Earthships

Crap, this arithmetic is wrong too. Let me go back to metric, but this time use the metric U value (5.6 times the Imperial)

1200 HDD in January is about a 40F gradient
= to 22.2C
My double pane with hard coat Low E has an American U value of 0.35, or 1.96 metric
Thus heat loss in watts is 22*1.96 = 43.5 watts, or about 1 kWh a day per square_meter.

OK, now I can see where Corwyn is coming from.
Now we can compare to triple pane:
U (imperial) is about 0.2 for fancy coated glass, or 1.12 metric
Now heat loss is 1.12*22.2 = 24.86 watts, a savings of (43.5 - 24.8) = 18.7 watts = 449 Wh a day per square meter.

How much heat gain have we given up ?
2 panes of clear glass in Jan in MI let in about 1800 Wh a day per square_meter;
3 panes will let in 2/3rds of that amount, or 1200 Wh a day per square_meter.

So, we reduce heat loss by 449 Wh per square_meter but reduce heat gain by 600 Wh per square_meter.

I think this is right, finally!
Which is better ? Double pane is cheaper, lighter, and has a (small) benefit in net solar gain compared to triple-pane, but would do much worse (almost twice the losses) in holding heat generated from inside the home from say a furnace (or from a ground heat pump.)

 

Sorry guys, you seem well on your way, and I am busy writing a paper for a local passivhaus forum, so I can't pay complete attention here.

Just on note on the above.

2 panes of clear glass have a SHGC of about 0.70
2 panes of soft-coat lo-e glass have a SHGC of about 0.45
3 panes of hard-coat lo-e glass have a SHGC of about 0.63
All GLASS ONLY values (i.e. not including frames)

 

Impressive, the triple-pane SHGC. My hard coat Low-E double pane are rated 0.62 SHGC! Who is the manufacturer ?

Improved coating tech aside, is it wrong to think that a double pane will have 50% more solar gain than a triple pane if the coatings are the same ?

 

This is an excellent intro article from the NREL:
http://www.nrel.gov/docs/fy01osti/27954.pdf

 

~ 800 watts, when facing opposite the sun.

Nominal radiation is 1000 watts, but for a variety of reasons 800 is a more realistic number.

 

Yes, Coatings are often only on one surface of one piece of glass, regardless of number of panes. There are some double coated triple panes, (for far northern latitudes) but generally just one coating is used.

 

Windows *can* be complicated, although once you understand U-value and SHGC the topic is less intimidating. Just keep in mind that the frame is yet another can of worms.

One piece of advice from Corwyn seems pretty easy to accept: Probably best to avoid window scraps of unknown quality and spec. I think I mentioned that I was able to find a local window maker who ordered my choice of glass for a final window price of about $12 a square foot.

 

Thanks. Who is the manufacturer of a triple pane, low U-value, 0.63 SHGC ?

 

Yes, Coatings are often only on one surface of one piece of glass, regardless of number of panes. There are some double coated triple panes, (for far northern latitudes) but generally just one coating is used.

 

No one. That is a generic window, from my spreadsheet. And again, it is glass only, actual windows with frames will differ.

 

Ok. Time to explain Lo-e. Lo-e stands for Low Emissivity. Emissivity is a measure of the percentage of radiant energy given of by a surface at a given temperature compared to a perfect black body radiator. Most things have an emissivity around 90-95%. This is how IR thermometers work, they measure the radiant energy, assume a certain emissivity, and calculate the temperature. Low emissivity materials (mostly shiny metals) give off less radiant energy than a higher emissivity material for the same temperature. So window manufacturers put a thin coating of metal on one surface of a piece of glass (for cold climates this is usually the outer surface of the innermost piece of glass). So that piece of glass is radiating more heat energy in the direction of the uncoated side, than it does on the coated side. This acts to send the heat back into the room, rather than outside. Now clearly (pun intended), the coating can't be to thick, or you wouldn't be able to see through the window, but it does act to reduce the light and solar heat that enters.

Hope this helps.

 

Yes, thanks.

Still confused about SHGC though; specifically, how a 3-pane with coating could reach a solar gain as high as 0.62
What is wrong in the following thinking ?

A single pane of clear glass blocks about 15% of radiation
Two panes would block 0.15 + 0.15(.85) = 0.2775
Three panes would block 0.15 + 0.15(.85) + 0.15(.72) = 0.3855

The coating is an additional block of some percentage on the inner-most glass. My sungate 500 hard coat Low E, double pane ends up with a SHGC of 0.62. That would suggest by my above reasoning that the hard coating itself blocks close to 15% of radiation. Or put another way, that the hard coat is equivalent to another pane of glass in terms of SHGC.

 

This Wasco website is informative. They are able to make a triple pane with a center glass U-value of 0.13 yet a SHGC of 0.56

Impressive!

 

The sun-room is not heated space. Unless you *care* that is is 40° instead of 30°, the heat that ends up trapped there (when it is cold) doesn't really help you any. You (I assume) want the heated space to be warm, so you should try to get all heat there (eventually).

Let's take a hypothetical situation. It is a cold night, your house is 68°F (maintained), and the outside is 0°F. By morning the sun-room will be 34° ( (68-0)/2 assuming half the insulation value (windows) is on either side of it). Let's say there is just enough sunlight that day to heat the sun-room just up to 68° by late afternoon. None of that solar heat ever makes it inside. It is like having a solar collector, but forgetting to hook up the pipes.

 

Please don't write it like that. Yes, you got it right, and I knew what you meant.

Two panes is 0.85 * 0.85 = 0.7225

Thanks.

 

Do we agree then that your earlier post that a generic 3 pane allows 0.85^3 SHGC is for clear glass without any coating ?