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Community and Q&A

Ratio for outsulation for roofs?

davidbailey | Posted in Energy Efficiency and Durability on

Hi all in the GBA community!
My apologies if this question has been asked before…
I often suggest to my clients, when they are in need of a new roof, and if their specific conditions (i.e. vaulted ceilings) warrant this approach, that we do what I like to call an Operation Knit Hat to their home. This is a couple to a few layers of exterior rigid insulation installed over their roof deck, strapped with 2x material (usually 2×4) with a new roof deck -usually 5/8″ t&g OSB- paper -usually Tri-Flex or an equivalent- and whatever roofing materials over that.
Some details:
Depending on the weather, budget, and the insulation material choice (e.g. XPS vs Roxul ComfortBoard) I may or may not include a roofing paper under the insulation.
I tape to seams of the insulation (not the Roxul, though….).
If I use Roxul ComfortBoard, budget is certainly less of an issue and I include a vapor permeable membrane (e.g. Solitex Mento Plus, or an equivalent) over the original roof deck.
There may or may not be any insulation in the rafter bays, but if there is, it’s almost always poorly installed and pest annihilated fiberglass.
All that said, and to get to my question, I wondering if the same interior-to-exterior insulation ratios apply to a roof assembly as they do to a wall. I build in zone 6A in northern and central Vermont and my understanding is that here I want to strive for a 60% interior to 40% exterior insulation ratio (more to the exterior being better). Does the presence of the cold roof, as created by the strapping and new roof deck, somehow change the dew point of the overall assembly such that the ratio rule of thumb I mentioned before doesn’t necessarily apply? Anyone feel like laying down a little science onto this question?
And just to say it, when I do this to a home I push the client to include as much exterior insulation as they can afford to do – this has, so far, always gotten their overall assemblies within the 60/40 ratio, which I feel good about. I’m asking more out of curiosity and for the future situation where a person wants to do better with their home but can’t quite afford to make me feel good in this way.
As always, thank you all so much for your input! This forum has really provided a truly excellent wealth of knowledge.

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  1. GBA Editor
    Martin Holladay | | #1

    You've got the right idea, but the wrong ratio. In your climate zone (Zone 6), for the type of roof you are describing, at least 51% of the total R-value of the roof assembly must be provided by the rigid foam layer.

    The ratio for roofs differs from the ratio for walls, for a variety of reasons, including the fact that the stack effect and night sky radiation puts roof sheathing at greater risk of rot than wall sheathing.

    With this type of roof, including ventilation channels above the rigid foam is useful but not required. The presence of ventilation channels doesn't change the ratio rules.

    All of these facts are explained in more detail in the following two articles:

    Combining Exterior Rigid Foam With Fluffy Insulation

    How to Install Rigid Foam On Top of Roof Sheathing

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