Choosing underlayment over 50-year-old subfloor


Old 10-31-12, 03:16 PM
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Join Date: Oct 2012
Location: USA
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Choosing underlayment over 50-year-old subfloor

I'm renovating and adding to a 50-yr old house in San Jose, CA. The subfloor is 1.5x3.5 planks laying side-by-side. I can see through to the crawlspace in between the planks. I'm considering several options to insulate and provide barrier between the crawlspace and the interior.
And I would like to hear your thoughts and experiences on similar situation.

1. Spread/roll liquid rubber onto the planks. Then put 3/8" plywood sheets on top as the new subfloor for hardwood or tile flooring.
2. Spread water sealer onto the planks. Then poly sheeting as vapor barrier.
Then 3/8" plywood sheets.
3. Put combo underlayment (plastic on one side and foam on the other) as
vapor barrier and sound reduction. Then 3/8" plywood sheets.

I would like to hear your thoughts and experiences on similar situation.
Any inputs would be greatly appreciated.
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Old 10-31-12, 07:59 PM
Join Date: Mar 2012
Location: USA
Posts: 305
This is what I would suggest:

1) Screw all of the planks with deck screws (not drywall screws) to prevent squeaking. Drywall screws are too weak/rust so don't use them.
2) For hardwood floor areas: I would put fiberglass bats in between the joists (get a bat thick enough to fill the whole joist bay) if you can access them in your crawl space. You would have to double check which way to face the paper for your area. If you cannot access then I would lay 6mm polyethylene as a vapor barrier over the floor and staple it. Over the polyethylene I would use an insulating underlayment like "Insulayment" (instead of rosin paper). It gives you a small but helpful amount of insulation if you don't have any. I would then install the hardwood floor.
3) For tiles areas - Not sure how you can insulate unless you can access from crawlspace. Screw planks with deck screws. Then put a layer of 1/2" exterior rated BC plywood on top of the planks. ONLY screw the plywood into the planks (don't screw them into the joists and don't put glue in between!). Put an underlayment for the tile on the plywood following manufacturer instructions (Ditra/Noble CIS/cement board/etc). If you install cement board make sure to apply thinset under the cement board (follow the instructions). Btw you don't want to have anything like Insulayment under a tile installation. The floor needs to be as rigid as possible. And finally you need to make sure your joists can handle a tile installation. Check out the Deflectolator at the johnbridge forum to see if your joists can handle a tile installation. Generally most houses need to have their joists beefed up for natural tile like Granite and Marble.
Old 11-01-12, 04:02 AM
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What is your joist spacing, how wide are your joists, and what is the total length of unsupported span of the joists? This is a critical factor in what subflooring you should use. I would use nothing less than 3/4" Advantech or similar subflooring, then apply the hardwood. Are your planks installed at an angle to the joists or straight across them? Insulate from the crawlspace with vapor barrier up to the living area. I would not install a vapor barrier between your existing planks and any subflooring you choose to install. It will tend to trap moisture between the plies.
Old 11-01-12, 03:28 PM
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Join Date: Oct 2012
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I think what I have is just the 4x6 10-12' long girders connecting the piers at about 4' center-to-center. Then the 2x4 planks double as joists and subfloor.

A co-worker suggests roof felt paper over the planks, then 3/4" 5-ply plywood, then another layer of roof felt paper, and then the hardwood flooring.

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