Dude, tune your windows (and other simple stuff to make a better building)

May 2009

I’ll have to start this sustainable line off with an apology. I was having a conversation on Earth Day with a builder for whom I have great respect. I was talking to him about building orientation and how important it is and he told a story about how he talked a couple into facing view windows to the west, to make the clients happy. These are the type of stories that make a green building consultant’s heart sink. So I chastised him. I felt bad and want to say I’m sorry. But then I think about it again and my blood pressure goes up. Its Earth Day 2009 and I have to teach a builder who has built off the grid homes how important good orientation is. If he doesn’t get it, then what about your typical builder/ developer? What about you?
So what is wrong with view windows to the west? In Colorado, where I live that’s where the view is and it makes sense, we have such nice mountains. NO! Not about our mountains being nice, no to the seemingly irresistible urge to make a home an energy idiot. West facing windows gain a lot of heat in the summer months. You need a lot of ac to make it work and probably shades to reduce that late day heat, even with your fancy pants low e-4 glazing. So now you have a view of your shades. These windows give you the view in the winter but you’ll have to subsidize it with a lot of heat, aka money. The home I live in was originally set 20° west of solar south and now I wish I got a crane and turned it before I put in an addition. 20° makes that much difference and the view is still there, really.
Studies have shown a 30% energy savings in just good building orientation. This is not cheap, it’s free! So when I am driving through the country and see houses sitting alone and their windows are everywhere but solar south I know their owners 401k is dedicated to keeping the home from being unlivable for the rest of the homes existence. Just to help dissuade rumors, the LEED commercial green building system does account for building orientation, but currently only if you chose to peruse it in version NC 2.2.
So let’s dig a little deeper. You know all this solar orientation stuff but what’s with the “tune your windows” part, and “dude?” I’ll have to apologize again, that was uncalled for. This subject gets me a little emotional. Tuning your windows simply means that with modern glass you can actually select the amount of solar heat that comes through the window. The big commercial buildings would be in deep trouble if they couldn’t dramatically reduce all the heat that hits the surface. They have glass that can cut 80% or more of the solar heat gain. This keeps the occupants from throwing file cabinets through the windows in overheated despair, thus resulting in the collapsing of the economy (even more.) The housing window manufacturers and dealers followed suit and sell their fancy pants low-e windows as energy savers.
So what’s wrong with low-E glass? It keeps the winter heat in and summer heat out. We are smart and our windows are south facing with good overhangs so in the winter we can heat our house up and the low-e glass will keep our rug from fading. NO! The window folks, I will refer to them to as ‘Dude’ for this conversation, are not correct. Dude will tell you about the U-factor, basically a measurement of how much heat escapes the entire window unit, and low-e eliminates fading (it doesn’t). Dude will talk about low-e2 glass, and how low-e4 glass is even better, aka more expensive. But look at the National Fenestration Rating Council (NFRC) sticker on that new window, over on the side, see it, next to the U-factor number. That’s the solar heat gain coefficient (SHGC) rating. In the building world you have to get used to acronyms. That number is basically a percentage of heat that comes through the window from the outside. Large commercial buildings are very keen about heat gain.
But you are building a house where you want heat gain in the winter and out in the summer. Summer is easy, limited west and east facing windows, overhangs to shade your southern windows. Winter is long here in Colorado and some free sunshine heat would hit the spot, but not according to the sticker on that new window. An average low-e2 window is reducing about 60% of that solar heat and low-e4 is up to 80%. The lower the number the more radiation that is blocked from the outside. Don’t be discouraged though, you have the ability to purchase glass with a different SHGC. Dude doesn’t talk about this but there are two types of low-e, a hard coat which has a low SHGC .30-.45, and a soft coat which has a higher SHGC, around .60-65 and hence more heat gain. So all you need to do is order the soft coat low-e with a high SHGC for your southern windows and a low SHGC for the other sides of the house. It may take a little more effort but you will be well rewarded with a smart house (except in Phoenix). Computer modeling could show that you may even be better off without low-e glass. For extra credit figure out the shading coefficient (and then email me at andrew@baosol.com, because I don’t get it).
How much southern facing window? Rule of thumb is 7% fenestration (window area) on the southern side, and if you really good at this green building thing 20% fenestration with distributed direct gain thermal mass.
Ok, next month is “Why green building is for the conservatives”, and bowing to the blogosphere and its rapacious appetite I’ll be adding an extra post or two a month to the sustainable line: blog such as “Is carbon a waste product?” and “Tax rebates out the window? True or False”.

Thanks to igreenbuild.com, Otherpower, and treenex.com for letting me banter on to their websites.


Andrew Michler, LEED AP