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Community and Q&A

Insulating wood I-joists with Roxul

user-1087436 | Posted in Energy Efficiency and Durability on

O.K., to repeat, I’m building a 1-story house in Zone 4C. Builder is using wood I-joists 16″ O.C. for the floor. After considerable thought and reading, including comments by Dana Dorsett and research by Washington State University, I decided that an open crawl space would work for me. However, I have insisted that we use good insulation: R-24 (commercial size) stone wool batts by Roxul for the floor insulation. This comes in 16.25″ widths. My builder says, Fine, but how are we going to shove them past the flanges at the top of the I-joist so that they’re snug against the floor? And would this cause voids? Good question. The most minimal TJI’s, with 1 3/4″ flanges at the top, would have 14.25″ between them at 16″ O.C. So I sent an email to Roxul in Ontario. They were useless. The girl that answered me scarcely knew what an I-joist was. Her salespeople, she said, had never heard of people insulating I-joists. And so I am asking the GBA community, which I guess means the long-suffering Martin. Does anyone have experience insulating wood I-joists with Roxul mineral wool batts?

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  1. GBA Editor
    Martin Holladay | | #1

    I have a great job, and I suffer very little. I also depend heavily on the GBA community to help me answer questions like this. I have never installed mineral wool insulation of any kind, so I have no direct experience to lean on.

    I do know that mineral wool comes in different densities and thicknesses. I assume that thin insulation -- especially insulation of a lower density -- will be more flexible than dense, thick insulation. So perhaps you just need to insulate your I-joist bays with two layers of mineral wool, and you need to choose the most flexible available product.

    I'd be happy to hear from experienced Roxul installers with opinions on your question.

  2. ROXUL Inc. | | #2

    Hello Gordon,

    My name is Karen Babel and I work for ROXUL Insulation. I am sorry to hear you did not have a successful experience with our contact us portal.

    I have spoken with our technical department and shared your situation with them and they would be happy to assist in any way possible.

    If you could please call our toll free number (1-800-265-6878) and request to speak with Max Maxim in technical, he would be pleased to assist you.

    Best of luck on your project and we look forward to speaking with you soon.


  3. GBA Editor
    Martin Holladay | | #3

    Karen Babel,
    I imagine that GBA readers would be pleased if Max Maxim posts the answer here, so we can all learn something.

  4. steveoneil | | #4

    I'm in the process of insulating an attic floor that has 2 x 8 floor joist and 1/2" strapping running perpendicular underneath them that holds the ceiling sheetrock for the level below. This creates a 1/2 gap under the roxul if i just lay it in the joist bays. So my solution is to cut a flat piece of roxul about 1/2' thick and put that between the strapping, then lay the full-size batts on top of that. This takes care of the potential void. Maybe you could do something similar. Your install would be more complicated because you'll be working against gravity, but if you cut it right, friction should hold it in place until you get the full size batt in to hold it all up. Get a big bread knife.

    Before I went DIY, an insulation contractor told me they could come in and use a blow-in roxul product. I didn't know one existed and he said that technically it doesn't, but they have source to get it before it is mixed with adhesive as a spray-on fire retardant in commercial applications. He said all I would need to do is install Tyvec over the floor joists and they would come in and blow the rock wool under it. I was all ready to go this route, but I never heard back from the contractor, despite trying to contact him with several phone calls and email. So I bought 87 bags of Roxul batts and this is how I now spend my "free" time.

  5. user-1087436 | | #5

    Thanks to all who replied, especially Karen Babel. I will definitely be in touch. The first question to answer is, is this a problem. Maybe it isn't. What we could do, I suppose, is buy the cheapest, thinnest unfaced fiberglass batts that we could find (in a 14.5" width), staple them between the flanges, then shove the 16.25" Roxul in after them between the webs, thus compressing the fiberglass and holding it in place. (How thin do they make fiberglass?) What I want to have is a ready-made product, not something that has to be modified by some guy working in a crawl space. We're talking about a 1900 sf one-story house, so that's a lot of crawling around.

  6. user-884554 | | #6

    There is no reason why not to use the rockwool batts as you had planned. As you point out, the issue is the 14.5" distance between the I Joist flanges. You asked 2 questions: 1) Is there a fiberglass product made in 14.5" width? The answer is no. I work for a large fiberglass manufacturer and am familiar with all competitive product offerings. 2).How thin do they make fiberglass? The answer is: That varies depending on the product application. Insulation typically used for thermal applications between framing members is at a minimum 3.5" thick. However, insulation made for the HVAC market (as duct wrap , duct board or duct liner) is available in thicknesses of 1 to 2". Your best opportunity might be with an HVAC contractor or distributor as a source for these products. Typically these products are available as board stock (duct board) or roll goods (liner & wrap). Regardless, you would still have to cut them to width. If I were you, I would look for the duct board product as it is fairly easy to cut with a standard utility knife or duct board knife. Make a template and cut as many as you need. The product is actually easier to work with than are standard insulation batts.The last bit of advice that I would offer is to be sure to mechanically support the rockwool batts in each joist space so that they remain firmly up against the above described fiberglass product. In order for this system to work, there can be no free air movement between the insulation and the decking of the floor above. Good luck.

