True felt tar paper / grade D paper
As Martin has written many times, “felt paper” used to be made from rag, but is now made from paper. Thus when it gets wet it gets super weak.
https://www.greenbuildingadvisor.com/blogs/dept/musings/install-stucco-right-include-air-gap
Is there a brand or source of “true” felt or synthetic “tar paper” in grade D, saturated felt, for use as a stucco underlay. What would this “better stuff” be called, and what certifications would it have? Is ASTM D226 the right certification? Are the words “organic felt” the right words to indicate this resilient quality? Does such felt still retain the quality of increasing perm with increasing moisture?
It’s wicked expensive to ever tear stucco off, seems worth an investment to get really good stuff under there.
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Replies
Bryce,
You can buy 100% rag office paper -- it's expensive, and it resembles the paper used to make dollar bills. But to the best of my knowledge, no one makes 100% rag asphalt felt.
"It's wicked expensive to ever tear stucco off, seems worth an investment to get really good stuff under there."
Yes, but today we would call the "really good stuff" a rainscreen drainage plane and WRB of your choice. I'm not sure where to find "classic" felt paper that is approved for walls, but consensus is that the rainscreen approach is best practice behind stucco.
Bryce,
The ASTM D 226 standard has nothing to do with whether the felt has rag content. (It doesn't.)
In August 2000 I wrote an article for the Journal of Light Construction on water-resistive barriers.
In that article, I wrote:
"Asphalt felt, which has been around for over a hundred years, was originally a true cloth felt. “A long time ago, they used rag felt, which was cotton,” says Dodie Webster, technical services manager at Tamko Roofing in Joplin, Mo., a manufacturer of asphalt felt. “But we can’t get cotton rags any more.” Since present-day asphalt felt is a paper product, the term “felt” is somewhat of a misnomer. “Unsaturated felt is basically composed of recycled corrugated papers mixed with sawdust,” Webster says....
"ASTM has established two standards for asphalt felt. The less stringent standard is ASTM D 4869, which requires Type 1 (#15) felt to weigh at least 8 pounds per 100 square feet. The more rigorous standard, ASTM D 226, requires a minimum weight of 11.5 pounds per square. Most lumberyards stock only lightweight asphalt felt with no ASTM rating. “We sell a lot of the lightweight felts, the non-ASTM #15,” says Webster. “It is probably our biggest seller.” This type of #15 felt sometimes weighs only 7.6 pounds per square."
Bryce,
Asphalt felt is a "smart" vapor retarder with variable permeance. Typically, asphalt felt has a permeance that ranges from 0.5 to 5 perms when dry, and up to 60 perms when wet.
If I had the luxury of doing the wall from foundation to roofline, yes. Rain-screen. But here I'm talking about retrofit where the additional thickness can't be accommodated without say, making the stucco less thick.
Do ALL asphalt felts have the variable perm characteristic, or is that the result of the paper fibers swelling? In other words would a synthetic roofing felt be different?
Bryce,
Synthetic roofing underlayments are a type of plastic (resembling housewrap more than asphalt felt). Synthetic roofing underlayments are not "smart" layers -- they don't exhibit variable permeance.
While most synthetic roofing underlayments are vapor-impermeable, a few brands (usually the expensive ones) are vapor-permeable.
Bryce,
I guess it really depends on your climate as to how serious it is, but stucco without an air-gap exhibits all the problems of other reservoir cladding, and can allow a lot of moisture to penetrate to the sheathing through building paper.