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Felt Paper Vapor Barrier

felt paper vapor barrier

Felt paper vapor barrier | Felt, or tar paper, has a proven track record of success as a water barrier. But what about water vapor coming from the inside of the building? Copyright 2018 Tim Carter

"It's important to realize this permeance rating was achieved just testing a single piece of felt paper on its own. Adding other materials on top of the felt paper could radically change the felt paper's ability to transfer water vapor back out to the atmosphere."

Felt Paper Vapor Barrier? You Bet It Can Be

You may want to know if felt paper is a vapor barrier or vapor retarder. It's a very valid question.

There's no simple answer unless you just consider the felt paper on its own.

What is a Vapor Barrier?

A vapor barrier is a product that would stop all water vapor from passing through it. It's better to use the term vapor retarder as there are many products that slow down the passage of water vapor through them.

A technical white paper produced in 2011 shows that #15 felt paper has a vapor permeance rating of 7. This measurement was derived by using the ASTM E 96 test.

felt paper vapor barrier

This white paper has very interesting information about the felt paper as a possible vapor barrier. Keep in mind felt paper is almost always covered by different materials that aid or block air flow. Copyright 2018 Owens Corning, Inc.

It's important to realize this permeance rating was achieved just testing a single piece of felt paper on its own. Adding other materials on top of the felt paper could radically change the felt paper's ability to transfer water vapor back out to the atmosphere.

How Does the Permeance Rating of Felt Paper Compare to a High-Performance House Wrap?

High-performance house wraps have much higher permeance ratings. You can check the specs yourself with ease but one that I'm using on my daughter's new home is Delta Vent-SA and it has the following ratings:

  • 31 perms using the ASTM E 96-05 Procedure A
  • 50 perms using the ASTM E 96-05 Procedure B

Those numbers are much better than the perm rating of 7 for typical asphalt-impregnated felt paper! Another key point is the closer the perm rating gets to 1, the less water vapor the product will transfer to the atmosphere.

Is Asphalt a Vapor Retarder?

It's a good idea to think about felt paper vapor barrier questions in the context of what comprises felt paper.

In its most basic form, felt paper is an absorbent paper that is saturated with liquid asphalt.

This same liquid asphalt has been used for decades as a dampproofing compound on house foundations to slow down water vapor from entering basements and crawlspaces. The normal soil around homes has lots of water vapor in it and this vapor will readily pass through untreated concrete or concrete block.

The asphalt that might be sprayed on a foundation can be pure asphalt or a mixture of asphalt with other ingredients. A thin coating much like you might paint a wall can have remarkable vapor retarding properties.

Karnak makes one product that has a permeance rating of 0.5.

You can scour the Internet and discover many other asphalt-based spray-on asphalt compounds that have a permeance rating of 1.0 or less. That should communicate to you that asphalt can be a very effective vapor retarder.

foundation waterproofing

The black goo is asphalt. The red arrow points to the thick coating where it's sagged. Damproofing is applied much thinner like a simple coat of paint. Some asphalt foundation products contain rubber or other compounds that allow the asphalt to bridge future cracks in the concrete. Copyright 2018 Tim Carter

Your own home may have asphalt sprayed on the outside of the concrete walls. This is why your basement doesn't smell moldy.

Felt Paper Under House Siding

The vapor barrier issue gets very complex depending on what covers the felt paper. Some exterior building products like brick and old traditional cement stucco allow water vapor to pass through them with ease.

Newer synthetic stucco is the exact opposite. It blocks the passage of water vapor from inside a home to the outside air in cold climates.

Vinyl siding allows for lots of air movement because of the manner in which the siding is designed and installed.

Fiber cement siding that's painted could be very problematic because latex paints can act as a thin vapor retarder.

Can Felt Paper Covering Wall Sheathing Create an Odor Inside a House?

The answer is yes. This column was inspired by a phone consultation I did with a homeowner whose house was covered with tar paper in 2012 when it was built.

