Passive House Windows: SHGC - Solar Heat Gain Coefficient

Passive House Windows: SHGC - Solar Heat Gain Coefficient

High Solar Heat Gain Coefficient (SHGC) Windows Facing Towards Sun
Perhaps less understood with energy efficient windows (outside of the Passive House community) is the concept of solar heat gain.

Windows with a high(er) solar hear gain coefficient rating--having a greater ability to harness heat gains from sun and allow those gains to be passed through the window unit into the interior conditioned space--are most desirable (in cold climates).  In part, that's the meaning of the term "passive" in Passive House.

The use of such windows reduces (or completely eliminates) the need to actively heat with mechanical/electrical equipment, such as a forced-air heat-pump or geothermal system; hydronic radiant heating; or combustion.

In warm(er) climates, the opposite would be true; one would want to minimize solar heat gain to avoid overheating.  In climates that are more temperate, the most desirable setup would be to have windows that have the ability to harness (for free) heat from the sun during the colder fall and winter months, but minimize the heat-transfer during the warmer spring and hot summer months.  It is in this climate region of hot and cold where managing solar heat gain is important and can be tricky.

In an apparent contradiction of thermal performance, the most desirable windows for cold weather are those that have high solar heat gain coefficients (high SHGC values) and low u-values.  Windows so designed are net energy producers, in that they generate more interior heat (through solar heat gain) than they give back during the evening (through heat transmission loss to the outside).

To accomplish this, it is required that one side of a building structure orient towards the arc of the sun throughout the day.  In the Northern hemisphere that would be South facing, in the Southern hemisphere that would be North facing.  Furthermore, it is expected that the largest amount of glazings (windows), in total square footage, should also be placed on that side of the building.  It is here where windows with (very) high solar heat gain coefficients (SHGC values) are to be utilized.

It is generally accepted that windows on the East and West side of the building are neutral with respect to their heat gain versus heat loss as each orientation would tend to get a little bit of solar gain during times of morning sun-rise and evening sun-set respectively.

It is on the "dark-side" of the building, in our case the North side, where the number of windows is minimized since they will be net losers of heat since as there is no way to gain any heat from the sun.  Of course, if there are rooms located at this orientation, building code requires a certain amount and size of windows to allow safe egress in the event of an evacuation (due to a fire).  In any event, the use of windows should be used sparingly.

There are different viewpoints as to the kinds of windows, with respect to solar heat gain, that should be employed at each exterior wall orientation.  Some believe that using high solar heat gain coefficient (high SHGC) windows can safely be used at every orientation (including the dark side), which could be at the expense of thermal insulation performance (u-value), depending upon the manufacture selected.

Another viewpoint is to use high solar heat gain coefficient (high SHGC) windows on the side getting the most sun as well as the East and West side and on the "dark" side low solar heat gain windows with the lowest u-values possible.

And yet there is another viewpoint which suggests using high solar heat gain windows only on the side facing the sun and low heat gain windows at every other orientation.

My suggestion would be to analyze your building lot (whether there are a lot of trees or not) and determine the amount of solar exposure each side of the building receives and then run a variety of energy modelings via the PHPP program with varying window u-value and SHGC combinations.

With SHGC, the higher the number, the greater the solar heat gain.  The holy-grail of windows for Passive House (or any house for that matter)  for cold(er)-climates are windows that would have very high solar heat gain coefficients (SHGC) and very low u-values (very high r-values).  Finding windows in our region with that combination has been challenging; compromises generally have to be made.

Something to consider with windows having a very high SHGC rating is the potential for over-heating in the summer time. There are several things to consider: 

First, it is less expensive to cool than to heat, so the cost to offset overheating via active cooling is potentially less than having to add heat to compensate for windows that have low(er) solar heat gains.

Second, during the summer months the arc of the sun is higher in the sky than in the winter months even though the days are longer, which reduces the impact of solar heat transfer as the angles are greater.

Third, overheating can be minimized or completely eliminated by using shading--either with large overhangs in roof design that cast a large shadow (which aren't as attractive in my opinion), thermal shutters or awnings, using natural shade with plant/tree material such a deciduous trees or shrubs, by using windows that have moderate SHGC ratings, or some combination of these.

One final thing I am going to mention in this article about high-performance windows is their VT rating, that is visual transmittance.  We had looked at Serious Windows.  Their strength is their purported very low u-values even though their solar heat gains were not as desirable for Passive House construction.  One of the attributes we immediately noticed is that they were more difficult to see through as they had a low visual transmittance (VT rating); everything looked a dark greenish/grey which was not very appealing.

In my mind the ultimate purpose of a window is to let LIGHT in.  We can focus so much on solar heat gain and heat loss, that we can overlook the most fundamental purpose of a window: to provide natural interior lighting.  Who wants to live in a dark home?  We sure didn't.  Brighter houses are healthier to live in, require less energy to artificially light, and can reduce the onset of SAD (seasonal affected disorder).

This is where my wife and I "broke" a cardinal rule (not the only time either) of PH design, we used a large(r) amount of windows on the North side than would have been optimal.  We certainly didn't want a home that was bright on one side and dark on the other--a bipolar house, so to speak. To offset this designed-in weakness, we made up for this self-imposed "handicap" in other areas.  That's one of uniquely powerful aspects of PHPP is that it allows you to experiment with different designs prior to developing the architectural/construction plans.