Glass 101

[Jim Adrian]

I have seen several window posts lately and thought I share one on glass properties. I have tried to break this down to simple terms and only hit the important parts of glass.  There are many variables to glass but only complicates things.  I know we have wide range of knowledgeable people on this site!  When people talk about windows they forget about the glass which has the biggest effect on a house or building vs the window frame or air leakage test. Sorry this post got rather long!

An insulated glass is made of up 2 pieces of 1/4" glass with a 1/2" air space.  Glass can come with a low E coating or not. Always go with low e coating on the #2 face of glass. The window/glass supplier knows exactly what this means.  The #2 face of the glass is on the exterior pane of glass located on the inside face. Argon gas is optional as some say the gas leaks out over time.  Low E coatings reflect invisible light range and heat. You decide if you want to spend money on this.  I feel you it money better spend on other properties described below then on argon gas.

In general never go cheap on windows as this is one thing in a home that seems to get replaces once in every 30-50 years. You need a product that will last/perform. This is the biggest area where a house will lose/gain heat/cold pending the season and make your hvac system work harder. Hvac energy consumes about 40% of your utility bill (typically) based on seasons. Do you want to throw money out the window with energy cost that continues to rise or would you rather save some by buying a better glass upfront?

Glass comes in a variety of colors as well. Typically in residential market the glass is clear or gray in color.  Commercially you will see more the greens, blues and greys on buildings.  Colored glass does cost a little more and a clear to grey glass you can’t go wrong with.  Color does play a part in the performance of glass.

The other thing to look at is the glass performance. This is more important than the window frame. Look at % of visible light (higher the more natural) 35% to 45% is a good target number. Winter and Summer U-value: the lower the better; this is the inverse of the R-value of insulation. A good target for U-value is 0.26 to 0.32, lower the better. Example 0.26 u-value is 3.8 R-value and we put R-15 or greater in the walls typically.

These last 2 glass properties you can pick either one to use. Solar Heat Gain Coefficient (SHGC) measures solar energy absorbed into the building. A good target number is around 0.33 or lower, again lower the better. Lastly the Shading Coefficient (SC), target around 0.30, again lower the better. There are 9 different variables to look at with glass but only look at these few listed to simplify things.

Ask the window or glass supplier for a product data sheet of the glass properties. They should be able to give to you with ease.If they can’t then you can figure you are getting poor quality glass.

The lower the glass numbers the more it will cost. But will reduce the utilities allowing the tenant to have money to pay rent. I believe this is one area where you pay for what you get. Now it’s up the building owner to decide on how much they want to spend on windows.  I know we are all here to make a profit so spending less means more profit, but in the bigger picture is this the right thing to do?  This is for you to decide but here are some tools to help you make a decision.

This link to Viracon has more info on glass and is one of the largest glass mfrs. Your local shop probably gets their glass from here. PPG Glass is another larger mfr but I’m less familiar to their website. Local glass shops by glass by the sheet and make their own insulated glass unit themselves.  They may provide similar shades of glass or similar low e films but the values work out the same.