  7. user-1087436 | | #7

    Thank you, Chris. That's sound advice, and a feasible option.

  8. wjrobinson | | #8

    Counter intuitive is sometimes more right than wrong.

    Joe L.... stated somewhere that insulation for floors should NOT touch the floor. And to me this makes perfect sense. The floor will be warmer because the floor and the air below it will all be close to the indoor room air temperature.

    What you need to do is make sure the PERIMETER of the floor is tightly sealed off with insulation.

    I believe with some searching someone can find where it has been stated to insulate as I am posting.

    So, no matter how you insulate Gordon, make sure to seal the bays up well with an air barrier and an insulation barrier on bay ends and below. Touching the floor is not important if all the rest is done right.

  9. user-1087436 | | #9

    AJ, you have hit the nail on the head. That is exactly the question I was going to ask next. Insulation works because it traps dead air space. If the joist pockets are sealed and insulated on the perimeter, and if the Roxul fits tightly, as it surely will, the air above it will be "dead air space." So why should it matter that the insulation is not touching the floor?

  10. Expert Member
    Dana Dorsett | | #10

    AJ: The whole cold floor thing has more to do with the temperature of the floor where the thermally bridging joists run. With a full cavity fill the joist edge will be cold at the floor level, even if the span between the joists are at room temp (which they are). If you have the upper 2-3" of joist surrounded by room temp cavity air the "coolth" of that thermal bridge is desributed over a much wider area, most of which is to the cavity air rather than the subflooring nearest the joist, and those cooler stripes all but disappear, giving the perception to bare feet of having warmer floor. (The average temp of the floor isn't really any higher, but the min-temp of the striping is MUCH higher.)

    A TJI has a lot less thermal bridging than a 2x joist, and the striping effect would be a lot less. And in zone 4C you're talking an under-house temp in low or mid-20s F on the coldest morning of the year, not -20F like you might experience in the Adirondacks or Quebec. It's never going to be a significant comfort issue in a 4C climate, even if you pumped the cavity full of 3-3.5lb cellulose (which is probably what I'd do, if the Roxul approach didn't work out for some reason.)

  11. alricb | | #11

    If you still want to fill that gap, they also make rigid rock wool boards. For 14.5" between I-joists, you could use 1" or 1.5" thick boards in 48" x 96" cut lengthwise or in 36" x 48" cut crosswise.

  12. user-1087436 | | #12

    Note to AJ Builder: Here is the Joe L. article you were talking about. He affirms that the floor is warmer if you leave a gap. There is a problem, though, and that is code. "Insulation must be in continuous contact" etc. But he discusses that and finds a way around it.

  13. Expert Member
    MALCOLM TAYLOR | | #13

    Gordon, I'll leave others to work out the insulating details you are asking about but would suggest not using TJs with small flanges just to minimize thermal bridging. Wider flanges on TJs make for a much better connection to the subfloor and also add considerable stiffness.

  14. Expert Member
    Dana Dorsett | | #14

    Listen to Malcolm! More than just improved structural capacity, wider flanges improve the cold-striping situation!

    Wider flange spread the heat loss over more floor area, so you'd have less severe minimums on the striping with and better evenness of floor temperatures. The flanges don't dramatically affect the thermal bridging- the webbing is the thermal bridge. The flange is just the heat/coolth spreader on the end.

    And again, this is a zone 4C climate which is quite temperate- not a cold enough for it to become a comfort problem. On a 14" deep TGI you have better than R15 even on that 3/4" stripe, which is more than the center-joist R of most existing homes with crawlspaces or pier foundations in climate zone 4C. AT R15 and a delta-T of 45F (about the 99% condition in that area) you're talking about a (45F/R15=) 3 BTU per square foot heat flux along that stripe, which means the stripe will be running about 1.5F below room temp, even if it weren't being spread by the flange. With the flange you're talking maybe 0.5-1F below room temp at the coldest point in the more-diffuse stripe.

    OK princess, do you REALLY think you feel a pea under that mattress?

    With 2x10 joists the R of the stripe is less, and the stripe 2-3x as wide as the web of a TGI, and in places where the design temp is -15F, for an 80F delta-T, yes, it's an issue.

