15 Pound Felt house wrap (C) Daniel Friedman

Roofing Felt Suitability as a Building Housewrap or Vapor Barrier
     

  • FELT 15# ROOFING, as HOUSEWRAP/VAPOR BARRIER - CONTENTS: Use of 15 pound felt as a vapor barrier or housewrap
    • Where to place or locate the vapor barrier in a building wall
    • Are vapor barriers required in building ceilings?
  • Solar Age Magazine Articles on Renewable Energy, Energy Savings, Construction Practices
  • POST a QUESTION or READ FAQs about roofing felt used as housewrap or for a vapor barrier on a building exterior
  • REFERENCES

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This article discusses the use of roofing felt or 15 pound felt as a building house wrap or vapor barrier and the permeability, moisture problems, and indoor condensation problems that may occur.

This discussion of vapor barriers and condensation in buildings in this article series begins at part I, VAPOR BARRIERS & CONDENSATION in buildings, (when and why condensation occurs inside buildings, explains the problems caused by excessive indoor condensation, explains how moisture enters building wall and ceiling cavities, and summarizes the best approaches to prevention of indoor moisture and condensation problems), continues with part II at VAPOR CONDENSATION & BUILDING SHEATHING (detailed questions and answers about various building wall sheathing and insulating materials and their impact on building condensation problems) followed by VAPOR BARRIERS & AIR SEALING at BAND JOISTS.

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Inadequacy of Roofing Felt as a Vapor Barrier for Asphalt Shingles in Hot Humid Climates?

As discussed at HOUSEWRAP PRODUCT CHOICES, adaped from Best Practices Guide to Residential Construction, by Steven Bliss, courtesy of Wiley & Sons.

Grade D building paper is an asphalt-impregnated kraft-type paper, similar to the backing on fiberglass insulation. Unlike asphalt felt, it is made from new wood pulp, rather than recycled material. Its most common use is under stucco in the western United States. The vapor permeance of Grade D paper is similar to asphalt felt. Its liquid water resistance ratings range from 20 to 60 minutes, as measured by using the boat test (see Water Resistance in "Water Resistive Barriers on Building Exterior Walls").

Because Grade D paper tends to deteriorate under prolonged wetting, the trend in three-coat stucco is to use two layers of 30-minute paper. Because the paper tends to wrinkle, the two layers tend to form a small air space, creating a rain-screen effect.

A separate Building Sciences Corporation report also elaborates the usefulness of placing a vapor barrier on the roof deck below shingles in hot humid climates. BSC points out that: [some paraphrasing -DF]

Unvented roofs with asphalt shingles in hot humid climates require a vapor barrier between the asphalt shingles and the roof deck. This is because asphalt roofing materials store water from dew or rain. Thus asphalt shingles form a water reservoir not unlike wood shingle or shake roofs. The report argues that this stored moisture is driven inwards [presumably as water vapor, not liquid water] when sun strikes the damp or wet roof surface, and it continues to argue that moisture is driven through vapor-permeable roofing paper, felt, and plywood or OSB roof decking, thus ultimately into the attic space

But unlike an asphalt shingle roof nailed [over felt] directly to a roof deck, a wood shingle or shake roof that has been installed using best practices includes a disposal path for water absorbed in the roof surface: an air space between the wood roofing and the roof deck, or the installation of wood roofing over spaced nailers or "skip sheathing".

In cool or temperate climates this does not present a problem because the combination of heavy wetting from due or rain i snot combined with solar heating at high outdoor temperatures, say the authors who go on to argue that that buckled roof shingles observed in the morning (caused by moisture migrating back up from the roof deck) relax during the day. But on an un-vented roof moisture driven inwards [through the shingles, roofing felt, and OSB or plywood roof decking] in hot humid climates, needs to be addressed.

This phenomenon can typically be ignored in climates other than hot humid climates because the combination of extensive dew formation and solar heating at high outside ambient temperatures is not common. In vented roofs, this is often manifested in the buckling of shingles early in the morning as the moisture migrates in to the roof deck sheathing and the joints close. This is followed by relaxation and opening up of the roof sheathing later in the day—the buckling disappears.

But in un-vented roofs in hot humid climates, the authors argue that water from the roof surface is drawn upwards in liquid form, by capillary action, between plies of overlapped shingle courses where it passes ultimately through the vapor barrier and through the roof decking to the roof cavity interior.

The driving force of moisture through the roof and into the building is by solar heating according to the authors.

[OPINION-DF: from exterior roof inspections at all times of day and seasons, we have not observed this time-related morning roof shingle buckling in the Northeastern U.S. nor in Florida, nor the Southwest, though the authors report the phenomenon. It is possible that the authors are not quite correct that daily buckling and relaxing of roof shingles can be ignored on a vented roof as harmless, since certainly the product is expected to remain flat, and flexing daily might reduce its anticipated wear life.]

With unvented roof assemblies, this inwardly driven moisture must be addressed. The preferred method is to prevent the moisture from entering the roof deck material via the installation of a vapor barrier.

[OPINION-DF: we argue at ROOF VENTILATION SPECIFICATIONS that un-vented roofs are not a best building method in any climate.]

Asphalt shingles are quite impermeable to the passage of liquid water directly through them. However the geometry of their installation allows wicking at overlaps. This inwardly driven capillary water is the source for the wetting of the roofing underlayment and roof sheathing. The material properties of shingles change under elevated temperatures and moist conditions due to their hygroscopic nature. The large vapor pressures resulting from incident solar radiation and the changed material properties are sufficient to drive moisture inward through the shingles. Roofing felts or underlayments vary greatly in their permeability to water vapor; the typical underlayment used under asphalt shingles in residential construction is quite permeable.

[QUESTION-DF: we note that the test chamber constructed by BSC was itself in an enclosed, air-conditioned space, and that the underside of the test chamber roof was at least in part exposed to the air conditioning. It seems possible that the reduced humidity and lower temperatures on the "interior-side" of the test roof may have contributed to moisture behaviors that vary from what occurs in the field. Attics and under-roof spaces such as in an un-vented "hot roof" cathedral ceiling are certainly not exposed to cool dry conditioned air. BSC may have addressed this concern but we did not find it in the referenced article.

The conclusion of the BSC report is an argument for installation of an impermeable moisture barrier underneath roof shingles, perhaps in place of the traditional and permeable roofing felt.

[QUESTIONS-DF:

  • What are the differences between the test roof and a roof's behavior in the field?
  • How does the impermeable moisture barrier under roof remain impermeable when perforated by shingle nails or staples? What will be the market effects of suggesting entire roofs be underlaid first with a moisture barrier unaffected by roof shingle fasteners such as ice and water shield?
  • Why not recommend that all roofs and roof cavities include ventilation in their design, not only solving the possible driven-moisture problem discussed by BSC but also providing for longer shingle life (cooler roof surface) and in cooling climates, possibly reduced building cooling costs?]

Technical note on roof ventilation and cooling cost:

As explained in Best Practices Guide to Residential Construction, chapter on BEST ROOFING PRACTICES:

Researchers at the Florida Solar Energy Center (FSEC) have found that adequate attic ventilation can modestly lower sheathing and shingle temperatures, and reduce an average home’s cooling load by about 5%.

Details about combining roof color, roof ventilation, and radiant barriers to reduce cooling cost are found at: ROOF COLOR RECOMMENDATIONS or select a topic from the More Reading links shown below.

More Reading

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