Proper Way to Insulate a Crawl Space

Answered by Jeffrey ~ September 15, 2010 ~ Comments

What is the proper way and what material should be used to insulate a crawl space? The floor of the crawl space is dirt and about 2 to 3 feet high. It is accessed by small passegeways in the foundation. We live in northern New Jersey and have fairly cold winters.

Maryann S. ~ Millington, New Jersey

Jeffrey Anderson

Hi Maryann, there are a couple of options for insulating a crawl space and whether or not your crawl space is ventilated can determine which method you should use.

Up until recently builders and architects felt that providing ventilation into crawlspaces helped prevent problems with moisture such as mildew and mold. However, the belief has been changing recently as they realize that during warm humid weather ventilation into the crawl space can actually promote moisture problems and during the cold months a ventilated crawl space can contribute to higher energy costs. Proper grade around the outside can do more to prevent moisture problems than a ventilated crawl space can.

So you should walk around the outside of your crawl space and see if you can locate any vents through the foundation walls. If you have vents, then you are going to have to use batt insulation up between the floor joists in the crawl space. It should be up tight against the bottom of the subfloor but not compressed. I would recommend an R-19, but you could go higher if you wanted.

One of the problems you're probably going to encounter is that there may be duct-work and water lines you're going to have to work around. The insulation should be below all duct-work and waterlines, but if you have duct-work that extends below the floor joists, just make sure the duct-work itself is insulated. It's very important that the water lines be above the insulation.

Insulation companies have cross braces that can be used to hold the insulation in place between the joists and they're easier to work with than a staple gun and also hold better. There should be some insulation companies around Millington, New Jerse you can get them from and you should also get a roll of vapor barrier. Once the insulation is in place you then install a vapor barrier at the bottom of all the floor joists.

If your crawlspace isn't ventilated, then it is a much easier process. You can either use the method I just described or you can insulate the inside walls of the crawl space with batt insulation or several inch thick insulating board. If you go this route, you should use an insulating contractor as they will need to shoot fasteners into the masonry walls of the crawlspace to hold the insulation in place. The same method is used for insulating an unfinished basement. I would recommend an R-19 insulation for this method, also. It is important that the insulation go up into what is considered the band board area of the crawl space. This is the area above the foundation walls but below the subfloor.

A vapor barrier should be used with the insulation in this method also. If you insulate your crawl space's exterior walls, you can even have an HVAC supply line directed into the area to help keep your floors toasty.

Whichever method you use it is important that all penetrations through the crawl space walls be sealed thoroughly and the doors at the passageways have weatherstrips to help prevent the cold from your New Jersey winter from getting in to the area.

Insulating a crawlspace is not always a fun job, especially on an older home where you may encounter all sorts of things down there. You may want to hire a contractor to take care of this project.

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