OOOOWEEEEE, I'm tying my brain in knots trying to optimize window selection parameters. A little knowledge is a dangerous thing and boy am I DANGEROUS!
I understand the meaning of the terms used to describe window performance:
U-factor is the "heat loss factor", the inverse of R-Value
VT is visual transmittance, how much "daylight" a window provides (and allows fading of fabrics, etc)
SHGC is solar heat gain coefficient. This is the percent of sunlight hitting the window that makes it through. This differs from VT as it includes the IR (heat rays) as well.
AL is air leakage. Important but with decent new windows not as important as U-factor and SHGC. I, of course, want to make your ears pop when I slam the front door, so low AL windows are GOOD.
This is where things begin to get more difficult. We are located near "the line" between the Central Zone with significant heating and cooling loads and the Southern Zone with most significant loads being for cooling. Any given year is unpredictable and we may have weather more alligned with the Central Zone or the Southern Zone so you must design to be OK in either situation.
I hope to kick off a discussion that would be of general interest to folks building a house, remodeling, or updating their windows through retrofit. I'll use my house-under-construction as a concrete example but would be happy to read about other folks and their ideas and chalenges regarding their new house, remodel, or retrofit.
A good web site for background info is:
http://www.efficientwindows.org/ where you can get window selection info for any state. I think the US DoE is behind this site so it is your tax money at work for you.
If my new house were a simple rectangular econobox with standard roof overhangs it would be a lot easier to pick windows. I have windows shaded by porch overhangs and windows with just a bit of roof (eave) overhang. Some are on the south side and some on the North. Some face NW, some SW, some SE, and one is in a west facing gable end with no effective overhang. I am thinking that different window technology might be optimum for different window situations of orientation and coverage (shading).
Ordinarily I wouldn't mix window technology in one room because of the different VT and appearance would make it look a bit odd. I'm guessing that windows on the north side could have different technology from the south side, even in the same large room and probably not be noticed. You sure won't see them both at the same time from outside and being on opposite sides of the house (north and south) I'm guessing that the same windows would look different. The light of a north facing window is cooler and more blue in color (higher color temperature say 7000 Kelvin) while a south facing window has warmer light, maybe 5-6000 Kelvin (just guessing not measuring.)
So, the same windows on both sides would actually look different, especially on a clear sunny day but not so different on an overcast day. I'm guessing the different window technologies would have similar differences and if the different windows weren't side by side would probably not be noticed.
I'm just guessing here because I haven't read where this topic was covered but I'm thinking that with windows well shaded by porch overhangs on the south side that I can use windows with higher SHGC and hopefully higher VT to get better daylighting as long as I have a good U-factor to keep the windw from loosing to much heat in winter. Then for windows that arent shaded, say above the porch roof in dormers, I should have a lower SHGC to cut back on solar heating in summer.
For the north side whether under a porch roof or not, there is not much radiant heating so SHGC shouldn't be a big deal so good VT should be easy to get and low U-factor is still desired so you don't loose too much heat in winter.
I know there are windows that are electrically adjustable from transparent to translucent for privacy and they are quite pricey. Maybe one day there will be electrically alterable or otherwise adjustable U, VT, or SHGC capability. Meanwhile, I have to build with what I can buy now.
Anyone have any thoughts?
[img]/forums/images/icons/smile.gif[/img] Pat [img]/forums/images/icons/smile.gif[/img]