Replies

1. Expert Member
   BILL WICHERS | Jul 18, 2023 06:24pm | #1

   Since the vast majority of the insulating value of an IGU comes from the gas fill between the panes, and not the glass itself, I doubt iron free glass would make any difference in terms of energy efficiency. Thermal performance should be pretty much the same either way, unless the low-E coatings were affected in some way (which I think is probably unlikely).

   I'm not sure you gain much for the extra cost here though. I'm used to iron free glass (Starfire) being used in aquariums where the glass is 1/2"+ thick, where you notice the greenish tint more. With the much thinner glass typically used for residential windows, I don't think there'd be much noticeable difference. I recommend you compare the two materials yourself before making the investment, to make sure you really do see enough of a difference to warrant the cost premium for iron free.

   Note that commerical windows use much thicker glass, so that's where you'd be more likely to see a benefit from iron free glass.

   Bill

2. 
   oberon476 | Jul 18, 2023 09:57pm | #2

   Low iron glass improves visible transmittance by about 4% over standard float glass and also increases SHGC by about the same amount. Using low iron glass does not affect the U factor.

   That 4% difference will affect the VT when using LowE coatings. Take for example a low iron glass coated with a triple silver LowE coating. While that coating would normally have a 66% VT when applied to standard float (in an IG unit), that same coating would have a 70% VT when using low iron glass. However even with the VT improvement when using low iron glass, that IGU would have slightly lower VT than an IGU using a dual silver layer with 72% VT. Granted the 72% would improve to 76% when using low iron glass.

   The slightly greenish tint of standard glass is much more noticeable when the glass is viewed from the edge rather than the face, and as Bill Wichers pointed out, the slight green hue is more noticeable in thicker glass than thinner - thus the popularity of low iron in shower door and other more aesthetic applications that require glass that is thicker, but also clearer, than required in windows.

   From a manufacturing standpoint there are reasons for either choosing sand with a higher iron content or adding small amounts of iron to sand that doesn't have the level of iron desired when producing glass. The primary reason is because standard higher-iron glass sand melts at a lower temperature than does low iron mix. Basically low iron glass takes more energy to produce making it a little less environmentally friendly and more expensive to produce than standard iron glass.

   All of the major glass companies offer low iron glass for shower doors and such, but availability for windows will take some research. It's out there but not generally widely promoted.