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Sealing Sub Floors

7103 Views 7 Replies 7 Participants Last post by  Floordude
I was speaking with my contractor friend Juan the other day and he was telling me that no matter what Hardwood species you use, you must always seal a concrete slab.. I was told that it is not necessary if you are going to install Engineered Hardwood, for it is resistant to the moisture that may come from the concrete. I'm confused.. do i need to seal the concrete sub floor or can i just glue the engineered hardwood directly over the concrete (since it is designed for that lol) ?
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You normally only seal sub-floors for solid hardwood or bamboo , i use Mapai WFM or Bostik MVP . I offer to seal the floors with sealcrete for engineered floors, you can buy it at Sherwin Williams & Kelly Moore . Home depot or Lowes might still sell it.

Sealcrete works well for engineered floors if you have a low to medium moisture content in your sub-floor and want a little insurance.

Mapai will guarantee the floor from failing for 10 years with a moisture reading up to 8 if you use their WFM sealer with new trowels.
I always seal a slab with two coats no matter what is going down. It's really cheap insurance.

Make sure that your cust. knows this, it will set you apart from other bids.
The engineered wood itself is not resistant, but the 6 mil poly you put down before the pad is.
I always seal a slab with two coats no matter what is going down. It's really cheap insurance.
What sealer do you use?
I have always a DPM of one form or another on crete floors when installing wood, (type of dpm changes with type of installation ect)

In 25 yrs never had a problem due to moisture except in one job

(friends house) road construction outside his house six months after install cracked all his floors, lots of mositure built up under the DPM sheet to the point it worked its way to the sides and back over the top of the DPM sheet and soaked into the laminate. when we took the floor up to find the cause we had to mop the floors, good job we still had our meter readings from prior to install to prove it was dry. never seen mositure like that before, two years later he is still trying to sort it out with the construction company.

If the concrete tests to be way below moisture emission or rH levels, there is no reason a moisture blocker membrane is needed, adhesive alone will work just fine for years, unless there is a flood. If moisture testing is close to the limit, use a moisture blocker, engineered or solid wood.
What are the limits?
3-lb. per 1000sq.ft. minimum of 3 test per the first 1000, and 1 additional for each 1000.

75%rH, internal slab humidity

4.5 on the upper scale of a Tramex concrete meter.

@ $150 per 5 gallons, and a spread rate of 30 sq.ft per gallon, I would call it "cheap" insurance. That is a $1 per ft. my cost, no mark-up, and that doesn't include application labor.
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