Correct way to insulate your crawl space

Answered by Jeffrey ~ April 13, 2011 ~ Comments

My house was built in 1994. I already have unfaced insulation in my crawl space running North to South. Can I put more insulation running East to West over the existing insulation? If so, what kind should I purchase? I was told by one sales person at Lowes that I can get R-13 (faced) and install it over the existing insulation with the faced part down. Then on another day another sales person at Lowes told me that I should install a moisture barrier and then I could use non faced insulation. I do not have a moisture barrier under my house and now I'm confused to what to do. The ultimate goal is to insulate my crawl space properly. What is the proper way to do it to keep my energy cost very very low?

Kenneth ~ Madison, Wisconsin

Jeffrey Anderson

Hi Kenneth. You can add additional insulation to your crawl space, but how you go about it is going to depend on what your floor framing is like down there. I would assume your floor joists are either two-inch lumber or engineered floor trusses, and their depth is going help determine how you should add insulation.

You don't mention what type of insulation you are currently using other than it is unfaced, but I'm guessing it's a batt insulation that you have installed between the floor joists, and it is resting up against your first level sub-floor. I'm also guessing that those floor joists run north to south under your home. If you have room between the bottom of the insulation and the bottom of your floor joists, then I recommend adding your second layer of insulation in those same cavities.

However, you do not want to compress the existing insulation or your new insulation to make everything fit. Choose an insulation that fits up there comfortably -- a little bit of compression is okay, but only a little. If your floor framing is ten inches deep and you have R-19 insulation installed, you should have about four inches left to fill to reach the bottom of the joist. If that is the case, an R-13 insulation should work nicely as it's normally about three to four inches thick.

If you have twelve inch floor joists, you might be able to fit another layer of R-19 in and you would end up with a very well insulated crawl space. I would not run the insulation east to west unless your current insulation already fills up the joist cavities. Regardless of whether the new insulation runs north to south in the joist cavities or east to west under the cavities, I would use unfaced insulation and install a moisture barrier underneath it.

You should be able to purchase moisture barrier sheeting in rolls from a home improvement store around Madison or from a local insulation distributor. It serves two purposes in your application -- preventing moisture from damaging your insulation and getting into your home and helping to keep the insulation in place against your sub-floor.

One other item I'll mention since you live in Wisconsin and it gets pretty cold there during the winter from what I hear. Make sure any water lines you have in your crawl space are covered by your insulation and if possible, by both layers. If you have any duct work in your crawl space, it should either be insulated duct or it should also be covered by the insulation you install.

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