How-To Videos

How to Insulate Under Floors in a Basement or Crawlspace

By: Danny Lipford

Insulating the space between the joists in a basement or crawlspace can help reduce your energy bills. Here’s how to insulate under a floor:

  1. Cut the insulation to length using a square and sharp utility knife.
  2. In older homes, where the spacing of the floor joists varies, you may need to cut the insulation to width as well.
  3. Install the insulation with the vapor barrier facing up toward the heated living area of the house. If both floors of the house are heated, use unfaced insulation.
  4. Don’t compress the insulation when installing it, but allow it to expand to fill the cavity.
  5. Hold the insulation in place using support wires beneath it. Cut to length if needed using wire cutters.


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17 Comments on “How to Insulate Under Floors in a Basement or Crawlspace”

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  • Jill Musteen Says:
    November 4th, 2018 at 3:16 pm

    Will placing the paper down not work as well when placing insulation under floors on crawl space

  • William Champenoy Says:
    November 2nd, 2018 at 6:56 pm
    In a basement space that is not heated and has a “double” joice (1890 home) I want to do two layers of insulation. The sewer pipe and bathroom plumbing is in the second joice level (or below the first Joyce level). Each joice level is running opposite of each day other. I put the first layer with the paper up. But do I place the second layer with paper down or up?


  • Official Comment:

    Thomas Boni Says:
    December 8th, 2018 at 4:06 pm

    Hi, B.,
    Danny says, “I would suggest using Rockwool insulation (unfaced), which is also known as mineral wool insulation.
    Thanks for your question!”

  • B. Castella Says:
    October 18th, 2018 at 6:34 pm

    How to insulate the top of the basement foundation and space between the floor joyces (spelling?) Will the backing be toward the inside of the basement or against the outside wall.

  • Peter Notehelfer Says:
    September 10th, 2018 at 5:02 pm

    How far apart to I put the wire spacers when holding up the newly installed insulation in the crawlspace under my house? And do I buy the spacers at the same place where I buy the insulation? Thanks for your reply . . .

  • Official Comment:

    Thomas Boni Says:
    July 7th, 2018 at 11:58 pm

    Hi, Clayton,

    That should be no problem at all. However, you might consider mineral wool insulation, which is a lot more resistant to moisture and mold/mildew. Rockwool is the brand of insulation that we use.

    Good luck!

  • Clayton Ebsch Says:
    March 26th, 2018 at 4:35 pm

    Is it okay to place fiberglass insulation between joists that are on a concrete slab with osb subfloor above? This will be heated living space. About 6 -7 inches between the slab & subfloor, with always dry, well drained conditions?

  • John A Cervo Says:
    January 5th, 2018 at 8:08 am

    Excellent video. My house has an x board support in the floor joists for support. Also electric wires. Do you cover up the electric wires? Do you run insulation up to the x support or cover that up with insulation? Putting the paper so it faces the basement floor makes it look a lot neater and complete, but we are not to do it that way? The paper side goes up to the floor and insulation shows? Please advise. Thanks for your help.

  • Anna Janssens Says:
    December 31st, 2017 at 1:51 pm

    It is my understanding the insulation goes on the walls of the crawl space as well as the floor of crawl space. NO insulation on the ceiling of the crawl space.

    Please clarify.

  • Louise Gauld Says:
    November 17th, 2017 at 8:23 pm

    We have an old house and want to insulate between the floor joists but there are wires and pipes running everywhere. Do we cover them with the insulation, insulate around them? I don’t want to create more problems. It’s very tricky to do

  • Randy Bevills Says:
    April 29th, 2017 at 8:17 pm

    Can you use plastic to hold up the insulation and use plastic on the ground to stop moisture?

  • Don Says:
    April 2nd, 2017 at 3:37 pm

    We are installing insulation under our crawl space. Do we take out the old insulation or leave it there?

  • Jane Lindsey Says:
    January 30th, 2017 at 9:20 am

    We have an old house and want to insulate between the floor joists but there are wires and pipes running everywhere. I don’t see addressed anywhere what to do when encounter these wires and pipes in the space we are trying to insulate. Do we cover them with the insulation, insulate around them? I don’t want to create more problems.

  • carldonath Says:
    August 22nd, 2016 at 8:20 am

    I want to put insulation on the floor under the house I watched your video and want to know if it would be alright to do what you did only I would like to put foil bubble wrap on last to hold the insulation up. is this ok or not please let me know. Thanks carl

  • rodney Says:
    July 22nd, 2010 at 7:09 am

    in a crawlspace if the insulation is facing out it will retain
    moister and mold and eventually fall out it has to have some kind of barrier to prevent this from happening??????????

  • jd spencer Says:
    February 1st, 2010 at 12:17 pm

    installing roll insulation in ceiling of basement which is under floors of living space my basement is heated but a smaller amount than up in living space (usually 66-68 upstairs and 58-60 at the same time in basement) my question is about the kraft paper facing, I think it should face the basement because of any moisture in basement. It has concrete floors (not covered with carpet or anything other flooring)it’s 22 degrees outside and the hardwood floors upstairs are pretty cool. Before the floors in the living space were warmer because I was heating with baseboard heaters in each room but this year the electric company raised the price 2 cents a kwh and I got a $300.00 electric bill so now I’ve turned on the natural gas furnace.

  • mike Says:
    January 19th, 2010 at 10:38 am

    Will isolating the basement by insulating the ceiling cause the basement to become colder? If I have to now open the heat run in the basement that I currently keep closed, that seems like it will offset any savings I get in the living area. Not sure.

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How to Insulate Under Floors in a Basement or Crawlspace