While we have discussed the importance of an insulated and conditioned crawl space in a previous blog post, the recent installation of the crawl space insulation at Bellevue's Cottage No. 1 at Oak Park has inspired us to revisit the topic.
Unlike the vast majority of homes being built today, this cottage will be "closed and conditioned", rather than "wall-vented" (wall vents are those small metal vents that are placed about every 12' around a typical foundation). There are two primary reasons to close and condition a crawl space: first, to control moisture that might contribute to mold growth and other long-term durability and air quality problems; and second, to reduce cooling and heating bills.
Crawl spaces have traditionally been vented in the hopes of drying out them out. Unfortunately, during the warmer seasons, outside air actually has more moisture than air inside the crawl space, so the "outside" air has zero ability to contribute to drying of the crawl space, as shown by the top two lines in the chart below (taken from data from 12 homes in the Richmond area.)
Source: EarthCraft Virginia
What's more, this moist crawl space air comes into contact with colder items in the crawl space, so on a Richmond summer day, hot humid air is making contact with colder ducts, air handling units, and floor joists, leading to...
Source: Flickr (Creative Commons License)
...condensation, which is no friend of the equipment or the wood products that support your home.
If you take a look at the chart again, you will notice that a closed and conditioned crawlspace--essentially a short basement--maintains a lower and more consistent humidity than a wall-vented crawl space, through a combination of air sealing and the supply of a small amount of conditioned air directly to the crawl space. Lower humidity and more consistent temperatures help prevent mold and other moisture-based problems that can chip away at your home's durability and air quality.
The second reason to enclose and condition a crawl space is to save money on energy. Ducts that travel through a conditioned crawl space, rather than a typical wall-vented one, lose less of their heat (or "coolness") to the crawl space because temperature differentials are reduced. Conditioned crawl spaces also tend to more effectively retain heat (or "coolness") within the house itself. For example, Bellevue uses 2" thick sheets of rigid foam attached directly to the foundation walls to form the bottom layer of the homes "thermal envelope", but most vented crawl spaces use fiberglass batts stuffed into the spaces between the floor joists. Unfortunately, these batts can insulate your home only when they are able to trap air effectively. Since they are exposed on one side, and gravity is constantly working to pull the batt from contact with the subfloor above, air cannot be effectively trapped and the insulation performs well below its advertised R-value (as the familiar cold winter floor over a vented crawl space can attest). The bottom-line: a conditioned crawl can reduce energy costs by 15% or more.
Source: Closed Crawl Spaces: Introduction to Design, Construction and Performance, Advanced Energy
This link has as comprehensive 80-page report on the benefits of conditioned crawl spaces, tailored specifically to the climate of SouthEast. For our part, Bellevue Homes is a firm believer that the conditioned crawl is another example of "quality doesn't cost, it pays."
