mdf swelling - no moisture

john77777March 17, 2008

I was reading another thread from a while back:


and became concerned about mdf swelling in a high summer heat environment. I'm not worried about moisture swelling, because I think the fact that the mdf I'm talking about is indoors (in a bedroom) would mean that the relative humidity is never outside the range which one might consider "normal".

But there are at least two weeks during the year when the outside temperature will reach or approach 110 degrees and most of the summer will be 95 degrees or so in the middle of the day.

I have a wall against which I built a desk. The wall is 12 feet (I mean 12.00000 feet). The mdf, in three pieces, spans that wall and is very, very snug (like 11.999999 feet). As mentioned above, there are three pieces.

According to one of the messages posted in the thread identified above (by jon1270) mdf will swell as much as 1/4 of 1%. That means about 3/8th of an inch in the case of my 12 feet of mdf.

Should I just take the middle piece and shave off 3/8th of an inch now to avoid buckling and just live with the gaps that are created by doing so (the corners can't be moved at this point)? Or does the fact that each piece has been very well sealed (sanding sealer all around and primer and paint on the top) mean that the mdf will not swell as jon1270 indicated?

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I do not work with MDF but I do work with hard wood and vaneer plywood. It is important to allow for expansion and contraction of any material. You do have yourself in a corner, pardon the pun. I would consider a way to allow for possible expansion.

    Bookmark   March 19, 2008 at 8:53AM
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Keep in mind that the 1/4 of 1% is based on a panel completely acclimating to a rather large change in relative humidity. How much does RH inside your home vary during the year? Do you have (and use) central air? Does your winter heating system include a humidifier?

The document I pulled those numbers from describes the behavior of unfinished materials in a controlled lab environment, not finished materials in a normal domestic environment. If your MDF desk is painted or laminated with something moisture-resistant then is will be very slow to acclimate to changing humidity levels; annual fluctuations will probably produce only a fraction of that 3/8" of movement. I'm not sure I'd worry too much unless you see a problem developing.

    Bookmark   March 19, 2008 at 4:44PM
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I've never had a problem with MDF movement causing buckling of any sort, even in unconditioned houses where humidity levels rise and fall more than in a conditioned house.

How is this MDF finished? Any finish at all? Primer/paint/poly/oil/latex?

I wouldn't touch it until it showed you that it needs to be touched.


    Bookmark   March 20, 2008 at 5:52PM
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Thanks for the input, folks. Appreciated. Yes, the house is air conditioned. I can't imagine that the inside relative humidity changes all that much while the house is closed up. However, we do like to open the windows in the summer at night to cool down the house and the air can be pretty heavy with moisture when that happens, especially just before dawn.

Given that it has been sealed thoroughly and then also primed and painted on top (and the sides, although that isn't much surface area), I guess I'll let you know at the end of the summer whether any problems developed.

    Bookmark   March 20, 2008 at 5:59PM
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