In the course of building in northwest Washington for the last 15 years, we at The Artisans Group often come up on the discussion of moisture and mildew control. With the coming of our rainy season we thought it was a good time to revisit the building science of moisture and its consequence- mold, fungus, rust and decay.
There are three primary reasons homes have mildew and mold issues, all have to do with moisture. No water = no life. Listed in order of importance they are:
1.) Dew points, condensation planes and "moisture sandwich" wall assemblies. This is why the closet that is on a exterior corner of my old mid century modern house ruins all of my clothes. I breath, shower and cook making warm moist air inside the house, this migrates to the walls and condences on any cold surface. Because the corner is full of studs which are conducting coldness thrugh the wall, the moisture condenses and allows mildew to grow. Also, paint is not air tight so suffice it to say that the moist air is also condensing on the back of my wall sheathing during certain seasons, creating in wall mold also. Our houses are carefully designed to avoid any dew point issues by eliminating any cold spots and by creating open assemblies with optimally located condensation planes.
2.) Environmental moisture, this is most commonly found when siding is applied directly to sheathing, with no rain screen, and wind borne rain is forced behind the siding and then trapped there with no way to dry out. This is very typical in American homes, and although the detailing is more intense and time consuming we feel strongly, from a longevity standpoint, that a rain screen is necessary in our climate. Having your siding attached to a rain screen allows air to flow behind the siding, protecting it against decay and allowing the wall to dry out.
3.) Encapsulating moisture during construction. Here in rain-blessed Western Washington, we plan for it to be wet 3/4 of the time we build, since that loosely correlates to our rainy seasons. Honestly we even get deluges in the midst of summer... So, where in some climates it is possible to construct a house entirely in the “dry season” here this is near impossible. To mitigate the effects of moisture on the internal structure of our houses we select materials that will not be damaged by moisture during the construction process. Then, once the building is weathered-in, we carefully monitor moisture content to make sure that the bones of the building dry out before we install any insulation or interior finishes to the building. If necessary, we use industrial grade dehumidifiers to bring any excess internal moisture to the surface and remove it. If wet framing and sheathing is sealed into the wall by the interior finishes there is virtually no way for the moisture to escape. It will sit in there, molding your house from inside out.
By keeping a close watch on moisture throughout the entire building process we can keep that beautiful wet mossy-ness outside your house where it belongs.
Posted on October 17, 2014