When does triple glazing make sense?
I’m designing a home that will be built in Zone 6 and I’m trying to decide between double or triple glazed windows. While the home will not be super insulated (R33 walls, R60 ceiling), it will have correct solar orientation, compact form, less than 0.6 air changes per hour at 50 Pascals pressure, reduced thermal bridging and efficient HVAC systems. Obviously, in this scenario, more insulation in the thermal envelope would provide a better ROI than triple glazing. However, in terms of thermal comfort, it’s a fairly small home and you end up sitting pretty close to large sheets of glass and I’m wondering if it would make sense to address the weak point in the wall (windows), or would adding more insulation to the walls and ceiling? A well sealed R33 wall in below zero temperatures will still be comfortable to sit next to, but a double glazed R3.5 window in below zero temperatures won’t be as comfortable to sit next to. Also, in regards to ROI, does triple glazing really only make sense in super insulated, low-energy homes?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
This is not the professionally calculated answer, but my general impression is that good double glazed windows are appropriate for most Pretty Good Houses.
If you want to try to calculate ROI, you have to do good heating and cooling load calculations, room by room (Manual J), using a lot of assumptions, and then make a lot of further assumptions about future interest rates, energy costs (and energy policies), etc.
Thank you, Andrew. According to my projected Manual J heating and cooling loads, the ROI for most triple glazed windows (using current energy rates) is well over 30 years. Obviously, there are a lot of variables and assumptions there, but based solely on ROI, triple glazed windows probably do not make sense in this scenario. My struggle is more with comfort and if double glazed windows will feel "drafty" in the winter and if triple glazed windows would lessen that issue. Also, while I prefer an "innie" window for many reasons, there's also something to be said for the relative simplicity of water management with an "outtie" window. Do you think I'll have to worry about condensation on an outtie window in this climate? How much of a difference does window placement in the wall make in Zone 6?
I don't think that properly installed windows have condensation problems unless there's another issue with the building. From what I can recall, placement of the window (in, out, middle) doesn't make a big difference until you get to extreme temperatures. There's a guy way up in Alaska (Thorsten Chlupp sp?) that has experience saying that up there, innies are preferably, but his area of Alaska is much more severe.
I think the bigger factor in deciding on innie vs outie is the type of wall you're building, the type of exterior insulation you're using (ie, rigid foam or mineral wool), where the drainage plane is, and what your building crew can build/flash with confidence.
I do not know the answer to your question of comfort, double glazed vs triple. By "drafty", I'm assuming you're talking about the effects of radiate heat loss when the sun isn't shining. The glazing experts will have to chime in on that.
I opted for triple glazed windows in my new pretty good house. The additional cost over double wasn't a big deal in the overall project cost. I wanted comfort and that's what i got. Being able to sit right next to a window and not feel any draft is really nice, although hard to quantify in financial terms.
Thanks, Stephen. What triple glazed window did you choose and what was your criteria?
I did Manual J calcs double versus triple pane, couldn't justify the added expense of triple panes. (I was already paying a premium for LoE-179 glass on the south side of the house.)
One other thing, at that time our Marvin rep said he hadn't ever sold triple panes. I think this has changed in the last few years, and triple panes are more common, even the window guys that advertise on TV now have them.
Q. " Also, in regards to ROI, does triple glazing really only make sense in super insulated, low-energy homes?"
A. No. The ROI on triple glazing in a given climate will be independent of the insulation level. If you are losing 50 kBTU/hour through the rest of the envelope, and you change one window and save 50 BTU/hour, you save whatever that 50 BTU/hour costs, regardless of what else is going on, same as if the rest of the load was only 5 kBTU/hour.
We enjoy our triple pane windows here in North Central Minnesota. Comfortable indeed during the winter. Quiet too, if muffeling outdoor sounds is important.
ROI? Over rated concept in my view. What's the ROI on a granite counter, or that fancy car in the garage? How can we achieve our goals with the monetary resources at hand was far more useful approach for us in building our new passive solar home.
Thank you for your post, Charlie. Makes perfect sense.
Thanks, Randy. I agree that ROI isn't everything and my main concern was about comfort, which I indicated in my original post. Would a well sealed R30 wall with R7 triple glazed windows in below zero temperatures be more comfortable to sit next to than a well sealed R60 wall with double glazed R3.5 windows? In my mind, the R33/R7 wall would be more comfortable in a small house with lots of exterior glazing. However, I'm wondering if anyone can speak to this from a building science perspective or from personal experience.
In sub-zero outdoor temperatures, you could dress to be comfortable sitting in front of either window. In the house with R60 walls and R3.5 windows, you would then be much too warm if you moved a few feet so you were no longer in front of the window. In the R33/R7 house, you would only become a little bit too warm when you move without removing clothes. (I am ignoring solar gain in this example.)
Thanks, Reid. I agree with your scenarios. Can anyone tell me if there's a big real world difference between a U.29 and U.14 window when your sitting a few feet from them on a negative degree evening, all other things being equal?
I've gone through the window research/shopping process, the house build, and living in the house. This is for a place in zone 6, double stud wall, outie windows, Alpen 525 series. These were chosen over double pane Marvin Integrity.
My overall advice would be to ignore ROI numbers and get the best windows you can afford. In cold climates, I do think triple pane and higher R value windows can make a difference in comfort and condensation resistance.
Brad,
You've gotten some good advice, and you have evidently already made your own return-on-investment calculations.
Another GBA reader, David Posluszny, performed some calculations for his location in Massachusetts, and concluded that the upcharge to pay for triple-glazed windows instead of double-glazed windows saved less energy than buying a few more PV panels with the same amount of money. Here is a link to his blog: An Affordable Zero-Energy House in Massachusetts.
