Double or Triple?

May 10, 2012 12:55 PM

Double or Triple Pane Windows?

We are replacing some windows in our townhouse, it sits at 9200 feet in the rocky mountains. Cold is -20F. Hot is 85F. Double and triple pane windows end up costing about the same (with in a few dollars). But the triple pane are known to have more hardware issues (they are heavier).

I am not an engineer, so the question is: Given Low E glass and similar construction are how much more efficient are the triple pane windows compared with the double?

posted by Fuzzy Dog to home & garden (11 answers total)

My first question would be, simply, which one will give you the most tax benefit?

With the double, you just have the one airspace, but with the triple, depending on brand and local building requirements, you have a bit more variety of options; aside from the increased sound and temperature barrier, you can have the middle layer tinted instead of/in addition to the exterior glass. And if you decide to get inset blinds now or in the future, the windows won't be so different; I know there's a bit of a look difference between single and double, but I don't know about double and triple.

When you talk to the salesperson, they should be able to give you the ratings on the windows that spec out the rated differences.

And, if the visual difference inside and outside is minimal, you might want to put triple pane on high, hardly opened windows that get a lot of sun time, and double on ones you're going to want to open more. A decent showroom will have examples of both, but a well hung window should would smoothly for years and years and years.
posted by tilde at 1:11 PM on May 10, 2012

An anecdote. In 1971 when my parents were building their house in Phoenix, my dad, fresh off of The Whole Earth Catalog, asked the contractor about using double paned windows for the Wall of Windows that faced North. The contractor laughed and said it wouldn't be worth the money what with energy being so cheap and all.

In 1972, there were oil shortages and energy wasn't so cheap any more. For the next 10 years the windows were covered with 1/2 sheets of styrofoam to keep the heat out of the house.

So I believe the take away on this one is, never underestimate the value of anticipating the worst case scenario.
posted by Ruthless Bunny at 1:38 PM on May 10, 2012 [3 favorites]

The general rule of thumb is (IIRC) 0.5-1.0 R-Value per glass layer and 0.5-1.0 R-Value per air gap, so 2-pane ~ 1.5-3 R and 3-pane ~ 2.5-5 R. This table seems to back that up, more or less (1/4" air gap, 2-pane=1.7R, 3-pane=2.5R, or 0.5R per gap/pane; 1/2" air gap improves this).

The other big takeaway from that table is how R-value improves (significantly!) with air gap size. I'd previously heard there was diminishing returns with air gap beyond a certain (small) gap size; this doesn't reflect that: thicker windows = better.

This is rough, and the gas-filled windows apparently get another R "point" from that feature... but the gas leaks out, and IMO probably a lot faster than the industry admits, so you're really paying for a short-term boost there, not 10 to 20-year savings.

I'd definitely go with triple-pane, low-E windows, if I were you.
posted by IAmBroom at 1:43 PM on May 10, 2012

This is rough, and the gas-filled windows apparently get another R "point" from that feature... but the gas leaks out, and IMO probably a lot faster than the industry admits, so you're really paying for a short-term boost there, not 10 to 20-year savings.

Anecdotal point: a firm I worked for refused to specify gas-filled double pane windows manufactured east of the Rockies (i.e. Marvin, otherwise a reputable brand) because they had sealing issues after being trucked over the mountains and getting exposed to the pressure differences; supposedly, the seals failed rather quickly and our clients would get condensation inside their windows. I don't know how accurate that assessment was.
posted by LionIndex at 2:00 PM on May 10, 2012 [1 favorite]

An expert opinion appears at the bottom of this page. I suspect your "number of heating days" is closer to Madison WI than St. Louis MO, so a U-value (=1/R) below 0.3 (R>3) is suggested.

That would also indicate triple pane, probably.
posted by IAmBroom at 2:04 PM on May 10, 2012 [1 favorite]

Oops: An expert opinion appears at the bottom of page 5 in this article....
posted by IAmBroom at 2:07 PM on May 10, 2012

Some of the biggest gains you will get with such a temperature range (based on living in Flagstaff, AZ at 7,000+ ft) is with adjustable awnings. Cloth ones made out of sail cloth are about the best and most durable (might last 10 years, then need replacement of the cloth-the frams should last longer). They can shade in the summer when you don't want the solar gain and can be retracted/removed in the winter when you do want the gain. If they cost the same, get the triple pane windows. Just don't go with the fancy ones with any hardware on the inside like mini blinds or such. You want an opening as near the floor as possible and as near the ceiling as possible to help air circulate so get ones that slide open side to side or casement, not up and down. Double hung windows also work great but are more expensive and complicated. And avoid sliding patio doors-they are bitch to seal good and hard to secure-it is really hard to beat modern outside doors.
posted by bartonlong at 4:32 PM on May 10, 2012

How air tight is the rest of the house? If you've got a house that isn't too well insulated, then it probably isn't worth the potential hassle of the triple pane windows. However, if the rest of the house is well insulated, then it probably is worth the difference.

Same thing with how many windows there are. Lots of windows, and I think you'll see a bigger benefit. Fewer windows, and it won't make much of a difference.

Third is the above mentioned awning situation. If you have awnings or trees that block the summer sun from getting in, but don't block the winter sun, you might be better off with glass that lets some of that radiant heat in during the winter.

But I also wonder about the cost difference, and why there isn't one. If double and triple paned windows are really that close in price, why do they even sell double paned ones anymore? Are the triple paned windows a cheaper model?
posted by gjc at 4:45 PM on May 10, 2012

gjc - from what (little) I understand in the industry (was only in the periphery for about a year), manufacturers don't just make up a bunch of windows and hope to sell them. They do keep a lot of stock components around, but are, when they can, trying to move to just in time inventory and manufacture.

They might have a lot of old DP stock left from which to make DP windows, or the TP might have gone through a redesign and reauthorization (engineering certifications) long ago and is nearing the end of the cycle?
posted by tilde at 8:26 AM on May 11, 2012

Triple paned windows are the future (in North America, they've been standard in Northern Europe for decades). If you're getting them for nearly the same price as double paned then triple is the obvious choice, assuming a reputable manufacturer with experience making triple paned. The efficiency rating is easy enough to check; you want a low U-factor (or high R-value, which is the inverse of U-factor).

You also need to pay attention to visible transmittance and solar heat gain coefficient. In a cold climate you want high SHGC on south facing windows, to maximize the amount of heat you gain from winter sun each day, whereas you want low SHGC on northern exposures. But if you don't have proper overhangs on the south facing windows you could end up with too much heat gain in the summer. Shades, blinds, and awnings could mitigate that. Do rooms, especially on the south side of your place get too hot in the summer?

Also, your altitude may preclude argon fill, in which case you need air fill windows.

I'd join Green Building Advisor and ask the experts there. Be sure to include all relevant information, like your house's orientation and geographical location.
posted by 6550 at 11:43 AM on May 11, 2012 [2 favorites]

Another product to look at is "triple element" windows, which have only two panes of glass, but centered in the airspace between them is a thin film, usually with a low-e coating. So you get most of the performance of the triple pane, but better visible light transmittance and less weight.

E.g. Serious Windows
posted by misterbrandt at 2:57 PM on May 12, 2012