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Is it practical to create an airtight barrier within a scissor truss?

Scot69 | Posted in Energy Efficiency and Durability on

First off, I would like to again thank you Martin and others helping me take my large home from using literally thousands of gallons of oil to last season using 8 tons of pellets. The advise was hard to take but true, people maybe questin but follow. I am building our retirement home in zone 2 in hurricane prone area and want to build a ventless roof to avoid windblown rain entering soffits and ridge. I would like to a 2 inch layer of closed with rockwool but can i create a good enough barrier within the truss or just use all foam, expensive of course thanks again

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  1. GBA Editor
    Martin Holladay | | #1

    Q. "I would like to [use] a 2 inch layer of closed [cell spray foam] with rockwool [batts] but can I create a good enough barrier within the truss?"

    A. First of all, 2 inches of closed-cell spray foam will definitely create an air barrier and a strong vapor retarder (with a permeance of about 1 perm). In your climate zone, 2 inches of spray foam is enough for a flash-and-batt roof assembly. So your plan will work.

  2. Scot69 | | #2

    Thank you do I add 2x up the truss side at the desired rockwool thickness to the ridge and apply osb and infill with 2x the gap left by nailers and tape. An added note the peak of the truss is 4 feet high and i want to run erv and hvac through remaining space and drywall bottom of chords

  3. GBA Editor
    Martin Holladay | | #3

    As I said, the closed-cell spray foam will create a decent air barrier. Your main problem has nothing to do with whether or not the closed-cell spray foam will work. The closed-cell spray foam is the easy part of the installation.

    The hard part is figuring a way to install mineral wool batts such that (a) they are in direct contact with the cured spray foam, (b) there are no voids, and (c) the batts don't slump over time.

  4. Scot69 | | #4

    Thanks I guess I funked my homework again, I thought I would need another barrier inside the envelope

  5. GBA Editor
    Martin Holladay | | #5

    It never hurts to have two air barriers. If you can find a way to install an air barrier on the interior side of the mineral wool batts, go for it. But I still think that the three biggest challenges are the ones I listed.

  6. Expert Member
    Dana Dorsett | | #6

    Properly insulating the roof deck with a truss is nearly impossible to do with batts due the detailed fit issues.

    A skinny-inch of closed cell foam is sufficient for dew point control in that climate, and the money not spent on the second inch of closed cell foam R6-R7) buys ~4" of open cell foam (R14-R15) bringing the total up to R20+. For another buck-fifty a square foot applied to open cell foam you'd be at or above the code-min R38 for zone 2. Open cell foam is it's own air barrier, and with a competent installer there will be no significant depressions or voids or thermal bypass channels that are all but guaranteed with a batt solution.

    The other way to go (and probably more expensive, due to higher labor costs) is to give it a 1" shot of closed cell foam followed by high density damp-sprayed fiber blown in mesh. With 1.8lbs per cubic foot density for fiberglass or 3 lbs per cubic foot cellulose an interior side air barrier isn't needed for maintaining thermal performance, since the high density sufficiently retards convective air flow through the fiber layer. To ensure that it sticks to the polyurethane for the long term the fiber insulation should be a type with water-activated adhesives, and damp sprayed, not dry blown, such JM Spider fiberglass, or any "stabilized formula" cellulose. Some stabilized cellulose is alleged to stick well enough to be able to skip the mes, but I haven't seen any third party data testing of that thesis, and getting a uniform depth might be difficult without it.

  7. Scot69 | | #7

    Thank you Dana it is amazing the knowledge you guys have. I didnt mention it but I was planning on filling the bay with fiberglass,I was thinking I could get better contact with it and then use the rockwool. I originally thought Iwould densepack it but as mentioned before by others that is not done down south much.On the walls would you put the foam in or out, I am planning on a rainscreen with another layer of plywood for ventalation and another layer of protection for windblown debris. Thanks again for your service to this site

  8. KeithH | | #8

    Ok, I'm not an expert like Martin and Dana so this is DIYer level advice. Repeat, Dana and martin are the experts, I'm not. Take it as such.

    I'm also a fan of Roxul and not a fan of blown foam so I like to offer my non-expert opinion for folks committed to avoiding large quantities of foam.

    If you aren't committed to wanting materials without fire retardants, I'd forget about the Roxul. Just too much fussy detailing. Spray foam and blown cellulose will be so easy (and thus cheap) comparatively. If you are committed to avoiding foam or cellulose, however...

    In my limited DIYer experience, you can use Roxul batts in your roof if you are doing the following:
    - Fully filling the depth of the joist
    - Using a standard spacing (16" o.c. or 24" o.c.) solid joist (you can deal with off sizes but the waste factor can rise quickly)
    - Installing a continuous air barrier membrane (Intello Plus or others) or solid surface (plywood, straps, drywall) using a mechanical attachment Just placing the batts, leaving a void behind your solid surface, and hoping for the best won't work. It may take years but you'll eventually have drooping batts that aren't as effective (I've done this experiment).

    I think these requirements aren't really any different than blown fiber products so I don't understand why they would be objections for doing it with Roxul. (except the standard spacing but then your framers, roofers etc are doing that anyways).

    However, with the batts, there is an additional practical requirement:
    -The required depth must be a stock size of Roxul Comfortbatt or a combination of sizes that includes at least 1 2x6 or 2x8 batt. The 2x4 batts only stay put under a deck long enough to install the membrane if there is a 2x6 or 2x8 batt underneath. So, for example, a 2x4 on top and a 2x8 batt underneath will stay put long enough to install your membrane or solid surface. 3 2x4 layers will not. There is a certain amount of squish forgiveness to Roxul comfortbatt thickness wise but it's not as much as width wise.

    Potential Nightmare With Trusses:
    If you have open web trusses and not joists, you have to figure out how you are going to deal with the truss web, which is definitely be here be dragons part of the map. If you are hiring an insulation crew and using open web trusses I'd forget using batts at all. I think I-joist trusses would be a manageable fit problem for Roxul but still require increased labor/install costs. .But if you are doing it yourself, it can be done. Without careful planning, however, you may end up with a nightmare of tape, nailers, filler roxul, filler blocking. I've done it and it was horrible. Self loathing required. It did work out well though so there's that.

    To summarize
    - Getting the Roxul batts to fit tightly width will be easy
    - Getting the batts to stay up long enough to install a membrane or straps is easy
    - Getting the batts to fill the depth is not too hard but requires planning and sizing that plays well
    - Dealing with the open spaces of trusses is really hard and painful.

    To be clear, IMO:
    They are easy to install if you are insulating :
    - the ceiling joists (e.g. vented gable roof with unconditioned attic)
    - walls
    - fully filled solid joists with a planned depth and a solid surface underneath
    - solid joist ends

    they are a potential nightmare to install if you are insulating:
    - open web roof trusses
    - open web truss ends (interior ceiling space for example) (assuming you want a 6 sided install)

    I hope this helps and await Dana and Martin's rebuttal. ;)

  9. Scot69 | | #9

    Thanks for your time Keith writing a response and taking that chance of being chastised haha. I am doing the work myself and I really did not want to use foam but I see no alternative as adding it to the deck would require me to resheave it in hurricane code. The idea of putting that much foam in scares me a little because I would be that guy in 50 that has a smell problem plus the green effect also.

  10. GBA Editor
    Martin Holladay | | #10

    Thanks for your useful comments. I have no reason whatsoever to contradict your logical conclusions.

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