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.

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-S and it has the following ratings:

• 69 perms using the ASTM E 96-05 Procedure A
• 120 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!

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 stop or slow down water vapor from entering basements and crawlspaces.

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 thing vapor retarder.

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.

 

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