We have a small, two-story wood-framed townhouse in NYC built in the late 1800’s. The top floor ceiling is vaulted to the roof deck in the front half of the house, and there is attic space on the back half that is unvented. Nothing is insulated. The roof is pitched and abuts the adjacent house on one side with an integrated “Yankee gutter” on that side. The surface area of the entire roof is only about 1,000sqft. The attic and air space in the vaulted ceiling room get very hot in the summer. We are about to replace the roof and it seems like a good opportunity to insulate. The current roof has been in place for >20yrs and there are no leaks.
From what I’ve gathered from reading here and elsewhere, our best (only?) option is to insulate the roof deck externally with rigid foam. I believe we are in zone 4a, and it seems code requires R-33 when insulating entirely above the roof deck.
Problem is, it doesn’t seem that any roofing contractors actually have any experience doing this kind of project. I’m also hesitant to make any major changes to a house that has survived 125yrs without any insulation or venting of the roof/attic
My questions are:
1. Any drawbacks to just replacing the roof shingles and leaving things as is (other than the attic/upper floor being hot in the summer and cold in the winter?
2. Any problem you see with doing external rigid insulation in this setting?
3. How do I find a reputable contractor that actually knows how to do this correctly?
4. How would you detail the foam at the junction where the house is attached to the adjacent building (which is taller than ours), where there is an integrated gutter
Thanks for your input!
Replies
You may also be able to use roof vent chutes (something like the Durovent rafter vent) below the sheathing between the rafters if you'll have the sheathing off. You'd need to do this in EVERY rafter bay, from eave to ridge, then add soffit and ridge vents.
Insulating above the sheathing would need rigid foam. Commercial roofers will be familiar with this, since most commerical buildings are insulated this way on the uppermost floor. It's a lot less commonly done on residential structures, but it's entirely possible to do. Normally polyiso is used for this application.
In regards to your specific questions:
1- You risk moisture problems. Anytime air can accumulate in a roof assembly, there is a possibility for moisture problems.
2- You may need strapping and/or a second layer of sheathing above the foam to provide a surface to secure the roof materials. You can't, for example, nail shingles directly to a rigid foam insulating panel.
3- Check with COMMERCIAL roofing contractors. Someone will probably be able to help you. Commercial roofers routinely deal with rigid foam above the roof deck.
4- Can you provide a picture?
Bill
Thanks for your reply. Great tip on the commercial roofer!
The way I would approach this in your shoes is I would hire a carpentry company to remove the old shingles and lay down the ridgid foam w/ 50% offset in both horiz & vertical directions. then I would have them install the roof sheathing and underlayment again with 50% offset to the sheathing joints and lapped per installation instructions. At the eave & rake edges I'd have them back up the seams with acoustical caulk between the layers of underlayment. This would be for any seams within 4' of the edges and between the sheathing and the underlayment before installing the metal drip edge. Once the underlying insulation layers, drip edge, step & kick-out flashings and and water control are done the roofer should be able to proceed with very few questions asked.
Michael,
I'm with you except for the flashing. It's a lot better if you leave that to the roofers for a few reasons.
- The flashing needs to be integrated into the shingle layers, and that much easier if they go in at the same time.
- It keeps the workmanship of the entire roof under one warranty. No arguing that leaks or defects are due to someone else.
- The flashing is outside most carpenter's wheelhouses in the same way the layers below were outside most roofers.
I’d prefer no sealant at the lower edges of the eaves, to avoid trapping any water in there that may have found a way in somewhere higher on the roof.
Bill