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How Much Floor Insulation Do I Need?




Arnold V. of Missoula, MT wrote:

What's happening Itchy?

I need your help. My wife is at me to get the floor insulated and I need to get it done before winter. I think I can insulate it myself but I need some advice on how to measure a floor for insulation.

Although measuring a floor is not difficult, I have good news and bad. The good news is that normally you can measure the floor from outside of the house by measuring the perimeter of the foundation. The bad is that once that's done, you're gonna have to climb on down into the crawl space.

Measure the length and width of the floor and multiply the two figures to come up with the square footage.

Next, grab a flashlight, tape measure and get on down into the crawlspace. There are two measurements you need to make:


1. Measure the distance between the floor joists. This will determine the width of the insulation you need to purchase.

2. Measure the depth of the floor joists. This will indicate the thickness of the insulation you will need. Fiberglass insulation is rated by its r-value - the thicker the insulation the higher the r-value. Many floors across the country are insulated with R-25. The chart below will help you determine the highest r-value you can install, based on the depth of the floor joists.


Insulation Type
per Inch
Cotton (Recycled denim)

You can see that if your floor joists were 8" deep, you could insulate with a R-25 fiberglass batt.

Grab your measurements and head on over to the home improvement store. Say you measured the depth of your floor joists as 8" and the distance between them at 15". You would be purchasing R-25 insulation that is 15" wide. Look at the bag of insulation and there will be a chart on it that will tell you how many square feet of the floor that that bag will cover.

Final step: Divide the total square feet of your floor by the coverage area of the bag of insulation to come up with how many bags of floor insulation you'll need.

Related Tips and Advice:

How Much Wall Insulation Do I Need?
How To Measure Attics For Insulation

How To Insulate A Covered Floor
How To Insulate Unvented Crawl Spaces


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