I have approx 500 sq. ft. of crawl space (under addition to original home). 15 x 35 ft. with 2 x 10 joist & concrete flooring ( no dirt ). Ventilated to outside is provided by 3 windows. The height is approx 2-3 ft of this crawl space. The foundation wall is of cinder blocks.

How do I keep the home above this crawl space warm in winter?

Do I insulate joist or walls or both?
Which insulation do I use, and how to secure it?
Should the ventilation be closed in winter?

  • This is still a hotly debated subject in the industry, but more and more are now recommending that crawlspaces should be closed and fully insulated--not vented. There's a lot of information on the internet about this.
    – DA01
    Commented Aug 20, 2015 at 2:57
  • I don't know whether it's safe to insulate the crawlspace itself, but surely you could insulate between the crawlspace and your home? i.e. insulate the "ceiling" of your crawlspace. (This is a comment based on logic, and not on any understanding of what code requirements might be for crawlspaces, in your locale or anywhere else.)
    – AndyT
    Commented Aug 20, 2015 at 10:16
  • It is 100% safe to insulate crawl spaces... And regarding the "crawlspaces should be closed" comment, that is completely untrue. Especially if you live in any kind of flood plain. You want to vent your crawl space. They make vents which open and close. i'll leave an answer below...
    – maplemale
    Commented Aug 20, 2015 at 19:53

2 Answers 2


It's becoming code in a lot of places to vent your crawl space. Pretty much everywhere if you live in a flood plain in order to get insurance. Whether flood plain or not, many areas are requiring them for everything new. Vented helps to prevent both mold and buildup of gasses like radon in your house. You can get vents which open and close according to the temperature (and water pressure in case of a flood) if you think that would help. People post mixed reviews as to the benefit of these products. Do your research if you go this route.

It is perfectly acceptable and I would say, the best approach to spray foam insulation (similar to "Great Stuff") in a crawl space that is properly prepared for it. It needs to be vented already. It needs to have certain things like pex pluming covered as the foam will eat it. The foam goes right up to your sub-floor adhering to the joists and down the walls a little. And it does wonders for your energy bill.

I owned a 4k square ft house in Montana in a flood plain with 3k square ft of crawl space that was vented with flood vents (automatic open and closing vents). I had the entire 3k square ft of space sprayed with 3" of foam insulation and it cost me about $2800. This was in 2008. -30 degrees Fahrenheit and the floor stays fairly warm. An added benefit is this does wonders for bug protection, completely sealing up your floor. This is the only way to go in my opinion.


The answer to this question depends upon the climate condition in which it is located. Moderate climate: insulate between floor joists and vent crawl space to promote air circulation and mitigate mold growth. Colder dryer climates: Insulate foundation and seal ground to prevent moisture coming up from ground. Thus keeping any pipes in crawl space from freezing.

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