I have standard subfloor + 1/2" plywood on top, to correct for joists going different directions, so that all hardwood boards can be laid one direction for a clean look and avoid sagging where they would go parallel to the joists. The sagging problem apparently solved - but would it still make sense to go through the expense of quality underlayment? Or the extra layer of plywood be just as adequate for sound/vapor/heat control? Thanks

  • What are you installing?
    – DMoore
    Jan 7, 2015 at 5:18
  • @DMoore - OP stated hardwood boards.
    – HasH_BrowN
    Jan 7, 2015 at 15:59
  • Wide engineered hardwood boards
    – Marius
    Jan 7, 2015 at 16:05
  • There are a number of variables that are unknown. Without those an answer wont be accurate. What climate, 1st or 2nd floor, accoutrements in the room (sound deadening), any moisture issues, owner or renter, and anything else that might be a determining factor in this decision.
    – HasH_BrowN
    Jan 7, 2015 at 16:06
  • This is owner inhabited, in Washington DC metro area, winters are rather mild. 1st floor, over small basement and crawl space which are quite moist as there is underground water seeping continuously. No sound deadening. My basic question is whether the extra layer of plywoood has any of the characteristics associated with quality underlayment - for sound/heat/moisture insulation. Thanks
    – Marius
    Jan 7, 2015 at 16:13

2 Answers 2


I would. I put underlayment for all engineered. I spend about .25 a sq/ft for my high quality rolls so not sure what you are looking at. The point of the underlayment for engineered flooring is to provide a surface for the wood to slide across (wood on wood gets stuck sometimes) and to decrease the noise between hardwood and subfloor.

Imagining your situation I feel like you might hear Clydesdale clicking and clacking if you don't install anything.


I believe the point of underlayment in your situation will be controlling moisture movement from below. That is, you want a vapor barrier under the floor. Now, the underlayment can achieve other things as well, such as providing a "loose" surface for a floating floor (as discussed by @DMoore), or enhance sound deadening.

I read your post and comments, and I'm still not 100% whether you intend to float the floor, or nail/staple it down. I have installed wide engineered hardwood both ways, and I far prefer nail install where feasible. Mostly that is because gluing the tongue/groove is way more time consuming (assuming your engineered is not click lock or the like) than nailing. But too, a nailed floor feels sooo much better than floating. But I digress....

Because this is a first floor, I assume sound deadening is not a requirement. So, for a floating floor, I would use a 2-in-1 underlayment like the Robert's product here: http://thd.co/1AGfcoZ. For a nail or staple down, use Fortifiber Aquabar B (http://www.fortifiber.com/aquabar_b.html). Either product will do the trick....

  • Thanks a lot for the response. It's tongue and groove. I think the contractor's intention is to nail it, need to check. The key point is that I have already bought the underlayment (Eco Ultra Quiet Premium, from Floor & Decor) and know all its good qualities - and wonder whether it is worth the expense now that we got what basically is the second layer of the subfloor. But I gather from your and DMoore's comments that it is still worth it, as the subfloor+plywood w/o any underlayment would be noisy and would not hold the hardwood as well as with the underlayment.
    – Marius
    Jan 7, 2015 at 22:19

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