Exposed wood ceiling cathedral roof
We live in an 80 year old horse barn converted to a beautiful home.
The upper half has a gambrel roof with exposed redwood reverse trusses under exposed ruff 1X12 sheathing . We need to re roof. currently there is no insulation. House is in SF bay area hills climate zone 3.
We heat with a pellet stove and no AC.
I was thinking of adding 2″ of rigid foam on top of redwood sheathing then 1/2″plywood under composition roofing.
I know this is far from ideal but it should be an improvement.
My questions are ;should I use any moisture barrier or backing on the foam or plywood? Will the shingles get too hot on top of invented foam?
Will rot from condensation be a huge problem? We keep the Windows open most of the year and every night.
Thanks for your input.
Danny
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That is very cool.
That construction should be fine
To the extent there is a moisture risk it would be to the plywood layer between the foam and shingles, the redwood sheathing is fine. The plywood is trapped between two impermeable layers, if moisture were to get in there the risk would be there would be no way for it to get out. You could mitigate the risk by putting some sort of spacing between the foam and plywood and a vent at the ridge.
This is common in SIP houses and goes by the name of "ridge rot" because it usually shows up at the ridge of the roof.
If budget allows, I would use a fully adhered membrane under the foam to seal up the board sheathing. Make sure this has an acrylic adhesive (ie Pro clima Adhero) as the more common rubberized asphalt ones will smell.
A bit of flex foam or caulk is also a good idea between the board gaps at the outside walls to seal up the gaps.
Would taping the rigid foam itself achieve the same results?
Thank you,
Smell and off gassing from the foam are a big concern. Is there a foam you would recommend? Polyiso? I have been trying to get a quote for Rockwool comfort board to see how much more expensive it would be but the local rep never comes through.
Most rigid has no smell, the only time I had issues with some roofing polyiso that was left exposed to the elements and that smelled terrible. Even that was fine once installed on the outside of the house.
The smell I was referring to is the adhesive from the peel and stick. Some of them can be very strong especially when they get hot. "the sealing up the board sheathing" I was referring is not to keep out smells but to reduce air leaks which tends to be the biggest heat loss in most houses.
Some foam do off gas their blowing agent over time. Most of the blowing agents don't have any smell, this is also such a slow process and the volume of blowing agent that it will disperse outdoors.
Rigid mineral wool is a much more expensive material and much harder install, I would stick to foam. If you are looking at greener options you can try wood fiberboard insulation (ie Gutex).
What would you think of putting Tyvek (or there's a Tyvek-like roof underlayment, I forget the name) down as the air barrier?
I was wondering if that might be a less expensive alternative under the rigid foam or if taping foil backed would do the same.
OK I'll look for an acrylic adhesive peal and stick. The pro clima doesn't seem available here.
For the 2" Polyiso most seems to have a foil face. Is that good and should it face the interior? Maybe it's two sided. Would taping the seams on foil faced insulation help as a air barrier?
Thanks again
The important part is the air barrier on over the roof deck, that is what you want nice and tight. If you can't find a peel and stick and are using a standard synthetic roofing underlayment, tape the seams on that. Taping the seams on the iso doesn't hurt but not a must either.
Not rated for roofing but I've used on roofs without issues is Henry VP100, relatively cheap and available acrylic peel and stick typically used on walls.
On important detail (at least around me with a lot of critters) is to protect the edge of the rigid insulation. This is best done by picture framing the roof with dimensional lumber ripped to the height of the iso. In your case, it might be simpler to bump up your rigid to 2.5" so you can use 2x3s on edge for this.
For radiant barrier to work it needs to face an air gap. This can be built but generally not worth the effort as it only R2 to the overall assembly R value.
I don't see why Henry VP100 wouldn't work. It isn't being used as a WRB or roof underlayment if it is installed under the foam.
Good point on protecting from critters.
New issues is there seems to be a fire requirement of a thermal barrier between Polyiso and the interior space. 1/2" gypsum would add a lot of weight to the roof assembly.
I found Dow Thermax that doesn't seen to require the gypsum but it is prohibitively expensive. :-(
dragonara,
I would check your code. Your existing ceiling may suffice. Under ours 1"x t&g qualifies as what your code calls a thermal barrier.
Existing “ceiling” is the 1”X12” redwood roof sheathing with large gaps so probably not thermal barrier rated.
Now I am reconsidering 2” of ROCKWOOL Comfort board. Not very high R value but it is non combustible and i understand it to not hold moisture. We would only gain about R 8.5 but considering there’s no insulation now might be the best we can get.
Dragonara,
One thing puzzles me. I don't see any roofing nails protruding from the exposed sheathing. Is there any chance there is another layer above you don't know about?
Given what you have to work with, your plan sounds like a sensible balance to me.
No hidden magic layer just felt and 4! Layers of composition roofing. They must have used 3/4 inch nails and actually there are some visible where they hit the edege of a plank. I will be happy to get all that additional weight off the roof. The BIG ONE (earthquake) could come anytime.
Thank you for the confirmation
I think you'll find the cost of the rigid mineral wool is much more than covering the roof deck with 7/16" OSB/CDX. The benefit is you can now tape the seams of the sheathing as your air barrier so you can skip the peel and stick.