I know that it may seem counter-intuitive, but you can actually lower the humidity in your crawl space by bringing in that wet air in the winter.  I will attempt to keep it as simple as possible by speaking in “generalities” as opposed to specific temperatures and humidity levels.

Lets take air that is very cold and saturated (as in raining), and bring that air into the crawl space that is warmer than outdoors.  When that cold wet air is heated to the temperature of the crawl space, not only can it easily hold the moisture in that air (and not rain in your crawl space) but it will mix with the warm humid air of the crawl space resulting in a net lower humidity of all the air in the crawl space.  As that air then moves out of the crawl space the humidity of the crawl space will thus be lowered.

Temperatures in the winter in the NW are generally favorable for this to happen and reverse slightly in the summer.  In this sense, general humidity and moisture levels can be expected to rise in the summer and then drop back down in the winter.  Of course without proper venting the moisture levels can build to far above acceptable levels, resulting in wood decay/rot and infestations of Anobiid Beetles etc.  And that, in a nut shell, is why it is a bad idea to cover those vents in the winter.

This same principle can happen in the indoor environment if we bring too much of that wet cold air into our very warm indoor environment and result in humidity levels inside the home becoming lower than desired.  This can be especially pronounced in two or three story homes where stack effect, induced by pressure/temperature differentials as well as by winds blowing by the house, come into play.  A simple way to think about this is that the house is having the air sucked out of it at the top and sucked in at the bottom (your house is acting a bit like a chimney).

This can be especially problematic with houses that have window inlets as part of whole house air ventilation systems. 

These air intakes at the windows are one of the ways to meet modern energy code requirements to bring fresh air into the home.  The problem with them is that in multi-story houses, they do not just bring fresh air into the home whenever an exhaust fan is running, they also continually vent outward at the top and inward at the bottom (due to stack effect) when fans are not running (see drawing that shows window vents at “1” and “2”).  This creates WAY more air changes in the home than necessary, wasting considerable energy, and it can lower the humidity in the home by the same principles that reduce humidity in the crawl space.

There are other factors that can contribute to loss of control of house humidity due to stack effect.  Openings anywhere in the building envelope can result in air moving in and out of the building where we don’t want it to.  This is why it is so important to keep the “Dotted Line” (see drawing) as perfectly sealed as possible so that we are not pulling crawl space air into the home.  In many homes a very large percentage of air that is being drawn into the home can come from the crawl space.  In the drawing we can see that penetrations like plumbing pipes (marked “V”) are sources of such leaks—but “V” should also be seen as symbolizing b-vents, chimneys, wiring holes etc. 

Inadequately sealed attic hatches and crawl space hatches can represent very large breaches in the building envelope. 

Obviously exhaust fans can represent breaches—but they are designed to be breaches.  Unfortunately, even when the fans are not running they can still have inadequate dampers and leak around the fan housings themselves.  Of course running them 24/7 would result in pulling all that cold/wet air into the home around the clock.

Can-lights or any electrical junction boxes in the building envelope—especially at the attic level—can represent significant breaches of the building envelope as well. 

While right now we are primarily discussing the effect of bringing cold/wet air indoors, we can obviously create problems for the home if this heated/moist air can find its way, or is vented directly into the cold attic space.

A real-life story of how this works happened on a recent inspection.  My client complained that he was not able to keep the humidity in the home high enough in the winter to protect some very expensive guitars.  I suggested that he correct the missing weather-stripping on the attic access hatch and close the upper window air intakes.  Humidity levels quickly returned to normal levels and the guitars are now happy.

Charles Buell, Real Estate Inspections in Seattle