Q: We have some windows that are about five feet high that have been installed in an apartment building. After construction was finished this fall, the heat was turned on and within a week or two we noticed that the window frames were warping, lengthwise, with the middle of the window bowing outwards. In fact, now after a month, we see a few cases where the warp has caused the window to crack. It would seem that the dry outside in Minnesota at this time of year would not be that much different from the inside, which does not have humidity control. What is happening?

A: The key rule or concept is that wood does not change its size or shape over a few hours, weeks, or months unless its moisture is changing. So, we know, assuming that the initial MC of the windows, throughout, inside and outside, at the time of manufacturing was uniform, that now the MC has changed. As surprising as it may seem to you, in Minnesota the outside humidity is rather uniform throughout the year, 65 percent RH is the daily average. This is equivalent to 12 percent MC in wood. In the wood business, we call this 65 percent RH condition (temperature is not a big factor) 12 percent EMC. So, the outside of the windows is now at 12 percent MC, even if painted.

The manufacturing conditions inside a wood manufacturing plant would oftentimes be 9 percent in the summertime in Minnesota, so it is likely that the windows were made at 9 percent MC. (Special note: Because MC is so important, it would be a good idea for a manufacturer and even an installer to check the MC of incoming wood supplies and finished products to assure that everything is at the correct moisture level.) Of course, the key is that the architect is the person who needs to specify what the correct MC level is for a given construction project.

Finally, during construction, with the moisture being released from paint, curing cement, drywall mudding, and so on, the humidity can be rather high (Some builders will turn on the heat to hasten the drying, and once the drying is complete, this high heat will assure a very low inside humidity, which means low wood MC.), once normal living conditions are obtained, the interior wintertime humidity will be 30 percent RH or lower unless the air is humidified artificially. This means that wood on the inside of the structure during the wintertime will be at 6 percent MC.

So, the inside of the window frame is at 6 percent MC and the outside is 12 percent MC. Differences in moisture mean differences in size; the inside, being drier will shrink and the outside, being wetter, will swell.

The solution in your case with such a large window is to use some sort of stiffening rod in the window to prevent the window from warping. The outside environment is something you cannot change, but the building designer would be well advised to humidify the interior to avoid extremely dry conditions (under 30 percent RH). (Of course, humidification means condensation on the windows and, if vapor barriers do not exist, within the walls as well, so some designers do not like humidification.)

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