Nowadays, the software used by energy modellers is focused on reducing energy consumption. High solar heat gain (SHGC) can be misinterpreted as a good thing because it reduces energy consumption. Working with an energy advisor to prioritize energy efficiency doesn’t mean you give up on comfort but it is important to understand the relationship between high solar heat gain coefficients, energy efficiency and home comfort.
High solar heat gain = Free heat?
The logic behind this idea is understandable: specifying a high SHGC for your project will mean ‘free heat’, allowing more of the sun’s warmth into your home during the winter. The problem is, we only need this free heat for 6 months of the year at most. Adding this extra heat to the sealed environment that is your home may keep you nice and toasty over the winter holidays, but your windows are going to act in the same way during summer. You’ll be forced to crank up the A/C all day long, and your electricity bill is going to be astronomically high. As we’ve seen recently, simply opening the windows isn’t necessarily a good solution.
The problem with high solar heat gain coefficients
When you see a SHGC specified as a number “or Higher”, how high can this number be? If the energy model is based on a summer cooling load at 0.32, then allowing a higher SHGC could result in a much higher cooling load than anticipated. It also means the A/C unit may not be sufficient to keep up, especially as we see our summers become hotter and longer.
Let’s look at different LoE glass that is available and what SHGC you typically get with them:
- LoE3 – 0.20
- LoE2 – 0.25
- LoE1 – 0.50
When reviewing the specification below, which LoE coating do you think would be optimal if the target is 0.32 SHGC? Would you go up, or down? Opting for a 0.25 glass will result in less A/C but a bit more spent on winter heating, and going up to a 0.50 will result in a lower energy requirement to heat your home in winter but require significantly more work from your A/C to keep you cool in summer.
This is the dilemma we face every day. Computer simulations may select a SHGC of 0.32, but can you actually buy this glass and, if you could, will it actually result in a comfortable home?
What are we seeing at Centra?
At Centra, we’ve seen specs with a SHGC of 0.50 and higher. The Window Experts consider this to be far too high. After all, there’s a reason greenhouses are made of glass. Like a greenhouse, the LoE1 glass with a 0.50 SHGC mentioned above lets in a lot of heat. The overheating issue will be compounded by the modern design of windows, doors, and homes overall, which is to make them relatively airtight. This traps the heat, which may be nice in winter, but makes for terribly uncomfortable living the rest of the year. 50% of the window retrofit business is replacing them due to overheating in the summertime and high solar heat gain. Even if you were to specify a 0.4 SHGC, the performance is likely to be similar to that of an older home. You’ll find yourself wanting to replace the windows again straight away.
When it comes to energy modelling and ensuring home comfort, putting an upper limit on a window’s SHGC is the way forward. 'Free’ heat may be nice in winter, but it’s going to make a home unbearable in the summer.
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