Hi,

I am new here, and I have tried to read as many of the acclimation posts as I can. But I haven't found one that closely matches my situation. So I'm hoping you can help.

I live in San Diego, about a block from the Pacific ocean. I am remodeling my house. It was originally built in 1959 as a single story home with a crawl space. I took it down to the studs, and added a second floor. We still have a crawlspace, and it is dry and well vented.

On the first floor, we kept almost all of the original subfloor, which is made of 1x6 planks on a 45 degree angle. I am not sure of the species, but I'd guess Douglas fir. Upstairs, the subfloor is 3/4" CDX plywood.

The house is nearly complete, and we have had our flooring (5" prefinished maple plank from Somerset) in the house for over five (5) weeks. It is stacked criss-cross with open boxes in the rooms where it will be installed.

On delivery, the moisture content of the maple was between 7.5% and 9%. The moisture content of the subfloor was between 13% and 15%, with lower readings upstairs, and higher readings downstairs.

We didn't run the AC (it is San Diego) for the first two weeks. At that point, there was almost no change in the moisture content of either the hardwood or the subfloor. But the humidity was pretty high inside, about 70%. So at our installer's recommendation, we started running the AC. We have it set at 67 degrees to keep it running (it's about 75 outside), and the humidity is now about 50% inside.

After three more weeks with the AC running, the moisture content of the hardwood is still between 8% and 9% and the subfloor is still between 13% and 15%.

The installer doesn't want to install it until the difference is 2% or lower.

I really need to move out of my rental and back into my house. So I have a few questions.

1. It's been five weeks (10% of a year at the wettest time of year), and I live in about the mildest climate on earth. Given the time the floor has spent acclimating, do I really need further acclimation?

2. How much would a 4 percent increase in moisture content (from 9% to 13%) affect the dimensions of the maple?

3. Given the moisture content percentages above, if we installed it as is now, what are the real risks?

4. According to the EMC data I've seen for San Diego in July and August, the EMC for wood is 14%. So it looks like the hardwood is too dry. Should I turn off the AC, and open the windows to bring in more moisture for the hardwood to absorb?

5. Is there any other way to drive the moisture content in the maple faster?

Thanks!

John

Your floor layer will be able to predict the expansion with a table available in several different sources for the change of moisture content for each percentage difference in maple.
You will find that your flooring will cup from expansion after it has been laid if you go ahead and lay it tightly now. With the predicted expansion that will take place in mind, washers can be placed between rows of flooring to simulate the expansion that will take place over time and the floor will even out after it has had time to acclimate in place. Even if you have a moisture barrier between the sub-floor and the flooring the atmosphere will add enough moisture over time to expand the flooring to fill gaps created by pulling the washers out. The expansion co-efficient is an engineering constant calculated for each species. You can count on it as an average, each board will expand some depending on the orientation of the grain. Quarter sawn has less expansion than plain sawn.
Perfection would be to leave enough room for each board to expand, by gapping enough laying the flooring loosely as it is fastened, for it to expand and then be tight after it has been sanded and acclimated after finishing.
I will sometimes spray the floor with a garden sprayer with water after the flooring has been laid to help acclimate before the flooring is laid, before sanding.
Sorry that you have this problem. It should be figured out before proceeding.

Pete, thank you for the reply.

My flooring is prefinished, so spraying after installation, before sanding isn't possible. Would misting the backs of the boards help?

Thanks!

Pre-finished flooring will take a lot longer to acclimate. Since the sub-floor is higher in moisture content than the flooring, it would not be good to mist or add any moisture to the underside of the flooring. It will take time to acclimate after it has been fastened in place.

Listen to your installer, sounds legit to me.

So your in a wet season and rest of the year it is very mild yet humid?

It seems your subfloors are doing what comes naturally during the season. Is the installer taking into account the different species of wood with a species correction chart or does his meter have it built in?

Thanks. I hear what you're saying. But the floor has been in a climate controlled environment for 6 weeks now (8 weeks total) with no change in moisture content. I am beginning to think there may be a difference in readings that does not take into account the difference between the plywood subfloor and the maple hardwood.

With the inside A/C running and the RH at 50% the moisture content of the hardwood flooring is going to be approximately 9%. If you want to increase the MC then you'll need to increase the RH inside the house. You mentioned a difference in subfloor MC between the lower and upper floors with the lower floor's MC being higher and right over a crawl space. Seems that the crawl space humidity level is contributing to this. Did the installer say anything about controlling the crawl space humidity? Seems like the A/C is controlling the hardwood MC and the crawl space is controlling the subfloor MC....especially on the first floor.

Thanks.

Yes, the installer asked about ventilation for the foundation. I have over 800 sq inches of venting.

But we are going to add 5 additional vents to nearly double the vent opening area.

Another difference is the subfloor on the first floor is the 1959 original plank ad opposed to the plywood.

I went back and looked at my records of the readings. It looks like it has stabilized. The subfloor has consistently been at 13-15% and the hardwood has consistently been at 9% for the past 4 weeks according to his meter.

I am getting to the point of saying it's time to put it in.

You can never wait to long when acclimating.

I appreciate the sentiment. And it may be true in an ideal world. But I see no evidence that the moisture content is changing in either the floor or the subfloor after over seven weeks of acclimation, which tells me that it has reached its EMC, despite the difference in readings. And at the rate I'm paying for rent, the real world dictates that it's time to install the floor.

You can always figure how much the flooring will expand and use a washer placed between rows every so often according to your calculations to allow for future expansion of the wood as it goes through a one year cycle of changing conditions.According to the difference of moisture content between the flooring and the sub-floor, the flooring will expand after if has been laid.

Gotta remember all meters run plus or minus 2% Got a different meter around?
I use pin and pinless.

https://www.wagnermeters.com/pin-moisture-meter/