South facing windows?

mjtx2February 1, 2012

I don't want to get overheated in a house but I like the *idea* of solar heat gain, especially in the winter. We're going to have a ton of windows on one side of the house, and they can face either due south or southeast or somewhere in between. They're going to be large windows, too.

For those of you with several south-facing windows, did you use low-e and low-U windows, or windows that allow more solar gain? Is there a happy medium? We live in central Illinois and it (usually) can get pretty cold here.

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A generic tip to try and prevent overheating is to have somewhere between 5-9% of glass area in relation to your floor area the windows are looking into. An example would be, say you have an open concept plan. Your kitchen, living room and dining room area all open and they add up to be 1000 sqft. I would recommend not to exceed 90 sqft in glass. If you do, that is fine, but then you need to start to think about other areas to reduce the possible overheating. By adding mass to the interior of the space, that will help to absorb more of the heat and allow your floor area to glass area to increase some. Mass can be things such as tile floors and other dense products to help to store some heat. However typically mass with direct contact with the sun is most effective.
Low E windows can be had in a soft coat or a hard coat. Most US window manufacturers use soft coat as it typically has the lowest u value as well as lower SHGC. However I typically specify hard coat Low E coating on my south windows to get as much heat in as possible. A good solar gaining window would have a u of around .31 and a SHGC of .5+. However if you are already in fear of overheating, then soft coats are probably what you will want to use.
Another thing to consider is shading of the glass in the summer. Properly sized overhangs can do wonders in reducing your AC costs.

Besides glazing, I would also encourage you to put focus on the shell of your home and how it is constructed. For your climate (I am thinking zone 5?) 2x6 framing with exterior foam and high attention to air sealing details would be a good starting point. 2x6 bays filled with dense packed cellulose or blown Spider (or sim) fiberglass. I highly discourage the use of fiberglass batts.

    Bookmark   February 1, 2012 at 8:57AM
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"Properly sized overhangs can do wonders in reducing your AC costs."

Lzerarc, is there a rule of thumb for sizing the eaves (or other overhang) to accomplish this?

    Bookmark   February 1, 2012 at 10:45AM
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Some people have 'rules of thumbs', which can be 1' up and 2' out, but that doesn't really cover very many window options.
The best thing to do is consider the height of the windows in relation to sun angles.

In my blog I have diagrams about half way down listed under 'Sun Studies'. I would recommend checking that out.

Here is a link that might be useful: My home blog

    Bookmark   February 1, 2012 at 11:28AM
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Big windows are nice. However they can be cold in that any window will allow more heat through than a wall with insulation. So I would be more worried about heat loss an heating costs for large windows.

Given that you do have large windows, heat gain in the summer can be fixed by overhangs which shade the windows more in the summer than the winter. (Sun is higher during the summer months in Illinos, Northern Hemisphere). The other solution if you site allows is to plant deciduous trees. These will give you shade in the summer and light in the winter. Think of them as nature's sun blinds.

Best, Mike.

    Bookmark   February 1, 2012 at 11:52AM
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Properly "tuned" and sized south glazing for heating climates can typically be net positive for heat gain. Meaning, they will let more heat in then what they lose to the exterior over the course of the heating season.
However if you have large windows next to sitting areas, such as a living room with a chair right by the window or a dining table, I would recommend you consider triple pane glazing with hard coat LowE coatings. This will lower the overal SHGC some, but also help to eliminate cold air off of a typical dual pane window since the inner 3rd pane will be close to room temp thus no draft/cold spot feeling.

    Bookmark   February 1, 2012 at 12:44PM
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We are a bit north of Indy and the rear of our home faces due south. Ours is a 'typical' spec built home with normal overhangs, double pane alumimum clad windows (Pella). We have large windows across the back of the house. The kitchen and bedroom windows are roughly 6 ft x 6 ft and the family room has about 14 ft x 10 ft of windows.

In winter the sun is low in the sky so the sun pours into the house reaching as deep as 16 - 18 feet - just wonderful! It is amazing how much warmth comes in even on the coldest days when the sun is out. (Here in central Indiana, we could do with a lot more sunny days in winter - it can be so dreary.) Yes, windows allow winter cold in more than walls do but I will take big windows any day. A house can be well-insulated with quality windows without sacrificing views and sunshine.

In the summer, the sun is high in the sky so we get very little direct sun into the house. I have 2-inch wood blinds so I tilt them just enough to block direct rays into the house. A properly insulated attic can reduce heat gain from that overhead sun.

An amazing amount of information on the Building Science website that is specific to your climate with scientific research on what works balanced with economics.

Here is a link that might be useful: Building Science: Enclosures that work - New Construction

    Bookmark   February 1, 2012 at 8:01PM
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Mjtx2-hi, I'm also in C. Il. I was here 6 yrs ago building and we just purchased property and are about to do it again.

We have lots of big back south windows and used a compass to place our house south. That is how much we wanted to take advantage of the southerly exposure. In the summer, I do partially close my silhouette blinds in my family room and breakfast room but otherwise no issues with heat. Just plain Anderson windows. Love my big front North porch that is shaded all day.

Our new property, will have a back to the East and I will miss my South back. This time I am hiring an architect to get exactly what I want and plan to try to use a lot of south windows. Good luck on your build.

    Bookmark   February 1, 2012 at 10:54PM
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