Unvented roof assembly in old barn retrofit
Assembly from inside: Old exposed rafters; old 1″ roof boards with gaps; new 3/4″ roof boards to fill in gaps; Intello x; 2″x10″ cellulose filled rafters; zip sheathing; 2 layers of 2.5″ polyiso, exterior layer taped; strapping parallel to rafters; horizontal strapping; standing seam metal with ridge vent
I am building a new roof over an existing mid 19th century barn roof (that roof is just roof boards and undersized rafters exposed from underneath). I calculated the rafter width so that I can insulate the new rafter bays with cellulose and then add two layers of polyiso above. I used ratios for my climate (zone 5), which dictate that 41% minimum R-value comes from foam, 59% maximum from cellulose. The rafters are 2x10s (rough sawn, with support midspan per engineers drawings), so 10″ thick cellulose (R-35). 2 layers of 2.5″ polyiso gives me 5″ (R-27.5 if I use R-5.5/inch allowing for thermal drift). That leaves me 44% in foam R-value and 56% in cellulose R-value.
All good theoretical BUT the old roof sags. It sags quite a bit in the middle of the building, about 6″ from the new ridge beam. That means I will have more than 10″ of dense-pack cellulose in the bays as you get to the middle and towards the ridge of the building. If I have 16″ of cellulose I would need 7 inches of polyiso at R5.5 to get my ratios back to good. The new roof is already built with the sheathing above the rafters in place. Am I going to have condensation issues?!
One idea I had is to stuff some less insulating material up into those bays that have a deeper cavity. The dense pack cellulose would get blown on top of this and would be less insulating than if it was all cellulose. not sure what that material could be.
Thoughts? Ideas? Another idea would be to add a third layer of polyiso, but that becomes pretty expensive and makes the install much more difficult.
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Replies
If the Intello is attached to the bottom of the new rafters it will hold the blown insulation in place. You say the new roof is already built so I guess you may not have access to the underside.
The R value ratios are always a bit squishy. If you make your ceiling air tight, you can always push the ratios a bit. You can also include a smart vapor retarder in ceiling which can help.
If you don't mind a bit of metric and a bit of learning curve, you can run your assembly choices through this calculator:
https://www.ubakus.de/en/r-value-calculator/
It gives temperature and moisture profiles through the assembly. For the outdoor temperature use the average (not the lowest!) temperature of the three coldest months in your area.
A couple of things. The main air barrier should be at the roof deck not above the foam. Taping the seams of the foam doesn't hurt but also not needed especially with staggered seams. This also lets you use fiber faced roofing polyiso which tends to be the cheapest insulation around me.
Since you have saggy rafters another, instead of foam, an option is to go for an overroof:
https://www.jlconline.com/how-to/insulation/why-and-how-to-build-an-insulated-overroof_o
You can use larger lumber up top and shim it to get a flat roof deck. It also adds a fair bit of structural strength to probably undersized rafters. The article uses rigid mineral wool above but you can use any fluffy insulation including cellulose. I've used regular batts for this without issues. You do have to include a vent space above these batts though which is generally easy by going up one size on the rafters.
The nice part with the overroof is if you use something like deep I-joists you can put all the insulation above the roof deck which will let you keep the nice old rafters exposed.
An "overroof" is indeed what I have. I took the old metal roof off, put a new layer of roof decking to fill gaps in old roof boards, wrapped the whole thing in Intello X, going all the way down to the sills (I cut the old rafter tails off). So there's my continuous air barrier. I put a new roof on by securing a ledger at the top plates and shimming that in such a way that it was on the same plane as my ridge. New structural ridge beam with LVLs. Notched rafters at the end to sit on the 2"x6" ledger. So the Intello is not on the bottom of the new rafters but rather resting on top of the roof deck.
The space will not be a living space except for the occasional guests, all plumbing is on the first floor and there will be an ERV/HRV installed. Would be interested in other thoughts but I guess I won't worry about it. I read Martin's article from 2017 and other comments here and there and it seems like my situation is less risky than most. A concern I do have is that because it isn't a living space I won't be keeping it at the 68 range all the time. I understand that the ratios are to keep the sheathing warm enough, so with a lower interior temp in the winter (let's say 52 at times?!) am I asking for trouble?
The moisture that you have to worry about comes from the occupants, each person puts out about 5 pints a day from breathing, cooking and bathing. Unless there is liquid water leaking somewhere, when the space is unoccupied the moisture content of the air should be the same as the exterior air. So air that is warmer than exterior temperature won't have condensation issues.
It sounds like your assembly is capable of drying to the interior. When the sun beats down on the roof it will create vapor drive toward the interior which will drive out moisture. The cooler it is inside the greater the vapor drive. I don't think you have anything to worry about.
If you have an overroof already, I would skip any rigid an insulate just with fluffy. If you don't have enough space above for condensation control, use either thinner batts bellow or strap out the overoof rafter to add height.
Cut and cobble rigid between the overroof is a lot of work and doesn't change the assembly R value much.