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Community and Q&A

Underlayment to use with hardwood

Mark Kessler | Posted in General Questions on


Wondering what the opinion is on an underlayment for a hardwood floor.
From what i understand the main reason for an underlayment would be to have Vapor retarder (not barrier!) to slow the water vapor (if any) from passing thru the hardwood to fast which could possobly cause cupping and other issuies. Please add or correct if i am wrong.

slab > 6mil poly > 1″ polyiso rigid foam > 5/8 advantec.

I think the advantec has a perm rating of 1-2

The question is do i need one? I was planning on using aquarbar “B” but the concern is the asphalt, are ther alternitives that are greener? should i be conerned with installing a petrolium product in my living enviroment? I know it is used alot for this but i don’t know if there are any health risks.

Thanks for the help!

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  1. User avatar
    Dana Dorsett | | #1

    With the 6 mil poly between the slab and foam there's no reason to use a vapor retarder of any type between the subflooring & hardwood. But you might want a layer of a plain kraft or rosin paper between the subfloor and hardwoods as a slip surface.

  2. Mark Kessler | | #2

    Thanks Dana, I kinda thought that was the case but i am no expert and wasnt totaly sure if the penetrations thru 6 mill plastic from the fastners could impact it, and i will only put the floor down once.

    Also seems like i would want somthing beetween the hardwood and the advantec so i was thinking the rosin but i have read some opinions that say that the rosin (or any paper) would actually draw mosture to the hardwood...seams far fetched to me..

    thanks, Mark

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