Best Insulation Materials for a Crawl Space
Top performance. Rigid foam insulation is ideally suited for insulating crawl space and basement walls. It offers high R-value, effective draft stopping and excellent resistance to mold and moisture damage.
Crawl space insulation has a much bigger impact on a home's energy efficiency than most people realize. Unlike a basement, that has both headroom and fairly easy access, a typical crawl space can be difficult to get into.
This is the main reason why crawl space insulation is often overlooked when people are aiming to improve energy efficiency and reduce heating and cooling problems.
Your feet and your nose get the message
If your house has a crawl space foundation, there are some very basic symptoms of poor crawl space insulation. During chilly winter months, floor areas above the crawl space will be too cold for you to walk on barefooted or with just a pair of socks on. Another sign of improper crawl space insulation is musty odors coming up from below. Using the wrong kind of crawl space insulation can actually increase the likelihood that mold and mildew will grow in the dark, damp crawl space.
Foam, not fiberglass, is the right choice
The old-fashioned way of insulating a crawl space was to install fiberglass batt insulation between the exposed joists in the crawl space. Fiberglass is cheaper than other types of insulation and also widely available. But it's tendency to absorb moisture, support mold, harbor mice and sag or fall out of place are all negative qualities that argue against its use in crawl spaces.
The best insulation material for crawl spaces is rigid foam insulation board. Instead of installing the foam insulation between floor joists in the crawl space, the foam board is installed against the crawl space foundation walls. Unlike fiberglass, rigid foam insulation does not absorb water, support mold or allow air to pass through (providing that joints between boards are sealed with tape).
Not just insulation, but air sealing as well
In a crawl space, insulating the crawl space walls with rigid foam insulation is usually part of an overall strategy to completely air-seal or "encapsulate" the crawl space. The rim joist around the foundation is sealed to eliminate air leaks. Walls are sealed and insulated with rigid foam. Any dirt-floored areas are covered with a durable plastic membrane, and moisture levels are reduced (as necessary) with waterproofing and dehumidification. These measures transform a damp, dirty, energy-wasting crawl space into a dry, clean, durable space that improves interior comfort and reduces energy consumption.