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Unvented and unconditioned attic, is this to code and is this a good idea?

Brian P | Posted in Building Code Questions on

New construction in climate zone 6a, north/central New Hampshire. House specs:
-r45 walls
-approx r70 attic consisting of loose fill cellulose in rafters above drywall (the ceiling air barrier)
-goal of 0.6 ACH @ 50 Pascal

The plan is for an unvented and unconditioned attic:
-sealed below at the drywall
-Advantech sheathing on trusses
-roof underlayment either RoofTopGuard (http://www.rooftopguard.com/en/home.html) or heavy felt with Grace Ice and Water first 3 feet on edges, strapped, then metal roof (very simple A frame roof, rectangular 24×32 two story house)

This is a setup that our (experienced) building crew has done before.

We’ve tried general web searches and searching GBA, but what I’ve run into is unvented conditioned attics. We’ve also tried studying the IBC/IRC codes, but are having trouble finding an answer.

With such (anticipated) airtight construction, is it code compliant to have an unconditioned/unvented attic? And if so, is this a good idea and accepted practice in the energy efficient building community?

Thanks for any help you can offer.

Brian

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Brian,
    I have several questions.

    First, you wrote that you want to install cellulose "in the rafters." But I'm guess that you meant to write "between the ceiling joists." Which is it? Are you insulating the sloped roof assembly, or the attic floor?

    My second comment concerns your choice of roofing underlayment. RoofTopGuard is a synthetic roofing underlayment that (according to the manufacturer's installation instructions, which are binding by code) cannot be installed over an unvented space. More information here:
    http://www.rooftopguard.com/en/installation.html

  2. Brian P | | #2

    Sorry, I got mixed up, yes the cellulose will be installed between the ceiling joists so insulating the attic floor.

    Thanks for the note on the RoofTopGuard, I did see something about that on the installation instructions, but wasn't sure how "binding" that was. The specification of RoofTopGuard as the roof underlayment can easily be changed.

    If we wanted an unvented and unconditioned attic (and if this is allowed by code), would there be a best practice for the roof underlayment, taking into account the Advantech sheathing and metal roof?

    Or ultimately, is best practice going with a vented unconditioned attic?

    Thanks.

  3. GBA Editor
    Martin Holladay | | #3

    Brian,
    Code requirements for attic ventilation (in the 2009 IRC) can be found here:
    http://publicecodes.cyberregs.com/icod/irc/2009/icod_irc_2009_8_sec006.htm

    The way I read the code, attics MUST be vented, unless you choose to locate the insulation at the sloped roof assembly (making the attic part of the conditioned space). Here are the relevant sections of the code:

    "R806.1 Ventilation required. Enclosed attics ... shall have cross ventilation for each separate space by ventilating openings protected against the entrance of rain or snow. ..."

    "R806.4 Unvented attic assemblies. Unvented attic assemblies (spaces between the ceiling joists of the top story and the roof rafters) shall be permitted if all the following conditions are met:
    1. The unvented attic space is completely contained within the building thermal envelope. ..."

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