Historic home roof insulation solution

[Dan McGonigle]

Hello! I’m in a bit of quandary here in regards to how best to approach insulating the roof of my 1842 brick home.

Simply put, we have a third story that we don’t use. We don’t use it because it is in a state of disrepair. It has been on the project list for years and my wife and I have decided to get serious about it this year. I have plans to renovate it, not as a living space but as a clean and comfortable area for storage and space for my wife’s various crafts. The current roof system is as follows: typical a frame, rafters, purlins, original cedar shakes nailed directly to purlins, standing seam metal roof applied directly over original cedar shakes. No roof insulation. I’m not sure when the metal roof was installed, but I have to assume it’s old. No ridge vent, no soffit vents, no roof box vents, no gable vents, no venting whatsoever. How can I insulate this? I’ve read plenty of things online regarding this issue, I’ve spoke with my local building inspector, a residential architect, and the cedar shake bureau technical rep. The idea of not installing vents and applying closed cell foam to the underside of the original singles (possibly with a contact barrier in between) is debated. Some say you can, others say you can’t. My options are to explore this route (waiting to hear back from a reputable insulation contractor) or to have a ridge/soffit system installed and use fiberglass batts.

That comes to my second quandary. Current code in my area (NJ climate 4a) states roofs must be insulated to R-49. This can be achieved with fiberglass batts at 14” thick. As far as I know, my 180 year old roof rafters are 4-6” thick. There’s no way that could work. And I seriously do not want to extend my rafters by 10 inches - it would have a huge spacial and visual impact on the third story (sloped walls/knee walls and chimneys coming through). Even with closed cell foam, using the max allowable thickness of 4-5 inches (I believe) I would fall quite short. I’m looking at the NJ rehab sub code which I believe states an exception in my case - stating that if recommended R value cannot be achieved due to space constraints, fill the available cavities. Which I interpret as put as much insulation as you can in there. But, unfortunately the inspector may interpret it differently and state I “could” achieve this by undertaking more construction and increasing the depth of my rafter bays.

Any thoughts at all would be welcome!

[Ron Selzer]

Years ago I subscribed to New England journal of Light Construction (https://www.jlconline.com/magazine/) they always had a lot of good information and seemed to be on the cutting edge. Would be worth a visit there to see what you can find. Then s much as i hate to do it it would be time to get an architect and the building inspection dept. involved or decide to do it with any inspections.
Good Luck
Ron

[Jim Matthews]

I wouldn't spray foam.

Rigid insulation board overlay can be serviced, or removed more readily. Some accomodations will be required to hide the increased depth at the gutter and gable ends.

https://www.buildingscience.com/docu...3-over-roofing

[Jon Nuckles]

My understanding, based on my architect, contractor, and the approval of plans for our remodel a couple of years ago, is that spraying closed cell foam completely prevents vapor penetration and you don't need to vent on the underside of the roof. I sure hope they were right!

[Ron Citerone]

Sub code sounds like it is worth running by the inspector.

[roger wiegand]

If it were me I'd go the unvented roof route (as we have in our current house and shop), fill the bays with closed cell foam, than 2" of rigid foam on the inside to stop thermal bridging, with blueboard and plaster to create your interior finished surface. (someone sells a product with blueboard glued to rigid foam for a all-in-one solution at minimal thickness.

An unvented roof can be problematic in tropical climates, shortening the life of roofing materials. I'm aware of no other downsides except for the inevitable "you can't tell where the roof is leaking". We've had one roof leak in 60 years, and it was pretty easy to find due to the branch sticking through the hole. I don't think this is a real world concern, at least if you maintain your roof.

At my son's house with a very low slope roof and no way to access the space to spray foam it needed a new roof anyway, so we put 8" of recycled polyiso rigid foam from commercial roofing on top of the existing roof, built up the edges, and applied a rubber roof over it. It has worked out extremely well.

My impression is that 90% of the naysaying on moving the insulating layer up to the rafters and going unvented comes from the guys who sell fiberglass and blown in cellulose. For us it got the HVAC equipment in the attic inside the conditioned envelope which was a huge win.

