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Community and Q&A

Thermal bridge

Chris Rorie | Posted in General Questions on

With regard to thermal performance in central Texas, how significant would the impact be (from thermal bridging) of the webs of site built, raised heel, steel roof trusses penetrating from an unconditioned attic thru 15” of cellulose to the corrugated steel ceiling below? There are a total of 78 webs. Both webs and chords are 3½” 20 ga. cee.

More or less than the impact a 3-0 X 5-0 .30 U factor window has in a well insulated shell.

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Replies

  1. User avatar GBA Editor
    Peter Yost | | #1

    Hi Chris -

    The thermal conductivity of steel is about 50 times greater than wood. Even in milder climates, thermal breaks with steel framing are worth considering from a thermal perspective. But perhaps more importantly, it's the potential for condensation that makes it worth it.

    There are three ways to deal with thermal bridging:

    1. keep the steel within the conditioned boundary
    2. locally insulate any steel to make the conductive pathway longer
    3. introduce thermal breaks: connector pads of much-lower thermal conductivity, slotted steel sections, stainless steel connectors/fasteners

    Will you be using your attic space for HVAC? If so, then #1 above is a good approach. To make #2 work well, you need a continuous air barrier "around" your 15-inch deep cellulose. For #3, check with your local steel supplier for whats sort of thermal break connectors might work with your assembly.

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