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Community and Q&A

Roof insulation plan — low slope — trusses

Alan Afsari | Posted in GBA Pro Help on

Prior roof questions —

Climate Zone 5 – low slope roof –

most of the roof is cathedral – 12″ I-joists – flash and batt (fill- with BIBS) approach.  From areas that use trusses and the interior ceiling is flat, do you do the same thing? (flash and fill – foam on undersurface of the roof deck with BIBS suspended below it). We would still use an unventilated assembly.


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  1. GBA Editor
    Martin Holladay | | #1

    The image that you attempted to upload didn't work. (That problem may be due to a site glitch on GBA -- I've had the same problem recently. I hope the problem is resolved soon.)

    If you use the flash-and-fill approach for your low-slope roof, that doesn't mean you have to use the flash-and-fill approach everywhere. But for the roof assembly that has a horizontal ceiling, I'm not sure what type of attic you have above the horizontal ceiling. You wrote that this assembly with a horizontal ceiling would still be "unventilated" -- and I'm not sure why, if there is an attic above.

    1. Alan Afsari | | #2

      Martin - the slope is 1.5:12. There is only about 11" for an air space and insulation where the roof meets the exterior wall. I didn't think there is enough room to ventilate it. I also thought the low slope has inadequate vertical difference to get air to move through an air/vent without a dog house on the roof - something we would like to avoid.

      Is my understanding of this inaccurate?

      Thanks, Alan

  2. GBA Editor
    Martin Holladay | | #3

    For the portions of your house with a horizontal ceiling, it would be safer to keep the flash-and-fill insulation along the roof plane, not against the horizontal ceiling. Otherwise, there is a risk that you'll get moisture condensing on your cold roof sheathing.

    Without any attic ventilation, this risk increases. All it takes is a few air leaks for your roof sheathing to be in trouble.

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