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Insulating warm side of scissor truss ceiling

Paulmcgee | Posted in General Questions on


I am looking for help/best practice for insulating the warm side of a scissor truss ceiling.

Possible idea:

-Vapor barrier
-1″ rigid foam

Do I need the vapor barrier if I tape the foam?
Do I need strapping?

Thanks in advance.

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  1. ohioandy | | #1

    Paul, more info needed.
    - where are you located? Even just climate zone helps--it makes a big difference.
    - is there already insulation present?
    - is this relatively new construction? Are you remodeling an old house?

    You should also type "cathedral ceiling" into the search bar of this site--great articles that probably answer many of your questions.

    1. Paulmcgee | | #2

      Thank you Andy,

      Vancouver Island BC, 1200m, zone 5/6

      New construction, just the bare trusses at the moment.

  2. Expert Member


    How are you insulating the rest of the truss space, and why do you want to add the foam to the roof assembly?

    - With 1" foam you have the choice to strap or not.

    - The BC building code wants a vapour-barrier on the warm side of the roof. It depends on the thickness and type of foam whether it meets this requirement. Taping makes no difference to the vapour-barrier requirement. It is used to air-seal the foam.

    1. Paulmcgee | | #4

      Hi Malcolm,

      -The rest of the truss space will be batt/blown in

      -I would like to add foam to help prevent thermal bridging as our trusses are 12" centers for snow load. We also have an HRV exhaust in the ceiling which will take some of the excess heat from the wood stove so will want to retain as much heat as possible at the 14' height.

  3. Expert Member


    Makes sense. Rather than worry about taping the foam as an air-barrier, I would cover it with poly. Makes the building inspector happy, and does no harm in your roof assembly.

  4. Expert Member
    Akos | | #6

    Trusses are much lower thermal bridging than dimensional lumber. Adding the foam, won't budge the assembly R value that much, I wouldn't bother with it. A good warm side air barrier is important, focus on that instead.

    Depending on the slope of your trusses, blown in insulation is much simpler as it fills around the truss members, hard to do neatly with batts.

    If you are blowing insulation, make sure your vent baffle is detailed well enough to keep the insulation out of the channel.

  5. Paulmcgee | | #7

    Thanks all.

    I am hoping that insulating the inside of the trusses will have some benefits, mainly helping to trap the heat rather than transfer it to the trusses.

    I am thinking of this set up:

    Vapor barrier
    Silverboard 1/2" reflective side inwards
    1x4 strapping
    1/2" drwall

    -The strapping will help with pot light installation (1/2 thick units) only a single hole for each pot light wire will go through the foam/barrier and can be easily sealed from above.

    -My understanding is the air gap between drywall and foam/reflective surface will apparently improve the r value.


    1. Jon_R | | #10

      For heat loss, focus on whole assembly R value and ignore ideas about thermal bridging.

  6. hlederman | | #8

    Paul -
    The attached pics show how we approached this in our construction. We had a 24" raised heel scissor truss, carried air/vapor barrier over top plate for continuous barrier, and used blown in cellulose insulation, supported by heftier strapping. This allowed us to keep all electricals inside the envelope.

    Hope this helps you.

    1. Jon_R | | #9

      Did the weight of the "before drywall" cellulose cause fastener holes (if any) in the Tyvek to enlarge?

  7. hlederman | | #11

    No, it was very quick and easy to do, and no issues were apparent from fastening the strapping.

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