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I have an A-frame roof (scissor truss)

Jason Fuhrman | Posted in GBA Pro Help on

I am using 2″ EPS as a baffle with 2″ of vent space (I am using 2″x2″ sticks of EPS glued to the underside of the roof deck for spacers). I am using “Great Stuff” to seal all the joints (where the baffle contacts the…)

I read somewhere to only put the “spacers” in the corners of the rafter space. I started this project before I read that, and I have some of the spacers more or less in the middle of the rafter space (not to one side or the other).

Any suggestions or concerns?

Please educate me.

Thank you

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Jason,
    If you have a few spacer sticks glued to the center of the ventilation channels, don't worry. It's no big deal. Other builders sometimes use that technique.

    You have two aims: You want to have enough support behind the ventilation baffle so that the ventilation baffle doesn't get crushed or deformed. And you want to leave the ventilation channel as open as possible (with as few restrictions as possible).

    One or two sticks measuring 2" by 2" are no big deal (as long, of course, as the sticks are installed parallel to the air flow, not perpendicular to the air flow). But if you can get away with just using sticks in the corners, without any sticks in the center of the rafter bay, all the better.

    For more information on this work, see Site-Built Ventilation Baffles for Roofs.

  2. Jason Fuhrman | | #2

    Thanks Martin. The whole cavity will look like this:

    1. 2" air space
    2. 2"EPS sealed with great stuff
    3. Between 8" and 15" of fiberglass
    4. 2" of EPS caulked with foam board adhesive

    Any concerns about this assembly in climate zone 5?

    Thanks

  3. GBA Editor
    Martin Holladay | | #3

    Jason,
    As long as all of the wooden components of your roof assembly are dry on the day that the work is performed, and as long as you make sure that your ceiling is as airtight as possible, your stack-up will work fine.

  4. Jason Fuhrman | | #4

    Martin:

    I have a lot of scrap pieces of EPS from when I cut pieces for my wall cavities. What do you think about filling the rafters with a combination of fiberglass and irregular shaped pieces of EPS? Everything else will remain as I described earlier.

    Thanks again

  5. GBA Editor
    Martin Holladay | | #5

    Jason,
    Q. "What do you think about filling the rafters with a combination of fiberglass and irregular shaped pieces of EPS?"

    A. I think it is a bad idea, unless the EPS pieces are installed in a continuous layer, with carefully sealed edges (sealed with canned spray foam), cut-and-cobble style.

    If you just throw a few chunks of EPS in along with your fiberglass batts, you'll end up with degraded performance from your fiberglass, and the EPS won't contribute in any meaningful way to the roof assembly's R-value.

  6. Jason Fuhrman | | #6

    Thanks Martin.

    If this was your house, how would you insulate the roof?

  7. GBA Editor
    Martin Holladay | | #7

    Jason,
    If this were my house:

    1. My first preference would always be for a house design with no sloped insulated ceilings. I would prefer all of the ceiling insulation to be on the floor of a vented unconditioned attic.

    2. If the house design required sloped insulated ceilings (cathedral ceilings), my first choice would always be to install as much of the insulation as possible as rigid foam on the exterior side of the roof sheathing. For more information on this approach, see How to Install Rigid Foam On Top of Roof Sheathing.

    3. If it was impossible to install rigid foam on top of the roof sheathing, I would always prefer a vented assembly to an unvented assembly. However, a vented assembly only works for houses that don't have dormers, skylights, valleys, or hips. If the roof has those features, options narrow considerably. If a vented approach is possible, I would insulate under the ventilation channels with dense-packed cellulose.

    4. If I had to create an unvented insulated roof assembly working only from the inside, options are extremely limited, so I would have to install closed-cell spray foam. I would install as little spray foam as possible -- only what is required to make sure that the interior surface of the cured spray foam was above the dew point during the winter -- and I would use cellulose to provide the rest of the needed R-49 or R-60 insulation.

  8. Jason Fuhrman | | #8

    Martin:

    Well, from what I can gather from your answer, I have the worst case scenario. My A-frame roof has the following features:

    1. A 12' wide dormer style roof protrusion on both sides of the house (one for the front door and one for the back.

    2. A 4' x 2' chimney that exits from the side of the roof.

    3. A 2' x 2' chimney that runs along the gabel end and the through the ridge of the roof.

    4. 2 side by side skylights.

    Given these features, how would you proceed to air seal and insulate the most economical and effective way possible?

    By the way, I can't express to you how much I appreciate your willingness to help me with this project. I learned a lot from reading your articles. I never dreamed I would have a chance to actually ask you questions (let alone read your answers).

    Thank you so much.

  9. GBA Editor
    Martin Holladay | | #9

    Jason,
    If you can't ventilate all of your rafter bays -- in other words, if you can't have an air inlet at the bottom of the rafter bay (usually a soffit vent) and an air outlet at the top of the rafter bay (usually a ridge vent) -- then you should approach this as an unvented roof assembly (Option #4 in the options listed in Comment #7).

    For more information on how to proceed, see How to Build an Insulated Cathedral Ceiling.

    Assuming that this is an unvented assembly, and assuming that you can't (or don't want to) install rigid foam above the existing roof sheathing, you have to install some closed-cell rigid foam on the underside of the roof sheathing. (Unless I missed it, you still haven't told us your geographical location or climate zone.) If you are in Climate Zone 8, you will need to install 5.5 inches of closed-cell spray foam. As the article explains, if you live in a warmer zone, the spray foam layer can be thinner.

    Once this spray foam has been installed, you can install some fiberglass batts, mineral wool batts, or cellulose directly below the cured spray foam (and in direct contact with the spray foam) to add enough total R-value to give you R-49 or R-38.

  10. Jason Fuhrman | | #10

    OK. I live in zone 5. I believe I need R20 before I can start the permiable insulation. So I guess I will have to use rigid foam or have a contractor spay foam it for me. That is not good news. More work and more money.

  11. Steve Knapp CZ 3A Georgia | | #11

    It might be possible to use reclaimed foam if you have good access. That would lower your material costs significantly.

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