Saturday, February 22, 2020

More about Windows

A note from a friend raised an important point:
The most efficient solar is the sun warming things, right?

That is not how Title 24 thinks, which can drive up energy use. Basically Title 24 assumes outside inputs are bad and the way to manage temperature is to seal a house from the outside - and as efficiently as possible use energy inside. This is based on a lifestyle assumption – that you need to live at a constant temperature. If you are OK with letting the sun in to heat your home and having temperature vary by 10º or 15º over the course of a day Title 24 works against you by limiting glass area while mandating sun blocking windows and roof insulation that prevents internal solar gain.
Yes, he's right and that's a long story. The good news is that Title 24 offers the alternative of demonstrating performance compliance through good design.

So you can hire folks who know what they're doing and prepare calculations which show compliance.  Here are a couple of short web pages explaining the basics:
The most challenging detail to grasp is the Solar Heat Gain Coefficient. Window manufacturers don't want to confuse people with too much information, so they train their sales forces to promote "one size fits all" and for God's sake, don't let the sun come in and fade your drapes and furniture!

So if the customer says "energy conservation," they will get low-e glass (good insulation) with the standard _low_ solar heat gain—which works well on the southwest or west sides of the home, but for windows on the south, very little heat will come in during the winter when it's needed.

You can read glowing reports (such as US DOE's Energy Star propaganda) that will confuse you even more. So instead, here are a couple of articles (one from Canada where there are 4 seasons) explaining why you will want to specify the high solar heat gain coefficient on your south-facing windows:
All the more reason to know your home's compass headings... North-South, East-West.