Dryer or wetter engineered hardwood planks for install into intermittently conditioned space?

Question

Engineered hardwood planks: 3/4 thick T&G white oak, 8" wide, 5mm thick top wood layer.

Subfloor is out of the equation: we're using a 100% moisture barrier underneath.

So our seasonal changes in condition will be purely from the air in the room.

This is a closed, finished, nicely built indoor space - tight, but not super tightly sealed.

BUT there is no HVAC.

We are in the bay area so we could have a foggy summer evening when someone leaves a window open to 100% humidity OR during the winter somebody runs a dehumidifier for a week straight.

The big issue is we cannot depend on any particular climate control. Someday somebody will leave the windows open.

Right now the planks measure out at around 9.5% which the floorers say is on the "high end of acceptable". They think I should dehumidify it down to around 8%. BUT if we do that we're installing it "dry" in a space where it's probably going to get "wetter". That sounds like a bad plan to me.

I think we should install it "wet" at the 9.5% number - then we are worried about somebody accidentally over-drying the space which is less common than somebody leaves all the windows open to the fog ... we're also going toward gaps instead of cupping. I don't want either but I would choose gaps over cupping ... I think ?

Am I making a good call to install slightly "wetter" into this barely-climate-controlled space ?

Answer 1

Since it is an engineered wood product, the movement between 8% and 9.5%, in my thought would be negligible. Take into consideration plywood, it is 6X wider than your floor and only needs an 1/8 gap, if that. Another personal experience I had was to measure the moisture content (MC) of the framing of a storage building in a lumber yard that had framing in an unconditioned covered area. I was expecting to be be around 10-12% MC, much to my surprise it was 8 1/2%. The building has been there in operation for decades. The wood I tested was a 2X4 guardrail of a loft storage area in the second level of the building. Not heated or cooled, same situation you will have.

With all that said, I would go with the dryer side, or at least let it acclimate for 2 weeks bundled or 1 week with the packages opened up. 9.5% may shrink with absolutely no consequence, but if the MC grew a little, I think it would "snug up" real nice. Do always keep expansion gaps around all walls ect., I believe it is required for all wood floors.

Answer 2

Bumbling through the literature, the 8% and 9.5% sound way off. At the minimum, I would use a moisture meter to test wood surfaces inside this area to confirm that the 8% or 9.5% align somewhat with current conditions.

Here's somebody who has correlated monthly temperatures with monthly relative humidity values to get month-to-month equilibrium moisture content values for San Francisco among other places: https://sites.cnr.ncsu.edu/wpe/wp-content/uploads/sites/35/2016/08/Equilibrium-Moisture-Content-of-Exterior-Wood-in-the-US.pdf. These numbers reconcile nicely with my wider range based on the FPL Wood Handbook's Table 4-2. According to the NCSU document, San Francisco's exterior equilibrium moisture content ranges between 14.4% and 16.5%.

I feel that monthly averages are appropriate because the 3/4" thickness will smooth out moisture content variability in comparison to noise in relative humidity. I further anticipate that an enclosed space like yours will somewhat smooth the relative humidity noise and therefore the moisture content variability. Between these mechanisms and a surface finish of some sort on your floor, your 3 nights of 100% humidity don't seem worth much worrying.

To rationalize your 8% and 9.5%, I looked up the NWFA guidelines and found that their specs are premised on an indoor relative humidity between 30% and 50%, where they seem to push responsibility for achieving this range onto the homeowner. There could be some mechanism for lower indoor relative humidity regardless of temperature and humidity controls. I can't imagine what it would be. Looking up the equilibrium moisture content values for the 30% to 50% and for temperatures between 60°F and 80°F, I see a range from 6.1% to 9.5%. The contractor's 8% target sounds like they're splitting the difference between these two values (or assiduously following manufacturer recommendations to minimize liability).

The NWFA guidelines contain a map that lists a range of moisture content values between 8% and 13% for coastal Oregon and California. I take this as confirmation that I was right all along :-). Seriously, though, I'm just bumbling through the literature here. If I were you, I would at least use a moisture meter to test the moisture content of wood surfaces inside this space to verify that your target moisture content is reasonably close. I'm not very confident that a surface reading from your wood flooring accurately reflects the wood's through-thickness moisture content, however. When it has been in a controlled environment for a few days without the moisture content readings moving much over those few days, then I would perceive the reading as potentially reliable.