foam board on south walls only?

landngarageJune 30, 2012

My building orients north/south, meaning only one gable end faces south. Plans call for 2x6 walls sheathed with 1/2 OSB followed by Tyvek wrap. It will be bricked. I hope to use some closed cell foam inside but fiberglass batts will do the bulk of the insulating.

I read alot of talk on this forum about "thermal bridging" and if I gather correctly the solution is often "foam board". I brought this up to my builder and he seemed dismissive. I am on a budget, pretty much spent my wad on all the brick and the insistence on 2x6 walls on 16" centers with hurricane straps and simpson ties... on every 2x6.

I live near the Gulf of Mexico and it is hot and sunny. Should I insist on foam board on the south gable end?

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david_cary

It makes little sense to do foam on the south wall only. In your climate, foam board makes little sense generally. Your issue is solar gain more than anything else. Thermal bridging when your difference between indoor and outdoor temps is only 20 degrees is not much of an issue.

You want to minimize solar gain and properly size your a/c to limit humidity. Those are your priorities.

Next would be limiting internal gains. No incandescent lights. Cook outside. Hang dry your clothes or put a dryer in unconditioned space.

Thermal bridging becomes more important when it is zero degrees outside and 70 inside. Is there some heat gain at 20 degree difference - sure. But it is very minimal.

    Bookmark   June 30, 2012 at 7:35AM
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landngarage

Thanks for the quick response. "Solar gain"... I need to learn more about that to reduce it...The gable end that faces south has four 3x5 double pane single hung windows, two to each floor. The builder has talked me into a silverish 26 gauge galvalume roof, rather than the dark grey I would have preferred. The brick goes up the entire gable end.

Thanks again!

    Bookmark   June 30, 2012 at 8:08AM
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david_cary

Solar gain is generally through windows. You limit the gain by limiting the East and West windows and put overhangs on the South windows. Shade trees also help. Blackout screens help. Low SHGC windows help.

But hurricane windows aren't great with SHGC - at least the cheap ones aren't.

    Bookmark   June 30, 2012 at 2:09PM
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landngarage

Thank you David. There's probably not much I can do on the windows for budget reasons. I hope to someday have some sort of roll down hurricane shutters for while I am off at work. (There aren't many downstairs windows on this building, it being primarily a garage.) I'll look into black out screens and the like.

Thanks again!

    Bookmark   June 30, 2012 at 4:31PM
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energy_rater_la

our hottest walls are the west walls. but
foil/foam sheathing should be on all exterior walls.
tape all seams, repair any holes before cladding.
tyvek is not necessary with the foam sheathing well sealed.
(unless doing stucco..then two layers of tyvek)
making the wall air tight and reflecting heat out
will allow insulation inside the walls to perform.
check out building a perfect wall for all climates
at buildingscience.com

interior air tight drywall.
recessed lights ICAT

put the open cell foam under the roof decking.
unvented attics survive hurrican winds.
also on buildingscience's website.

look for windows with solar heat gain coefficients
of.30 or less. or solar screens.
window world..of all places.. has affordable
hurricane resistant windows.

size the hvac system for the improved structure
not just the average stick build. bigger is
not better when it comes to humidity removal.

darker roofs are hotter, reflective is better
but white is best.

best of luck

    Bookmark   June 30, 2012 at 6:49PM
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