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Community and Q&A

Durability of Vented Roof with Loose-Fill Cellulose

AdamPNW | Posted in Energy Efficiency and Durability on

My plan was to build a vented, scissor-trussed roof with r-49 loose fill cellulose.  However after hearing Jo Lstiburek (on GreenHome Institute) talk about how venting can actually ADD moisture to a roof, I’m wondering if this is still the most durable assembly for a marine climate zone 4c (long wet winters, short dry summers).

Additionally, the Pacific Northwest National Laboratory and BetterBuilt NW put out a paper about growing risk of wildfires in the PNW and the increased fire risk of vented assemblies.

So, assuming I have a meticulously smart membrane air-barrier (such as Intello) at the ceiling plane, AND a vapor diffusion port at the ridge, AND a dark colored roof…is it possible to have a durable unvented assembly even though there is loose fill cellulose (ie. space between the insulation and roof sheathing)?
Unfortunately I cant easily switch to rafters with dense-pack due to my floorplan.


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  1. Expert Member


    That breaks down into two related questions.

    The first is whether your code would allow such an assembly. It wouldn't here in Coastal BC.

    The second is whether in practice it would be safe - and the answer to that is far from clear. It might work in certain circumstances on certain sites, although we have no evidence it would - and quite good evidence that vented roofs in the PNW perform fine, with the exception of some occasional minor moisture damage due to night sky radiance.

    It's not something I'd be temped to try. Marginally safe assemblies that rely on perfect conditions often become risky over time, and not something I'd recommend including in a new build.

    1. AdamPNW | | #2

      Thanks Malcolm, that makes plenty of sense.
      Do you have any experience with dampers on a ventilated roof, for fire safety?

      1. Expert Member
        MALCOLM TAYLOR | | #3


        No. Although we live in a heavily forested area, we are fortunate to also be in both the coastal fog zone, and on the lower portions of the sloping terrain, which significantly reduces the risk here, so I haven't spent the time I otherwise would have learning about wildfire abatement strategies for houses.

        From what I've read here in GBA over the years California seems to be in the forefront of those discussions, and publishes good technical recommendations on materials and venting.

  2. Expert Member
    Michael Maines | | #4

    Anything is possible, it just depends on your tolerance for risk. Most of us pros involved in designing and building homes can't afford high levels of risk on someone else's home. I prefer vented roofs in most situations, in part because they are the easiest way to use materials with low up-front carbon emissions, but if you're building in a wildfire zone (which I also wouldn't do) then you need to make choices that are safe for you and your home. A traditionally unvented roof, using foam, is probably the least risky approach in a situation like yours.

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