I'm desperately looking for some advice on how to properly insulate the crawl space. I live in eastern Massachusetts where it has recently been in the teens and 20s for low temps in winter. I have a large crawl space under my kitchen and 3 season porch ranging in height from 1' to 4' with no venting to the outside. I recently air-sealed the entire rim joist with an expanding foam and added a heavy mil plastic sheeting over the dirt floor, making sure to overlap and seal all edges. We also insulated the copper pipes in the space with closed cell insulation tubing and taped all joints to prevent the pipes from freezing. The kitchen is still very cold.
I would like to ideally insulate the space in between the floor joists and possibly the exterior brick foundation walls but are getting conflicting info on what to do.

  • Could use a poly spray foam kit, but the max R-values I have seen are only in the 7-10 range and its not anywhere close to warm enough outside to prevent it from failing. Everything I read says not to use apply when below 60 degrees or it will not cure properly. Its barely in the 30s right now! Also, the exterior walls are not all that clean and the foam might not stick.

  • Floor joists are not evenly spaced and range from 17"OC to 22"OC so batt insulation is going to be a pain.

  • Could use blown-in insulation but how do I get it to stay up there? Someone told me to use a plastic sheeting across all the joists with furring strips for extra support and then simply blow the insulation into each joist cavity, but I am worried that the plastic sheeting is too much of a vapor barrier and will cause mold inside the joist cavities. If the crawl space is already moisture sealed, am I OK? Do I need some sort of barrier up inside the cavities on the bottom of the floor?

Also, do I really need to insulate the brick foundation walls and will this show a big difference? Because the dirt is not evenly distributed and some areas are tight to get into, getting rigid foam board would be nearly impossible.

Please help!!!

  • Have you determined why the crawlspace is cold? Is it vented to outside air in the winter? Jan 11, 2023 at 10:05

5 Answers 5


I am also in the Northeast and have had to tackle this type of situation a few times. There are several ways to attack it.

In your case, the best solution, but unfortunately the most expensive would be to use an open cell foam kit to fill the joist bays. They are fairly easy to use and give you about an R-4 rating per inch. They are a two tank kit with a hose and wand. Here is an example product: Touch N Seal 1000 Kit Open Cell Spray Foam Insulation

These type of kits start around $400, depending on the square footage and depth required.

A cheaper method would be to use 24 inch R-19 unfaced blanket insulation cut to your custom widths. You can use wire springs to hold it in place and keep it from falling. I would then cover the entire area with 4 to 6 mil plastic, stapled to all the joists. Tape the joints, if any. Moisture should not be a major problem since you have already done what sounds like a good job sealing the dirt floor. In the summer months, some ventilation in the crawl space will also help avoid excess moisture.

Rigid foam glued to the foundation walls would also help if the area is fairly air tight. If there is free air flow, insulating the brick walls would be a waste of time and money.

  • 1
    Thanks Shirlock. My only issue with the foam kits is that it says it needs to be 70-90 degrees to install. I guess we can wait until summer, but I really wanted to do something now since we have 3+ more months of cold! Its encouraging to hear you say to use the plastic & add'l insulation, as thats what we were thinking. Thanks for your help! Feb 5, 2013 at 5:56

You could use rigid XPS foam. Usually you get about R-5 per inch of foam and the foam itself comes in 1", 1 1/2" and 2". It usually comes in 4'x8' sheets which you could cut to size in order to properly fill your mixed dimensions. You can secure it with adhesive (PL-300 - must be for XPS foam) or screws/washers.

Insulating the brick foundation walls will likely make a big difference too. Building Science has all sorts of great information on the different ways to accomplish this.


There is netting that is used to hold loose insulation up; it could be attached under the joists and then loose-fill insulation added.

  • A picture, or link to an example product might be useful.
    – Tester101
    Feb 4, 2013 at 12:12
  • Thanks Eric. Ill definitely look into that. That might solve my issue of moisture buildup Feb 5, 2013 at 5:57

I'm currently insulating my bosses crawl space and we decided on paper backed R30 stapled up with the paper facing down as to provide the means to attach it. Problem is R30 while providing a far more energy efficient means of insulating is too thick and protrudes from the ceiling putting extra pressure the staples which we fear will cause the paper to rip off over time . Solution : install 6 mil plastic with staples over the insulation.


The paper side is a vapor barrier and normally should face living space, not external space. Also this can trap moisture within the insulation if the crawl space gets humid. Better to use spray foam or coated rigid insulation.

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