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Community and Q&A

Insulating a parallel scissor truss roof

Tim Brown | Posted in Energy Efficiency and Durability on

The new side of my reno and expand project will have a 8/12 cathedral ceiling with parrallel scissor truss. I am planning R80 so 24″ of batt or blown insulation in the truss space.

My question is…. since this insulation will be applied from the inside (well after the roof sheathing and then shingles are applied) how do I provide a space between the underside of the roof deck and the top of the insulation?

It seems impossible with a sprayed product unless I build a channel or sorts to provide that airspace…like 2×4 laid on their side on either side of the truss with OSB cut and nailed onto the 2X to provide a top limit to the insulation and a 1 1/2″ airspace.

Am I lost or is that the only way?

I don’t want to build a HOT roof as they are not commonly done or understood here (Kenora Ontario Canada zone 7)… how do I build a vented roof…

Thanks

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Replies

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

    Tim,
    The installation of ventilation baffles is a fairly routine procedure. There are many commercial products you can use; a popular one is the AccuVent.

    To create deeper ventilation gaps -- many experts advise that gaps should measure at least 2 inches deep -- the usual method is to make site-built ventilation baffles. This can be done by installing 2 inch by 2 inch "sticks" in the upper corners of each rafter bay, followed by sheet goods of your choice (thin plywood, fiberboard, or rigid foam).

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

  2. User avatar GBA Editor
    Martin Holladay | | #2

    Tim,
    Although I understand that you are planning to install parallel-chord trusses, not scissors trusses, you may still be interested in the discussion in this previous Q&A thread: Insulating scissor truss.

  3. Doug McEvers | | #3

    Martin has described the process, furring strips on the top of the rafters and sheathing attached to form the air channel. Pay attention to the wind wash barrier at the bottom of the parallel chord trusses, this should be a continuation of the wall sheathing. Before insulating I will install the interior finish ceiling (drywall or other) leaving about 24" open each way at the ridge. The insulating contractors will have an easy time blowing insulation and they will net the last open area to finish the job.

  4. Tim Brown | | #4

    Thanks...looks like its easier than I imagined (feared)

    Thanks for setting me straight and the advise

  5. Aj Builder, Upstate NY Zone 6a | | #5

    Great advice given by our GBA advisor.

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