Windows and Passive Solar

Well it's been one month since my last entry and I must say I'm feeling a bit guilty.  Things slowed down a bit during the Pennsylvania Home Show (Bridlewood has a booth there every year) and we've been dealing with a whole lot of decisions.  But basically I've just been a bit lazy about posting updates.  Anyway, a whole lot has happened since my last entry so let's get started with the update.
In my last entry, the roof had finally gone up so that the house is now reasonably safe from the snow.  Well, believe it or not, one month later and the weather prediction for today is a whopping 74 degrees!  Since so much has happened since my last entry, I'm going to keep things simple and cover them one at a time.  This first entry will focus on windows.  Right after the roof went up, they started putting in windows and at this point they are all in place with the exception of the sliding glass doors.  I must say, it seems so much more like a home with the windows in.

House with windows (Click image for more photos.)

When you are trying to build an efficient house, windows are an important decision.  Windows are the weak point in terms of energy flow.  Compared to the walls and the roof, windows have very low R-values.  R-values are a measure of insulating power (or resistance to heat flow).  Windows are where the heat flows out of the house in the winter and into the house in the summer.  From a purely energy efficiency standpoint, it would be better not to have any windows in your house, but of course no one would want to live in a house like that.

Well after research windows for quite some time, I must say that window technology has come a long way.  Typically, a single pane glass window has an R-value of 0.85 or so.  When compared to the insulation used in a wall (typically R-12 for a 4-inch wall), you can easily see how windows can be problematic.  Of course, it's never as simple as it seems.  For example a 2x4 stud in a conventionally framed wall has an R-value of about R-4, so studs lower the R-value of the wall considerably.  That's one of the reasons why you can't just rely on the R-value of the insulation in the wall.  You really need to consider the "whole-wall" R-value, which includes the windows and everything else (and of course, this neglects air infiltration completely).  But I digress...

As I was saying, window technology has come a long way.  Today, almost all windows being produced are double-pane and "low-E" (low emissivity).  Double pane simply means there are two panes of glass instead of one.  This immediately more than doubles the insulating value because there are two panes of glass.  In addition, the gas layer between the glass also adds insulating value and it turns out that it is better to use something other than air (Argon is a common choice).  Low-E windows are a terrific (relatively) new technology that helps the windows perform even better.  Basically, specially designed coatings, such as aluminum oxide, are applied to one or more surfaces of the glass.  These coatings are designed to reflect infrared energy (heat) so that the windows retain more heat in the winter (and keep it out in the summer).  A typical double-pane low-E window (argon filled) will have an R-value approaching 4 -- a significant improvement as it now rivals the R-value of the studs in a wall.

So if double-pane windows are so good, why not use triple-pane or quadruple-pane?  Well, you can.  But the increase in glass adds to the weight and cost of the windows.  A new breakthrough that holds tremendous promise is thin-films suspended between conventional double-pane windows.  These thin films create multiple gas layers that improve performance significantly.  For example, Southwall Technologies boasts a "superglass" with an incredible R-value of 20!  Of course, as with many insulation claims this cannot be taken at face value because it does not incorporate the window framing into the equation (which lowers the value significantly).  These windows are also tremendously expensive and at some point it doesn't make financial sense to invest in such high-cost windows (at least, when you're on a budget like most of us).

So where is the point of diminishing returns?  For most people, double-pane low-E windows are going to be the best bet.  For us, this was fine for the windows on the north side of the house.  But because our house is designed for passive solar heating in the winter, the general "low-E" windows produced in the United States are actually a poor choice for the south-facing windows.  You see, in the United States, the federal government has issues a tax credit for energy efficient windows.  The is a great decision.  Unfortunately, this decision was made with all the forethought that is typical of a governmental decision (in other words, very little).  In order to qualify for the tax credit, the windows must have a "U-factor" of 0.3 or less and an "SHGC" of 0.3 or less (explained here).  The U-factor is simply the inverse of the R-values I talked about earlier.  Thus, an R-value of 4 has a U-factor of 1/4 or 0.25.  Great, so low U-factors mean high insulating capabilities.  That's what you want.  The problem is the SHGC.

SHGC stands for Solar Heat Gain Coefficient.  Quite simply, the Solar Heat Gain Coefficient tells you  how much of the sun's energy penetrates the window.  And if you want to take advantage of the sun's energy to heat your home in the winter, you want as much of this energy as you can get.  After all, this is FREE energy!  Thus, you want a Solar Heat Gain Coefficient that is high, not low.  A low SHGC window is great for southern climates that are mild in the winter and hot in the summer.  But for northern climates, a high SHGC (and low-E) window is ideal for south-facing windows.  Of course, you do need to be careful because in the summer, the sun's energy has the potential to make your house uncomfortably hot.  This is typically handled by designing appropriate overhangs to block the sunlight during the summer.

Now, since the government has issued this tax credit for windows, this is basically the only kind of window you can find.  We found it very difficult to find appropriate windows for passive solar heating in the United States.  (Incidentally, Canada has a rating system for windows that takes both U-factor and SHGC into account a single Energy Rating (ER) value.  So you can get good passive solar windows in Canada pretty easily.)  Fortunately, our architect and builder worked diligently (at our request) to make sure we found appropriate south-facing windows for our home.  Although it was by no means simple, Anderson windows allowed us the option of swapping the glass in their clad windows with Cardinal Low-E 179 glass.  This glass has a U-factor of 0.28 and an SHGC of 0.7.  These windows will let in more than twice the energy compared to windows that would qualify for the tax credit.  Now we can just sit back and let the sunshine do its job.