Replies

1. Expert Member
   Dana Dorsett | Mar 28, 2017 05:29pm | #1

   Since you want/need a drying path for the OSB and already have a true vapor barrier under the subfloor,use #15 felt for the underlayment. When it's bone-dry #15 felt is fairly low permeance, but if the OSB has a moisture content high enough to support mold or rot it would raise the moisture content of the felt to bring it above 5 perms.

   For more money you could also use 1/8" cork, as long as it it's not the type with a vapor barrier (it's sold both with and without), which will also make the floor a bit quieter (not that you'd notice much with your stackup.)

2. 
   Silversledder | Mar 28, 2017 10:01pm | #2

   I was not expecting building paper. It's readily available. Just read some on-line articles and didn't appreciate its permeability behaviour until now. Although an acceptable choice I have a few more questions:
   I wonder if it's sufficient to cushion noise between laminate and subfloor?
   Any chance one might smell the asphalt paper?
   Any concern with longevity?
   Will felt paper feed mold?(assume not else it wouldn't be suggested)
   Notwithstanding felt paper will do,Is the cork a better choice? Area in question is 400 sq ft.
   Thanks for your help.

3. GBA Editor
   Martin Holladay | Mar 29, 2017 04:41am | #3

   Grant,
   I assume you are talking about flooring underlayment (something like UL plywood), not a membrane (as suggested by Dana).

   The answer to your question is to use the type of underlayment required by the flooring manufacturer. When you call up the flooring manufacturer (or look up their installation instructions online), make sure that you check whether the manufacturer approves below-grade installations.

   -- Martin Holladay

4. 
   Silversledder | Mar 29, 2017 07:42am | #4

   Maybe a terminology thing on my part. I was talking about something I would lay between the OSB sub floor and the laminate flooring to keep the floor quiet. All retailers seem to have a 3 in1 roll out product that provides vapour retardance, cushioning and noise reduction. I was concerned about sandwiching the OSB between such a product above it and the poly layer on concrete below it.
   The manufacturer's instructions say it is suitable for basement and goes on to say an "approved vapour retarder" should be placed on the sub floor. In my build up I deem the vapour retarder as being the poly on the concrete floor, topped by the extruded 1.5" extruded polystyrene so am of opinion I don't want another vapour retarder layer above the OSB. So Dana's answer made some sense to me, though I still had questions about the felt paper.
   One thing that comes to mind is whether those all in 1 roll underlays are truly impervious or if they also transmit some perms like felt paper. I ask because I noticed some stated they provide sub floor moisture resistance which would imply its not vapour impermeable.

5. Expert Member
   Dana Dorsett | Mar 29, 2017 04:10pm | #5

   Many flooring manufacturers will insist on a vapor barrier type of underlayment, but the polyethylene under the subfloor is providing that protection.

   Cork flooring underlayment can be had in a number of thicknesses, starting at about 1/16", both with and without vapor barriers. At 1/8" or thicker they do a decent job of quieting the floor. If adding thickness to the floor is problematic, #15 felt works fine as a slip surface under most flooring, but it'll be a noisier floor. Hopefully the manufacturer is fine with cork underlayments(?). (I suspect none would approve #15 felt, but I've seen it used under laminate flooring on a plywood subfloor over a vented crawlspace on a project in coastal Oregon, and it seemed to hold up just fine. No problems had appeared by age ~5 years when the house was sold.)

   Most of the vapor barrier underlayments for flooring are true vapor barriers (<< 1 US perm), and create a potential moisture trap for the OSB.