  15. user-1087436 | | #15

    Thanks for the thoughts. The i-joists are spec'ed at a normal 1 3/4" wide, which is 1/4" wider than a 2x12. I'm not going to ask him to redraw the plans over that. I'm concerned about insulation. This morning I talked with the rep at Roxul. I won't summarize. They are going to be posting his conclusions, probably today. In the meantime, we're thinking of just packing the joists with cellulose. We're already doing the attic and the double-stud walls with cellulose, so the thought is, Why not go the whole hog and do the joists as well? My concern, of course, is containment underneath.

  16. Expert Member
    Dana Dorsett | | #16

    Even 5/8" exterior grade gypsum (DensGlass Gold) on the bottom nailed with ring-shank 8" o.c. would support 14" of 3.2lbs density cellulose (a minimal dense-pack, appropriate for your climate). OSB or plywood sheathing underneath would have HUGE capacity.

  17. user-1087436 | | #17

    Dana, thanks for that. It's an 11 - 7/8" joist, 16" O.C. Somewhere you (or someone else) talked about using something called Canvex. (???) What about 4x10 sheets of 1" duct board?

  18. Expert Member
    Dana Dorsett | | #18

    I've never dealt with it, but at 0.04 perms Canvex would be a TERRIBLE choice for putting on the exterior-side in your climate- it would be a true vapor barrier on the COLD side of the assembly. (see the bottom line of the spec.)

    Any material used on the under side would have to be rated for exterior use, and ideally at least semi-permeable to water vapor (more than 1 perm.)

  19. user-1087436 | | #19

    Dana, if you're saying it has to be rated for exterior use, does that rule out netting reinforced with battens? After all, it's not really exterior. There's not going to be any UV damage from the sun, and it's not going to get rained on.

  20. captainlabrador | | #20

    We have the same question - can Roxul Batts fit between 16" O.C. I-joists?

    Gordon Taylor mentioned that Roxul was going to post their own response, but I do not see anything?


  21. LucyF | | #21


    I've spent a good part of the past few weeks crawling around under the new house insulating with Roxul in an open crawlspace. We didn't use I-joists though. You can easily cut the Roxul a little narrower to fit the space where the flange is.

    Also I think the width of Roxul is 15.25" not 16.25".

    If you want to leave the gap between the floor and the insulation where the flange is as suggested by Dr. Joe Lstiburek, it is highly unlikely that any inspector is going to tear out 9-10 inches of insulation to see if you filled the cavity completely with insulation. I'd rather have the entire space filled with insulation though.

    In the main part of the house we used R-23 Roxul (5.5") and a layer of R-15 Roxul (3.5") to fill the floor joist bays. On our small enclosed porch we used the 5.5" roxul with 2" leftover Roxul Rockboard to fill in the 7" floor joist area. The Rockboard is a PAIN to use that way because you have to wedge it in between the joists against gravity. I'd do it again if I had to, but I would dread it again.

    I like the idea of using mineral wool because of the fire resistant properties, ability to shed water, vermin aren't attracted to it, and I can install it myself.

    We will install a smart membrane over the mineral wool to air seal it.

  22. nvman | | #22

    Hi Lucy,
    Sorry for posting so late to this thread.
    I too am thinking of insulating with roxul but for a cathedral ceiling constructed of I-joists.
    Gordon is correct. The dimension he quoted, 16.25", is for a steel stud batt.
    And like Gordon, I placed a request for information with Roxul.
    I received a reply but very short and only a regurgitation of the information on the website.
    Furthermore, it came from a "business development trainee" so I have a feeling that their experience is very limited.

  23. RobertMaxim | | #23

    Reference answer # 1 and #3

    I am Robert "Max" Maxim and I work for ROXUL Technical Insulation (industrial division) and previously worked as tech support for ROXUL Inc. (commercial and residential division).

    ROXUL batts in "steel stud" sizing should be used with TGI's and engineered joists; insulation batts designed for steel studs are slightly wider than batts sized for regular dimensional lumber and as a result work perfectly for positioning at the webs (rather than flanges) of engineered joists. If you wish to position the batt flush with either the sub-floor or the drywall you can easily cut a notch along the edge of each steel stud sized batt to accommodate the flange.

    For acoustics, having a small dead air space at the top of bottom of the insulation is preferable to pushing the insulation against the sub floor.

    For more information call ROXUL Tech Support (1-800-265-6878)

  24. ethant | | #24

    Has anyone at GBA followed Max's suggestion and installed Comfortbatt in TJI using steel stud sizing? I just spoke with Roxul, who informed me that the Steel Stud version of the Comfortbatt comes in 16 1/4 and 24 1/4 inch widths, as apposed to the standard wood stud sizing of 15 1/4 and 23 inch widths.

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