New poster here hoping for some help from the experts, because my maple flooring is not behaving like it's apparently supposed to..

I am installing ~ 600 sq of 3 1/4" Muskoka 3/4" maple in my Southern California living room. First floor installation on plywood subfloor over ventilated crawlspace. I'll put down some Aquabar B first.

The flooring arrived from the Muskoka at < 6% MC (the lowest my moisture meter reads). The subfloor is currently around 8-9% MC, depending on where I measure. I have been acclimating the flooring for a couple of weeks now to get it to within 2% of the subfloor, as recommended. So far the flooring is still below 6%, so I am waiting.

Now, I have seen charts showing that for my coastal area, the annual indoor EMC can be expected to vary from 8 to 13%, and I've read that I should aim for installation MC to be somewhere in the middle of that expected range, i.e. around 9% MC. Short of bringing in some serious humidification into the room where the flooring is currently acclimating, I am afraid that it'll take me till summer to get anywhere near that 9% MC point.

So I started estimating how much my flooring might expand, if I installed it now (at 6% MC) and then have the EMC rise to 9% or more during the summer. Using the literature I have seen, it suggests that with an expansion coefficient of 0.0033 for maple, a 3.25" board should expand about .0033 * 3.25" = 0.0107" per 1% MC change, or .032" for 3% MC change. Given that I have a width of about 100 boards, that would imply an expansion of over 3 inches for the entire room! Good luck leaving expansion space for that!

So I decided to run some experiments. I took some floor boards to the basement area, where the humidity was around 60% (compared to indoor humidity of 40%), and the resulting EMC was 12%. I measured the initial width of the samples at 6% MC, and recorded it. After a week, the MC of the samples has risen by 2%, to 8%, which means that their width should have changed by 0.020" or so. However, the measured change in width was no more that 0.002-0.004", so about only 1/10th of the predicted growth.

I then placed some samples into a home-made greenhouse, a plastic container with some water at the bottom, with the board elevated above the water. I placed this in the sun, and within a few days, the board had reached a MC of 12-13%. This should have made the board grow by 0.060". However, the measured change in width was again only about 0.002".

So what is going on here? Why is my maple not expanding like it is supposed to with the MC change? And can I put my maple down now without waiting for it to acclimate to 8-9%, if it's going to expand by only 1/10th of the predicted numbers?

Any thoughts would be appreciated.

Maybe I can try to answer my own question... The flooring is prefinished, and I have been making the MC measurements on the back side (unfinished), while taking the width measurements on the other (finished) side. So perhaps this accounts for the difference, if the prefinished side is at a lower MC than the unfinished side. I'll make some MC measurements on the finished side tonight and see if that explains what I am seeing.

Sounds like your having fun experimenting.

So your indoor rh is 40% right now? And it gets up to 60% in the summer? Don't you have an AC unit? Thats awful high humidity for in a house isin't it?

It's sort of fun for now, but the goal is to decide how to proceed, and I don't see the flooring getting to 9% MC any time soon . So if I can convince myself that it won't expand as much as the books suggest it will, then I can put the floor down. If not, then I'll need a Plan B.



Yup, currently it's around 40% indoors. I don't know what it gets to in the summer time, I am just going by those Forestry Dept. charts that show annual EMC in different areas. In our area, the climate is very mild and moderate (60-80F from coldest to hottest), but the humidity can change from 10% when we get the desert winds to 80%'s when we have the coastal fog. We get coastal fog a lot in the summertime, so I can envision 60% indoor humidity, since the actual temperature will not be high enough to require use of AC. Does that make sense?

It sounds like you're using a pinless meter. If so I would try to get my hands on a pin meter and see what the MC is in the center of the board.

I'm using the Lignomat Moisture Meter Mini-Ligno S/D http://www.amazon.com/gp/product/B000VIMGIA/ref=noref?ie=UTF8&s=hi&psc=1, which has pins. I try to stick the pins in as far as they'll go, though they don't reach the center of the board; maybe about 1/8" deep at most.