Crawl Spaces

Every once in a while I have the pleasure of inspecting a crawl space.  Upon exiting, I usually get a host of comments and in some cases a little shock of how dirty they can be.  In Wisconsin most houses are built with a full basement due to the fact that you have to excavate below the frost line for the foundation footings and you may as well go the rest of the way for the full basement.  Besides,  the basement IS the best place for a man cave.

Though homeowners rarely think about that small space under their houses, builders should give crawlspaces careful consideration. Some of the most common—and damaging— building problems stem from moisture in crawlspaces. Poorly constructed crawlspaces can contribute to mold problems, indoor air problems, and inefficient operation of the home’s mechanical equipment. Carefully considering design and construction of crawlspaces, as part of an overall quality construction strategy, can reduce problems for both builders and homeowners.

Now for the big question, vented or unvented.  To explain this in simple terms, the diagram shows the differences and identifies them as heated vs. unheated.  Crawl spaces used to be built with vents to the outside air.  The thinking behind that is it will cut down on moisture and humidity with the moving air through the vents.  The heated crawlspace has insulation on the floor and walls of the space.  The unheated version has the insulation under the floor, leaving the space to get cold and create a list of potential problems like the ones mentioned.  The moisture contributed to the conditions in the first picture.  In the winter time, the floors would be very cold and any duct work run through there would make the mechanical systems inefficient and cause high utility bills, not to mention the possibility of a frozen water pipe bursting.  By closing up the vents, and moving the location of the insulation and opening the crawl space to conditioned (read heated) the benefits start to become evident.

To have a conditioned crawl space, we have to have certain criteria met just like a full basement.  The foundation walls need to be built on footings with drain tile and a sump pump.  The wall system must not leak air or water.  A drainage system for roof water to be collected and drained to a correctly sloping grade so the water flows away from the crawl space.  Next, a continuous and sealed high quality 6 mil polyethylene vapor retarder on the ground in the crawlspace and protect it with a concrete scratch coat or a layer of gravel.  The purpose of the vapor barrier is to keep the moisture in the ground where it belongs, IN THE GROUND.  The walls need to be insulated to the proper thickness for your area.

Benefits of unvented crawlspaces.
Crawlspace vents effectively become the “determined hole” in a home. In winter, warm air exiting the attic or ceiling creates a negative pressure at the lowest openings in the home. This negative pressure causes cold air to enter through the crawlspace vents and rise through holes in the sub floor to the first floor. This phenomenon creates the “cold floor syndrome” prevalent in cold climate homes with vented crawlspaces.

The cold air can also carry unwanted moisture and gases from the soil, and odors from the crawlspace into the home.  In unvented crawlspaces, the walls, instead of the floor, should be insulated to limit heat transfer from outside to inside the crawlspace, and vice versa. When properly built, an unvented, insulated crawlspace helps to eliminate problems with comfort, odor, and health associated with mold, mildew, and bacterial growth.

Once the crawlspace is insulated and unvented, it should be treated the same way as any other space in the house—supplied with a certain amount of heated or cooled air from the mechanical system. Conditioning the crawlspace can help the mechanical components that are housed in the crawlspace (the air handler, duct work for the first floor zone, and plumbing line, for example) operate more efficiently and last longer because they are not subject to temperature and humidity extremes.

Ideally, the crawlspace temperature should be about 55°F to 65°F year-round. All ducts should be well sealed with mastic to prevent unwanted air leakage. This air leakage increases energy consumption because the outside air has to be heated in the winter and cooled in the summer.

Will these improvements increase construction costs? Probably not significantly. You can save money when replacing underfloor insulation with wall insulation, because the square footage of crawlspace walls is typically significantly less than the square footage of the first floor. The required insulation R-value is generally lower for crawlspace walls than what is required for floors.