Background: I am getting site-finished hardwood flooring installed in my house. Last week the installer started with the 1st floor area, which includes a living room and a hallway. After carpet removal we found out that living room had old 1/2" Oak hardwood floor underneath, and hallway had a 3/8" particle board underlayment in part of the area whereas rest of the hallway had plywood. This house was extended a few years ago, which is why (I think) the old part of the house subfloor is deckking and new part seems to have plywood.

New hardwood for 1st floor is 3/4" TG solid red-birch. The installer surprised me when he said that he would come in with the wood and install it right away without any acclimation period. Having read so much about importance of acclimation I was a bit concerned. But he insisted that it is simply a luxury and not an absolute requirement. He said that after laying down the wood we need to wait 2-4 weeks before sanding and finishing, which is the most important part. I was not convinced. I asked him to check with the moisture meter.

The subfloor (both deckking and plywood) was at 15%, old hardwood floor and particle board was 9%, and new hardwood was 8-9%. I was surprised because he told me those numbers upfront *before* measuring. Everywhere I read it says that the subfloor should be 12-14% and the new hardwood should be within 4% of the subfloor. If that is the case, WHY IS THE OLD HARDWOOD FLOOR AND SUBFLOOR ARE 6% apart?

According to him the new wood already is @ 9%, which is what it is going to remain at after installation evident by the old hardwood floor moisture content. Therefore no need to wait for any acclimation. Anyway, the old oak floor and particle board was removed due to height and other concerns. Now the whole subfloor is around 15%. Is it OK to install the new hardwood which is 9%?

Check his refrences ... I wouldn't do it without 'proper' acclimation ... PERIOD . When you check those refrences GO LOOK AT HIS WORK FOR YOURSELF especially if the floor has been in for at least six months. Then judge for yourself. I do quite a bit of birch and it likes to shrink in many cases in our climate, and head joints can lift slightly (and sometimes dramatically) against any headers that there may be.
Hope this helps.
William Burnam
Heritage Hardwood Floors
Coeur 'd Alene, ID

Your installer is trying to finish and collect a check as fast as possible.

15% is high for a subfloor!!!!

9% is about right for my region of the world.

With Oak 4% is acceptable, with Maple Birch, Elm and Hickory/Pecan, you want that figure closer to 2% difference in flooring and subfloor MC.

The subfloor is probably about 40+ years old and I live in the house with HVAC in place and temp setting between 66-70 (night and day). This is in SF Bay Area. What I could not understand is-
In all these years if the sub-floor is @ 15%, that is probably what it is going to stay at. Why is the existing hardwood floor @ 9%, which sits right on top of the sub-floor?

Have you had any wet trades (painter, ceramic tile, etc...) working in the house just prior to the wood delivery? May have some temporary effect on your subfloor if they were working while it was exposed?????
Still, if your subfloor "lives" at 15% then you ought let the birch "catch up" to those conditions in the house. 6% varience is too much. Yes he will probably use moisture barrier, unless he's from the stone ages and only uses resin paper, but one thing is for sure, more H2O under the finished floor than above the finished floor equals taco planks. (CUPPING) Especially if you are installing wider plank.
Regards,
William
PS Did he get moisture readings from more than one place in the subfloor? Did he test more than just one board in the stack of new stuff?

The reason you have a 15% subfloor is because the ground or space under the subfloor does not have proper moisture retarders in place now.

Has the installer even looked under the subfloor to see why it is at 15%.

I know I would!!!


The ground under the subfloor(crawl space) needs to be covered with a poly moisture retarder.

Not sure if you have basements in San Fran, but it too could be the problem.


8% - 13% average, is the moisture content swing you could experience during the year, unless you get it under control. A 6% difference in the swing.


All I can say is if the floor is laid right now, your going to be crying the blues.


I hope he isn't planning on it to cup and is expecting to come in and sand the cupping flat!!

No nothing of that sort.



Now that makes sense! I know that there is nothing in the crawl space covering the dirt. There is some ventilation in the crawl space though.

Since the installer does not seem to be concerned about the moisture content of the sub-floor, I guess I can put in some moisture barrier myself. What is a recommended covering? I believe I should be able to buy it at HD or Lowes.

Also, this guy is putting in some craft paper over the sub-floor. Does that help? I remember that he mentioned to me that he is putting in some craft paper instead of felt paper...

The flooring is yet to be laid because the installer called in sick and its been six days so far that the wood is sitting in scattered in bundles all over the room. I hope it would help in acclimation.

BTW, say I add a poly sheet or whatever to cover the dirt tonight. How long would it take for the sub-floor to react and get in the acceptable range? I can turn on heat at 70 round the clock...

Thanks to all of you for your valuable insight. This is the kind of information I was looking forward to. Thanks again.

I have had to cover more then one crawl space!!

6 - 10 mil poly, over lapping at the seams and held around the edges & seams with rocks & a little dirt.

Here are some of a crawl space I did last year. Yes that is mud on top of a french drain that is just for looks! Who ever put that in, stole from these people.

It hit 15%, and I was able to bring it down to 11% in a week and a half by running dehumidifiers and fans, after covering the ground soil with poly.

I hope he isn't planning on it to cup and is expecting to come in and sand the cupping flat!!



As a rookie I did that and it wasn't a couple months later that I got to do it again after humpty dumpty set in. If a floor is sanded flat while cupped and is not stabily dry it WILL give the opposite effect once the boards lay back down again. OOOPS!


Good call on the crawl space moisture. I was thinking too much like a finisher and not enough like an installer.


Good Luck
William

Yesterday my new installer got a moisture meter (a very good one). It turns out that actually the sub-floor is at ~9%. I'd think it was that crappy moisture meter because I do not think that the sub-floor came from 15% down to 9% in a matter of last one week or so.

Installation will commence tomorrow. I am excited!

The MC could have dropped that much if a moisture barrier was installed on the ground. Moisture barriers in crawl spaces are a requirement for solid wood flooring.

I like how you said ""Yesterday my new installer ""

Have you brought it to his attention, about the crawl space?
It still should be covered with a moisture retarder.

Yes, your subfloor can gain and loose MC in a week, if conditions are right.

Wood can cup in one day, it can also uncup in one day if the MC is lost.

Crawl space has not been covered yet. My installer looked around the house and said that water goes away from my house which is a good thing but is going to cut some more vents around. It will help in moisture to escape instead of getting trapped. He said that poly sheet in crawl space would help but is not a must given the dry weather around here in CA and looking at the dirt in the crawl space which looks pretty dry esp for this time of the year when we get a lot more moisture than any other time of the year. Still I plan to put a poly sheet by myself. He told me that the way to put poly sheet is to wrap the poly sheet underneath the subfloor and staple it around in the crawl space instead of laying it right on top of soil.

I'm laughing so hard, I'm crying over here ---->



According to NWFA and NOFMA spec, it is a must to cover the soil.
Stapling to the underside of the joists is a formula for mold and mildew, as you have now created a dead space without air flow.


Ask to see the spec, in the NWFA or NOFMA installation manual and specification book, he should have with him on every job.


I carry my book everywhere, just for the case in point. When someone else has blown smoke up, a consumers butt. I can whip out my BOOK and show them the documented specifications on the subject at hand.

The see I'm not blowing smoke to pad profits, as the last guy there was.

Well, your new installer is wrong. The correct way IS to lay it on the ground.