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Winter sun through Low-E glass, summer heat later
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How do you weigh U-factor against solar heat gain coefficient when choosing Low-E glass? A lower U-factor helps limit heat transfer, but it doesn’t tell me how much winter sunlight the glass lets in, or how much summer sun it blocks.
I’d expect a south-facing window with useful winter sun to call for a different balance than a west-facing one that gets strong late-day sun. Seasonal shade matters too: deciduous trees or exterior shading might reduce summer gain, but they won’t help much if the glass is exposed when the sun is low. How much do orientation and shading change your choice between higher- and lower-solar-gain Low-E options, and what do you prioritize when heating and cooling needs pull in opposite directions?
2 replies
I’d be cautious about assuming south-facing glass should always favor higher solar gain. In a cooling-dominated climate, or where an overhang doesn’t block the high summer sun, the extra summer heat can outweigh the benefit of winter sun. Orientation helps frame the choice, but climate and how well the shading works matter just as much.
- West-facing glass is a tougher shading case: an overhang that blocks high summer sun may still let low-angle afternoon sun through. If exterior shade can’t block that late-day exposure, lower-SHGC glass may be the more dependable way to limit heat gain.
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