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Community and Q&A

Hardwood Flooring Vapor Retarders

JohnJones171 | Posted in GBA Pro Help on

Hi all-given this crowd tends to be very knowledgeable about vapor retarders and such, I’d love to hear your thoughts on the NWFA (National Wood Flooring Association) requirements to put a vapor retarder between the subfloor and the hardwood floor. I understand if this was below grade, or sitting on top of a crawlspace. But for standard main floor install over a full basement that is conditioned where humidity is controlled, what is the point?

The two most common products on the market aside from 15lb felt (which I’m not sure I want to use-health reasons??-not even entirely sure) which I am aware of are Silicone Vapor Shield and Aquabar. Both have perm ratings of approximately 0.7 or thereabouts once nailed. That is pretty tight, and makes me nervous putting that type of retarder between two layers of wood.

I’d love to know your guys thoughts on this. I’m obviously concerned if I don’t follow the NWFA retarder guidelines, it possible my void the warranty of my floor. At the same time, I don’t want to cause any subfloor moisture issue (which shouldn’t occur I wouldn’t think, as humidity is controlled and I’m above a conditioned basement). Thoughts? 

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  1. creativedestruction | | #1


    The primary purpose of the vapor retarder between hardwood and subfloor is to form a slipsheet plane to reduce creaks and squeaks. The grain of the hardwood will be counter to the grain of the subfloor, be it old diagonal boards or new plywood or osb. Seasonal moisture cycling will cause differential movement between the two surfaces, hence the need for the 'slippage' plane.

    Asphalt kraft or felt or rosin paper have variable permeance based on relative humidity, like wood. They won't trap moisture. Follow the recommendations.

    1. JohnJones171 | | #2

      Thanks Jason. Asphalt #15-is this safe to use inside? I've heard potentially it may off-gas somewhat.

      Also, rosin paper does not qualify under NWFA guidelines. While it does provide a slippage plane, it does not provide any vapor retardant characteristics-NWFA requires a perm rating of greater than 0.5 and less than 50 (although it seems some manufacturers require less then 10 perms) if memory serves. Asphalt felt like you mention would qualify, as does those other products I mentioned-but they seem very tight which concerns me. But not sure if that concern is warranted.

      To clarify-I was considering using waxed kraft or red rosin-but that would not qualify as an effective vapor retarder. So if the group feels that using something like 15lb felt or the Aquabar product would not cause any issues, that would be the safer route for warranty and such.

      1. creativedestruction | | #3

        I used #15 felt under hardwoods in my addition mostly because I had extra on hand. The smell dissipates quickly. I didn't measure VOCs over time but this is an old leaky house.

        I don't give permeance a second thought in this application, personally. For a VOC-free option you could go with "QuietWalk".

  2. charlie_sullivan | | #4

    I wouldn't use asphalt. No reason to expose yourself to those VOCs.

    I think the reason for a vapor retarder is that sure, you might keep your basement conditioned, but will a future resident? And they are making recommendations that apply broadly, including terrible basements. I think that using something that doesn't quite meet the requirements is fine, given that your scenario is better than the worst case they are planning for.

    1. JohnJones171 | | #5

      Thanks Charlie. I found an old link where Martin suggested that using a vapor retarder product between the subfloor and hardwood could cause problems. But that Q&A was dated somewhat. Perhaps the real question I should be asking is whether the Aquabar at a perm rating of 0.87 could cause any subfloor moisture issues. I'm in Zone 5, and you make a good point about how future homeowner's may not control humidity like I do-1970s style house.

      1. charlie_sullivan | | #7

        I'd opt for silicone paper rather than aquabar, to avoid the asphalt. But I don't think either would cause moisture problems--that's enough permeability to avoid anything problematic happening.

    2. creativedestruction | | #6

      I would agree with Charlie. My thought runs to "what problem is the low vapor permeance requirement trying to solve?" Zero perm can cause issues; trapped moisture at wood is ugly. Anything more than 10 perm means the subfloor or the hardwood act as the vapor retarder, which is fine. The pores 'open up' and dry when they need to dry. They stay closed under normal conditions less than 80% RH. Just about anything paper-based should have similar characteristics (okay this is a stretch considering manufacturers can do dumb things).

      Rosin paper has been used under hardwoods for a century because the permeance doesn't really matter here as long as it's not zero. Trying to stop (or intentionally slow) vapor drive via permeance characteristics is a fool's errand when we're not talking about airtight layers.

  3. JohnJones171 | | #8

    Thanks guys for your input. Just not sure why the market doesn't create a product that doesn't contain asphalt that allows a little more drying if needed for a nail down application. Not everything needs to be so tight. Less then 1 perm is a little much....thanks for your thoughts.

  4. Expert Member
    Michael Maines | | #9

    On the last wood floor I installed I used this silicone paper: The floor was partly over a basement and partly over open air, with closed-cell foam insulation in the floor joists. I was more concerned with the difference in drying between the two portions than in having a vapor retarder under the flooring.

    In the past I have used felt paper but I wouldn't do that today for VOC reasons. I have used rosin paper to supposedly create the slip sheet people have mentioned, but I am now convinced that if there is a dry basement, the main advantage of using any underlayment is to ensure that the floor is clean and clear of debris. I used to include a note to that effect in my specs but have decided that it's not a battle worth fighting the installers on.

    On the floor I installed over silicone paper, shortly after the project was done a sink overflowed and flooded the floor. I kicked myself for not allowing drying to below, but fans, a heater and a dehumidifier dried things out pretty quickly with only minor lippage in a few areas as a result. That's a worst-case scenario and it convinced me to not worry about what goes under the flooring, unless you know that you have a damp basement.

  5. JohnJones171 | | #10


    Any idea what the permeability would be on a typical waxed paper. More out of like this It's not perm rated, so I assume it probably is pretty high. I've seen pretty much every underlayment there is at this point...

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