Lowering Crawlspace Humidity and Conditioning the Air
There has been a lot of discussion about conditioning the air in an encapsulated crawlspace. With the brutally hot and humid weather this summer, we have been inundated with people calling about crawlspace moisture problems and a lot of discussion about crawlspace moisture problem solutions. In the years we’ve been in the business, crawlspace encapsulation is the most practical and cost effective solution we’ve found. And doing it yourself can save thousands.
Crawlspace encapsulation sometimes seems counter-intuitive to me. You’ve got a moisture problem in your crawlspace so you need vents to let the moisture out. Right? As it turns out, most of the moisture in your crawlspace does NOT come from the soil or through the foundation walls. The moisture in your crawlspace comes from the outside through foundation vents and miniscule unseen openings like gaps between the foundation wall and sill plate and the seal (or lack thereof) around the crawlspace door. Those foundation vents that were designed to let moisture out of your crawlspace are actually the primary source of crawlspace moisture.
So the solution is to encapsulate (seal) the crawlspace. The International Building Code permits crawlspace encapsulation provided the crawlspace air is conditioned. How do you go about doing that? One solution is to add a dehumidifier. That can be expensive initially. The internet is full of crawlspace dehumidifiers costing a thousand dollars and more. These are fine machines and I am sure they do a fine job dehumidifying a crawlspace. But in addition to costing a lot initially, they also cost a lot to operate on a day to day basis – usually about the same as running a refrigerator.
So, what are the alternatives? We’ve found that the most cost effect means of dehumidifying and conditioning a crawlspace is to add a small baffle vent to the existing HVAC system. There are a couple of prerequisites. First, the HVAC air-handler needs to be located in the crawlspace or the HVAC has to be at least ducted through the crawlspace. Not sure? If your HVAC registers are located in the floor of the living space (as opposed to the ceiling), your ductwork is in the crawlspace. Your system also needs to have central air-conditioning.
Won’t condition the air cost a lot? Hardly. The EPA and the US Department of Energy recommend adding conditioned air at a rate of 1 CFM/per 50 SqFt of crawlspace area. That’s not much at all. A typical home with a well-designed HVAC system will blow approximately 80 to 120 CFMs into a small bedroom. Assuming that our hypothetical bedroom is in a hypothetical home with a 2,000 SqFt crawlspace, you will only be adding one-half to one-third of conditioned air to the crawlspace than you’d be adding to condition a typical small bedroom. That will add very little load to your system. We often recommend encapsulation and them monitoring the crawlspace humidity for a few seasons. Studies by various public utilities have shown that ductwork in most homes leak on the average of 30%. That’s more than enough conditioned air to add to the crawlspace.
If you’re going to take this route and add a baffle vent, it’s best to have it done by a licensed HVAC contractor. That will assure you get the recommended air flow and assure that the vent is added with the minimum of leakage. Remember that 55 to 60 percent or less relative humidity is the recommended level. Relative humidity above this level will encourage mold and mildew growth.