I insulated the crawl space under the newer part of my kitchen a couple of years back, and it seems to me to have made a bit of difference in temperature control in both summer and winter.
I'm sure the spiders and crickets that live there would readily agree. To others in the house, however, there is minimal difference in the winter - perhaps a degree or two - but summer, with the shades closed during the heat of the day, is more noticeably comfortable.
I do get a lot of crawl-space-insulation questions, however, and as the heating season approaches, I thought some advice based on my experience and some materials from the U.S. Department of Energy, CertainTeed, and the North American Insulation Manufacturers Association might help you cut energy costs.
Energy tax credit: Materials, yes; labor, no. (Still, I recommend hiring a professional to do it correctly.) Go to www.irs.gov to determine eligibility.
Ventilated or not? Critical point, according to the energy experts and insulation manufacturers. Most building codes mandate vents to help keep crawl spaces dry. Many experts today favor unventilated crawl spaces or, at best, closing the vents on new crawl space after they've dried. Why? It's tough to keep a ventilated space warm in winter. Conversely, if humid air is vented into a crawl space in warm weather, dehumidification is nigh impossible.
If the space is unventilated: Seal and insulate the foundation walls, not the subfloor. Reasons? Less insulation is required - government experts say about 400 square feet for a 1,000-square-foot space with three-foot walls. In addition, pipes and ducts are within the conditioned portion, so they don't require insulation for energy efficiency or protection against freezing. And air sealing between the house and the crawl space is less critical.
Reasons not to insulate: Insulation damage by creatures and water. A radon-mitigation system will require ventilation of the crawl space to the exterior. The crawl-space access door must be in the home through the subfloor unless an airtight, insulated access door in the perimeter wall is built and maintained. The space must be built airtight, and the air barrier must be maintained.
What's an air barrier? House wrap or parts of the structure such as drywall and sheathing. Barriers halt random air movement through building cavities, reducing leakage that can raise heating and cooling costs almost one-third.
