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Community and Q&A

Insulating options for open-web floor trusses

jroy | Posted in GBA Pro Help on

Would greatly appreciate the collective advice from the readers of GBA on the following:

I have 18″ open-web floor trusses installed in a unconditioned garage with a conditioned room planned for above. Have had many opinions from insulation reps on how to insulate. Foam, flash & batt, blown cellulose/fiberglass, sprayed rock wool, and fiberglass batts.

I am leaning towards blown cellulose, but have concerns about settling over time. The garage walls will be insulated with blown insulation but I am not planning to heat it. Not sure what is the most feasible solution that balances economic concerns with insulation needs. I am located in zone 5.

Thanks for your thoughts on this…

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  1. GBA Editor
    Martin Holladay | | #1

    First, let's make it clear that fiberglass batts are the worst possible insulation choice for this application. In many garages similar to yours, the fiberglass batts are basically useless, leading to comfort complaints from the occupants in the bonus room upstairs.

    Many builders in Canada have switched over completely to the use of spray polyurethane foam to insulate bonus room floors, and this has greatly reduced callbacks for cold-floor complaints.

    I think that your plan to use cellulose will work well, as long as:

    1. The cellulose is installed by an experienced cellulose installer with a good reputation.

    2. The cellulose is installed to a minimum density of 3.5 pounds per cubic foot, using the dense-pack method.

    For more information on the dense-pack method, see How to Install Cellulose Insulation.

  2. user-577475 | | #2

    I looked at a similar situation with plywood web joists (TJI's). It is pretty much impossible to get bats to fill in all the spaces with either type of system. I got prices for foam and the contractors all said anything over 3"-4" was a waste of money even though this would not give the minimum recommended R-value for my area and was pretty expensive even for that amount. I decided to go with a foam flash coat at the perimeter to air seal, and blown in cellulose. Even if it settles I will have a good R value and the cavity will be on the warm side (floor) in a theoretically sealed cavity. If I get concerned in the future, I figured I could alway make some holes in the gypsum board ceiling and add more cellulose. Since cellulose does not in theory have a reduction in R-value when "dense packed", I'm assuming there will no loss, and perhaps an increase, in performance. I would appreciate any other thoughts on this as this is a pretty common construction methold in my area and I could find little informaton on how to insulate it. By the way, I'm using radiant in floor heat above the garage just so there is no problem with the cold floor normally associated with a room above a garage. I also have the ability to heat the garage with a in slab radiant hydronic system, but really don't want to spend the money for heating this other than when needed.

    1. JayMart | | #16

      This is my exact situation to a T (radiant in floor as well as garage). What did you end up going with?

  3. jroy | | #3

    I too looked at foam and had the same problems as Corian pricing for depths of 3" & 4"(was also told any more was a waste of $$$...but the price point for a flash coat was very expensive and still didnt meet minimum required levels.Im having 1" t&g rigid installed around the perimeter prior to installing any insulation.
    I have the option to use attic guard plus as an alternative to cellulose and I wonder what people here at GBA think about blown fiberglass as an alternative option to cellulose.

  4. bdrfab | | #4

    Martin, How feasible is it to dense pack in an open web truss? Would it be better to staple plastic/cardboard to one side to create a more defined cavity?

  5. GBA Editor
    Martin Holladay | | #5

    Good question. I'm not sure; I'll ask Bill Hulstrunk.

  6. user-723121 | | #6

    The important point here is to fully insulate and air seal the rim joist area. Even if the blown insulation did settle a bit, the thermal boundary would be continuous, the space between the floor and the insulation would be warm. Insulate the rim joist area with tightly fit rigid insulation and cover the framing with a layer of rigid.

  7. dankolbert | | #7

    Re: Aaron's question - I've never blown open web floor joists, but I have blown in both double-stud 12" thick walls and in site-built 14" open-web rafters. Careful installers shouldn't have a problem doing a good job.

  8. user-869687 | | #8


    If you're concerned that 18" deep dense-pack over insulweb might sag against the ceiling drywall, you could install 1x4 furring below the insulweb. Run the furring parallel to the trusses so there's no concern it might droop, and this will reduce the number of staples you need in the fabric.

  9. Foamer | | #9

    I am curious how your various options are pricing out. More specifically dense pack cellulose vs. spray foam. In my neck of the woods, R-30 foam would cost between $2.50 and $3.00 per square foot but i don't know how that compares to 18 inches of dense packed cellulose. The cellulose will give you massive r-value while the foam will keep the insulation where you want it (against the floor) and leave plenty of room for anything you may want to run in your trusses. Are the savings worth it?

  10. homedesign | | #10

    I thought Doug McEvers comments were good.

    The snipit Illustration is from A Lstiburek paper about crawlspaces....but he does comment about conditioned rooms above garages.

    I would think that the rimjoist and the garage ceiling should be detailed for airtightness.

  11. jroy | | #11

    I was given a base price package for R38 fiberglass batts for the garage ceiling with upgrade options (listed below)..

    garage ceiling (blown cellulose) R70 +$225
    garage ceiling (closed cell foam) R21 +$!,385

    seeing that my R value for cellulose was over 300% greater & the price difference was substantial, it didnt make much sense to go for the foam...

  12. GBA Editor
    Martin Holladay | | #12

    You asked, "How feasible is it to dense pack in an open web truss? Would it be better to staple plastic/cardboard to one side to create a more defined cavity?"

    As it turns out, I was visiting a job site in Washington state last Thursday, were I saw some open-web floor joists in a crawl space that are being prepared for a dense-pack insulation job. The builder is using Insulweb fabric to compartmentalize the open-web trusses. I took a photo:

  13. bdrfab | | #13

    So it would seem it can be done both ways. Thank you Martin.

  14. jroy | | #14

    Is there any thermal advantage to encasing the wooden members of the truss in insulation on all sides instead of just on the sides??

  15. GBA Editor
    Martin Holladay | | #15

    If you are asking whether a layer of rigid foam under the floor joists would reduce thermal bridging -- the answer is yes, it would.

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