, the metal will be fine. I do suppose that the powerlag screws that I had planned on using need to go through the strapping of course, through the two layers of foam board, and through the 5/8 inch layer of Zip, and then maybe 2 inches into the rafters, would that be your advice? And if I use the 4 foot wide roofing underlayment directly over the foam board, how do you recommend that I attach it? And what is your recommendation for covering the exposed ends of the foam board? 1 am using the Zip sheathing on my walls, I've removed everything down to my studs. So Zip, then 2 inches of rigid polyiso foam board over the Zip, and it could butt up against a 2 inch overhang of the foam board on the roof, which would leave the end of the roof foam exposed, or let the roof foam end even with the wall, and let the foam on the wall go up to the bottom side of the Zip on the roof overhang? Again, Much thanks for your help.
The amount of embed for the screws depends on your local code and wind load. The screws will be pretty long. Installing through 4" foam is not the easiest, make sure you mark out your rafter center lines. I normally drive a screw out from the inside beside the rafter at the ridge and use that as a reference. Check from the inside when done as there will always be some shiners.
I've normally just tape the underlayment down with some Tucktape but any tape that will stick will work. The underlayment also gets stapled to the framing around the edge. You only need to hold it in place till the strapping goes on.
For the edges, I like to frame around the perimeter with wood, this means some ripping to match the angle at the eaves. The whole thing then gets covered with custom bent metal.
For your wall to roof transition, just need to make sure your air barrier is continuous. Rest of the details there depend on your overhangs, siding and the look you are going for.
Thanks again Akos for your help. One more question. My house was built on a conventional foundation on concrete blocks, with vents, and a crawlspace underneath the floor. I am removing the brick on the exterior walls down to my concrete footing. So the concrete block walls and the rim joist sitting on top of them will be accessible. How would you advise me to waterproof and air seal the blocks and rim joist? And then should I put the Zip sheathing and the Polyiso foam board low enough to cover the concrete blocks? So I can have an airtight conditioned crawlspace that is clean and dry. Thanks again.
Generally you would want some flexible membrane to handle the transition from wall to foundation. Check the manufactuters data sheet as some membranes require a primer for proper adhesion to masonry. If you are using ZIP sheathing, follow their details as it will be different from a standard housewrap install.
Thanks Akos. With all of your help I think I need to get to work. All of the research and building science that ya'll share is so helpful and useful to those of us who are trying to make our older homes more energy efficient and sustainable. Thank you for your advice.
Tommy,
Glad to help out. Post some pictures of your progress. Always good to see the final results of these projects .
Akos