Fiberglass Insulation in a Crawl Space
Insulating a crawlspace with fiberglass has been the preferred insulation method for decades. The floors and foundation of the crawlspace were insulated because the crawl is vented to the cold winter air. Cold floors, freezing water pipes, and higher heating costs are all a result of ventilation. Fiberglass installed in between the floor joists is very loose and cold air can pass up through and around it. Since fiberglass insulation is more effective in a closed cavity where cold air cannot move around it, it is not very efficient in between a floor joist whether or not the paper side is up or down. Installing the fiberglass vertically on the crawlspace walls is a problem because it will become saturated with moisture from the foundation wall and ground floor. Fiberglass has no insulating value when wet.
Fiberglass is a bad insulation practice for a below grade crawlspace. Crawlspaces are generally damp, and since they are vented, the outdoor air is different from the crawlspace air causing moisture to condensate on many surfaces. The insulation absorbs the moisture from the crawlspace air like a sponge. The wet fiberglass raises the moisture content in the wood and causes the insulation to fall. When the moisture content in the wood rises, mold growth and wood rot occurs.
The resin of the insulation and the paper face is made of organic material and is what building scientists consider to be “mold candy”. In other words, mold thrives on the organic material in the insulation.
Crawl Spaces SHOULD be insulated but NOT with fiberglass. According to building scientist and independent organizations such as Advanced Energy and Habitat for Humanity, the right way to deal with a crawl space is to seal off the ground floor and foundation with a crawlspace liner and seal off the outside air completely. The next step would be to condition the crawlspace. Once this is done the crawlspace is just like another part of the building (like a basement).
The first step is the completely seal the crawlspace. Once the crawlspace is dry, the next step would be to add additional insulation.