Energy Efficient Windows- How well insulated do they need to be?

windowWhen thinking of energy efficiency in our off grid house, we paid a lot of attention to creating a design that would utilize passive solar.  The final version of our design has the majority of windows on the south side with minimal windows on the other sides of the house.  In the end we had 141 square feet of windows in our 1500 square ft house.  These windows are strategically placed in relation to the house overhang to shade out the hot summer sun and allow the sun in during the cold winters. How well the windows work depends mainly on their insulative value and the amount of the sun’s energy they allow in for heat.  So how well insulated do they need to be?  How much “sun heat” do they need to allow in? Would big box windows with the energy star label be enough?  What are my options?  This is the bunny trail that we went down while trying to figure out what the best option would be and yet not break the bank.  Here is what we found and our personal decision.

In the world of window insulation, there are 2 common terms you will hear; U factor and Solar Heat Gain Coefficient (SHGC).

  • U factor is referring to the amount of heat/cooling that leaks from inside your house through your window to the outside.  The lower the U factor = less amount of heat/cooling escaping out of your house.
  • SHGC or the Solar Heat Gain Coefficient is a representation of how much of the sun’s energy is coming into your house through the window.  Obviously, more sun means more heat.  The lower the SHGC = less solar heat coming inside your house.  The tricky thing here is that sometimes you will see where the SHGC is labeled as “center of glass SHGC” which is referring to the sun blocking properties of the glass glaze in the windows alone.  If it does not specify that, it is referring to the entire window (frame and glass glaze).

It is important to note that choosing between single, double, or triple pane (which refers to the number of layers of glass that are inside your window) also effects these properties.  The more layers you have, the lower the U factor and SHGC is going to be.

A typical Big Box Energy Star window that is double paned will have a U Factor of about 0.3 and a SHGC of 0.3.  A Specialty Window that is also double paned while have a u factor of between 0.02- 0.14 and a SHGC of 0.4- 0.5.  With this information, it is obvious that specialty windows do have the ability to give you more insulation.

So our next question was… so how much does this effect our house and our pocket book?

With our eye on efficiency and insulation being important, better insulated windows sounded great to us.  Sign me up.  So how much do they cost?  At the big box store we estimated that we would spend less than $5000 for Energy Star rated double paned windows to equip the entire house. We took our same windows to several specialty window stores for an estimate.  They all came back about the same amount… about $20,000.  Yes.. those zeros are in the right place.  Specialty windows would be 4 times the cost.  As an added bonus we also found out they are NOT eligible for tax credits.  That’s right, apparently the government thinks we all live in Florida.  Wow.  I was shocked.

Would the specialty windows be worth the extra expense?  Would we bite the bullet and make the investment?

The answer to this question gets muddy.  At this point we can share what it meant to us, but your decision in your own house must be dependent on where you live. If you live in an area with extreme weather (either extremely cold winters or extremely hot summers) you need to be sure that the window you choose gives you the best insulative properties.  Since our farm is in North Carolina, we do not have extreme weather for any extended period of time.  We have cold (to us) winters and hot, humid summers… but not anything considered extreme.  For us, we just could not make financial sense of spending 4 times the price of Energy Star windows .  The extra money we would pay for the highly insulated windows would take far too long to recoup in saved energy bills.  Now with that said, if we lived in an area of extreme weather the benefit of these windows might pay for themselves.

Here are some other resources that might help you make the best decision.
• Energy Star has come out with a chart and a description for every area in the United States that gives recommendations of the best insulation for your area.
www.EfficientWindows.org is a fantastic resource with great charts and clear explanations of everything windows.

So what will you choose?   Have you made the investment in Specialty Windows?  Please comment below and let us know your thoughts. 



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