Moisture content for exterior doors

Determining the appropriate MC for lumber that will be exposed to vastly different humidity levels. November 14, 2001

How dry does wood need to be to build a door? It will be exposed to low %RH on the inside, and high %RH on the outside? I'm in western North Carolina.

Forum Responses
Get the lumber to 12 percent and then seal the outside of the door, including edges top and bottom. Now seal the inside.

Here is the problem: when the sun shines on the door on the outside, the EMC will drop to 2%. At night with fog or high RH, the EMC will be 25% or more. Typically, the EMC will average around 12%. Your customer will have better performance if there is a big overhang to protect the door from sun and rain. The inside might be more constant at 6% to 9% EMC.

The idea of sealing the door so that the MC cannot change is a creative thought, but we do not have any sealers that provide such protection. (Wax is perfect, but would not work for a door. Products called sealers do not really seal moisture. Three coats of spar varnish would probably be the best option.) Of course, there will be color issues as the wood fades and UV light changes the color. If you do seal it, seal the edges too.

Gene Wengert, forum technical advisor

So a final MC of 12% would be good?

That is too high for the interior side but is probably (depending on location, sun, etc.) okay for exterior.

Gene Wengert, forum technical advisor

What readers need to remember is that finishes like polyurethane may resist liquids, but these finishes do not retard the transmission of water vapor as a wax base product would do, such as Anchorseal.

Spar varnishes work better than floor or furniture polyurethanes for exterior because they are a bit softer than other varnishes and can shrink and swell with temperature changes that occur on exterior surfaces. The extreme temperature changes can make the harder finishes crack and fail sooner.

Seal the end grain with wax--Anchorseal (it's just latex-emulsified wax). If you seal the outside with a film, varnish, varathane, etc, it will either crack, curl or grow mold (discolor). Build a roof or covering over the door. Keep your exterior seal thin--let the door breathe. Don't put a film on it--a skin. Think of the door as a membrane between the interior and exterior--it's a skin, it's got to breathe. It folds every time it opens. Try an oil or sealer exterior that has a mold inhibitor.