Low Iron Glass – Do You Need it?

Lately, incorporating low-iron glass into building projects has become more popular. But what is low-iron glass, what does it do, and is it worth the additional expense?

Article written by Barry Litt, Principal, Tower Insulating Glass, LLC

Iron (ferric oxide) is the ingredient in clear glass that gives it a greenish edge. Low-iron glass is made using less iron. Some of the other ingredients are silica (sand), soda ash and calcium carbonate. The silica used to make low-iron glass has about 1/10th the amount of iron than ordinary clear glass has, and is primarily sourced from Wisconsin. 

The lower iron content makes the glass clearer. How much clearer? 

While each manufacturer has a slightly different formula for manufacturing, on average monolithic 6.0mm (1/4”) low-iron glass has a VLT (visible light transmittance) of 91%, while clear glass has a VLT of 89%. Surface hardness and scratch resistance are the same. 

The original application for low-iron glass was table-tops. If your den or dining room table had a beautiful rug beneath it, the colors would present brightly through the clearer glass. 

Over time, low-iron glass slowly found its way into architectural applications where the expectation was the view out — through your window, door or wall — would be enhanced.

Today, most structures are built with insulated glass. The VLT for insulated, uncoated glass (2 lites of 6.0mm glass) is 79% for clear, 84% for low-iron. A difference yes but still, in my opinion, only marginal. Because low-iron glass has a greater VLT, one of the side-effects is that it doesn’t reduce passive solar gain (SHGC) as well as clear glass, an important feature for projects located in Zones 1-3 on the IECC (International Energy Conservation Code) map. Conversely, low-iron glass is preferred in Zones 5-7 where the greater SHGC is an advantage during the long, cold winters. 

When coatings such as Low-E (Low Emissivity) are added, the performance (VLT & SHGC) changes which may make the benefits of low-iron glass superfluous and the additional cost unnecessary. Also, low-iron glass is slightly more reflective than clear glass. Not meaningful during the day but problematic in the evening where an anti-reflective coating would enhance the view from inside the space.

Finally, is low-iron glass worth the additional cost? 

A careful analysis is required but it would be fair to say that incorporating low-iron glass into your project doesn’t necessarily make the project more energy efficient nor aesthetically more appealing, and may not justify the added expense.

Call Tower: (516) 887-3300. We're happy to assist you in selecting the right glass product for your next project.