The debate over open and closed crawl space systems.

Whether or not you should open or close the vents on your crawl space has been argued back and forth for years. I have spent the last few weeks pouring over blog posts, articles and scientific publishing’s trying to make sense of this paradox to some avail. For the sake of the lay reader or DIY folks I’d like to start with a brief tutorial on the basics of crawl space moisture science or Psychrometrics.

Relative humidity is the measure, in percentage, of the airs potential to hold moisture. This means, if the air can hold 20 grams of water total but currently holds 10 grams, then the relative humidity is 50%. This is due to the air only holding half of its potential moisture. Absolute humidity is the actual measure of water in a specific amount of air. Generally, absolute humidity is measured in grains of moisture per pound of air, or GPP. The higher the temperature of the air the higher its potential to hold moisture, or, warm air holds more water than cold air. This is most easily explained by chapped skin in the winter months when its cold and the air is dry.

Moisture moves toward warm air. For every ten degrees the temperature is rises the airs potential to hold moisture is doubled. This means if air at 70degrees holds 30gpp at 50%RH then air at 80 degrees will hold 60gpp at the same 50%RH. This is very important when knowing when to vent your home to the outside air and when to leave the windows or crawl space vents closed.

OK, open system or leaving your crawl space open to the outside air. A very large portion of the homes in our southern IL area have this type of system. We open the vents in the spring to dry out the “hole” and close them up in the winter to keep the pipes from freezing. Problem is, when we open the vents in the spring the humidity outside is generally much higher than the level in the crawl space and we introduce large amounts of moisture to a cool, dark, still are of our homes. This can be devastating to the health of our homes due to potential microbial growth and wood rot. Now, don’t let it be said that having foundation vents doesn’t have it benefits. There are several times a year when it can be very beneficial to open these vents. For this reason I am a proponent of humidistat controlled powered closing vents. They are attached to a humidistat and open on the days when the humidity is less outside than inside and introduce dry fresh air in to the “hole”. Open systems just like closed or encapsulated systems need to have a good 6mil or thicker vapor barrier installed not less than 6 inches up the foundation walls, piers and should be overlapped not less than 12 inches at the seams. The use of a dehumidifier on an open system is not recommended as you would not be able to keep up with the outside air constantly adding moisture to the crawl space area.

Closed systems consist of a thick 12mil or greater vapor barrier being applied up the foundation wall to within inches of the top as well as covering the piers. The vents will either be removed or permanently sealed so that the crawl space is now completely separated from the outside air. Often there is conditioned or process air pumped into the crawl space from the hvac system. When this is done there is positive pressure put on the crawl space and there should be a vent for either return air or shared air with the living area of the home. Another option is to have a dehumidifier installed to condition the air and maintain healthy RH. The closed system is very effective and a good option for those who can afford to have it done. A few of the draw backs that concern me are the air in the crawl space is shared with the living are of the home when utilizing the hvac unit to control crawl space air. Not having the ability to exchange air when using a dehumidifier can cause stale air and promote some odors. If, there should be a water intrusion from the living area of the home such as a leaky toilet drain or broken ice maker line the crawl space would be turned into a not so nice pond and could fill to the bottom of the floor joist damaging not only the floor system but potentially causing damage to the foundation from the weight of the water. Just to be clear, I have worked dozens water losses where the volume of water that ran through the home exceeded ten thousand gallons.

When it comes to insulating your crawl space, my only recommendation is to use 4 inches of foam board insulation on the foundation wall leaving approximately 2 inches exposed at the top of the wall for termite inspection. All areas that open into the home or outside should be sealed using spray foam insulation. Use no less than one inch foam board glued or calked into the cavities of the box sill. Leave the floor system exposed so that it can be seen for proper maintenance and good ventilation can occur. Question, if you have spray foamed the floor system and have a water loss from the living area affecting the floor, how do you dry the floor decking and joist that have been encapsulated in spray foam? I’m not real sure either except to scrape away the spray foam and that sounds like an extremely expensive job.

As is often the case when there are two opposing points of view on a topic the answer can be found somewhere in between. I call it a hybrid system, one that uses the best of both, like what my parents did with me. If we use the information from the beginning of this article we know that the air outside can be used at the right times to help not only dry but freshen the area of our crawl space using a controlled vent system. When used in conjunction with a dehumidifier these controlled vents would stay closed on the days where the humidity is higher than that of the crawlspace and opened on the days where it is less, allowing for less wear and tear of the dehumidifier and less electricity usage as well. Allowing for an air exchange to take place. When the winter months approach the vents would be sealed with foam board insulation from the inside and while there the crawl space can be inspected for plumbing leaks and the dehumidifier serviced. The one thing that is consistent with both systems is a good quality vapor barrier, run up the foundation walls and piers at least six inches. Thank you for taking time to read this!
Yours In Service,

Eric Speakman
Leading Edge Restoration