Insulating a Floor Underneath the Crawlspace

To insulate in between the joists underneath the floor in a crawlspace is definitely dirty and annoying work, however it can make a big difference in terms of keeping a space nice and warm / cool. I'm turning a small shed into a workshop and wanted to make sure that the floor wouldn't be the source of letting all the energy out once I heat or cool the space, so to insulate it made a lot of sense. In this instructable I'll go over the process that I used the add insulation underneath the joists.

(For a better perspective, make sure to watch the video that goes over the entire process!)

Step 1: Insulation

For this space I decided to use 1.5 inch Rigid foam insulation, and I picked up four sheets to cover my under 200 sqft building.

Step 2: Cutting to Size

I began with measuring the distance in between the joists. It measured 14.5 inches, which meant that cutting the boards to 14 inch widths was a good idea. I first used a jigsaw for the cuts, however a razor blade knife also works great.

Step 3: Cutting to Size

The individual sections can't be too long, or else you can't fit them inside. I found it easiest to cut them to about 31 inches long. I began by adding some construction adhesive to the pieces and then secured them in between the joists with 2 in. long 16 gauge nails in a nail gun hooked up to a compressor.

Step 4: Clothes

Getting under a crawlspace isn't not exactly a clean and pleasant experience, so it's nice if you can wear clothes that you don't mind getting dirty! I wore nylon rainpants, jacket, hat, boots and gloves, and that made a huge difference! Make sure the area is clean beforehand (I used a leaf blower and rake), and another option is to place cardboard pieces on the ground before crawling under to make it a bit more manageable.

Step 5: Preparation

If you want this job to go smoothly, preparation is key! Cut a bunch of sections to size, add construction adhesive and then place them around the house so you can easily access them when you're underneath. Make sure your air compressor is pumped up and that you've got plenty of nails in the gun! The less trips you have to make crawling in and out of the crawlspace, the faster you'll be able to complete the job.

Step 6: Final Section

Once I was done placing the 31 inch long sections within all the rows, it was time to measure and place the last piece on each. I did this all at the end, and could do it from the side, so I didn't even have to crawl under completely!

Step 7: Spray Foam

To fill in any small gaps, I used expanding spray foam. This way I didn't have to be too precise when cutting the foam pieces to size, since any gaps could be filled with the foam product.