Replace the roof before insulating roof rafters with spray foam?
I am just about to make my vented attic, unvented, i.e. raise the thermal barrier to the roof. I plan to use 2” of closed cell spray foam next to the roof deck and 9” of open cell to complete the 2×6 cavity and to overfill the framing (in an attempt to arrest the thermal bridging of the rafters).
In doing my homework about potential problems with spray foam, I’m most concerned about water getting at the roof deck from the top through the shingles. The problem, as I understand it, is I won’t know about the leak. Without the foam on the underside of the deck, I might discover the leak. With the foam, I may not discover it until a rotten roof fails. Therefore, I feel obligated to have the roof replaced now, so that any suspect boards can be replaced. In replacing the roof, the project gets pricey, but I want to do it right. (The roof looks ok but it is 15 years old)
Assuming my reasoning is sound about replacing the roof, I’d be grateful for advice on what kind of shingles and underlayment to use. I’m ok if my new “hot roof” will make the shingles deteriorate faster, but I’d like to find a way to protect the boards from water leaks as much as possible.
I’ve never seen this recommended, but should I ask the roofer to extend the water and ice shield to the ridge line, in otherword, rubberize the entire roof? Or, is there another preferred method I should consider to ensure the deck boards stay dry?
This is a 1500 square foot ranch in Massachusetts (Zone 5). Roof has a 5 on 12 slope.
Thanks very much for your help!
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Replies
{Not a Pro, but]
Can someone please shake this man's hand for think of the roofing and the insulation as an integrated system!
I have to restrain myself when the dozens of people post that they are looking at insulating their roof 'but it is a brand new roof'
ARGHHH!
I would look into insulation board over rather than spray foam, as it may be less expensive, but YMMV
Thanks for thinking I'm not crazy :) If you look at BEGREENER's comment, this COOL-VENT product seems perfect. To condition my attic, I think I need to have both the foam on the underside and insulation on the top side. This may be insanely expensive, but if you are going to protect your house, I think you have to start at the top of the house.
As long as you have enough exterior rigid, you don't need any interior foam. This means you can insulate with regular batts in the rafters. You still have to watch the ratio of rigid to fluffy, so in zone 5 with R21 batts in the rafters you need R14 exterior rigid. If you want a bit more R value you can go with R24 batts and R16 rigid.
The one challenge with exterior rigid install is your typical roofer won't know how to do it. You would need a roofer to strip the shingles, install I&W as temporary protection plus to air seal. You would than have a carpenter install the exterior rigid and facia extensions. Finally have the roofer come back and install the new shingles. This would be more cost but you would have a spray foam free house.
Thanks for your comments Akos. Good information and helpful in managing my expectations.
Former spray foamer here ...
If you are going to replace your roof, why not just put poly-iso boards on top to create a thermal break?
https://hunterpanels.com/products/roof-products/cool-vent/
I like it. Thanks for your help!!
Yep. What do all of us home owners with spray foam do in the future when roof sheathing has to be replaced?
Attic is roof+ insulation.
Cathedral ceiling = Roof+insulation+drywall demo.
Great that the pros here know how to make a lasting roof at 100%, but no home owner can manage 100% perfect maintenance or stop acts of water/nature on the roof.
If there were one item the green community should rally against, it's spray foam.
You may want to look for a roofer who uses Atlas roofing products, as they have had an exterior insulation roofing product for many years (search for acfoam nail base). It's similar to the Hunter product previously mentioned. This way you have a single manufacturer for underlayment/insulation/decking/shingles that will warranty your roof assembly, and you can insulate the interior with a lower cost option.