fancy glass worth it ?

[RustyShackleford]

I am adding a room to my house. North side, so I'm tempted to get really well insulated glass. The operable windows are bought and installed, but I've got some fixed glass too, and wondering how fancy to get. But when I do the arithmetic I'm not so sure it's worth it.

The basic options are low-E/argon filled with an R-value of 2.04 and plain ole' insulated glass (two panes of regular glass with air in between) with a R-value of 1.69.

My area has about 3000 heating degree days, which is 72,000 degree hours. I have about 40 sq-ft of the fixed glass I'm buying. So I believe the btu's lost through the glass in a heating season is:

72000 degree-hours * 40 sq-ft / R

The units of R (reciprocal of U) are degree*sq-ft per btu/hour, so the units work.

So the difference between R=2.04 and R=1.69 is about 300,000 btu.

I'm going to install a mini-split ultra-efficient heat-pump:

http://www.fujitsugeneral.com/wallmountedRLS2_specs.htm

The 12RLS2 has an HSPF of 12 btu per watt-hour.

So providing that extra heat will cost me about 25 kilowatt-hours. For a season.

For my regular electric rate this is less than $3; for the peak billing program, and I plan to consume no electricity during peak, it's a little more than a buck. For summer-time, the degree days are only about 1500 and the SEER is 25 (same units as HSPF, btu per watt-hour).

It seems like a no-brainer. Anyone want to check my math ?

 

[jebatty]

Not going to check your math because window R or U value is more than energy savings.Evaluate everything you want the windows to do and then make your decision.

We installed R-8 rated windows throughout the entire house knowing that they would never pay for themselves in energy savings, but the payback has been in comfort and lack of needed maintenance. Aluminum clad outside and stained/varnished wood inside. My wife can sit next to a 30' span of glass in the dead of winter, howling winds and -35F outside, and she feels warm -- no cold air wash, and the glass is warm to the touch on the inside. Priceless. Also, no condensation and after 19 years no need to re-stain or re-varnish any of the windows. We do have energy "savings" in the form of one small wood stove in the living room which heats the entire 1500 sq ft house, and I feel pretty certain that the windows help to make that possible. The wood stove has kept up with the coldest temperatures northern MN can deliver, including record breaking lower than -50F a few years ago (my thermometer bottomed out three nights in a row on that cold snap).

 

[woodgeek]

Your math looks aok to me. On a pure BTU loss and cost calculation the two products you found the difference is pretty small. But jebatty makes a great point. There are intangibles and comfort issues. Are you going to be sitting near under this window in the winter? Radiant coolth would makes some people boost the thermostat a degree or two, costing you a few% of your total heating bill.

I think your outcome is comparing R-1.7 to 2. If you can find cheap, higher R products, it might come out differently. And pay attention to fit and install to minimize air infiltration...a leaky R-2 will be a lot worse than a tight R-1.7.

 

[Tramontana]

I am curious where you obtained the insulation values for your calc's.

Are these manufacturer's claimed performance or NFRC test numbers? You mention these are north facing lites, what is SHGC for seasonal exposure? Summertime heat gain in morning and evening will impact your cooling loads.

Also, you may wish to factor increasing energy cost into your modeling (if you haven't already). It may impact your life cycle cost over expected duration for the install?

Cheers!

 

[RustyShackleford]

Thanks for the thoughtful replies folks (and for checking my math, I knew I could count on you for that, woodgeek ...

Jim, where the heck did you find R=8 windows ? Did not know such a thing existed. Yes, you make a good point about comfort, although my climate is nowhere near as frigid as yours. Below 20 (above, fahrenheit) ain't too common here anymore, and single digits is a major news event.

Woodgeek refers to black-body radiation out the windows, I believe; the same thing that makes frost occur on cars when the air temperature is above freezing. That might be a reason for the low-E type I guess. Yes, I was comparing R=1.7 to R=2. These are fixed pieces for a custom installation (some glass corners, and a trapezoidal transom). We will place close attention to sealing around the panes and frame.

The R values were given by the local shop that's making the custom sealed units. I am a little surprised they quoted only R=2.04 for the low-E argon-filled version, since the operable factory-made windows I installed (Marvin Integrity) have a sticker that says their U-value is about 0.3.

Solar heat gain should not be an issue, since the room is on the north side of the house, and lower than the main house to boot. So I think low-E probably makes no sense, though perhaps it would reduce the outward radiation that woodgeek mentioned. Probably my money is best spent with larger air spaces between the glass, which is pretty cheap too. I doubt if thicker glass helps much, though I will probably go with 3/16" tempered for the outer layer of the pieces on the corners (since they're so exposed to damage).

These are the best forums around, thanks to people like all of you ...

 

[Frozen Canuck]

You have some good advise here. Don't make your decision based solely on cost recovery formulas. Had the same decision several years ago, went with buy as much quality as the budget allows route & have never regreted it. A one time hit $$$ wise & years of comfort & a happy wife to look forward to. Coldest here since install has been -47 according to the thermometer on the deck. Not even any frost in the corners at that temp. We went with triple pane polar units from a shop about 1.5 hrs away. Maintence to date, annual cleaning/checking seals, cranks, locks etc. If I had it to do over today the only change I would make is to go with a fibreglass frame as that tech has come along way in windows.

 

[dougstove]

I put in good quality double pane, argon filled, vinyl windows 5 years ago, but the let contractor talk me down from triple pane low-E, since the nominal difference in R-value did not warrant the extra cost.
For all the reasons other posters have mentioned, I really wish now I had gone with the higher grade, since in winter we have condensation and cold wash.

 

[RustyShackleford]

Thanks folks, and although I hear your point about not JUST considering btu cost-recovery calcs, most of you making that point are from places where it's just ludicrously cold IMHO Like I said, single-digits is an event here, below zero is probably an every 10-20 year phenomenon; a typical January overnight-low is in the 20s. (That's Fahrenheit, for you Canucks ...) Here, tonite, you guys would probably be out barbequeing in your short pants; I'm sitting by my BlazeKing.

 

[Frozen Canuck]

Yep hear ya Rusty, however higher quality units will also give you more summer comfort. You know when you can fry an egg on the sidewalk!

 

[woodgeek]

Yup. standard low-E have low solar gain and are good for AC climates as well. Prob not an issue facing North.