The Best Way to Insulate a Floor


Floor insulation should take place before the sub floor has been screwed down into place. That way, with bare floor joists, the insulation can easily be installed. However, if the floor is in place (and can't easily be removed) it is possible to install the insulation from underneath. The insulation itself comes in rolls containing five lengths, each length roughly 8-feet long. Depending on the geographical location of the house, the insulation will have a different "R" value (the higher the "R" value, the higher the quality of insulation): check with your permit office for the correct "R" value for your area.

Things You'll Need

  • Insulation
  • Tradesman's knife
  • Straight edge
  • Tape measure
  • Staple gun
  • 1/2-inch staples
  • Caulk and caulk gun
  • Insulation hangers
  • Roll the insulation out between the floor joists (before the sub floor has been installed), starting at one end by the wall. Have the vapor retarder paper facing up, with the fiberglass underneath. Use a staple gun to staple the lips on the side of the vapor retarder paper to the joists at the top, every 10 inches. The insulation should be level with the top of the joists. Continue in like fashion until the entire floor is insulated.

  • Caulk the joints between the stud walls and the floor all around the room, once the sub floor has been installed. This will also help insulate the floor/room.

  • Apply insulation to the underside of the floor joists if the sub floor has already been installed and can't be removed. Depending on the location of the house, the vapor retarder paper can face up, or down. Check with the local permit office for your area. If the retarder paper faces down, roll it out between the joists, starting at one end. Staple the insulation, but make sure the paper is level with the bottom of the joists, not the top. If the vapor retarder faces up, start at one end of the joists and push the insulation up between the joists. Push insulation hangers up underneath the insulation every 15 inches, but don't indent the insulation with the hanger more than an inch.

Tips & Warnings

  • Insulation is made of fiberglass (usually either pink or white) and vapor retarder paper on one side.
  • Insulation comes in 16- or 24-inch widths. Though most floor joists are positioned 16 inches wide (on center), check your joists before purchasing insulation.
  • Wear rubber gloves, a dust mask and a long-sleeved shirt to install insulation.
  • Cut the insulation by placing the insulation paper-side down on the floor. Rest a metal straight edge on the insulation at the point where you want to cut. Run a tradesman's knife several times against the straight edge until the insulation is cut through.
  • Insulation hangers are pieces of flexible wire that are slightly longer than the distance between the two joists. When you push them up between the joists, they bend and the two ends dig into the sides of the joists. Make sure that the bend faces up, as this will hold the insulation in place.
  • Insulation hanger come in two sizes that fit 16 or 24-inch gaps: make sure you purchase the correct length for your job.

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  • Photo Credit spiraling insulation image by jbattx from
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