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Community and Q&A

Foam free vented flat roof?

ShaunQC | Posted in Energy Efficiency and Durability on
Yes, I know, ‘just don’t do it.
I’m adding an ADU to my house and it must have a flat roof (1/4 / 12) given how it’s attaching to the existing house. It’ll have an EPDM membrane. I’m in CZ 6 (southern Quebec, near Newport, VT). I would like to avoid foam (and other materials with higher embodied carbon) above grade as a principal but realize a safer assembly is more environmentally friendly if it saves having to make repairs. I’m aiming for R60. There will be no mechanicals in the attic space. Maybe only a chimney and a plumbing vent passing through if I can’t put them out the side.
My options:
1. Vented roof: I know in these parts sentiment around this idea is quite opposed, however, as others have pointed out, they can work in this neck of the woods if done correctly. So, taking Joe Lstiburek’s advice as detailed in Martin’s article, I would have 24” open-web deep trusses with 16” of cellulose and an 8” vent space. I’d have soffit vents around the exterior bringing in air above the insulation. And I would use three cupolas / doghouses spaced out in the middle of the 900 sq ft roof. (Like these by Ventilation Maxium (

Joe Listburek says this will work. And there are the many examples of them not so far away in Montreal, but has anyone had personal experience building such an assembly? Since all the drying would be to the exterior would a smart vapour barrier on the interior be excessive in this case and poly would be fine as VB/air barrier? I’m not in a particularly windy area so wind-washing is less of a concern.

 – CDX
 – 24” truss
 – 8” vent space
 – 16” cellulose
 – Vapour barrier
 – 2x service cavity
 – drywall
2. If that assembly is too risky then I’m considering unvented with foam (I know I could use wood fiber but the cost in my area is steep and mineral wool is on par with closed cell for embodied carbon). To respect the ratio of exterior to interior, I could either do 6” of polyiso on top of the roof (R30) or 5” of HFO CcSF under the roof decking (the pricing difference is negligible, I think). I’d like to use dense pack cellulose for the other R30 but it seems difficult to net it up into the trusses. Michael Maines mentioned elsewhere you could weave the insulweb through the trusses. Has anyone done this, any tips? Otherwise we could strap batts, though I like that idea less from labor/strapping angle. 
 – *6” polyiso
 – CDX
 -* OR 5” CcSF
– 8” dense-pack cellulose
– smart vapour barrier
– 2x service cavity
 – drywall
Thanks for any inputs. I know this has been discussed often but I just want to get some clarification. 

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  1. Expert Member


    No recent experience, but what you (and Joe) describe uses the same mechanisms as the typical townhouse roof I grew up with in Montreal. I don't see any reason it wouldn't work.

    The potential danger with poly in climates like southern Quebec is during the cooling season, when they become wrong side vapour-barriers. That cooling period is too short to be problematic in the way it is in warmer climates.

  2. BSBot | | #2

    Your considerations for insulating a flat roof for an ADU in Climate Zone 6 (southern Quebec, near Newport, VT) with a focus on environmental impact and performance are commendable. Both vented and unvented roof strategies have their merits and challenges, especially in colder climates where insulation and moisture management are critical. Let's explore your options:

    1. Vented Roof with Cellulose Insulation:
    Your approach to a vented roof assembly, drawing on Joe Lstiburek's advice, is a viable solution, particularly with the use of open-web deep trusses, cellulose insulation, and cupolas/doghouses for ventilation. This setup aims to maintain a cold roof surface, minimizing the risk of ice dams and allowing moisture that penetrates the assembly to be carried away by airflow.

    Moisture Management: Given your climate zone, a smart vapor barrier on the interior might still be beneficial to manage vapor diffusion effectively during the colder months while allowing for drying to the interior during warmer periods. A class II vapor retarder could offer a balance between moisture management and drying potential.
    Ventilation Effectiveness: Ensuring effective ventilation with soffit vents and cupolas is key. The design should facilitate adequate air movement to carry away moisture without causing wind-washing of the insulation. Properly designed and spaced vent openings can achieve this.
    2. Unvented Roof with Foam Insulation:
    An unvented roof using foam insulation, either externally applied polyiso or internally applied HFO-blown closed-cell spray foam (CcSF), provides a compact solution with high R-values. This approach reduces thermal bridging and controls condensation risk by creating a warm roof assembly.

    Assembly with Foam and Cellulose: Combining foam with dense-pack cellulose adds complexity, especially in maintaining a continuous insulation layer and managing the potential for moisture accumulation within the cellulose layer. If opting for this route, careful detailing to ensure an air-impermeable insulation layer close to the roof deck is critical.
    Installation Considerations: Weaving insulweb through trusses for dense-pack cellulose application is a technique that requires precision but can be effective. Ensure the webbing is tightly secured to prevent sagging and allow for a uniform density of cellulose throughout the trusses. Consider consulting with insulation professionals experienced in this method for specific tips and best practices.
    Decision Factors:
    Environmental Impact: Your preference to avoid high-embodied-carbon materials is admirable. Given this, thoroughly weighing the long-term durability and performance of the assembly against the initial environmental impact is essential. Materials like mineral wool and cellulose have lower embodied carbon and are effective insulation options.
    Cost vs. Performance: While budget constraints are a consideration, the long-term energy savings, durability, and comfort provided by a well-insulated and air-sealed roof can offset initial costs. Metal roofing, although initially more expensive, could provide long-term benefits in terms of durability and performance.
    For your ADU in a cold climate, both the vented and unvented roof options have potential, with specific considerations for moisture management, environmental impact, and structural requirements. The choice between them should factor in long-term durability, cost-effectiveness, and alignment with your environmental principles. Consulting with a local building science expert can provide further clarity, ensuring that whichever method you choose, it is optimized for your specific conditions and needs.

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