Hi to all pros. I know, I know...the best way to restrict moisture vapor emissions is a concrete sealant, like MVP. You've all recommended a floating sub floor installed over a product like MVP, and I concur.

However, I called Bostik's tech dept., and they said they will ONLY guarantee the bond, and not against moisture if I don't do a direct glue down of my solid planks over the MVP using their urethane glue. WHAT? After the NWFA, Ray, Gary, and kls all said the same thing- that there must be a plywood sub floor installed above the concrete so the solid planks can be nailed into it. The techs said because Bostik had not tested their products for this situation they will not guarantee against moisture with a plywood sub floor installed over their product, but if I glue my solid planks onto the concrete (provided that the manufacturer recommends a glue down, which most will not), they'll guarantee it! This makes no sense whatsoever.

I also spoke to a Taylor tech, and he said their systems are only designed for a direct glue down as well.

Now what? I had decided to go with MVP until I heard this. Since MVP is only about 2 years old (according to the tech), I'm nervous about not having a guarantee, but I don't see any safer alternatives. I don't want to attach the sub floor to the concrete, but I do want to glue down the first of two plywood layers with the Bostik's Best over the MVP. Can anyone with extensive experience with floating substrates help me?
It seems that although all of you have recommended the floating sub floor method, most of your state the you prefer attached sub floors on your own jobs. I am wondering why...

Is there anyone in the Westlake Village area of So. CA that wants to take this project on or can anyone give me a recommendation?

Karen

Let's start here.You seem to have a rather unusual situation. A concrete slab that is emitting considerable amounts of moisture vapor. I recall you saying something about 17 lbs. over 1000 sq.ft. in 24 hours. That is nearly 6 times the normal allowable amount. Then you asked us for the type of subfloor that would BEST keep your plank floor isolated from that moisture. Well, I still stand by my recommendation. That a floating subfloor IS the best way to isolate itself from the moisture in the slab. This does not have anything to do with what I prefer. I prefer a solid feeling subfloor but in your case, because you want to attach solid wood plank to a substrate that is basically unacceptable, I'd recommend the floating subfloor as having the highest chance of success. You will not get a guarentee from anyone as your situation has failure written all over it. Any experienced contractor would either walk away or not warranty the work. Having said that, I still think your floor could be done successfully IF you follow the recommendations I have outlined for you. There are other concrete sealers that will reduce vapor emmisions. And for that matter, you could always go low tech and have a vinyl floor installed first, followed by a 8 mil polyethylene vapor barrier, float the subfloor, lay #15 asphalt OR Aquabar, then nail the walnut flooring. Or skip the Aquabar and glue and nail the flooring. Moisture levels and the success level could be monitored as each phase of the vapor barrier is installed to check to see if it is functioning properly.
I'm not surprized at Bostick's remarks. This is what they have said for years. Does it make sense? Not really. Sort of a catch 22.

Gary: You must be mixing my situation up with someone else's. I don't know anything about the number of pounds of moisture in my slab, it was never tested with a calcium chloride test. The only reading that has been done so far was with a surface concrete moisture meter, which read 4.5 % moisture after the floor had been removed. The original test was done with the floor intact, the engineered wood had 18-20% moisture content with HVAC turned off. If that translates into the results you stated,then I stand corrected.

Gary or anyone else:I forgot to ask, does the data I have given you change your opinion that my floor is doomed to fail, or do you think I still have a chance at using the Bostik products successfully

Although I have understood your newest recommendation, I feel most comfortable with the first layer of plywood being glued down with the Bostik's Best over the MVP to help with void problems. I am not trying to challenge you, I am trying to understand the process so I can pursue the best approach when I hire someone. I already tried trusting the "professional", and I ended up getting shafted. Once I can trust the method, then I can feel more secure that the system can succeed. Thanks for confirming that I should test moisture levels after each process is complete. This will determine whether or not I should finish with solid wood, or go to another medium completely.

I very well have gotten mixed up with the various posts I have responded to.
It seems hard to imagine the wood having a 20% MC when the slab is only a 4.5. Be that as it may, at this point, even 4.5 is about 1&1/2 times the recommended amount; and that's for engineered, which is much more stable. If you wish to use MVP and glue down your plywood subfloor, I say go for it. After it is all done, moisture test the plywood. It should be no greater than 10% MC and stay there. It should work but you never know till you try. BTW, when I have installed a vinyl floor first, then glued and nailed a plywood subfloor over that, using Franklin 811 Advantage, I have not ever had a call back over moisture problems. I have yet to use MVP and Best to install a plywood subfloor over concrete. I have used MVP and BEST to install an engineered. It was fine but the slab was dry as well. I do very little work over concrete as most of my flooring jobs are finish in place over raised wood subfloors. I have done the plywood over slab and as I said, when I used a vinyl vapor barrier, I've had no problems.

Thanks, Gary. BTW, is there a reason why installers have switched from the old tried and true method of wood floor cutback adhesive layered twice with asphalt felt? According the the manufacturer, it is rubberized, protects against moisture and vapor transmission, and contains no chlorinated solvents. The NWFA still approves this method, but I don't see standards written for urethane based underlayments.

