GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

Community and Q&A

Red Oak Hardwood floor on plywood above crawl space in hot humid climate

NikoFL | Posted in General Questions on


I am about to install red oak hardwood floor in my living room. The local installed wants to put 30 pound felt paper on the plywood subfloor to control moisture levels in the hardwood floor over time. In theory this makes sense however underneath the subfloor I have a crawl space that is encapsulated and humidity controlled. I also am planing to install a liquid applied air barrier (Pro Clima Viscon) on the underside of the subfloor to completely separate the crawl space from the living space. Viscon has a perm rating of 1.

My concern is that I then have two vapor retarders (Viscon on the underside and felt paper on the top side of the subfloor) and that it could cause moisture issues in the subfloor. 

Vapor barriers are pretty complex to get right and you can easily get it wrong. So my questions is – can anybody comment if I am introducing too much risk with the viscon on the underside or should I look into not using the felt paper between subfloor and hardwood floor.

thank you so much!

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.


  1. user-5946022 | | #1

    Why are you trying to separate the living space from the crawl space if you have an encapsulated and humidity controlled crawl space? A little bit of sealing for fire protection is good, such as foaming the gaps in the holes through which pipes and wire go, HOWEVER
    Typically, in climate zone 3 (humid) you would NOT install any air barrier on the underside of the floor between the living and crawl space IF the crawl is encapsulated and humidity controlled. This seems like inviting problems. Your air barrier should be at the perimeter walls of your crawl, and on the ground of your crawl. You would only install an underlayment under the hardwoods. The purpose of that underlayment is to reduce squeaking.

    Also, these days, hardwood installers typically use synthetic underlayment. I would be very concerned about any installer using felt, as felt is asphaltic, which is not good in a tight house. One consideration is to upgrade the underlayment to a very very thin foam type material - a sound dampener underlayment, or just typically "upgraded underlayment." It does not matter that the crawl space is not a living area - it will make the hardwoods less squeaky in the living space when you walk on them and slightly more resilient.

  2. Expert Member
    Michael Maines | | #2

    When installers want to use a membrane under the flooring I no longer bother arguing about it, though I think it does nothing in most cases. I agree with the comment above about using tarpaper; that is not a product meant for interior use, only for exterior. Red rosin paper is a more traditional option, though its original intent was likely to keep dust from falling between floor boards than for some notion of it preventing squeaks or slowing moisture movement.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |