To Vent or Not To Vent

by Dave on August 22, 2009

by Matthew Peterson

By far, the most common type of foundation in the mid-Atlantic region is the crawl space. For a number of reasons, this is a functional cost effective construction method. For those of us new to the idea of a crawl space, here is a brief explanation: the floor joists of the home sits on a concrete block or poured concrete foundation wall that lifts the home off the ground anywhere between eighteen inches and two feet above ground over the soil below. Standard construction practices dictate placing vents in the foundation wall to allow air to flow through this space. Conventional thinking is that it will help reduce the moisture build up from the earth below. In order to insulate this space, the sub-floor would then have fiberglass batt insulation stapled to it to regulate the air flow between the crawl and the home. This is basically an ‘unconditioned’ crawl space.

It has been found that in a typical crawlspace two things occur that affect air flow. The air flowing through the crawl space (warm air in the summer and cold air in the winter) seeps through the floor because batt insulation is ineffective in the joist spacing to prevent air flow. The second issue is a result of running all of the HVAC duct/trunk lines in the crawl space. With an unconditioned crawl space, these ducts are supplying air to the home that is either much warmer or much cooler than the space they are travelling to (based on the season). Both these issues make the conditioning of the air in the home much less efficient – actually, it is the biggest air loss in a home built this way – which costs the home owner more money.

In order to combat this inefficiency, it has become more common to use a ‘conditioned’ crawl space. From a foundation standpoint and structural perspective, they are virtually the same. The difference is found in the venting of the space. A conditioned crawl space is, basically, a shallow basement. The vents are removed from the foundation walls and the moisture barrier that sits over the soil is covered with a 2” poured concrete slab. Supply lines from the HVAC unit are vented directly to the space. This keeps the temperature of the crawl space very consistent and close to the interior temperature of the home, regardless of the season. This helps reduce moisture build up on the duct work, reducing humidity levels, and allow the heating and cooling system to supply air flow through the duct work in the same temperature it will be received in the interior of the home.

The point of this type of foundation condition is to completely seal off the crawl space from the moisture and air influences happening outside the home. Taking this sealing to the next level, all the wood members need to be sealed and glued to prevent leakage between the crawl space and the house space as well as between the rim board and top of concrete foundation wall. All the penetrations (plumbing, vents, electrical) need to be sealed with expanding foam. Although they are similar in temperature, this sealing will be beneficial for both the crawl space and the interior of the home.

Additional benefits come from controlling the moisture infiltration to the crawl space. The issue with trapped moisture has become a hot topic over the past few years. You may have noticed when you sign a Homeowner’s Insurance Policy, there are all kinds of documents related to mold, the existence of mold, or the preventative measures to reduce mold. Mold is harmful to your health – especially to those that have allergies related to the various forms. A conditioned crawl space is quite effective in reducing the chance of mold forming. From a structural perspective, moisture can cause damage to the house, as well. Seldom would a builder use “weatherized” joists on the underside of the floor over the crawl space. This means that the wood under the house could be compromised by moisture – causing it to rot, deteriorate and, eventually, fail.

The idea of a conditioned crawl space is an excellent way to control heat loss/gain, improve the efficiency of your heating/cooling system, improve your indoor air quality and add to the durability of your home. All these items add up to a win/win. For little additional cost, the utility costs of operating the house will decrease on a monthly basis and peace of mind will be gained. If you have any questions or comments on the above writings, please do not hesitate to contact me.

Matthew Peterson is the Principal of Element Design Group. He can be reached at matthew (at)

This column first appeared in Coastal Sussex Weekly, July 16, 2009.

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