Replies

1. Expert Member
   MALCOLM TAYLOR | Oct 25, 2015 06:07am | #1

   Bryce,
   Synthetic underpayments are available in a variety of perms - but as you have found, the most common ones are impermeable. Tiles are one of the few roofing types that benefit from higher perm underpayments allowing some drying to the exterior.
   The requirements for capped nails only applies to the fasteners used to apply the underlayment, not the roof, so that shouldn't preclude using any of them.

2. GBA Editor
   Martin Holladay | Oct 25, 2015 06:23am | #2

   Bryce,
   Here is a link to an article that should answer all of your questions: Synthetic Roofing Underlayments.

   If you prefer to access the article from the Fine Homebuilding web site, use this link: Synthetic Roofing Underlayments.

   Here is the summary:

   1. Asphalt felt is somewhat vapor-permeable. The vapor permeance varies by brand, but in general, asphalt felt has a vapor permeance of 0.56 to 6.0 perms when dry, and up to 60 perms when damp.

   2. Most synthetic roofing underlayments are vapor barriers (see the image below). A few synthetic roofing underlayments are vapor-permeable; I will post a table listing these vapor-permeable synthetic roofing underlayments in a separate comment, so that the two images don't bleed into each other.

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3. GBA Editor
   Martin Holladay | Oct 25, 2015 06:26am | #3

   Bryce,
   The image below shows synthetic roofing underlayments that are vapor-permeable.

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4. 
   severaltypesofnerd | Nov 02, 2015 01:08am | #4

   Back to the original question: are there reliable perm ratings for organic (paper) and synthetic tar impregnated felt?

   What are tile roofers using foam using?
   What are tile roofers using wires using?
   Which products are actually available (GAF DeckArmor seems unavailable in my western region. Delta-Foxx has very limited USA distribution, and requires a patch at each nail hole.)

   Note also the white or grey color of the synthetics looks really bad under mission tile. A course of starter shingle or felt may be necessary to match the dark look of the existing tar paper.

5. GBA Editor
   Martin Holladay | Nov 02, 2015 06:02am | #5

   Bryce,
   Q. "Are there reliable perm ratings for organic (paper) and synthetic tar impregnated felt?"

   A. For more information on this topic, see All About Vapor Diffusion.

   In that article, I wrote:

   "According to ASTM E96, permeance can be determined using either Method A (known as the desiccant method or the dry cup method) or Method B (known as the wet cup method).

   "Building codes that reference permeance values require that these values be determined by Method A, the dry cup method. Materials with very low permeance values (close to zero) are vapor barriers; materials with high permeance values are called “vapor-open” or “vapor-permeable” materials.

   "It’s fair to say that procedure A measures the vapor permeance of a material when it is dry, while procedure B measures the vapor permeance of a material when it is damp. The permeance of many materials (including asphalt felt, plywood, and OSB) is variable: when these materials are dry, they have a relatively low permeance; when they are damp, their permeance rises. (Some people refer to materials with a variable permeance as “smart retarders.”)

   "If you try to create your own table of permeance values for building materials by collecting information from multiple sources, you will soon discover that different sources provide different values for common materials like #15 asphalt felt or 7/16" OSB. There are three reasons for this:
   Materials produced by different manufacturers have different characteristics.
   The permeance of some materials varies depending on moisture content, making permeance measurements difficult even when laboratory technicians try to use consistent test procedures.
   The ASTM E96 procedure is difficult to perform and often produces inconsistent results.

   "William Rose, a research architect at the Building Research Council at the University of Illinois, elaborated on this last point in his landmark book, Water in Buildings. Rose wrote, "It is very difficult to conduct this test [the ASTM E96 test] well, and it is easy to wet the specimen in doing the wet-cup test. ... Unfortunately, results from ASTM E96 are not particularly dependable. Several round-robin test of water vapor permeance have been conducted, resulting in a wide range of values.""

   * * * *

   In the table accompanying that article, I noted that #15 asphalt felt has a permeance of 0.56 to 6.0 when tested according to the dry cup method, and "up to 60 when damp." If you research this issue, you will find other published values; the differences in published values can be attributed to the factors I listed.

   Confusingly, some published permeance values for #30 asphalt felt (which is more rarely tested than #15 asphalt felt) are nearly identical to the published permeance values for #15 asphalt felt. This is unlikely, but is evidence of the variability encountered in this type of testing.

   For the published permeance values for synthetic roofing underlayment, see the images I posted in my earlier comments.