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Community and Q&A

What’s the best all around roofing/insulation solution for Zone 5?

Roger Smith | Posted in Green Building Techniques on

Hello to the community. I am in northern Nevada zone 5. This is an arid valley with normally low humidity but it can get some really high winds… upwards of 80 mph. The house I just bought was built in the 70’s. There is about 6 inches of blown insulation in the attic but none under the roof sheathing. I do plan on bringing the attic insulation up to at least R40. The attic has two small gable vents but zero soffit vents. The roof is completely failed cedar shakes with a lot of exposed sheathing throughout. Obviously I plan on re-roofing asap with winter coming on but this is also an opportunity to go green on the most critical part of the house – the Roof.

So what is the recommended roofing solution for this climate and circumstance???

1. I plan on using the new asphalt shingles with the reflective material. Owens Corning Duration Premier Cool shingles are one such product.
2. Additionally should I use:
2a. Sheet rigid foam (polyiso or EPS [or a combo]) under the shingles? If so how much?
2aa. Would I need an air gap?
2ab. Would a roof liner or membrane (in addition to the roofing felt) add anything of value?
2b. Insulation between the rafters under the sheathing? If so How much?
2c. or both??? if so, How much and what composition.

I am sure lots of folks are facing some combination of problems of a similar nature so I hope this inquiry sparks a lively discussion.

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Replies

  1. Steve Knapp CZ 3A Georgia | | #1

    Do you have a simple roof? Are you willing to convert the attic to an unventilated space? If yes on both counts, then exterior rigid foam would be a good strategy. Martin elaborates on this topic in this article: https://www.greenbuildingadvisor.com/articles/dept/musings/how-install-rigid-foam-top-roof-sheathing

  2. GBA Editor
    Martin Holladay | | #2

    Roger,
    Before you can develop an insulation plan, you need to decide whether you want to have a vented unconditioned attic (by far the best approach, as long as there is no HVAC equipment or ductwork in your attic) or an unvented conditioned attic (the approach which is often used for homes with HVAC equipment or ductwork in the attic).

    Here is a link to an article that describes what you need to do to insulate a vented unconditioned attic: How to Insulate an Attic Floor.

    Here's a link to an article that describes what you need to do to insulate an unvented conditioned attic: Creating a Conditioned Attic.

    One way to create an unvented conditioned attic is by installing rigid foam on the exterior side of the roof sheathing. Here is a link to an article that tells you more: How to Install Rigid Foam On Top of Roof Sheathing.

  3. Expert Member
    Dana Dorsett | | #3

    In your climate using a cool roof shingle will end up increasing energy use, and increasing the average moisture content of the roof by lowering the average temperature at the roof deck. You'll also find that with R44 in the attic a cool roof shingle barely moves the needle on cooling load. Use advisedly, and only with a vented upper roof deck.

    In a zone 5 climate with an insulated roof a minimum of 40% of the total R has to be on the exterior of the structural roof deck for dew point control. At the IRC code-min R49 the prescriptive is R20 on the exterior, R29 under the roof deck, but that's a minimum- more is better. Using 4" of polyiso strapped to the roof deck by 2x4 furring through-screwed to the structural deck with 6.5-7" pancake head timber screws 24" o.c. would give you something on which to mount an OSB or plywood nailer deck on which to apply the roofing felt & shingles. If you use a self-healing membrane (eg Grace Ice & Water Shield) between the foam & roof deck you won't end up with moisture wicking down the screws to the structural deck.

    At 4" the labeled R of the polyiso will be 24, but the mid-winter performance will average roughly R20 (a bit less at the coldest temperatures), but that's enough dew point control for R30 batts in 2x10 rafters, or compressed (to about R25 performance) in a 2x8 rafter, or an R23 rock wool batt in 2x6 rafters. Any of the above will meet code-minimum on calculated U-factor basis (U0.026, maximum.)

    Using reclaimed roofing polyiso will usually save 2/3 or more of the material cost of the foam. While 4" is a standard thickness, there is an advantage to using two layers of 2" (also a standard thickness), with seams overlapping. If need be the bottom layer can be cap-nailed in place at loose spacing to secure it while applying the second layer. Taping the seams of both layers with housewrap tape is recommended.

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