How to Insulate a Floor Over a Crawl Space

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As energy prices keep rising, many homeowners are trying to find better ways to save both money and energy in their homes. Owners of older or underinsulated homes, and those in warmer climates where insulation is not as compulsory as it is in colder locales, may find that adding insulation to the underside of the floor over their crawl space can help them achieve their savings goals.

Things You'll Need

  • Calculator
  • Tape measure
  • Floor insulation
  • Utility knife
  • Insulation supports
  • Staple gun
  • Rake or shovel
  • Plastic sheeting
  • Tape
  • Gloves
  • Goggles
  • Mask
  • Estimate how much insulation and materials you will need. Do this by measuring the length and width of areas to be insulated and figuring out how many square feet of insulation will be necessary to complete the project. Don't forget to measure the distance between your floor joists. Typically, joists are installed 16 inches on center and will measure roughly 14 to 15 inches in between. Knowing the average space between the joists will determine if you can purchase precut insulation or whether you will have to cut each piece separately to fit.

  • Determine which kind of insulation you will need for your project and what way your vapor barrier---the paper side---of the insulation will face. This will vary and is based on your location and climate. This kind of information can be found at your code enforcement office or materials supplier, or by visiting the manufacturer's website. Also decide whether to purchase batts or rolls (again, this will vary according to your project and joist spacing). Once you have your estimations and materials list, purchase all needed items.

  • Put on your protective clothing, eyewear, mask and gloves. Install the batts of fiberglass insulation a piece at a time according to the manufacturer's instructions, being careful to put it in with the vapor barrier in the correct way, and secure in place with either wire or nylon supports, wood furring strips or by stapling the paper to the floor joist. Make sure the ends of the insulation touch the outside band joist. Do not pack the insulation tightly but do be sure to have it snug against the subfloor. Do all the areas with full pieces first.

  • Cut pieces of the insulation to fit any odd or small areas and install. Secure these pieces as best you can by either trimming the insulation supports to size or stapling to the joists. Make sure to put insulation into all of the off-shaped areas that were left after the large pieces were installed, including those around HVAC vents and plumbing piping.

  • Clean up the entire area under the house after all insulation has been installed. Use a flashlight to double check that all insulation is secure in place and nothing is hanging down or loose. Rake or use a shovel to level the dirt on the floor of the crawl space then put down a 4- to 6-mil polyethylene clear plastic sheeting on the ground. Be sure to affix tape to all seams to hold the plastic in place.

Tips & Warnings

  • Make sure you crawl space is free of debris and/or trash before beginning this project. For unventilated crawl spaces, consider insulating the walls instead of the floor. Estimate higher to avoid extra trips to the supplier.
  • Ventilate the crawl space during installation if possible. Have any water and/or HVAC repairs performed prior to starting. Wear eye, hand and body protection at all times. Wear a respirator to protect your lungs.

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