Huge, minimal glass surfaces that merge indoor and outdoor spaces are the ultimate trend. But architecture clashes with the scorching Greek sun: a large glazing unit facing south or west turns the living room into a greenhouse.
Very dark tinted glass cuts the heat but darkens the home in winter. How do we let light in while keeping heat out? The answer lies in two indices: Light Transmittance (Lt) and Solar Heat Gain (g-value).
The Light Transmittance (Lt) index measures the percentage of visible solar radiation that passes through the glass. Measured in percentage (%), it is straightforward: Lt = 90% means near-perfect transparency, while Lt < 50% means a noticeably tinted pane.
Lt = 90%: Perfectly clear glass. Almost all light passes
through.
Lt = 70%: Excellent for modern energy glass. The home
is fully bright.
Lt < 50%: Visibly dark or reflective glass. Reduces
glare but requires earlier use of electric lighting.
In homes with few or small windows (e.g. north-facing), choosing glass with high Lt is critical to maximise natural daylight. For large south- or west-facing glazing, the focus shifts to g-value, but a good glass achieves both simultaneously.
The sun doesn't only send light. It sends massive amounts of invisible thermal energy through infrared radiation (IR). The g-value (Solar Factor / SHGC) shows how much total solar energy (light + heat) enters the building.
g = 80% (0.80): An old single pane. 80% of the sun's
heat passes unhindered into your living room. A nightmare for the air
conditioner.
g = 40% (0.40): Excellent "4 Seasons" (Solar Control)
energy glass. Blocks 60% of solar heat before it enters the home.
g < 30%: Extreme solar control. Ideal for office curtain
walls, but potentially too dark for a home.
Historically, a low g-value (coolness) required dark, reflective glass, which drastically dropped Lt. Modern energy glass solves this with nanotechnology coatings (soft coating): microscopic silver particles act as a filter, allowing light to pass while reflecting infrared heat back out.
To know whether the glass you've been quoted is truly top technology, engineers use a simple formula: Selectivity Index = Lt / g-value. We want this number as high as possible (lots of light, little heat).
Lt = 60%, g = 50% → Index: 60 / 50 = 1.2. Average performance. Lets relatively little light through and doesn't cut enough heat. Typical of older-generation energy glass.
Lt = 70%, g = 35% → Index: 70 / 35 = 2.0. A crystal diamond! Lets 70% of light in (abundant brightness) while blocking 65% of heat. An index near or above 2.0 is considered the "Holy Grail" of glazing.
Not every window needs the same glass. Smart design is based on each opening's orientation.
Never hit by the scorching sun. You need standard Low-E energy glass with high Lt (≥ 70%) to capture every drop of natural light. The g-value is less of a concern here. Focus on insulation (low Ug) and maximum brightness.
This is where summer overheating strikes (especially west, after 3 p.m.). Invest in Solar Control Energy Glass (4 Seasons) with g ≤ 40%, keeping Lt above 60%. Add external shading (roller shutters, awnings, brise-soleil) for full protection during the hottest months.
Morning sun is pleasant, but harsh summer daybreak can overheat bedrooms. Energy glass with a moderate g (45–50%) and high Lt (65–70%) is the golden balance. Pair with an external roller shutter for a complete solution.
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