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Meeting commercial energy codes for slate roof insulation

user-7206197 | Posted in General Questions on

I am detailing a commercial slate roof and I am having a hard time finding information on how best to insulate a slate roof while meeting commercial energy codes. The roof insulation must exceed 2012 IECC R value by 10% to meet a local green code provision. The project is a classroom building in the
4A climate zone.

The original roof detail has continuous insulation entirely above the roof deck with 5” poly iso insulation to achieve R value of 28. I have read Martin Holladay’s article “How to Install Rigid Foam on Top of Roof Sheathing” Do the assemblies discussed in the article apply to slate roofs or should the insulation be below the roof sheathing? Does the slate roof need to be ventilated or installed on battens if the insulation is above the roof sheathing?

Also, the roofing contractor has substituted the specified 2 layers 30lb asphalt impregnated roofing felt  with SBS modified bitumen underlayment. NRCA mentions the modified bitumen underlayment can act as a vapor retarder that may cause problems with ventilation and moisture. The project does not have an eave condition so I do not think we have to worry about ice dams which is what the product seems to be for. Is there any good reason to stay with the SBS modified bitumen underlayment?

Slate roof assembly detail is as follows:

Slate roof

Underlayment (see question above)

5/8” CDX plywood sheathing (recommended by NRCA for plywood)

5” poly iso insulation – 28 ci R value

Air barrier

5/8” plywood sheathing

Light gage metal trusses 24” o.c.

Thank you.

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  1. GBA Editor
    Martin Holladay | | #1

    First of all, can you tell us your name? (I'm Martin.)

    I advise you to take advantage of the resources provided by the National Slate Association. You'll probably want to buy their book, "Slate Roofs Design and Installation Manual" ($90).

    Most slate roofs are installed over vented unconditioned attics, not insulated cathedral ceilings. That said, I'm sure that it's possible to come up with details that will work.

    These roofs can easily last 100 years, so you need top-notch detailing. Above the insulation layer, you'll want roofing underlayment, followed by a well-ventilated air cavity at least 2 inches deep. Then you can install your slate -- ideally over skip sheathing.

  2. user-7206197 | | #2

    I want to keep the insulation at the roof level because there will be duct work below. But this is not a cathedral ceiling condition. There will be flat metal roof deck below the bottom cord of the trusses then a dropped ceiling.

    The contractor is concerned about attaching the slate through 5" of insulation. If the slate is anchored to the skip sheathing, then would the skip sheathing be anchored to the plywood sheathing below the 5" of insulation?

    I have an older version of the The Slate Bible and 5th addition of NRCA. Insulation was not mentioned as much as I wanted in those publications. I will look into the Manual you recommended.

    Thank you for the quick reply.
    - Lindsay

  3. GBA Editor
    Martin Holladay | | #3

    I was envisioning the installation of 2x4s on the flat above the rigid foam. These 2x4s would be installed either 16 inches on center or 24 inches on center, so that there is one 2x4 above each rafter. These 2x4s would be installed with long screws extending through the rigid foam to the rafters below.

    Because the 2x4s would be installed on the flat, they would create ventilation channels that are 1.5 inch deep. If you want the ventilation channels to be 2 inches deep, you could buy rough 2x4s from a sawmill, or you could create 2x2s (full dimension) on a table saw.

    Above the 2x4s you could install 1x4 skip sheathing -- old-school style -- for a traditional slate installation. Use solid copper or stainless-steel nails to attach the slates.

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