Why the glass looks greener…

When choosing glass for a building project, the primary factors to be concerned with are visual transmittance, the solar heat gain coefficient, and the U-value.

There is a good deal of discussion on U-values, as this pertains to the insulative properties of the window.  Closing the information gap on solar heat gain, and visual transmittance, on the other hand, can take a little more digging.

The Visual Transmittance rating is a measurement of the percentage of visible light than can pass through a glazing unit.  It is typically abbreviated with the letters VT.

The Solar Heat Gain Coefficient is a measurement of the percentage of radiant heat coming from the sun that can pass through a glazing unit.  It is typically abbreviated as SHGC.

These measurements are shown as decimal numbers by most window manufacturers, somewhere between 0 and 1.  The higher the decimal, the greater the percentage of solar heat, or light that can pass through the glazing unit.

There is a positive correlation between the two, but the first pertains to visual functionality, and the second pertains to building energy performance.

A higher VT rating (better visibility) is almost always preferable, but the SHGC preference may be higher or lower, depending on the needs of the project.

In colder climates, a high SHGC is usually preferable, as it allows for passive solar heating.  When designing this kind of building, proper orientation and shading become important aspects of the design.  The basic concept is to have primarily south facing glass with an adequate roof overhang.  This way, when the sun is moving close to the southern horizon in winter, the house gains heat, and when the sun is moving over the top of the house in summer, the heat gain is minimized.


What is different about high solar heat gain glass?

Glass with lower iron oxide content will have higher VT and SHGC ratings, and is, therefore more suitable for passive solar heating in cold climates.  In Europe, this glass is readily available.  In North America however, low iron glass is not as easy to come by and is more expensive.

On the other hand, glass made from silicates (sand) with the standard iron oxide content will have a green tint to it, that is more noticeable the thicker the piece of glass.  Look at the glass from the side, and it will appear very green.

This kind of glass is somewhat ambiguously called ‘clear float glass’ or ‘standard clear glass’ and is the industry standard in the US.

There are two factors that determine how low the iron content in the glass is.  One is the amount of iron oxide in the raw silicate used in manufacturing, and the other factor is how much iron is removed in the process.  Removing most of the iron from the glass would weaken it, so scientists came up with a way to add something else for strength that is more clear.

Low iron glass is more available in Europe as a result of higher demand.  It’s not that raw silicates naturally occurring with low iron content are entirely absent from North America, but instead, the lack of availability is attributed more the lack of manufacturing.

Why is there a low demand for clearer glass on our side of the pond?  Because the industry hasn’t really been scrutinizing glazing options until more recently. As a result, manufacturing infrastructure isn’t in place to offer it at an affordable rate like our European counterparts.  This is indicative of the greater spiral of resistance to revolutionizing the building components industry here in North America.

Though these ratings are applicable to transparent materials only, many North American window manufacturers will often publish whole window VT and SHGC values.  This makes North American windows looks considerably worse on paper at first glance, and also makes comparing windows on either side of the pond a little more tricky.  If possible, getting just the glass values for these numbers will give the most accurate and informative comparison.

When we compared tilt/turn windows, using glass from Europe, with a comparable US made tilt/turn window, the superior VT and SHGC ratings are evident.

European glazing unit with three 4mm panes of glass:
                                                                         SHGC                     VT
Standard option:                                              0.49                        0.71
High SHGC option:                                         0.62                        0.73
(Low SHGC option also available)

Other glazing units used in US-made tilt/turn that looks similar to Europe, but uses three 3mm panes of glass:
                                                                        SHGC                     VT
Standard option:                                              0.37                        0.63
High SHGC option:                                         0.56                        0.70


Note that even though the domestic glazing manufacturers use thinner glass, they still have less desirable ratings in regard to visual transmittance and solar heat gain.  Most of these windows have a greenish tint to them, compared to the clearer low iron glass option.  Keep an eye out for the difference, and be sure to compare the values, as the glass almost always looks a little greener on our side of the pond.

By Alba Briggs

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