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Community and Q&A

Insulating a scissor truss

Myrtleboone | Posted in Green Building Techniques on

When insulating an unvented scissor truss roof with loose cellulose, ~24″ heel height and ~ r-30 spray foam against the underside of the roof decking, can I leave an increasing airspace as I work my way up to the peak? In other words, I want a consistent thickness of cellulose (~18″) starting from the raised heel up to the ridge. Can the remaining space between the cellulose and spray foam be a air space or am I risking condensation somewhere in the sandwich? This is assuming air barriers at the roof decking level and bottom chord. BTW: Zone 7-8. Thanks.

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  1. GBA Editor
    Martin Holladay | | #1

    I don't like your proposed plan, for a variety of reasons, including the possibility of convective currents and air leaks under the driving force of the stack effect.

    Your insulation should be installed as a single layer of material, without any intervening air.

  2. Myrtleboone | | #2

    Thanks for your comment. So I will need to fill the entire void with insulation? For those that use common trusses, are they forced to use a vented roof as it would be unrealistic to fill the entire void with insulation?

    Do you agree on the spray foam under the roof deck to make it air tight, with the blown-in cellulose up to the foam? With your experience, what would most cost effective way to insulate the roof decking, above with rigid foam sheets, or below with the appropriate r-value (R-30 for Zone 7) closed cell spray foam?


  3. GBA Editor
    Martin Holladay | | #3

    There are at least two ways to insulate this type of roof, and only you can decide which way is the least expensive. You can discover which way is cheapest by getting bids from contractors.

    It's hard (or impossible) to use cellulose with scissors trusses. If you wanted to insulate your sloped roof assembly with cellulose, you should have used common (sawn lumber) rafters. So it's probably too late for cellulose.

    Your two best choices are probably to insulate from above with one or more layers of rigid foam (or nailbase, or SIPs), or to insulate from below with spray polyurethane foam.

    More information here: How to Build an Insulated Cathedral Ceiling.

  4. Myrtleboone | | #4

    Why is it impossible to insulate scissor trusses with cellulose-is it because of the increasing amount of cellulose required up towards to peak? Would this be a different story if I used a cathedral truss? BTW, I am only in the planning stages-no trusses built yet-I want to design a roof that allows for a vaulted ceiling that I don't have to vent.

  5. GBA Editor
    Martin Holladay | | #5

    Yes -- the problem with a scissors truss is that you need a very large volume of cellulose at the top -- so much cellulose that it would be expensive to install and might cause the drywall to sag.

    It would be easier to insulate a parallel-chord truss with cellulose than a scissors truss.

  6. Myrtleboone | | #6

    Am I on the right track to build an unvented cathedral ceiling like this?
    Metal roofing
    1/4" spacer
    Advantech roof decking
    CC spray foam under the roof decking (~R-30 for Zone 7)
    Blown-in cellulose from cc spray foam down to bottom chord
    Air barrier (possibly taped OSB for support above)
    5/8" sheetrock

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