FWIW, our inspector was thrilled we were doing anything to upgrade the insulation in both houses. Code does not require you to do the impossible in an old structure.

In the future, when it is time to replace the roof you can do a couple of additional improvements, add more insulation above, and/or add a new layer of sheathing set up 3/4" off the old roof with soffit and gable vents to keep your new roof cool-- this will certainly help with summer AC loads.

[Jim Becker]

IF the roof deck wasn't old cedar shakes and was plywood or solid boards laid out as a closed deck, I wouldn't hesitate to use closed cell spray foam because it solves almost all of your issues...you can get to R-49, you get a full vapor barrier, you get a much stiffer structure, you do not need venting because inside the spray foam is considered conditioned space, etc. It also wouldn't interfere with anything exterior that has to be maintained for historical preservation if that's a factor, either. The same would be true if you had the underside of the steel on the purlins as the spray foam eliminates condensation issues on the underside of the steel.

BUT...you don't have that situation because of how the roof was previously renovated. The cedar shakes should have been removed prior to installing the steel for both functional and appearance reasons. (steel doesn't like uneven surfaces under it and can project the same to the top surface) So the best solution would be to retrofit with a ridge vent and vented soffit, have baffles in the bays for air flow, fir out the roof joists to accommodate high density/high R-Value fiberglass batts and if that doesn't get you to your required R-49, use foam insulation board cross ways over the augmented joists to get the R-value and tape the joints for air infiltration and vapor before doing drywall or other coverings.

That said, Roger is also correct that you may be relieved of being fully code compliant as a retrofit rather than new construction, and with furring out the joists you should be able to get an R-38 with the best material available in, say, 12" of space. All the rest applies, however...you do need to do the venting or you're going to have moisture issues.

[Andrew Hughes]

I would reach out to a local Roofing company. I say this because I’m a roofer and always felt like inspectors and architects were sometimes full of baloney.
Since I’m a west coast roofer I have no experience with east coast residential practices. But I’ve done commercial roofs that were supposed to be good for snow and ice dams conditions.
Dens glass sheeting with a very good peel and stick membrane.
Good Luck

[Steve Demuth]

I would absolutely go with sprayed in closed cell foam. If there are concerns about the purlin-shake confection against the foam, layer in 1" closed cell board between the rafters firmly attached to the purlins, and then cover that with foam.

My roof is a crazy concoction of non-continuous roof boards originally put down for wooden shingles and plywood, all overlain with the actual roof. Ten years ago we cleaned up the inside, extended the rafters and blew in 10" of closed cell foam before drywalling the entire mess to make a cathedral ceiling. The roof is completely unvented, as are the gable ends down to the loft floor. We have had absolutely zero moisture issues and the closed cell foam made the roof (which was probably structurally borderline before) so strong you could drive a truck on it. It's like one giant torsion box with molecular-scale webbing.

Edited to Add: I also have second house build a couple of years ago. It's 100% sprayed-in closed cell foam insulated, and is, while not airtight, sufficiently close that moisture condensation can be a problem on the colder inside surfaces (windows, e.g.). We have external ventilation with heat recovery unit that typically runs a 10% duty cycle to compensate.

[Mike Soaper]

You might consider contacting Ron Herman for a consult, he's a 3rd generation master housewright that has also done period work on historic houses.

I've enjoyed several "Woodworking show" seminars by Ron, here's some links, hope it helps.

Antiquity Builders of Ohio

Ron Herman - The Woodworking Shows

[Malcolm Schweizer]

My home was built in 1836. When I pulled the tin to replace it, I laid down treated ply over the original 1” pine and then put rigid foam insulation. The difference was amazing. The attic during mid-day is barely warmer than outside.

I have done a lot of research on spray foam insulation and if I were not reskinning the roof, I would go with that. It is in itself a moisture barrier. As stated by others, it also adds some structural rigidity.

[Tom M King]

Why not insulate at the ceiling joist level. We found a 1980 standing seam roof over an 1851 Cypress Shingle roof. I wouldn't spray foam against the wooden shingles. Not that it matters, but I doubt they're Cedar. Not sure about what was used up there in 1842, but around here everyone that doesn't know better also calls them Cedar Shakes.