For six years they never had a problem with odor inside the home. Their house is exposed to the blistering infrared rays of sun all day along the New Jersey coast. I know for a fact that some of the exterior surfaces on their home, those facing east, south, and west, can get too hot to touch on cloudless days in June.

felt paper vapor barrier

This is the house on the New Jersey shore that had felt paper under the fiber cement siding for six years with no odor inside. As you can see the sun punishes it with lots of infrared light. Copyright 2018 Tim Carter

Fiber cement siding covered the felt paper. The siding was improperly installed and was removed in early November of 2018. The siding contractor then covered the felt paper with CertaWrap, a modern air barrier.

The homeowners immediately noticed a petroleum odor when for six years there was never a problem.

Infrared photography of my own home makes the case that the CertaWrap was causing the problem.

felt paper vapor barrier

This photo shows how hot exterior siding can get. The surface of the door was 165.2F. Fiber cement siding will get just as hot. It will transfer that heat to the underlying felt paper with no trouble at all. Copyright 2018 Tim Carter

The fiber cement siding could easily reach temperatures of 160F+ and transfer that heat to the felt paper beneath. Previously there was enough air transfer under the siding to vent the petroleum gasses to the exterior.

The Certawrap forced the odor inside. The proof is indisputable.

Can Felt Paper Cause Wood Rot?

Yes, felt paper can cause wood rot if it begins to block water vapor from getting to the outside air. The wood oriented strand board (OSB) that's nailed to the studs can have water vapor condense and become liquid water inside the wall. This water is what fuels fungi that cause wood rot.

Years ago in old homes that didn't have insulation, balloon framing, and were drafty water vapor could evaporate before it caused wood rot. Water can easily get trapped now inside an insulated wall cavity.

Should I Use Felt Paper On the Outside of My Home?

I'd use felt paper on the outside of a shed, barn or other non-heated structure.

My choice on a heated home or building would be a more modern air and water infiltration membrane that readily passes water vapor.

Why Wasn't Felt Paper a Problem on Older Homes?

Old homes, and I'm referencing ones constructed in the late 1800s and early 1900s, rarely had insulation. Many of them had balloon framing which created a chimney from the basement up to the attic in between exterior wall studs.

The old houses were drafty. Water vapor in the houses could readily escape to the atmosphere. Any water vapor that might have started to condense on the backside of the tongue-and-groove 1x6 sheathing would rapidly disappear because of the air movement in the empty wall cavity.

Modern platform construction creates a sealed chamber at each floor level between exterior wall studs.

felt paper vapor barrier

Here's a typical 2x6 exterior wall before anything is put in the cavity. Note how the top and bottom plates block air flow. Once the wall is packed with insulation and drywall is installed, condensation starts to feast on the wall studs and OSB sheathing. (C) Copyright 2018 Tim Carter

The cavity is packed with insulation. There is virtually no air movement.

Should water vapor condense in a modern exterior wall cavity, it can lead to mold, mildew, and wood rot in short order.


Felt Paper Vapor Barrier? | It Depends on a Few Things | AsktheBuilder.com
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Felt Paper Vapor Barrier? | It Depends on a Few Things | AsktheBuilder.com
Is felt paper a vapor barrier? It seems like a simple question, but the answer is complex. It depends on what covers the felt paper.
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20 Responses to Felt Paper Vapor Barrier

  1. Tim,

    As you said, "It really pays off to pay attention in science classes in school." Yes it does. It also really pays to stay current in your understanding of building science. A little knowledge is a dangerous thing.

    • Yes, Bill, I agree. Lots of people like the folks on the conference call had just a little knowledge - enough to make them very dangerous. I'll share in another newsletter the fact that most builders on the New Jersey shore don't install vapor retarders on the inside face of exterior walls.

      And if you look at the weather almanac for New Jersey it can get damn cold there in the winter. What a shame builders and remodelers are not required to get a formal education in the trade.

      How do you stay current on building science? I attended an all-day conference in Portland, Maine not too long ago where several top national forensic building scientists presented. I also get Dr. Joe's newsletter.

      • Hi Tim,

        I just wrote a longish comment on this topic, hit send, and got an error message. Oh, I hate when that happens.


          • Bill, there are plenty of reasons why your long comment could have gone AWOL. Sometimes my server can be cranky, plus I moderate all my comments to keep at bay all the SEO nectar-sucking vampires. But this time I think the problem might be between you and my server as you can see your follow-up comments are posting. I'd reach out to your fellow computer science professors at Ball State and see what they say. Please come back and share their list as many of us would love to know where all the gremlins hide out there!

