Thursday, November 19, 2009

Do not use fiberglass insulation in a crawl space

Insulating a crawlspace with fiberglass has been the most common insulation approach in crawl spaces for decades. The floors or the walls of the crawlspace were insulated because the crawl is cold in the winter due to the open or closed vents. This cold air causes cold floors, freezing water pipes, and higher heating costs. While it does assist in insulating, it is not efficient at all. Fiberglass in a crawlspace joist system is very loose and cold air can pass up through and around it. Since fiberglass insulation is only effective in a closed cavity where air cannot pass through it, it does not function properly in between a floor joist whether or not the paper side is up or down. Installing the fiberglass on the crawlspace foundation walls has the same inefficiency because air can pass in between the pieces.

I just touched on why fiberglass is inefficient, but now I want to explain why I HATE fiberglass insulation in a crawlspace. Plain and simple, fiberglass insulation is mold candy. Fiberglass is the worst choice for a below grade crawlspace. Crawlspaces are generally damp and humid, and since they are ventilated, the outside air is always different from the inside air causing moisture
to condensate on nearly every surface. The insulation absorbs the moisture from the air and the ground like a sponge. The fiberglass saturates with moisture raising the moisture content in the wood and causing the insulation to fall. When the moisture content in the wood rises, mold starts to grow 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 loves to grow on the organic material in the insulation. Also, fiberglass has next to nil insulating value when it is wet.

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 durable vapor barrier 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 (i.e. basement). Now the walls could be insulated with foam board or spray foam depending on the outside grade of the crawlspace. In many cases, insulation will not even be needed because the cold air that is being sucked in through the vents will be sealed off. See my post "How to insulate crawlspaces" for more information.

It is a rare occasion that I am in a crawlspace with fiberglass where the fiberglass isn't wet, condensating, moldy, or falling down. I have a database of photos to prove it. Just look at some of them in the slideshow above. If you still want to insulate your crawlspace with fiberglass, please comment and tell me why.

If you need professional advice on how to insulate your crawlspace, give us a call @ 1-877-409-2837 or SIGN UP ONLINE for a FREE CONSULTATION.

Thanks for reading the rambling thoughts of a foundation inspector,
Larry Ralph Jr

Crawlspace Insulating Series:
Cold Floors above a Crawl Space
Foundation Vents are causing higher heating costs
Do not spray foam your crawlspace foundation walls
How to insulate crawlspaces - Encapsulate and Condition
Insulating above-grade crawlspaces - spray foam the walls



  1. Great blog - really interesting. I have very rarely even thought about our crawl space...until recently, when a feral cat decided to enter and live in ours. The smell is SO BAD. My husband went under there with a hudson sprayer full of vinegar - which I am sad to say only made it worse. ( I read that vinegar neutralizes ammonia?)Anyway - Pros are coming out at the end of the week. I do not envy you your job if you have ever had to deal with smells like this! We are in WA State, or we would give you a call. Thanks for the interesting read, and all of the best to you! :-)

    1. We just purchased a home with broken crawl space vents. It was infested with feral cats while it sat unoccupied for 3 years. Most of the insulation was on the ground, and used as a toilet or nests. 8 hours of removing all the insulation and vapor barrier got rid of the smell. Then new vapor barrier, and new insulation. Smell is completely gone. Didn't read this blog first though. Now I worry about moisture problems. I may block off my vents completely now.

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