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Insulating truss joists 16″ on center — Best way?

WEG | Posted in Energy Efficiency and Durability on

Batt insulation for 24″ o.c. The 2x lumber is 23″ wide. The truss joists I’m looking at are 2×4 lumber 24″ o.c. The space between the trusses is 21.5″. Does anyone know if R-19 batts are available in 21.5 or 22″? Seems like squeezing a 23″ wide batt will not work well.

Also the tiger wires used on 2x lumber will not work well on a truss. Is there another method of holding the batts up to the floor?

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  1. GBA Editor
    Martin Holladay | | #1

    Fiberglass batts that are 23 inches wide can certainly be used to insulate bays that are 21.5 inches wide. The batts can handle that much compression.

    However, I don't like the sounds of your plan -- to install fiberglass batts between floor trusses, and hold them in place with "tiger wires." Your description raises two questions:

    1. Where is this floor? Is it above a crawl space or basement? If so, (a) you don't want any fiberglass batts in these environments, and (b) you should be insulating the walls, not between the floor trusses.

    2. Are you planning to install an air barrier? If you don't have an air barrier, the batts are close to worthless. And if you have an air barrier, you won't need the "tiger wires."

  2. LucyF | | #2


    I've had some recent experience insulating under a floor for a house on piers. Our space was on 16" OC though. It's difficult. We used a number of ways to keep the batts in place.

    Here's how we kept the Roxul 5.5 inch deep batts (which are above and not seen) and the 2 inch Roxul Comfortboard in place.

    I seriously air sealed every bay before we installed the Roxul.

    I am also working on a small cement block building. Here's one way I kept 23" denim batts in place. It's not perfect and I will cover them with my leftover 2' x 4' Roxul Comfortboard later. See photo at the end of the post.

    I would recommend either Roxul mineral wool insulation at that area or perhaps denim insulation. Denim is not itchy so it is so much more comfortable to install when you are crawling around a confined area. Roxul is a little better for warding off pests.

    Martin is flat out right about sealing around a crawlspace rather than the floor, the area you have to cover is so much less if you do the sides rather than the entire floor area. You will be much happier if you do that.

  3. GBA Editor
    Martin Holladay | | #3

    After that photo was taken, did you complete the installation by adding an air barrier (maybe plywood or OSB) under the joists? Or did you just put up a sign reading, "Come on in, mice and raccoons"?

  4. LucyF | | #4

    That's funny, Martin. I neglected to say that the picture is the ceiling inside the cement block building which is very well sealed. I did not vent it because I am going to redo the roof with a layer of foam on top then a metal roof with an air gap on top.

    The porch floor on the house we just finished is sealed with a very well-taped DA membrane from 475 building supply. The problem here isn't so much raccoons as POSSUMS. We built the house where my brother had a trailer and he had possums in one closet. If he opened the door to shoo them away, they would back talk at him before they would move.

    Here's a photo of a possum convention on my porch, stealing food for my only feral cat.

  5. WEG | | #5

    Subject is a vented crawl space with concrete floor and poured concrete walls. Because it is vented I didn't consider insulting the crawl space walls. Occasionally when snow is deep it melts and comes in through some of the vents, so customer wants to keep it vented. I guess I could do R19 fiberglass and run strapping thu truss joists to hold insulation up to sub floor. The sub floor would be the air barrier in this case.

  6. LucyF | | #6

    You can seal off the vents and make them air and water tight. You might be able convince the homeowner that this is an easier and more effective solution and probably less expensive. Some homeowners even if they are not doing the work themselves, are very invested in how something is done and will research building issues and be receptive to "new" ideas that result in a more energy efficient home. Maybe your homeowner will be responsive to some of this information.

    Martin's article on building an unvented crawlspace is excellent. He also has photos that clearly demonstrate why you want a sealed crawlspace.

    Here are two articles from Building Science, the experts on building safely and more energy efficient.

    The best article with good photos is "New Light in Crawlspaces"

    "Crawlspace Insulation"

    And if your homeowner is not receptive to converting the crawlspace to a conditioned space, then perhaps the best solution is the one in "New Light in Crawlspaces" where you install the fiberglass batts and then cover it with a sheet of foil faced polyisocyanurate which will perform several functions - hold up the fiberglass batts, air seal, and provide more insulation to keep the floor joists warm and prevent them from mold and rot.

  7. GBA Editor
    Martin Holladay | | #7

    If the vents are now letting snow enter the crawl space, why does the customer like the vents? Does he enjoy having snow in the crawl space?

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