Floor joist and crawl space insulating method

Floor joist and crawl space insulation are important for conserving a lot of energy. Many homes are built without insulation between the floor joists and the floors are cold. If you want to have warm floors or economical heated floors, you should insulate the crawl space.



One of my least favorite parts of the whole building process is installing the insulation. Modern day insulation and insulation methods are much better than they were three or four decades ago.

Fiberglass insulation was a nasty, itchy product. Today, it’s not bad. There are several methods of insulating your house, but we will stick with the most economical and the easiest.

There are a few considerations to make before you get started. First of all, you should consider the local climate. Insulation comes in many forms, but the least expensive is the fiberglass rolls, so I’ll only cover that kind.


installing fiberglass insulation



Batted insulation comes in different widths and lengths. The two standard widths for wall studs are 16” on center and 24” on center. Insulation is made to fit snugly into the walls, so both widths are available almost anywhere.

The money spent on insulation is always money well spent. Three decades ago, the standard wall thicknesses were the width of a 2×4 stud.

We know better now, so most walls are made with 2×6’s instead. This allows more insulation which is a good thing in both the summer and the winter.

Insulation is measured by its R-value. The higher the R-value, the better, and usually the thicker the insulation. Here are some examples of R-values and insulation thicknesses. The climate map is a general guideline. Local codes will be slightly more demanding.


insulation chart



Most codes have a minimum of R-11 for the walls, but if you have 2×6 walls, you will be better off with R-19. It will make a lot of difference in temperature extremes.

You can see the R-values according to board thickness in the graph in the illustration below. Some insulation bats have a small flap on the edge to staple so it stays in place. Other rolls might need to be held in place with plastic.


applying insulation



It’s really important to insulate the floors as shown in the drawing below. Some might not feel it necessary to insulate the floors, but it makes a big difference in heating and cooling a home.

The crawl space insulation is the same as floor joist insulation unless you want extra insulation, then you’ll need to hold the insulation on by stretching plastic over it.

Insulating a Gambrel roof is different than a normal rafter roof because you will need to roll R-30 or R-38 into the rafters as shown below.


rolled insulation



Next, wall and ceiling insulation


Back to insulation main page


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