Hardwood tumescence: flooring issues and seasonal RH changes.
April 11, 2013 12:43 PM Subscribe
I have plans install a hardwood floor during a time of year when my house will be at its driest. Before I nail down these boards, I want to allow for seasonal changes in relative humidity so that the floor doesn't swell up and self-destruct come summer. On the other hand, I want to minimize gaps. How can I best guesstimate summer conditions, specifically indoor relative humidity?
Further details: I live in Anchorage Alaska. We've been experiencing late winter conditions, and indoors it's as dry as it's ever going to be (25% RH). The government tells me to expect summer outdoors relative humidity to range, roughly, between 60% (mornings) and 80% (afternoons) during the three or four months of the year when I will not be running heat or a/c. Should I space my boards and plan expansion gaps assuming a maximum mean 70% RH, indoors?
Of course every home is different, but my house, a ranch with a crawl space, seems very dry up top and down below (I've only lived here a few months). The house is NOT very airtight judging by past heating bills. Can I reasonably expect indoors conditions to track outdoor seasonal changes? Or should I just forget about it and install a good dehumidifier and set it to 50% RH?
It may be also be relevant to mention that I've perhaps unwisely accepted delivery of 1400 sq. ft. of 3 1/4 in. flat-sawn hickory. Hickory, I've learned, has a high expansion coefficient.
Further details: I live in Anchorage Alaska. We've been experiencing late winter conditions, and indoors it's as dry as it's ever going to be (25% RH). The government tells me to expect summer outdoors relative humidity to range, roughly, between 60% (mornings) and 80% (afternoons) during the three or four months of the year when I will not be running heat or a/c. Should I space my boards and plan expansion gaps assuming a maximum mean 70% RH, indoors?
Of course every home is different, but my house, a ranch with a crawl space, seems very dry up top and down below (I've only lived here a few months). The house is NOT very airtight judging by past heating bills. Can I reasonably expect indoors conditions to track outdoor seasonal changes? Or should I just forget about it and install a good dehumidifier and set it to 50% RH?
It may be also be relevant to mention that I've perhaps unwisely accepted delivery of 1400 sq. ft. of 3 1/4 in. flat-sawn hickory. Hickory, I've learned, has a high expansion coefficient.
Best answer: This depends on what the condition of the wood is in. If the wood is somewhat wet (relative to your house), installing them as is should probably be fine. If the wood is bone dry, like it was stored in an unconditioned space, then you probably should calculate some kind of gap.
It also depends on how it is being installed. If it is traditional tongue and groove, nailed at every course, then you have to be careful. But if it is a newer engineered wood that floats, you should be fine installing it with the recommended edge gaps.
The indoor space can be drier than the outdoors because you are taking 20 degree air with a RH of 50%, and warming it up to 70 degrees without adding any water. This will make it want to suck moisture out of everything.
If it was me, I'd get a decent humidity gauge and gather some data. You know that air conditioned space is going to be drier than the outdoors, so your high limit is probably 70 or 75% humidity. I know that's when I start feeling uncomfortable. And your lower limit is whatever your house is right now.
If you get the wood indoors and up to room temperature, and start running humidifiers for a week or so to get the RH up to 60 or so, I would think that would be the best way to go about installing it. The wood will be close to its largest, and there shouldn't be all that much shrink as the RH goes down.
Maybe install a vapor barrier underneath, too. The finish will be a vapor barrier of sorts on the top, but if the bottom sucks up more moisture, you might get cupping.
posted by gjc at 5:47 PM on April 11, 2013
It also depends on how it is being installed. If it is traditional tongue and groove, nailed at every course, then you have to be careful. But if it is a newer engineered wood that floats, you should be fine installing it with the recommended edge gaps.
The indoor space can be drier than the outdoors because you are taking 20 degree air with a RH of 50%, and warming it up to 70 degrees without adding any water. This will make it want to suck moisture out of everything.
If it was me, I'd get a decent humidity gauge and gather some data. You know that air conditioned space is going to be drier than the outdoors, so your high limit is probably 70 or 75% humidity. I know that's when I start feeling uncomfortable. And your lower limit is whatever your house is right now.
If you get the wood indoors and up to room temperature, and start running humidifiers for a week or so to get the RH up to 60 or so, I would think that would be the best way to go about installing it. The wood will be close to its largest, and there shouldn't be all that much shrink as the RH goes down.
Maybe install a vapor barrier underneath, too. The finish will be a vapor barrier of sorts on the top, but if the bottom sucks up more moisture, you might get cupping.
posted by gjc at 5:47 PM on April 11, 2013
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posted by axismundi at 3:44 PM on April 11, 2013