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Community and Q&A

Windows for Deep-Energy Retrofit

ERIC WHETZEL | Posted in Green Building Techniques on

I’m working on a deep energy retrofit near me. I’m concerned that the project as currently drawn has too much glass on the southern facade (kitchen-family room). Almost 40% of this wall area has glass. It does have ample overhang protection in the kitchen, but nothing, so far, above the family room windows.

There are also corner windows in the kitchen and family room (one in each space) that I worry will make overheating even worse, especially in winter, but also in spring/fall.

If going with less glass proves unacceptable to the homeowner, is going with low solar heat gain coefficient (SHGC) glass enough to counteract too much heat gain and excessive glare in fall-winter-spring? Or does a low SHGC also need to be combined with low visible transmittance (VT) in order to be effective?

If a combination of low SHGC and low VT is required, what are the aesthetic implications with this type of glass? Are there any concerns? Any other issues to worry about?

I’m also concerned that if they incorporate blinds/curtains that, over time, they’ll just end up being left closed most of the time, defeating the whole point of having all this glass in these rooms (in effect, becoming just poorly insulated sections of wall).

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  1. charlie_sullivan | | #1

    For heat gain, SHGC is an all-in number. You don't have to also look at VT to find the total solar heat gain. You only need to consider VT visual purposes like seeing out, daylight on the plus side and glare on the minus side.

    To think through the overheating potential, it would help if you said the climate. I saw all those VTs and thought vermont, but I don't think you actually said where the project is.

    As you got to very low SHGC you will start to have subtle tinting to the glass, ranging to obvious as VT goes really low. It's good to visit a showroom that has samples to get sense of how dark it is, and the subtle color differences.

    1. ERIC WHETZEL | | #2

      Thanks for the info, Charlie.

      Project is in suburban Chicago, climate zone 5.

  2. ROBERT OPALUCH | | #3

    Really someone needs to do heat loss and solar heat gain calculations. They are not difficult (really just arithmetic, preferably done using a spreadsheet on a computer for doing all the calculations and modifications efficiently; or using a computer application specialized for calculating heat losses or solar heat gains).

    Generally, VT<0.4 is too grayed out, and not aesthetically pleasing.

    You probably know this...VT and SHGC are correlated. High SGHC will be high VT and vice versa, but not an exact relation.

    Higher VT or SGHC and U-values are related as well. You won't get highly insulating glass (low U-Values/high R-value) with highest VT and highest SHGC, and vice-versa, but again not an exact relation. Depends on materials used (argon fill? Krypton? distance/gaps between panes? etc.)

    An important consideration is solar access. During winter months, the sun is low on the horizon to the south. In general, other buildings, stands of trees, etc. must be approximately twice as far to the south of the window as the height of the object in order to get the winter sunlight to reach the window (not be shaded by the buildings, trees etc.). Otherwise, in December and January, those buildings or trees will shade the south-facing glazing, so you don't get direct solar gain when shaded. Some measurements are needed to evaluate solar gain potential.

    Wintertime climate matters a great deal. Sunny dry winters are good for solar gain, overcast winters not good. Chicago isn't ideal for wintertime solar heat gain, but not the worst area either. Its moderately overcast during winter months. See this article for details:
    Note that Chicago is listed in all the Tables and Diagram. Chicago isn't a great area for wintertime solar heat gain through south-facing window glazing, EVEN IF you have good wintertime solar access (sun low nearer the southern horizon during Dec and Jan). But south-facing windows certainly are much better than east/west/north-facing windows for getting solar heat gain in winter and avoiding solar gain mid-summer.

    Finally, its better if the wintertime solar heat gain can be stored (or moderated) by a significant amount of thermal mass. So the temperature fluctuations inside the home range from high 60's F to under 80F on days that are sunny all day long in mid-winter. Things like a finished concrete slab floor, tile, stone, brick will absorb heat when the room gets warm, and radiate heat back into the room when the room gets cool overnight and during overcast periods. Otherwise, you have a solar-tempered space that gets heated up during the day, but no heat remains through the nighttime. So less glazing and solar heat is desirable if the interior has minimal thermal mass (carpets, wood, not any masonry).

    I'm assuming that all that south-facing glass isn't just there for wintertime solar heat gain. Is there a view to the south? Or are they planning to install nice landscaping in that direction? Or?

    As you correctly point out, overglazing doesn't just lead to mid-winter mid-afternoon leads to much higher heat losses during wintertime. Not a problem during sunny days, but a problem at night (its dark about 16 hours/day during winter). A good compromise might be to put more windows on the south side, but keep the area of the glazing to be about 10% of the floor area, not 30% or more. Less glazing needed on other walls facing north, west and east. Taking into consideration the views etc.

    1. ERIC WHETZEL | | #4

      Thanks for the input, Robert.

      The 'too grayed out' effect is what I'm worried about.

      And you're spot-on regarding Chicago winters --- we definitely have more than our fair share of bleak, steel gray days (they seem to come in bunches). Even so, when the sun does shine it is possible to enjoy some nice passive solar heating. One surprise in this regard, in our own house, is how ample glazing to the south can improve those gray days with a significant daylighting effect.

      Unfortunately, there are real limits. We managed to keep our south-facing glazing to about 15% of total wall area, which has worked really well for us. Even so, there's a couch in the family room that isn't much fun to sit on during sunny days from late December to early February. As a result, seeing up to 40% glazing for a south elevation obviously has me worried.

      And no, there isn't much of a view, at least nothing that would justify so much glass. It's aesthetics driving the current design. Unfortunately this has the potential to undermine occupant comfort. If this happens, it's likely most of this glass will end up covered with blinds or curtains in order to avoid excessive heat and glare. And in the winter, which would be the ironic part.

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