  2. Tim, this was a very informative article. You stated that
    "Fiber cement siding that's painted could be very problematic because latex paints can act as a thing vapor retarder." Does this mean that you would not recommend replacing vinyl siding with Fiber cement siding on a house due to possible moisture problems?
    I was planning on doing this on a laundry room extension on my brick ranch house. Now I'm not sure if that is a good idea.

    • No, it's fine to replace the vinyl. I'd just be sure I'd have a drainage plane membrane on so the fiber cement has some air space behind it. Or, you can always install thin 1/8-inch lath (think paint stirring sticks) over the membrane and then the fiber cement on the strips so you have a nice air space behind the siding.

  3. Hi. We are renovating our 70 year old house with foam insulation in 2x4 outside walls. What vapor barrier to use. We live in northern Illinois. Thanks

      • You told Marty (who had exterior walls insulated with foam) to use the LOWEST permeance rating vapor barrier he could find.

        But was Marty's question referring to an inside or outside vapor barrier? I would think that on the outside of his house sheathing, it would make sense to have a high vapor permeance house-wrap, so his sheathing could easily dry out if it became wet. I would use SIGA Majvest which has 50 or 60 perms.

    • Tyvek has different products some with drainage planes to work with different exterior claddings. It's important to realize that exterior membranes are almost always part of a system.

  4. Great article Tim! Love all your responses too! Just referred some colleagues to your website.

    T'is the season right?!

  5. Tim,

    You state that "The fiber cement siding could easily reach temperatures of 160F+ and transfer that heat to the felt paper beneath. Previously there was enough air transfer under the siding to vent the petroleum gasses to the exterior.

    The Certawrap forced the odor inside. The proof is indisputable."

    I don't understand this statement: "Previously there was enough air transfer under the siding to vent the petroleum gasses to the exterior." Please, if you would, provide cross section of this exterior wall before the failure of the siding and after, with the installation of the Certa Wrap between siding and Builders Felt. Why was there more air transfer before? What am I missing here?

    • Bill,

      You're a pretty smart guy. You used to teach this stuff. As for me producing a drawing for you, all I can go off are the photos the homeowners supplied. You've got a typical wall with 2x studs, covered with OSB that originally got 15# felt over the OSB. Then fiber cement on that.

      The only difference now is what you see in the photo above. The siding contractor came in and ripped off the fiber cement and proceeded to nail and tape CertaWrap on top of the felt paper. Does this help you?

      My only guess as to why a drawing might help you is because per your email to me you've never done any fieldwork. You've spent your entire career attending conferences and lecturing in the classroom up next to the chalkboard. There's nothing wrong with that, but it can, in my opinion, provide a bit of a myopic view of how things really work out in the real world.

      I'm guessing you've never installed fiber cement siding on an entire house or even a room addition. Please correct me if I'm wrong. There's no substitute for years of hands-on experience working with building materials.

      If you had installed thousands of linear feet of fiber cement as many of us have, and if you'd built frame walls with today's studs, you'd understand why air can get out to the atmosphere from underneath the fiber cement siding.

      I urge you in your retirement to go out to some building sites in central Indiana and see if you can gain some insight as to what's going on when you start to sandwich one material on top of another using modern materials. It doesn't happen as you see in the gorgeous line drawings in your textbooks.

      As always, I enjoy your comments and keep them coming!

  6. I have a 120 year old house in South Louisiana. I appreciate your article because I have been wrestling with whether or not to use what we call still tar paper as I install my new fiber cement siding, and I'm guessing it's not the end of the world either way as long as I have airflow. I am also more aware of the pros and cons of fiber cement, and not sure if it would choose it again. I love how it needs less paint, but I'm also now aware of the increased weight load on old studs. Now reading that it can reach 160 F and transfer heat into the wall, I worry that I'm baking the old studs. Does felt paper help prevent heat transfer?

    • No,it doesn't stop heat transfer. You've got nothing to worry about. Apply critical thinking skills here. How many 100-year-old building near you have tar paper on them and the studs haven't turned into black cinnamon rolls?

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