Raining and 35°—-why shouldn’t I cover my crawl space vents in the winter?

It is OK to leave them open—that is how it works in the Northwest.

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.

humidity1Lets 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.

humidity3There 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


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Whole house ventilation

As homes have gotten tighter and tighter to conserve energy, we have created indoor air quality issues.  Stale air can carry all kinds of particulate as well as allow for elevated carbon dioxide levels.  Furnishings, pets, molds, dust mites and hundreds of other elements in the home can make for poor indoor air quality and affect the way we breathe and thus our over all health.

This post is about whole house ventilation and how we use it to improve indoor air quality in new and remodeled homes—often required in many jurisdictions and certainly in Washington State.

In some homes the whole house ventilation is achieved by integrating it with the furnace operation.  With this approach air is pulled into the duct work by the furnace fan and distributed to the home through the duct-work the same way heat is distributed.  This is a good way to do it because it results in fresh air being distributed evenly throughout the home.

Other homes have exhaust fans (perhaps laundry or bathroom exhaust fans) that pull the air into the home.  In these homes there may be little vents in the window units that create an actual opening in the wall to pull air in.   This picture shows the air intake at the top of the outside of the window.

Window air intake

Window Air Intake

There are other means of doing this as well, including HRV’s (Heat Recovery Ventilation units).  These units have the ability to pull fresh air into the home while the outgoing air passes over the incoming air.  The heat of the outgoing air transfers to the cooler incoming air reducing heat loss dramatically.  These units are very effective and even required in some jurisdictions.

But back to this story.

I did a one year warranty inspection next door to a home that I had done a purchase inspection on.  On the home of the one year warranty inspection I found the same defect that was on the other house—and the whole development may have been this way.

This first picture shows the duct-work of the air intake coming off the side of the furnace return air duct and the location of the electronic damper that shuts off the duct-work when the furnace is in heating mode.

Air intake damper

Air Intake Damper

This next picture shows the intake pipe termination at the roof.  What is wrong in the picture?  Note that the cap has a back draft damper in it.

Air intake

Air Intake With Damper Installed

This is not the correct cap.  When the furnace is trying to pull in fresh air it is just pulling the damper tighter—-no fresh air is drawn in.  A simple enough fix—-but not functional the way that it is.

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