"Shakes" is probably a 20th Century term. Back then, they were all made by hand, including splitting, but they were called shingles.

Here's a link to the page on my website about that roof we found. You can see the old roof in the first three pictures, which include one edge of the metal roof on top of the shingles. http://historic-house-restoration.co...sShingles.html All those pictures are thumbnails. You can click on one, and get a full sized version.

Historic Associations don't like spray foam, but they're fine with the way we do it by wrapping a vapor barrier over, and between ceiling joists, and blowing in insulation. The blown in stuff can be removed later without damaging any of the original parts.

It was not unusual for these old wooden shingle roofs to last 150 years. These days, they've engineered the life out of them by interlacing rows of tar paper. If one is put on according to specifications by the Cedar Shingle and Shake Bureau, they typically last about 35 years. The old ones would have lasted longer than that, had the fasteners not started to fail. The reason there were enough buildings left standing to save in the restoration of Colonial Williamsburg was because they still had the original Cypress shingle roofs on them.

[Jim Becker]

I would absolutely go with sprayed in closed cell foam. If there are concerns about the purlin-shake confection against the foam, layer in 1" closed cell board between the rafters firmly attached to the purlins, and then cover that with foam.

That's an excellent point. It's not quite as good as foam directly to the roof because there's going to be a small air gap "just because" between the insulation and the roofing materials that can hold moisture, but it's still going to be a whole lot more efficient than other insulation materials.

[Dan McGonigle]

IF the roof deck wasn't old cedar shakes and was plywood or solid boards laid out as a closed deck, I wouldn't hesitate to use closed cell spray foam because it solves almost all of your issues...you can get to R-49, you get a full vapor barrier, you get a much stiffer structure, you do not need venting because inside the spray foam is considered conditioned space, etc. It also wouldn't interfere with anything exterior that has to be maintained for historical preservation if that's a factor, either. The same would be true if you had the underside of the steel on the purlins as the spray foam eliminates condensation issues on the underside of the steel.

BUT...you don't have that situation because of how the roof was previously renovated. The cedar shakes should have been removed prior to installing the steel for both functional and appearance reasons. (steel doesn't like uneven surfaces under it and can project the same to the top surface) So the best solution would be to retrofit with a ridge vent and vented soffit, have baffles in the bays for air flow, fir out the roof joists to accommodate high density/high R-Value fiberglass batts and if that doesn't get you to your required R-49, use foam insulation board cross ways over the augmented joists to get the R-value and tape the joints for air infiltration and vapor before doing drywall or other coverings.

That said, Roger is also correct that you may be relieved of being fully code compliant as a retrofit rather than new construction, and with furring out the joists you should be able to get an R-38 with the best material available in, say, 12" of space. All the rest applies, however...you do need to do the venting or you're going to have moisture issues.

I had read that closed cell foam has a maximum thickness spec. Such that it cannot be installed in greater thickness than “x” (I believe it varies, maybe based on climate?). Did I read that wrong?

Yes, historic preservation is a huge factor - not only for me but for my towns historic preservation committee.

I think that if I furred the rafters out more than a few inches. I’d lose so much space, it almost wouldn’t be worth it. Personally, I don’t care about not achieving R-49. Obviously I want it to be reasonably climate controlled but the historic preservation of my home is far more important to me than energy efficiency. Maybe I’m a bit crazy

The more I think about ridge/soffit the more I warm up to the idea. Although I’d hate to see my old roof with a big ridge cover on the top, mixing new with old. Same goes with the soffit vents. That also allows for an easier roof job if ever needed. If down the road this roof gives out, my hearts set on cedar shingles.

[Dan McGonigle]

I would reach out to a local Roofing company. I say this because I’m a roofer and always felt like inspectors and architects were sometimes full of baloney.
Since I’m a west coast roofer I have no experience with east coast residential practices. But I’ve done commercial roofs that were supposed to be good for snow and ice dams conditions.
Dens glass sheeting with a very good peel and stick membrane.
Good Luck

Thanks! Consulting with a few roofing companies is on my to-do list