This just occurred to me last night while reading Mickey Moore's column in the latest FCI magazine. I was remembering that when natural reflections first came out, they had you use 6 mil under the hardwood, over a wood subfloor, when stapling. I just installed a 3/4 solid job over an existing vinyl, that had a 1/4 multiply underlayment, over the plywood subfloor. Why is that ok? I called the tech guy from the mill, and was told this was acceptable procedure. So why is it OK to install naildown over vinyl flooring, but not 6 mil poly? Thanks.

Hi Steve:

Good question! Maybe somebody can provide a good answer. I wonder why Bruce doesn't mention it anymore?

Stevo,
I don't recall such a recommendation for Natural Reflections, but my memory is hardly infallible.

The reason 6 mil poly will not work where existing vinyl or gp-15 will has to do with diffusion. 6 mil poly has no capacity to assimilate water vapor. Because of this, any hole you poke in it will act as a portal for vapor to rapidly(and I mean rapidly) migrate through that perforation. Felt-backed sheet vinyl and roofing felt, on the other hand, have the ability to absorb moisture and thereby retard that migration.

Let us remember that water vapor is one sneaky and greedy little devil. One way or the other, every molecule of water seeks to someday become a raincloud. By nailing over a medium that allows for vapor diffusion/dispersal, you give the vapor a more convenient place to go.(for a little while) It is an equilibrium deal .

I know I have commited a scientific sin by attributing human properties to water vapor, but it is the easiest way to explain it.

Wet goes to dry. Give the vapor somewhere else to go besides the floor.

CHU

p.s. Felt WILL NOT ameliorate an existing moisture imbalance between subtsrate and finish product.

That makes sense, but it is still holding moisture. So why don't subfloors rot when vinyl is laid over them? I can't sleep, pondering this dilema. And in the future, please remember that everyone but you, ( and maybe Gary, Perry, and Ernesto, dont have a clue what "ameliorate" means...
I need to go lay down now, my head hurts. The thought of my felt doing whatever that word means, makes me uneasy.

That is the whole point, Steve. They HOLD it.(to whatever capacity they have) An amount of moisture that could produce rot would be much more than they could hold.

You already know this. Have you ever seen a floor that showed no cupping and was rotted underneath? How could that possibly happen?

As for why subfloors don't rot under vinyl, it goes back to diffusion. Vinyl is not waterproof. Vapor will move through it. It just takes longer.

CHU

p.s. Ameliorate: To make or become better; improve.

Natural Reflections did have some type of sheet plastic for a staple down installation. It had the Bruce logo on it but I'm not sure of it's composition and perm rating. I have no idea what they recommend now. Remember, for many, many years, Bruce would not join NOFMA and did not necessarily abide by NOFMA guidelines or grading standards. They considered themselves "too big" for NOFMA and felt they would be charged excessive dues, fees, etc. Of course, later, they quietly joined. The NWFA's position on installing over plastic is it promotes subfloor rot and is not recommended. Vinyl floors are felt backed and diffuse moisture, as Chuck pointed out. Apples and oranges.

Now I understand the vapor transmission properties of felt and vinyl but I am a little curious is not most underlayment for floating floors have a poly back on it? I am assuming that it must be thin enough so it passes a little vaper as well? Or is it because there is no staple passing through the barrier that it works?

Ok, why not roll out felt and the the poly?

The felt would do the same as the felt in vinyl, would it not????

Lets not forget too that vinyl floors are fully trowel glued and that glue goes a long way in moiture protection and seals the punctures pretty good too.

I hear Lunesta is a pretty good nighty night drug

So is vodka...