Passive Homes - Amazing...

Oh, don't say zero! I know what you are getting at, but while I would love to get infinite film coefficients for gas-to-gas exchangers it just doesn't happen no matter how hard I try. Realistically you would probably get around 10 Btu/hr sq.ft externally with a really stiff wind. That works out to R 0.1 worst case. Inside, you might get half of that, say 5 for an R of 0.2 even accounting for increasing convection as delta T rises with the increasing outer coefficient (greater difference in surface temp.) A pane of glass would be an R of about 0.02 if I calc'ed it correctly. Total R ~ 0.32, still abysmal.

 

While it aint a true Zero,,, it is for all intents and purposes, for the layman Zero!

Give me the U-Value of an R-value of .32 and it is a huge heat loss issue and most people ( present company excluded) don't get it!

Icarus

 

If I remember correctly, window manufacturers get full Energy Star certification by going to coated windows that reflect IR and UV rays from coming inside the house in addition to using triple-layer panes in some cases. The new windows at my house have IR/UV blocking and a thicker double-pane glass, hence the lower utility bills and considerable reduction of outside noise.

 

I'd love some advice on windows I am thinking of installing on the south side of my house. I live in the southwest high desert -- climate is similar to Denver I think, with dry hot summers in the 90's F in the day and 70's at night for about 3 months a year, and 4 month long winters where daytime temps are 30 - 50 F and night 20 degrees cooler.

The windows are for the view, and to passively heat the house in the winter; but if they heat up the house in summer it is going to make our current practice of doing without AC a real argument between me and everybody else.

 

Couple of points, to Sacto, Even with energy star ratings windows still are a big loser of heat. I don't think there is a window (double glazed) out there that gets R-3 even with coated glass, exotic gasses etc.

To Sagebrush,

Consider the winter heat gain as an advantage. Maximize solar gain for heat, and worry about summer gain in another way. Remember, with proper design a window can have 100% sun in the winter and 0% in the summer. Properly designed overhangs, portable awnings etc can serve to shade in the summer, as can proper Plantings.

Another thing to consider is when you can get and need your solar gain. In the Pacific NW for example, there is little merit to design for Nov-Feb, since sun is so rare, but by designing for March-May, and Sept and October, you can dramatically SHORTEN your heating season. My house for example, with lots of south glass, an attached passive solar greenhouse on the south side, won't need real heat until sometime in mid October, and we will stop lighting fires sometime in March. (An 1880's farm house that has been extensively upgraded over the years. The roof glass of the greenhouse allows direct sun to the back of the adjoining room(s) in December, but the way it is built only allows sun in the greenhouse itself in the summer,,, no direct sun comes in the main house.

Icarus

Give you latitude and I can give some calcs.

 

There are various factors and it varies by your climate region. Since I don't have model numbers or documentation for my windows I can only guestimate based on what I can observe and measure and match them to the most likely models from the manufacturer site. They seem to match the criteria for energy star based on their construction and coatings.

 

Icarus,

Simonton's performance data has their bottom end windows listed as R-3.0 for double hung, double glazed, argon filled, vinyl. Their Low-e version is listed at R-3.3.

With the temp outside at 14 F presently and a the wind gusting against my window at about 20 mph (vs. 40 mph measured earlier in the evening) the glass temp feels about right for the R values above (around 50 F by touch, perhaps a little warmer...I don't have a contact TC at home.) I compared to a single glaze (?) french door's glass w/storm door attached, as well as a small sheltered single glaze window in a non-heated basement area. The door glass was about midway between the other window glasses in in temp, and the basement glass is running above freezing even though the space itself is 45 F according to my TC--I can tell the glass surface is ~33+ because of a tiny hint of condensation on the glass can be felt inside, I've seen it frost up at lower outside temps, and that was before I insulated a wall that was warming this space considerably.

 

Shawn,

My original point was that even with the best windows on the market, getting to ~R-3 is about the best we can do. Adding window quilts is a way better investment that a third layer of glazing. Or adding insulation in the attic.


Icarus

 

That will certainly be handy during the coming Ice Age, eh? :_>

 

Sagebrush,

Here is a link that give you some sun angles over the year and over the day, quite handy,

UO SRML: Sun chart program

Good luck,

Icarus