Rigid roof , dry to the outside?
stephenr
| Posted in General Questions on
Hello,,
Maine, zone 6. Please consider this vented roof assembly, moving from the outside to the inside….sheathing, 2 inch vented space, 4 inches of polyiso, 2×8, rafters filled with cellulose, membrain and drywall. Soffit and ridge vents, interior HRV….
In the assembly above, does it matter that the thickness of the polyiso will prevent drying to the outside?? This assembly is being considered because headroom is at a premium in the single gable, 2nd floor of this farmhouse.
thanks for input.
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Replies
Your foam layer R value is supposed to be 51% or more of total R value to keep the face of the foam warm enough to prevent (most) condensation.
Thanks Tyler. Just to be clear, the description above is for a vented roof assembly and the foam is only thick because the homeowner wants to conserve as much headroom as possible. Does the 51% rule apply to vented roofs in the same way that it applies to unvented roofs?
Stephenr,
The roof venting will help keep the sheathing dry, but not prevent condensation on the inside face of the foam if it isn't thick enough.
Most interior air moves through the batt insulation until it hits the foam. If that face of the foam is too cold, the moisture will condense - whether the roof above is vented or not.
As malcolm says, you still want to hit the correct ratio.
Is the foam cut and cobbled in between the rafters? I read your description to be a vented over roof with 2 layers of sheathing.
Thanks for the replies.
I guess I was thinking of the foam as a really thick baffle, you know those 1/4 inch styrofoam thingies that you put above fluffy insulation to prevent it from intruding upon the vent space. I have done 1/2 inch polyiso as baffles for and didn't consider the potential for moisture forming on the inside.
Its an old farmhouse that has 6x6 rafters on 7 foot centers and 3x3 purlins on 4 foot centers. We plan to slap the rigid to the existing rafters, from the inside, no cut and cobble, which should create a 3 inch or so (nothing is uniforn in this house) vent space in the rafter bays between the sheathing and the rigid. Inside of the rigid, rafters (dimensions to be determined) will be in contact with the rigid and the bays stuffed with fluffy. The SI has specified a structural ridge, on posts and a truss, and i am trying to come up with a good insulation plan for the assembly. The sheathing and roofing is not a part of the remodel plan and will not be touched.
stephenr,
Thin foam baffles differ in two respects:
- The air in the vent channel above is often warmer than below the baffle, which limits condensation.
- They are not continuous, so small amounts of moisture can be moved through the structural members on each side by sorption.
That said, they rely on a good interior air-barrier to stay safe, and baffles made of permeable material are a lot less risky.
Your continuous layer of foam is has neither of those attributes. I think it would be safest to respect the minimum thickness and ratio requirements.
Thanks for the clear response, well explained. The client is leaning that way as well.
Just curious, however.....if we went with an unvented roof assembly in the same space, we would cut and cobblle, say 7 inches of rigid up against the sheathing, lay the rafters and then get 7 inches of fluffy (assuming 2x8's), so we would get the same r value but save on the headroom.. It seems like a better solution in this case.
...the unvented assembly above has the sheathing (old barn boards with gaps) exterior of the rigid. would this be a problem?
stephenr,
Un-vented cut & cobble assemblies are risky too. https://www.greenbuildingadvisor.com/article/cut-and-cobble-insulation
Why not include a 1" vent channel and fill the whole thing with dense-pack cellulose (r-53), or all mineral wool batts (r-45). Those are both simple, and yield a well performing assembly from a moisture standpoint.
So, with 7 feet between rafters, we would need to cut say, 1.5 inch strips and lay them parralell with the rafters and fasten them to the inside of the barn board sheathing to create the vent channels. We would then apply say, 1/2 inch polyiso continuous between the existing rafters to finish the channels and then stuff the new rafter bays with dense pack cellulose. Does this sound about right?
stephenr,
With that spacing between the rafters I would try and find something permeable - thin plywood or fiberboard - to use. If you are going with batts it doesn't need to be very strong at all. You could even use house-wrap.
Thanks Malcolm. I neglected to consider the 3x3 purlins, spaced every 4 feet, that would prevent this from being easily done. The membrane, or luan, would need to be at least an inch off of the purlins to allow air to move behind. I guess we could add batts at that point, but we would need 13 inches of them to reach code. Its starting to look like a spray foam job, although I would hate to do that.
stephenr,
I'm really reluctant to use spray foam too, but it does seem like it is the best option given the complexity of the roof - and compared both in terms of performance and the work involved, it seems a better choice than rigid foam.
Thanks again. Before I end this thread, one question, please. The structural engineer uses this insulation method that seems a little off to me. in a standard vented roof assembly, he will install 1-2 inches (i think i have the measurements correct) of EPS on the interior of the fluffy batts to bolster the r value and act as a vapor barrier. Its his go to method. Other than being more of a pain to install than a smart membrain, does EPS perform well as a vapor barrier in this application?
stephenr,
An inch of EPS is between 2 and 6 perms, so it acts as a class 3 vapour-retarder, which is about the same as two coats of latex paint on drywall. It doesn't vary in it's permeance like a smart membrane, but that's not typically a problem on vented roofs.
An inch means you can still attach the drywall through it. Two would need strapping.