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Besides giving you a wonderful view, high-VT windows can save energy because you need less artificial lighting. Producers set the VTs of windows as contrasts with the quantity of visible light which would pass through an open gap in the wall the same size as the window. VT may be expressed as a “whole-window” worth including the impact of the framework. The most important thing is your ability to see through the glass, not the framework, so make certain that you receive the VT of the glass, not of the whole unit.

The VT in residential windows extends from a shady 15 percent for some tinted glass around 90% for glass. To most people, glass VT values over 60% appears clear. Any value below 50 percent starts to appear dark and/or reflective. Dariush Arasteh, staff scientist at Lawrence Berkeley Laboratory, warns, “People have very different perceptions of what’s very clear and what’s a tint of colour, particularly when they look through glass at an angle.” Look at a sample of glass outside and assess for yourself before you choose to purchase the window.

It is warm in Sunlight

Producers have used shading coefficient (SC) to explain how much solar heat their windows transmit. An entirely opaque unit scores 0, and one pane of glass scores 1 on this relative scale. A transparent double-pane window scores 0.84 since it allows 84 percent as much heat to pass as one pane of glass.

Solar-heat-gain coefficient (SHGC) is the new, more precise tool that’s replacing SC to explain solar-heat gain. SHGC is the fraction of solar heat that goes through a window. It, too, uses a scale of 0, for not one, to 1 for 100 percent of available light. The key distinction is that SHGC is based on a proportion of available solar heat as opposed to on a fraction of what comes through a single pane of glass.

Glass coatings are formulated to select certain wavelengths of energy. It’s likely to have a glass coating which blocks long-wave heat energy (non SHGC) while allowing generous levels of visible light (large VT) to enter a house. A low SHGC can decrease air invoices more than if you raised the insulative value of your window with an extra pane of glass. I suggest that a SHGC under 0.40 for hot climates. In cold climates you need both high VT and higher SHGC. I Suggest that an SHGC of 0.55 and above from the North. In swing climates like Washington, D.C., picking a SHGC between 0.40 and 0.55 is reasonable since there’s a trade-off between heating and cooling loads. If you enjoy wearing sweaters and despise being overheated in the summer, then a low SHGC might be the option for you.” Pick the mix of glass coatings which works best on your climate and vulnerability.