Insulating a scissor truss

Hi all. Looking for opinions on how best to insulate a scissor truss. The attic in question will have have energy heel trusses with a bottom pitch of about 3/12; Soffit and roof vents with baffles. I'm concerned that just blowing insulation will end up with less than optimal insulation levels at the high point of the ceiling, from both gravity and wind wash. I've heard some idea of encapsulating the insulation? Any thoughts? Thanks.

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  • Scott,

    Is this new construction? What is the depth of the truss (ceiling to roof deck)? Is this a shed roof?

    Michael

    • Hi Michael. Its a gable roof. Depth of truss is about 5 feet, with about a 1 foot heel. What's your thought?

      • I'm not a fan of vented (open) attics/roofs.

        If your air barrier is well established to the interior of the walls (with Intello or the like), it is continuous and contiguous, then just blow the attic. If the air barrier is not well established, you could encapsulate the attic blow under an Intello Plus vapor open barrier. But the smart money is on taking the time to align your interior air barrier well, align your insulation barrier and if possible consider a second layer of exterior WRB (such as Mento) just under the exterior insulation or siding. This would allow you to lock out that attic air flow.

        Just remember if you build it tight you will require balanced ventilation in the form of an HRV or ERV.

        .02

        Michael

        • Interesting ideas. The walls are zip sheathing, taped at all edges, with a rain screen between the zip and the siding. I think it should be quite air tight. There is a soffit and ridge vent on all of one side and part of the other (this is an addition being married onto an existing house as a cross gable)

          Hopefully it will be tight and I do expect to need some continous mechanical air flow.

  • 5015117684?profile=RESIZE_930xHi Scott,

    Here you can see the truss in my roof. The skylight is flush with the pitched roof. I have a perforated radiant barrier under the roof rafters which is also stapled to the outer edge of the top plate to keep the insulation out of the soffit in addition to reflecting radiant heat. I filled the space between the ceiling and radiant barrier with cellulose in the lower portion of the ceiling and when it got to be 18" deep I just kept that depth. No problem with any wind wash or sliding. Hope that helps.

    Thom

    • Curous about that light fixture, if you take it down, and throw it away... Yeah just do that.

      • Thanks, Todd. That's such a helpful comment. I'll get right on it. Continue being helpful. You'll go far in life.

        Thom

         

    • Thanks, thom.  Like the idea. Kind of double duty using the radiant barrier.  I'd be ok with the webbing, I think.  Waiting to talk to their insulation guy to discuss options with him.

  • Consider a nail-base panel attached to the underside of the truss package. This will eliminate thermal bridging of your bottom cords as well as allow for a continuous air barrier once the joints are sealed. If your panel is thick enough, it also acts as your vapor barrier. The added batt (or similar) increases the thermal rating to the desired level. We use this detail a lot with good results.

    Al

    • Thanks for your reply. As I'm doing a drywall ceiling though, and blown insulation higher than the bottom chord, not too worried about thermal bridging. In some cases that would be a great plan.

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