Tuesday, 12 June 2012

low-e and trasmittance

In this post I write about the low-e glass mentioned in the post glass kind. About this topic there is a very interesting article on greenbuildingadvisor.com:
""[..] During the 1980s, glazing manufacturers perfected spectrally selective coatings that made it possible to produce low-solar-gain insulated glazing. During the 1990s, as builders in hot climates learned how these coatings reduced cooling loads, low-solar-gain glazing took an increasing share of the U.S. market.

Most builders prefer to order just one type of glazing. Window manufacturers share the same interest, since they prefer to promote a limited number of glazing options. As a result, low-e insulated glazing with a low SHGC is fast becoming the industry norm, from the Canadian border to the Gulf of Mexico.""
As it is already known, the effects of this coating is to decrease the factor U of the glass, and therefore the transfer of heat.

They can be applied in two modes: one is called in line -CVD- and takes place during the creation of the device. The result is called hard coat: it is very durable and can be positioned externally in contact with the weather (though almost never happens). The second way of coating is done in a separate phase of the production of the glass (off-line)-MSVD-. The result, said soft coat, is much more vulnerable than the previous, and it can deteriorate even in contact with air. So this is usually placed between two glass panes. This method is much more flexible, and gives the possibility to obtain more products aiming at ​​the ideal glas.

Problem: the SHGC. Knowing that a window has a layer of low-e tells us nothing about the solar factor, because it is a different matter than the factor U (low-e layer acts with longer wavelengths). For this, we have an other factor called "Light to Solar Gain" (LSG), obtained from the relationship between light transmittance and solar factor. An higher value means that the glass lets through the visible but not infrared. If this value is at least 1.25 the device is called "spectrally selective glass" [1].
Question: It is always better than a low solar factor? Or in other words, it's always better a soft coat or an hard? The answer is "no, not always." A selective glass is ideally best, but everything depends on climate. From the article we read:
 ""A window can have low-and low SHGC and represent a good opportunity for a home in Florida, or a high SHGC and work for a home in Minnesota.""

 From the article, however we understand how the thing isn't acknowledged by some manufacturers or sales agents.

Now we see transmittance graphs of some low-e glasses produced by the same company.
Low-e with hard coat

Low-e with soft coat

Low-e selective with soft coat


blu line =  Low-e with hard coat
red line = Low-e with soft coat
yellow line = selective Low-e

With soft coat the transmittance is almost exclusively within the range of the visible.

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