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Community and Q&A

Truss roof with multiple hips/valleys soffit to roof vents

user-833660 | Posted in Energy Efficiency and Durability on

Hi all. I have been reading some of the roof venting strategies and have not yet figured out my answer. We are in the middle of building our new home which was constructed with trusses for a multiple hip (courtyard in a ranch style house) roof.

The question is this – I am going to use about 24 inches minimum blown-in cellulose for about an R-60 or better rating, but I am not yet sure about using soffit to open space (ridge vented) pass through structures. Since my entire ridge line will be vented, and the soffits will also be vented, what is the reason or do I need to put in air paths from the soffits to the upper truss area?

The R60+ blown in insulation and superior sealing of the home should keep any moisture/heat escape to almost none so what is the reasoning for soffit to roof ventilation in my situation?

thanks all in advance.

steve d.

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  1. GBA Editor
    Martin Holladay | | #1

    Mark Dickerson / Steve D.,
    You didn't mention your climate, so I don't know if you have snow in your area. However, here are some reasons to provide ventilation channels between your soffit and your ridge vents:

    1. Since you are using air-permeable insulation, such ventilation is a code requirement.

    2. The ventilation channels may reduce the likelihood of ice-damming.

    3. If your ceiling air barrier is imperfect, or is ever accidentally disturbed in the future, the ventilation can help keep your roof sheathing dry.

    By the way, I hope you ordered raised-heel trusses so that you have deep insulation at the perimeter of your attic.

  2. user-833660 | | #2

    We are in northern Mass. and the trusses have over 2 feet at the walls of insulation space. no attic - just truss space. I was wondering if I should perhaps install a vapor barrier between the living space and truss space (plastic sheeting stapled to trusses prior to sheet rock)..

    thanks again.

  3. GBA Editor
    Martin Holladay | | #3

    Most codes require a vapor retarder, not a vapor barrier. I don't recommend the installation of polyethylene in your climate. However, you certainly want to be sure that your ceiling has been air-sealed.

    More information here:

    Vapor Retarders and Vapor Barriers

    Forget Vapor Diffusion — Stop the Air Leaks!

  4. user-833660 | | #4

    Thanks much!

  5. user-833660 | | #5

    I continue to think of more questions - For my trusses (cathedral ceiling-no attic). outside of sheet rocking properly, I have the ICF walls (12 inches - 3"+6+3" foam/concrete/foam) and direct attached trusses. I am building an interior wall system do I do not have to cut the ICF foam out and for my own ease of building, but what would you say is the best air sealing technique for ICF to truss systems? I want this place very tight - I do have pre-done passthrough for EHRV system pipes.

    thanks again.

  6. GBA Editor
    Martin Holladay | | #6

    You can create an air barrier at your drywall ceiling or at your roof sheathing -- your choice. Once you have chosen your air barrier location, you have to detail the roof/wall intersection so that the air barrier is continuous from the poured concrete in the center of your ICF wall and the air barrier in your roof assembly.

    If your wall has a top plate, you need a gasket or sill-seal between the concrete and the top plate. Then you have to seal all the cracks so that your wall air barrier and your roof assembly air barrier are continuous.

  7. user-833660 | | #7

    Thanks for the advice again. Our ICFs have wet set truss ties and there is no top plate. We used a foam liner between the cement and trusses. As for the sheathing, the roof is a "cold" roof so the sheathing is going to be ventilated from the soffets to the ridge line. So my blown in cellulose will sit on top of the sheet rock in the truss spaces. I will concentrate on sealing the truss attachment points with foam, and of course the sheet rock.

  8. user-1012653 | | #8

    just a heads up, has anyone else had issues with gyp spaced (assumed) 24" oc with 2' of cellulose and popping drywall?

  9. user-833660 | | #9

    I am putting strapping every 2 feet, so in essence there will be 2X2 foot openings in which the blown in cellulose will sit on the gypsum board. Perhaps I might staple mesh between the trusses for support. Just not sure yet.

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