I imagine you and most other installers put in many floors in the past using this "old" method. Did you experience moisture problems with this system? Thanks

For one thing, the smell and long dry times plus the difficulty of getting those older "cold bond, asphalt cut back adhesives." They are a dinosaur and the distributors just don't handle them much, at least around here. The system entailed applying a full trowel of "cold bond" asphalt adhesive and then laying a layer of #15 lb. asphalt felt. Then you repeat this process with more cut back and more felt followed by more cutback and then nailing the plywood down. A lot of labor and asphalt (it was CHEAP). It does work, sort of. How you gonna find all that cutback is beyond me. I suppose roofing tar could substitute but you're still stuck with the odor of tar (asphalt ). Since time is money, most contractors would prefer to continue moving along instead of waiting for adhesive to cure and flash off. It's a system that has seen it's day.
Furthermore, that asphalt adhesive gets brittle over time and loses it's bond. I've torn out plenty of older parquet glued down with that where you could just lift the blocks off the slab. The thing the cut back asphalt had going for it at the time, it was the ONLY game in town and it was cheap. Things have changed. NOFMA and the NWFA are a little slow in getting their recommendations up to present day materials and procedures. In their manuals it will say one thing. Go to a seminar or class and you'll get a completely different story.

Thanks for the quick response, Gary. There are three distributors that sell it in my area, made by The Pure Asphalt Co. in Chicago. The specs say it is and rubberized and waterproof , only when layered twice with roofing felt. It has no chlorinated solvents. Gary, the jobs that you saw fall apart... were they layered with the felt twice?

If I take the shot with one of the urethane systems and don't nail the sub floor to the concrete, what are the chances that the plywood could get pulled up with the solid finished floor when it expands and contracts, if it will be nailed to a glued-only plywood? Since a few of you have raised the issue of piercing the vapor barrier with nails,and the moisture following the nails to the plywood, I'm bewildered. I worry about the longevity of a vapor retarding system that hasn't withstood the test of time, but I need to choose, and I need to choose this week, no mistakes this time

Can any of you tell me about your successes with the glued down (no nails) plywood sub floors? Or have you done so few of them that you don't have any feedback yet? I understand the process and the theory, but want to know about the results from you experts in the field.

Thanks for all your help!

TO NAIL OR NOT TO NAIL......THAT IS THE QUESTION

I have heard that bostiks is really good and know what they are talking about.Have used it to glue down wood a few times and have never had a prob. The mvp is a newer idea but bostiks knows thier stuff and what ever they say I would do.

Karen,

Bostick will only guarantee there products used in the manner that they have tested. It does not mean that the accepted procedures are null and void it just means that Bostick has a tested methodology that works for them. As well it means that Bostick will now be selling you glue for full trowel glue down.

The most bulletproof installation is a floating sub floor it is sandwiched between a layer of poly paper on the bottom and felt paper on the top. Vinyl, MVP, concrete sealer, or Cut-back underneath the poly paper is only going to serve to make it that much more resilient.

The reason that most installer prefers to attach the sub-floor is that it has a much more solid feel. But as well most installers will work to correct any moisture issues before installing. The recommendation of a floating sub floor is for a high moisture install. Which most installers would walk away from or not warrantee and do a floating sub-floor install. In all reality an attached sub-floor will most likely correct your moisture issue and bring the numbers within norm but it will require a lot of testing with a moisture meter to verify after the sub-floor is installed. The only way to guarantee it blind is with a floating sub-floor.

I think I've got it! But I need your blessings...Gary, kls, Floorguy, et al.

Here goes...

1.) I want to loosely lay 2 layers of 8 mil poly on the concrete in opposite directions, taping overlapped seams together with packing tape, running it up to the baseboards.

2.) I would like to lay 2 layers of 3/8" CDX plywood in opposite directions, screwing them together with 1/2" decking screws.

3.) Staple 1 layer of 15# felt to the top of the plywood layers, and blind nail the 4 1/2" planks with 1 1/2" cleats to the plywood.

4.) Then test, test, test the plywood for 2 weeks to see if it stays at or below 10-12% moisture content. If it's stable, and my planks are within 2-4% of this range, proceed. If not, change direction in finished flooring.

Will this work?

I need at least 3 of your blessings, and I'll leave you alone! If you all concur, and this ends up working out in the end, I will let all of you know!

Yes you should use 1/2 ply but 3/8 will work. As well you can get some cheap concrete sealer from the home store if you want some added protection. Or just cover the concrete or plywood or both with a coat of plain old oil based polyurethane it’s cheap and it will knock the moisture down some. It does about the same as polyurethaneing the bottom of the boards but it is a lot easier. It really comes down to what is going to give you peace of mind. I understand that this floor has failed already (IIRC). So it is natural to be a little gun shy but the floating sub-floor method is absolutely the most moisture resistant method of installation. The moisture is no going to get through all of the layers of protection designed into a floating sub-floor.

The problem I see is fastening the two layers of 3/8th to each other without puncturing the moisture barriers. 1/2" fastener isn't much.

1/2 with a layer of 3/8 on a 45º. Or at least opposite directions.

I concur with the other guys. I'd feel better about using 1/2" CDX and it doesn't but $.50 a sheet more. That's 50 cents a sheet. Or 1&1/2 cents per sq.ft. It's easier to handle, will lay flatter and feel more substantial. I also like the idea of sealing the slab with something IF you are going to float the subfloor. Latex paint will NOT work. Polyurethane would perhaps. Two layers of 8 mil polyethylene taped and lapped up the walls. No way moisture could get by that unless it's punctured.

Can you suggest what type of concrete sealer? Do you know any brand names you can suggest? I'm thinking something with elastomeric qualities,brush application.

If I have height allowance,I will try for one layer of 1/2" plywood, but as it is, 3/4" net sub floor is pushing it. I was going to test plywood scraps to see if the screws grab well and don't come through before installing. Can I install full sheets if they lay flat?

Thanks for the guidance.