Tricky crawl space insulation question. I don't understand insulation!!

[cosine88]

Last year I had the side walls of my crawl space insulated with spray foam.
The ceiling of the crawl space already had insulation batts(still does).
The guy spraying the foam said he'd charge $950 to remove the insulation batts. For $950 I will happily crawl in there, get dirty, remove it and dispose of it. So I told him not to.
The guy also said I should remove the batts, but didn't quite explain why. What I don't understand is why I need to remove them, what do I gain from having my floor uninsulated? Are there pros/cons for summer or winter?

- A couple of batts had to be ripped out for other reasons. You can see them in the attached pic.
- I live in Boulder, CO. Not hellishly cold, but we do have actual winter.

[Fishindude]

Fiberglass insulation on the underside of your floor system, between the joists just creates a place to trap moisture which can lead to mold, rot, etc.   It also provides places for mice, etc. to live.   

Insulate the crawlspace sidewalls good and that will keep the cold from getting in, so the underside of your floor should not get too cold.

[lthenderson]

I will second Fishindude's comments. Although not a perfect moisture barrier, insulation backing can act as such and so you are essentially leaving a pocket of warm moist air in your crawl space that can develop mold.

[ncornilsen]

Looks like you have a conditioned crawlspace. Great in areas where Radon isn't as much of a concern.

best practice is to insulate the sidewalls and 2ft across the crawlspace floor, after creating a moisture barrier over the dirt. you then pipe 50CM/1000sqft of conditioned air into the crawlspace.

I have this setup, and my floors are nice an warm no matter the temperature outside. I have zero insulation in my floor joists, and I could eat off the floor of my crawlspace.

-Nick

[cosine88]

Looks like you have a conditioned crawlspace. Great in areas where Radon isn't as much of a concern.

I have Radon mitigation installed already -- that white plastic sheet was installed for that, and also I have one of those fans that releases to the exterior of the house. The foam on the walls of the crawlspace was installed AFTER the radon mitigation.

best practice is to insulate the sidewalls and 2ft across the crawlspace floor, after creating a moisture barrier over the dirt. you then pipe 50CM/1000sqft of conditioned air into the crawlspace.

I don't understand the 2ft across the crawlspace floor, is that a 2ft perimeter? And I dont understand the conditioned air part.
FYI the furnace is in the crawlspace, so it is more efficient with some heat.

I could eat off the floor of my crawlspace.

Next level!