“Standard 30# asphalt ” felt vs “ASTM 30# asphalt felt” (ASTM D-266)
I’m in a hot humid climate. Using plywood wall sheathing with 30# felt as the WRB, then a gap and then brick veneer. I was looking for a “better quality” 30# felt. The supplier tech support advised that most lumber yards stock what is referred to as “standard 30# felt” (actually ASTM 4869 at 16# per square). They also make what is referred to as “ASTM 30# asphalt felt” (actually ASTM D-226 at 26# per square). So here’s the question—-Is this D-266 similar to the 30# felt made and used decades ago, and thus better to use? My main question is the perm rating—-I’ve not been able to find that number. I don’t think it should be an issue, but I’m paranoid about accidentally putting in a vapor barrier in the wall of a hot humid climate. Any help out there would be appreciated.
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Replies
John,
Many of your questions are answered in an article I wrote a few years back for the Journal of Light Construction. Here's the link:
"Choosing A Sheathing Wrap."
Different types of asphalt felt have different weights. Here are some data on the weight of felt types per 100 square feet:
Weight per 100 square feet:
Unrated (non-ASTM) #15 felt: 7.6 to 8.8 lbs.
ASTM D 4869 #15 felt: 8.0 to 9.7 lbs.
ASTM D 226 #15 (Type 1): 11.5 to 12. 5 lbs.
Unrated (non-ASTM) #30 felt: 15.7 to 19.9 lbs.
ASTM D 226 #30 (Type 2) felt: 26.4 to 27.3 lbs.
Other relevant information from the article:
"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. Over the years, asphalt felt has also gotten lighter. “In the old days, it used to weigh 15 pounds per 100 square feet, but not anymore,” says Allen Snyder, product engineer at CertainTeed, a manufacturer of asphalt felt. The main reason manufacturers make light-weight felts is because they’re cheaper. “The whole issue comes down to price,” says Ed Todd, technical manager at Atlas Roofing, an asphalt felt manufacturer in Atlanta. “This is a price-sensitive product,” Todd says. Manufacturers now call their product “number 15” felt instead of 15-pound felt, and it weighs anywhere from 7 to 14 pounds per square.
"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 light-weight 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. There are a few regions where ASTM-rated felt is widely available, however, because of code requirements. “The most stringent felt market in the U.S. is Florida,” says Ed Todd, of Atlas Roofing. “In Florida, at a minimum you must have ‘ASTM D 4869’ on the wrappers.”
"Asphalt felt is also available in a heavier version, commonly called 30-pound felt. This #30 felt is available in both the unrated grades and the ASTM-rated felts. ASTM standards refer to #30 felt as Type 2. The lightest unrated #30 asphalt felt is still heavier than the heaviest ASTM-rated #15 felt, making it a logical choice for concerned builders.
"Asphalt felt has a permeance of only 5 perms when dry, but a much higher rating of 60 perms when wet. Fans of felt note one of its advantages over housewrap: If water gets behind felt — either due to a flashing leak or condensation from solar-driven moisture — the felt can soak up the liquid water and gradually dry to the exterior."
PERM RATING
Martin, thanks for the response--very helpful. I'm assuming the perm rating varies amongst the various grades. Where can I find the perm rating for 30# ASTM D-226-Type II ? I'm just trying to make sure it's a sufficiently large number to not have to worry about it being a vapor barrier.
John,
According to John Straube, published permeance values for #30 asphalt felt range from 0.5 perm to 3.0 perms. Straube suggests using 1.75 perm as a reasonable average value.
Since the permeance of all asphalt felts increases with rising humidity, asphalt felt is a "smart" vapor retarder that allows drying when humidity is high.
USE 1/2 THIS AMOUNT FOR 30# D-226 ?
Martin,
There are apparently two distinct 30# specifications (one approximately half the weight of the other).
Since 30# D-226 is twice the weight of standard 30# (ASTM D 4869), should I use 1/2 the above perm numbers for the 30# D-226 ?
John,
You're looking for precision that doesn't exist. Individual samples of asphalt felt from the same roll will differ in permeance due to differences in paper thickness and asphalt applications, which are not uniform. One manufacturer's product will differ from another's. Furthermore, the permeance of any piece of asphalt felt varies tremendously with humidity changes.
#15 asphalt felt has a permeance of about 5 perms. #30 asphalt felt has a permeance of about 1.75 perm. Both products range widely in permeance. I think that's about as accurate as you're going to get -- or are likely to need.
John Walls,
You have asked an interesting question....
Perhaps Martin is trying to tell you not to worry because of the "smart" properties of the "paper".
Still an interesting question.
If a certain thickness of a material has a particular perm rating.....
What happens as the material gets thicker?
Is the relationship a straight line?
Martin,
Where can we find good charts concerning permeance of materials?
I looked in my EEBA book and poked around at GBA and BSC....
Only to find general and vague values.
Maybe I am just not looking in the right place?
John Brooks,
ASHRAE Fundamentals, section 22.14, Table 9: Typical Water Vapor Permeance and Permeability Values for Common Building Materials.
I have the 1993 Fundamentals Handbook. If you don't own it, it's worth having.
Martin,
All I really want to confirm is that 30# D-226 felt paper will not be a vapor barrier in a hot-humid clamate wall system. John B, I came to the same conclusion as you----It will not be a vapor barrier primarily because it is hygroscopic.
I understand that there is no precise perm value and that there is considerable variation in the product. However, on average I would anticipate, from all the above, that it would be about 1perm (or less) d.c. and probably 10 or so w.c. The hygroscopic property of the felt should keep it from being a vapor barrerr (similar to how plywood would function).
Martin, I very much appreciate your input. Please let me know if I have misinterpreted any of your comments. Kudos to you and GBA---this is a great web site.
John B, there is a chart that BSC published (Designs that Work---Building Materials Property Table) that has a foot note stating that perms are roughly inversely proportional to thickness (except for coatings)---doubling the thickness, halves the perms.
How many linear ft are in a 2sq 30# felt
Anonymous,
2 square = 200 square feet
#15 asphalt felt is sold in rolls that are 36 inches wide
200 square feet of asphalt felt requires 67 linear feet of asphalt felt, assuming no overlapping
Repair a 1938 barrel tile roof, I've found the underlying felt paper in pretty good shape.
It's quite frankly more tear resistant, compared to the modern ASTM #30 roofing tar paper I picked up for patching. I assume this is because the original used rag or true felt, and the modern is paper.
Does anyone make a quality premium rag based asphalt felt now?
Something suitable for copper (not plastic cap) tile nails?
Best to verify ASTM and weight/square with the manufacture’s product data sheet before ALL purchases (and bring the printout to show the supplier) as many distributers are unaware of these issues and will claim that ‘this is the product that everyone else uses’, when it indeed is inferior to the job required. Avoiding undocumented manufactures’ products helps police this industry.