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Overdoing south-facing glazing

JTyler | Posted in Plans Review on

My current plan includes roughly 148 sqft of south-facing glazing on a 1400 sqft house. We have some nice views, and some small interior spaces that I want to open up with large windows. The house is in zone 6 at ~45* N latitude.

I have relied on the annual analysis calculator at http://www.susdesign.com to determine that I want a 3′ eave overhang 1′ above my 5′ tall windows. However, there is a 10′ wide glass door that will receive too much sun.

Can I keep this much south glazing and have a comfortable house? What additional steps should I take? Should I choose low SHGC glazing for the door? Can I rely on some combination of partial interior shutters and deciduous plantings if I am having overheating problems, or am I just poking a bear with this much glass?

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Replies

  1. GBA Editor
    Martin Holladay | | #1

    Jim,
    Your south-facing glazing area is 11% of your floor area. That's definitely on the high side.

    Most passive solar design guidelines advise that a house without extra interior thermal mass (located where sun will hit it) should have south-facing glazing that is no more than 6% of the floor area. Some passive solar designers say that 7% is OK without extra thermal mass; in high latitudes, some passive solar designers even accept 8%.

    If your design includes extra interior thermal mass (usually a concrete slab or a dark-colored tile floor on the south side of the house), you can increase the area of south-facing windows up to 12% (some experts say 14%) of your floor area.

    These are the old guidelines from the 1970s and 1980s. Much has changed since then, however. I urge you to read this article: Reassessing Passive Solar Design Principles.

  2. JTyler | | #2

    Thanks, Martin. I have read the article; actually, it's what moved me away from my original plan of a mud-job and tile in favor of lower mass floors for a more responsive house. Of course, that was before replacing some windows on the plan with the larger glass door. I do have a masonry heater directly in line with the glass door, but it is ~10.5' away, and so may not do me much good in the summer months when the sun won't reach it.

    Deciding not to do a mud-job tile install was a big relief for me - as a first timer, I see a lot of potential for hard-to-fix error. Would a dark tile over backer board be beneficial, even with the comparatively low additional mass?

  3. Expert Member
    MALCOLM TAYLOR | | #3

    Jim,
    Rules of thumb are great starting points, but need to modified to fit individual circumstances. The house I built this summer has 38% south glazing. However it sits on a waterfront lot which in the summer is often enveloped in fog, and by late afternoon invariably has a sustained onshore breeze. Avoiding overheating is as simple as opening two windows. Winter heat loss through the glazing is mitigated by our mild PNW climate.
    You could say these are rare fortuitous conditions, but if we had stuck religiously to the accepted formulas we wouldn't have benefited from them.

  4. brp_nh | | #4

    We are in zone 6 (NH), similar size house, thick concrete slab as first floor, and approx. 12% south facing glass (high SHGC). Eaves and window overhangs are just under 2', designed so we get full winter sun and block sun as much as possible in mid summer. No interior shades. More limited glass on east/west/north with low SHGC.

    We are comfortable during the couple of summer hot spells we get, I'd define hot spell as a few days 85f+. But, we do have a ductless mini-split and can run that on the lowest dehumidify mode to keep the downstairs 68-70f during those times. Upstairs can get a couple/few degrees hotter. We had a week of hot weather in September that was like mid summer because the overhangs aren't as effective.

    It's tough to give advice because we don't know your definition of comfort and the details of your house. Having to plant trees after the fact doesn't seem like a good idea. Shutters/shades probably wouldn't help in the summer anyways? You said the door was a change from windows, maybe revisit that? Will you actually use the door and it's purpose in the full heat/sun of summer?

    Sounds like if you're worried about it, don't have a high mass floor, and don't have AC....you might want to dial back the south facing glass. Just thinking out loud, it's your call.

  5. JTyler | | #5

    I appreciate the thinking out loud - especially with you having completed a project fairly similar to the one I am starting. Exchanging the lift and slide door for a 3' entry door and a 4' x 5' window, plus shaving 1' off the width of two other 4' wide windows in another room, puts me at 7% south glazing. It also removes the large section of glass that would have been exposed below overhang shading in the summer. As much as I was enjoying youtube videos of big lift and slide doors, I think this is a smart move. Thanks for the time guys.

  6. charlie_sullivan | | #6

    Yes, in the coming weeks I'll be replacing a vintage sliding glass door in my house with a side-by-side window and entry door. As a retrofit, that might be hard to justify but as a design choice in the initial construction it makes a lot of sense to me.

  7. varocketry | | #7

    What is a "big lift and slide door?"

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