Saturday, October 1, 2011


This section demonstrates how the sun will enter the house.  The windows were sized many to allow maximum heat gain during the winter and shade the glass completely during the summer to reduce heat gains. 

The sun is lowest in the sky on December 21st (about 25 degrees) and highest on June 21st (about 72 degrees).  To allows maximum full glass winter gains, there needs to be a compromise in the overhang length in regards to the summer sun angles.  4 degrees offset seems to make about the most sense.  This allows approximately 1 full month of sun on the glass in the winter (4 degrees above the winter solstice = about 29 degrees) and then 4 degrees below the summer solstice makes for approximately 68 degrees.  This allows for 1 full month of shading on the glass during the summer.  Using these angles, I am able to size my overhangs and heights of the windows to find the ideal placements for the windows. 

To increase the heat gains through the glass, glass with the highest SHGC (solar heat gain coefficient) values will be selected.  Typically, however, glass with high SHGC typically will not meat Energy Star ratings.  This is where I think Energy Star makes little sense, as correctly sized windows with high SHGC on the south in our heated climate will perform far better then a typical dual pane, lower u window.  However if triple pane windows are selected, they can be "tuned" by using hardcoat LowE coatings on several of the glass faces.  Typically, hardcoats will produce the maximum heat gains, while softcoats result in the lowest.  Most windows will be softcoat as standard as they are ideal for achieving the lowest u value as possible, which Energy Star wants.
Window companies such as Inline and Fibertec windows (as well as many others) allows you to 'tune' the glass and specify the coatings for each surface, for each window.
Using Inline thermal performance values, 2 options for the south would be a dual pane or a triple pane option.

Fixed windows
Dual pane numbers:  SHGC= .56,  U= .30
Triple pane numbers:  SHGC= .44, U= .19

Obviously the dual pane option gives a higher SHG value, however the triple pane does also come with a much lower u value. A comparison between the estimated heat gain difference (of .12 SHG) and heat loss (u .11) will have to be compared to the price difference to see if the triple pane option makes sense on the south side.

Comparitively, similar windows with softcoats are:

Fixed windows
Dual pane numbers:  SHGC= .33, U= .26
Triple pane numbers: SHGC= .28, U= .17

What does all of this add up to?  Well, it is estimated that the south glazing and high SHGC will supply approximately 28% of the required heating for the entire house during the winter. If the house were designed to be more passive solar utilizing storage mass, the heat the sun would provide would increase to over 50% of the required heat during the winter...all for free. 

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