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How To Insulate A New Unfinished Floor

Randy K. of Spokane, WA wrote:

Hey Itchy,

I plan on doing my own home addition and I want to do most of it myself. The floor will be framed with joists. What's the best way to insulate the floor?

Man, I have some fond memories of insulating floor joists. Getting my shins banged up, knocking my head up against them...well you get the picture. Just take it slow and easy and you'll be okay.

Before you get started you will need to have the insulation supported so it doesn't fall through the floor joists. One method is to staple twine underneath the joists, going back and forth across the bottom of each joist run every 12". Another method is to buy some wire hangers. In the insulating field we call them "lightning rods". These are easily installed by slightly bending them and placing them across the bottoms of the joists. Finally, another method is to purchase some lathe and nailing it across the underneath side of the joists.

After you get the insulation supports installed into the joists, it's time to insulate. You need to determine the R-Value needed for your climate. Make sure you know the square footage of the floor area you're insulating before you purchase the fiberglass batt insulation. Your home improvement will have charts for various brands of fiberglass insulation sold and will help you determine how much you need.



Get on your protective clothing, dust mask and goggles. Start at one end and lay the batt insulation into the joists. If you're using a vapor barrier, make sure the kraft paper side of the insulation blanket is facing up where the floor will be installed above it. Continue insulating along each run of the joists. Use a utility knife to make your cuts. Be careful not to cover any vents that may exist. If there is any plumbing running through the joists insulate on both sides of it.


Make sure you don't leave any voids or gaps and stuff these areas with loose insulation. After you insulate the floor, you should  cover it with whatever flooring materials you are using as soon as possible to prevent the weather from playing havoc on your work.

Hint: Some people have good balance and can walk the joists to get the insulating job done. I would recommend using a sheet of plywood by laying it on top of the joists as a work platform and move it as needed.

Related Tips and Advice:

How To Insulate A Unfinished Floor Framed With Post And Beams
How To Insulate A Floor With A Vented Crawl Space
How To Insulate Unvented Crawl Spaces

How To Insulate Your Attic With Fiberglass Batts or Blankets
How To Install Blown In Insulation In Your Attic
How To Insulate A Concrete Wall

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