Cedar Breather with Asphalt Shingles
Kiley Jacques
| Posted in Green Building Techniques on
Has anyone used these two materials together on a roof? If so, any advice?
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What problem are you trying to solve by combining them? I cannot think of a reason to use cedar breather under asphalt. I also think it would help the wind peel the shingles off.
Thanks, John. I'm writing on behalf of an FHB podcast listener who has a large new roof to detail in climate zone 2. He says the material and labor costs of furring strips for drainage/venting of the roof deck above the unvented conditioned attic are not in the budget. The Cedar Breather is his solution but he has concerns about using it with asphalt shingles.
Shingles rely on an adhesive strip on the back to stay on the roof. This needs to adhere to something solid, it can't be a mesh. Any strong wind and the shingles will be blown off.
Even if you do install it, it simply won't work. The vent space is too small and obstructed by the fibers to allow for any air flow. I seem to remember the BSC folks did a test roof with mesh breather for an unvented cathedral ceiling and it made no difference.
The proper solution is a real code compliant vent channel or vapor diffusion port.
Just picking nits here, but want to clarify the record. The adhesive strips don't stick the shingles to the substrate - they stick the shingles to each other. Nothing sticks them down except the nails. And the adhesive is on the top, not the bottom of the shingles. The cellophane strips on newer shingles are on the bottom to keep them from sticking together in the bundles. That said, I wouldn't consider installing Cedar Breather under asphalt shingles.
I may be confused, but why does he care about venting under the asphalt shingles? I think there have been plenty of tests showing that asphalt shingles do fine over unvented roof assemblies, even in hot climates. Specify high-reflectance shingles to keep them cooler. A vapor diffusion port would be helpful to release humidity from the roof assembly if vapor permeable insulation is used.
Cedar Breather primarily aids drying by creating a capillary break. I don't think it adds any appreciable ventilation to a wall or roof assembly.
I'm not sure I'd want to walk on an asphalt shingle roof laid over mesh. There is a good chance it would give enough to pop the roofing nails through the shingles.
Adding a mesh substrate would also void the shingle warranty.
I don't have any personal experience with asphalt shingles over cedar breather, but I have plenty of experience with asphalt shingle gone wrong. Seen too many installations on spaced sheathing, presumably when someone replaced the original cedar shakes without laying down some solid sheathing for the asphalt shingles. The asphalt sags into the spaces, looks like crap, and doesn't shed water as intended. By the time I am involved there is plenty of moisture damage below.
My guess is that since the cedar breather has some give to it, best case scenario is that the roof ends up looking bumpy. I can imagine situations where part of an architectural shingle is stiff (where there are two laminates) and bridges a low spot in the underlying, lumpy shingle, defeating the self-sealing mechanism, leaving the roof subject to wind damage and voiding the warranty.
Just for kicks I pulled up a set of GAF installation instructions for one product line.
Under a section titled Roof Decks: "Use minimum 3/8" (10mm) plywood or OSB decking as recommended by APA-The Engineered Wood Assn. Wood decks must
be well-seasoned and supported having a maximum 1/8" (3mm) spacing, using minimum nominal 1"(25mm) thick lumber, a maximum 6"
(152mm) width, having adequate nail-holding capacity and a smooth surface. Do NOT fasten shingles directly to insulation or insulated deck
unless authorized in writing by GAF. Roof decks and existing surfacing material must be dry prior to application of shingles."
While its not directly applicable to cedar breather, I would say that cedar breather is not smooth, and if GAF has to approve fastening to insulation in writing I got to imagine they would want a say in this as well.
All of the asphalt manufacturers are behind the times on unvented roofs. They still spread fear of the boogey man that is an unvented roof when the color options they provide have a greater impact on shingle life expectancy. Though if I were in their shoes, why would I want to give up an easy way to deny a bunch of warranty claims?
I'm a fan of over-venting a roof--in fact I'll be doing that soon to my own roof. But venting needs to at least meet code-minimum requirements of 1" space, and more is better, and roofing should be installed in a manner that meets manufacturer warranty requirements. Cedar breather does neither; it provides a capillary break and drainage plane.