First, it might be quite costly (several thousands of dollars). Second, and much more importantly, the sealing of a crawlspace is likely to degrade the IAQ (Indoor Air Quality) in your living space.
Rather than sealing a crawl space and creating an IAQ problem in the homes living space, there are less expensive methods to manage crawl space humidity. In the United States, the number of those with allergies is rising about 25% per decade, and asthma cases alone have doubled in the last 20 years.
The rise in those having allergies is more common in developed countries, and it is suggested there must be something in modern, urban life that promotes allergy. Some studies indicate a direct link to the increase in air conditioning and tighter homes. People may save a hundred dollars per year in energy, yet spend thousands on medication and life style change, so, there is no question that the homes IAQ is, by far, the overriding, single most important issue.
Proponents of sealing and not ventilating a crawl space have stated , venting a crawl space will either add moisture to, or remove moisture from, a crawlspace depending on the moisture content of the ventilation air compared to the desired conditions of the crawl space. Venting with dry air reduces the moisture levels in the crawl space, while venting with moist air contributes moisture. In this respect, such proponents are absolutely right-on correct!
So, even though it might cost a homeowner $6000-$8000, they suggest a closed, sealed crawl space because the possibility of venting with moist air can make the crawl space wetter.
An optimum alternative that might achieve the best outcome could be to ventilate the crawl space but with a ventilation strategy that would know how much water vapor is in the outdoor air and know how much water vapor is in the crawl space air. Armed with that knowledge the ventilator will be able to make an informed decision whether or not ventilation is going to be helpful or hurtful.
It is evident a strategy like this would permit a crawl space to be ventilated, to dilute and replace stagnant air, odors and gases such as radon, yet not bring in wetter air into the crawl space. A strategy like this will be more effective than other ventilation strategies (like a timer or dehumidistat) because it is selective about not bringing in wetter outdoor air, whereas the timer or dehumidistat is not selective.
Returning to the question of sealing the crawl space and using a dehumidifier to control moisture, there is no question that sealing a crawl space and using a sizable dehumidifier will reduce humidity in the crawl space, but, if sealing the crawl space can compromise the homes IAQ, you ought to reconsider.
Heres why.
1. Even if you are able to seal the crawl space well, you will need a sizable dehumidifier. It will need to be set well below a mold threshold limit because other surfaces may be colder. The lower setting is needed to prevent those colder surfaces from exceeding the mold threshold in the now stagnant crawl space.
2. A dehumidifier mentioned in article requires 30 times the power of other technologies.
3. A dehumidifier will need occasional maintenance.
4. When sealing walls and piers, how do you seal completely without creating a hidden path for termites? It means, of course, you cant completely seal it.
5: The crawl space has become a stagnant area, which, of course, is never cleaned, so odors and gases will be there and they will migrate into the living space of the home.
6: Under the vapor barrier it is always 100% relative humidity, so mold and bacteria is going to grow. And the mold and bacteria spores will migrate into the home.
With the home IAQ in mind, it becomes apparent that some sort of ventilation is needed in a crawl space. The ICC code for the usual passive ventilation setup required one sqft of opening for each 150 sqft of crawl space. Further, the ICC code stated that if a vapor barrier was installed over the ground surface only 10% of the venting area was required (ICC 1804.6.3.1). This would mean that only 1 sqft of opening would be needed for a 1500sqft of crawl space. Clearly this code wasnt working, so the ventilation rate was arbitrarily increased, and, now, today some state codes recognize the strategy of comparing inside/outside water vapor.
So, with IAQ being so important, we can define our goal in a crawl space project: remove/replace stagnant crawl space air and extract moisture from the wood mass in the crawl space. This makes a case for mechanical equipment.
Dilution and replacement of crawl space air with fresh, outdoor air is easily understood, but how do you extract moisture from the wood mass in the crawl space? You must reduce the vapor pressure at the surface of the wood, and you may do this by either of two ways. One is by dehumidification, another is to blow air across the woods surface, better yet, blow drier air across the woods surface.
So lets get down to the quick of things. From the homes IAQ perspective you should ventilate your crawl space, but using a dehumidifier and venting would be like setting a dehumidifier out on your front lawn, and then paying the electric bills.
A better system would use an intelligent mechanical ventilation system that, (1) keeps the crawl space closed during unfavorable conditions, (2) opens whenever the outdoor air water vapor content will lower humidity in the crawl space, (3) moves a lot of the lower humidity air to extract moisture from the wood mass. The result is, that as you are diluting & replacing stagnant odors & gases, and radon in the crawl space with fresh, outdoor air, you are also extracting moisture from the wood mass.
Further, an intelligent system might perform other functions, such as to not ventilate when its freezing outside, and, if the crawl space gets too dry, the ventilator could also automatically reverse its strategy to pump moisture into the crawl space to further stabilize wood floors by maintaining the wood in an acceptable range.
There is one inexpensive, automatic ventilator with all the characteristics described above. Its called the Smartvent.
Author has 20 years as GM with four manufacturing companies, 35 years in machine controls, 15 years of which are HVAC controls with special emphasis in humidity control.
Framing 2 X 4 Skylight Well
