GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Audio Play Icon Headphones Icon Plus Icon Minus Icon Check Icon Print Icon Picture icon Single Arrow Icon Double Arrow Icon Hamburger Icon TV Icon Close Icon Sorted Hamburger/Search Icon

Community and Q&A

Insulating scissor truss

Tom Smith | Posted in Energy Efficiency and Durability on

Hi, I am building my first house, what a project that has turned out to be but that’s for another day. Anyways, our great room has scissor trusses for a nice pitched ceiling. I would like to have at least 24″ of insulation to keep things nice and toasty in the winter. There is a 4′ overhang outside on either end. I nailed a 24″ high plywood leaving a vent space above where the red is on the layout. My question is what material and how to go about it. To use the high density cellulose is has to be contained by some mesh? Any input appreciated.

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.

Replies

  1. User avatar GBA Editor
    Martin Holladay | | #1

    Tom,
    First of all, here is some general advice to all GBA readers: you should nail down your insulation details before you specify and install your roof trusses. Insulation decisions shouldn't be last-minute affairs.

    It can be tricky to insulate this type of scissors truss, which is why I am not a fan of scissors trusses. Many types of fluffy insulation like to slide downhill, and the braces on the trusses make it difficult to install InsulWeb or similar membranes to contain cellulose.

    Gravity can work against you. It's always easier to insulate a flat ceiling than a sloped ceiling.

    There are several possible solutions, and the solutions depend to some extent on the steepness of the ceiling slope and the clearance available between the ceiling and the roof sheathing.

    One approach is to build an unvented roof assembly. This can be insulated above the roof sheathing with rigid foam insulation, or below the roof sheathing with spray polyurethane foam.

    Another approach is to use cellulose insulation, and to mound the insulation higher than usual, especially at the top of the slope -- in other words, install extra insulation -- in hopes that the extra insulation will make up for any slumping.

  2. User avatar
    Armando Cobo | | #2

    All trusses need to be braced together. Do so installing 2x4 bracing perpendicular to all scissor trusses at the bottom of both W5s to help contain some of the insulation slide as well. 4/12 pitch is not that steep, so you should be safe with cellulose. Use minimum 5/8 ceiling drywall to hold the weight of the insulation.

  3. Aj Builder, Upstate NY Zone 6a | | #3

    Vented roof
    bottom of trusses rigid foam taped.
    fill trusses with cellulose

  4. Tom Smith | | #4

    Hi, I did a lot of research and came up with InsulSafe SP is a fiber glass wool blowing insulation from CertainTeed. Does not settle and stays in place. I think this is the best option???

  5. User avatar GBA Editor
    Martin Holladay | | #5

    Tom,
    Your suggestion will work. I'm not sure it's the best option -- cellulose would be my first choice -- but blown-in fiberglass will work.

  6. Malcolm Taylor | | #6

    Go Logic architects, whose work has been frequently featured here at GBA, routinely use scissor trusses with cellulose. You can see a good cutaway drawing at this recent blog:
    https://www.greenbuildingadvisor.com/articles/dept/green-building-blog/cold-climate-collaboration

    .

  7. Tom Smith | | #7

    Cool picture, I see that cellulose should work. I am just curious why cellulose is the standard vs blown fiberglass wool? From what I read, cellulose settles up to 20% and wool only 4%. Would I assume dense pack cellulose should be the go to for the walls?

  8. User avatar GBA Editor
    Martin Holladay | | #8

    Tom,
    Q. "Why cellulose is the standard vs blown fiberglass wool?"

    A. Cellulose has a higher recycled content. Cellulose has a lower embodied energy. Cellulose does a better job of reducing air leakage. Cellulose can redistribute and store moisture, and this property can help even out fluctuations in moisture content due to temperature differences.

  9. Charlie Sullivan | | #9

    Settled cellulose has higher density than before it settled, and will stop convection better. Yes, you need to put in extra to plan for settling but what you end up with is not a bad thing.

    The picture in #6 looks a little iffy to me. The first 2 feet of cellulose near the eaves is held back from the roof, allowing ventilation space, but the next 2 feet looks like it would be hard to install with the close clearance shown and nothing holding it back. I'd think that vent baffles going up further would be a good idea.

  10. User avatar
    Stephen Sheehy | | #10

    We just did this in m y house. Put insulation baffles on the underside of the roof sheathing running well past where the top of the two feet of insulation would be. Installed air barrier membrane on underside of lower truss chord, then strapped it and drywalled. Then blew two feet of cellulose into space, accessing from an outside access port cut into the top of one gable, just under the peak.

  11. Bill Dietze | | #11

    You can view an example of the same scissor truss with loose cellulose in some Hammer and Hand Videos. Same 6/12 and 4/12 pitch arrangement.
    see
    https://www.youtube.com/watch?v=-zDqqfYbmqw
    and
    https://www.youtube.com/watch?v=ACDZ3gT7M4Q
    and
    https://www.youtube.com/watch?v=Pmx1GAHp1pc

Log in or create an account to post an answer.

Community

Recent Questions and Replies

  • |
  • |
  • |
  • |