bump

Thanks for your comments! I mistyped. No room heaters; the whole house furnace was running almost the whole time that the planks were acclimating and is running now. (fwiw we found strategic placement of the dehumidifier near the few spots that were 19 brought them down to 17 and then furnace brought it down further.) So the dehumidifier kick-started the process and the furnace ran after that. Fwiw, my readings of 15 were on the day the dehumidifier was removed, and we had 5 more days of the furnace running after that.

I thought the furnace was to run at our normal temps (we never set it to 72)—it’s been set in the high 60s, which means when I check sometimes it’s 70 or 71. The flooring guy said the MC of the planks was about 9%. He said his experience has shown that the best thing is to have the planks acclimate a good long time, installed, before finishing. His opinion is that the planks will absorb more moisture installed than sitting in a pile. Thoughts? thanks.

MC in a subfloor should not exceed 12%. Flooring is delivered between 6-9%. The flooring should never have been installed on top of wet subfloor. Unless, a roll on moisture retarder was applied. Only then you can get away with installation over a high MC rate. Roll- ons have a rating of 15 lb and 85% humidity Resistance.

I am afraid your gapping is only beginning. It’s going to get much worst. , once it reaches equilibrium.

I know he laid something in between. What do you do when 80 yr old subfloor (not new, not wet from the lumberyard) is at 14% MC?. Since it’s been on site 80 years isn’t it likely to stay at that moisture content? Does adding insulation under the floor make any difference?

Use Aquabar underlayment for homes with plank subfloors. What type of subfloor for the new section?

I'm going to guess the area underneath the 80 year old subfloor is 'damp'. Either a damp crawl space or a damp basement. If it's a basement then a dehumidifier running 24/7 for the rest of you home's life (30-40 years?) will fix it. A crawl space is much more difficult/expensive to deal with.

Thanks. No the area in my crawlspace is perfectly dry. His opinion was that if I had a dampness issues, the MC of the original subfloor would be higher.

The gaps in the images are from millwork. Your flooring was installed over a wet subfloor. It will expand, not shrink. Hopefully, there is sufficient expansion gaps along the perimeters to prevent buckling. You will notice a lot of over and under wood or slight cupping. When it finally dries out, your floor will be loose and squeaky.

Thanks. Yeah that what I thought—if the subfloor was too wet the planks would cup and then get sanded down.

Sounds like I should have take moisture readings closer to the installation day. I had spent Fri night-Tues (2 weeks ago) moving the dehumidifier around in rooms with new subfloor, concentrating on the few spots with 19. what I saw was 19-> 17 and then with the furnace running it came down to 15 within a day. After the dehumidifier was removed, the furnace ran another 6 days. Any guesses as to how much the MC might drop in those 6 extra days? Thanks

1-2%

The individual who needs to verify that the moisture content of the hardwood strip flooring and the subfloor meet the manufacturer's installation guidelines is the installer--not the homeowner.

While the interior environment in your home may be controlled, it sounds like you have a vented crawl space. The conditions (temperature and relative humidity) in a vented crawl space can vary depending on the exterior temperature and relative humidity. The equilibrium moisture content (EMC) of your subfloor (and to some extent your hardwood flooring) will also vary as a function of those conditions. The USDA's Forest Products Lab has compiled EMC data by month for various locations in the U.S.:

https://www.woodweb.com/knowledge_base_images/zp/equilibrium_moisture_content.pdf

If you look at the data for Fresno, CA you'll see that the EMC varies by month from a low of 7.8% to a high of 16.6% Trying to get the EMC of your subfloor below the EMC for the given exterior conditions wouldn't do much good if the crawl space is vented.

The best case for stable hardwood floors is to maintain relatively constant conditions throughout the year. To this end, I suggest you consult with a local building scientist about the benefits and cost of converting your vented crawl space to a sealed, conditioned crawl space. By keeping the conditions in the crawl closer to those of the conditioned living space, you should have less seasonal movement due to changes in relative humidity.

Thank you for that link. I’ve only had a chance to skim it but it’s very interesting. Of the towns listed, my town’s climate is closest to SF, with EMC that varies between 13.4 and 15.2. So not the big swings seen elsewhere. For March, it’s 14, the same reading as the subfloor in the original part of the house.

I’m reluctant to convert this to a sealed crawlspace. The house has survived 80 years without it. (and I had Doug Fir floors in the original part that were doing fine, albeit as beat up as you might guess for 80 year old Doug Fir floors)

I’m getting a bid for insulation under the floor. My architect said it wasn’t needed (was not part of the original plan) but said if the bid isn’t too high might be worth it.

thanks for your comments and thoughts.

Your climate may be mild enough not to warrant insulation under the floor or on the walls of a conditioned crawl space, either. If that's the case, I'd recommend you forgo insulation and spend the money to seal up the vents and install a fan to introduce a continuous flow of conditioned air from the living space at a rate of 1 cfm/50 SF or more.

I would expect the original hardwood floors in your home to behave differently (i.e., to be more stable) than wood harvested for hardwood floors today.