In my experience, sitting beside a triple-glazed window is more comfortable than sitting beside a double-glazed window when outdoor temperatures are below zero. On the other hand, maybe all you need is a long-sleeved shirt or a sweater. I agree with the advice that you should buy the best windows you can afford.
Thank you for your comments, Martin. Calculations that include future energy cost are always guesses at best, but most of my attempts at calculating the cost of double v. triple glazing in my scenario has shown payback in the 30 year range, depending on the windows selected of course. However, as I stated in my original post, ROI isn’t the deciding factor and comfort will probably play a bigger role in this decision.
Also, thank you for including the link to David Posluszny’s blog, ‘An Affordable Zero-Energy House in Massachusetts’. It was a very interesting read and I especially enjoyed reading the comments section, which discussed the balance between energy efficiency and design. I’ve wrestled with that issue on my project and I’ve tried to balance the two, but I have leaned more towards design in many instances and the windows are a prime example. I choose to put a lot of glazing on the South Elevation (see attached) not only to take advantage of solar heat gain, but also to take advantage of the views and to connect the house to the landscape. The downside is that using triple glazing on these large windows will be very expensive. Double glazing might make more economic sense, but could be uncomfortable to sit next to at certain times of the year. I haven’t made any final decisions on windows yet, but I do know that I won’t compromise the size or amount of glazing on the home to potentially increase energy efficiency.
Brad- I used Intus UPVC tilt-turn windows, about U-.12. We looked at a few others but Intus was the best combination of price and quality. The availability of a local dealer, Performance Building Supply in Portland ME, was a plus, especially since they came out to the house and spent several hours showing my GC the best way to install them in the center of the wall. We are very happy with them.
I'm in the "get the best windows you can afford" camp. ROI is, in my opinion, not a particularly good factor when you'll be living with the windows for many years. Maybe a better $$ analysis would be how much more $$ per month you'd be paying for better windows over the next ___years.
One other comment: We tried to end up with a reasonable mix of openable and fixed windows. If i had it to do over again, I might lean even more toward more fixed windows. They are less expensive and allow for more flexibility of furniture placement. Since we had never lived with air conditioning, I was expecting to leave all the windows open all summer. In fact, the mini-splits provide A/C and although we didn't use it a lot, it was used during the occasional really hot day, when otherwise, we'd need to leave all the windows open.
From your drawing, it looks like the big windows on the south side are mostly fixed anyway.
Thanks again, Stephen. I've looked at Intus as well. Did you research any wood/aluminum-clad versions at all? Energate has a dealer in my state and I've also heard good things about Zola out in Colorado. It looks like the glazing and performance are probably fairly similar with most of these companies, but the other aspects of the window along with service might be the deciding factor.
Brad- we got quotes from Zola and Klearwall, which were significantly higher than Intus. Our architect had experience with Intus and that was also a factor. I think the relatively weak Euro may have made a difference in the quotes.
Brad, for wood/aluminum consider also Loewen. The exchange rate may make Canadian windows a good deal for US customers right now too.
Brad,
Looks like it is going to be nice house! One thing that would effect window pricing under our (Canadian) code would be the lows sill height of your windows. Up here they would have to be tempered. It might be worth checking if that is true for you too.
Thank you, Charlie and Malcolm for the help. I'll look at Loewen again, but last I checked their sizes of triple glazed glass was very limited. Marvin's triple glazed window sizes are very limited too. The nice thing about the Euro window companies is that they can make triple glazed units in very large sizes. And yes, they would have to be tempered or laminated under my local building code too. Any Euro-styled Canadian window companies that I should consider?
We chose Inline Fiberglass windows from Canada. We are using mostly triple pane casements with a few fixed windows in places that we would be unlikely to ever open anyway. We haven't received the windows yet, so I cannot endorse them beyond specs and price which were excellent compared to the U.S. made choices.
Hi Brad, I am building a pretty good house on the plains of North Dakota, border of zone 6/7. I have thick walls with 6" of exterior foam. I chose European triple-pane tilt/turn windows for my house. Granted I have not experienced them yet in the warm/cool house as it's still under construction, but I can tell you that there's nothing I have seen that even compares with the quality and air sealing details that these windows provide. Number of panes is only one part of the equation - air sealing is another important factor. I have seen decent quality double pane sliders leak air like crazy, especially in our area where 30 mph winds are still considered a "slight breeze." These European windows have triple seals and frankly, I would have to take a prybar and very excessive force to even begin to have air leakage. I also ordered my two entry doors from the same company. Same thing there - triple seals, multiple locking points, triple glazing, etc. Pricing was approximately double what I would have paid for middle-of-the-road US double glazed windows and doors, and that included shipping from Poland. Check out http://prossimo.us for more info. Ben was super helpful throughout the entire process from start to finish.
Thanks, Reid and Adam. I'll have to check out each of these window manufacturers. I've never seen a fiberglass or UPVC windows from any of the German manufacturers in person yet. How do they compare in look and feel to their equivalent wood/aluminum clad versions?
The choice to use triple glazed windows should be based on comfort. The body loses heat to a colder surface, such as a pane of glass. In my home the temperature at the interior surface of the single glazing with storms was about 45o. The Integrities I replaced them with brought it up about 10o. The closer you get the temperature of the glass to the temperature of the walls, the less heat your body will lose to the glass and the more comfortable you will be. So mine are still cold, but not as bad as they were. the main problem with triple glazing right now is that the major players haven't stepped up to the plate to mass produce good, affordable windows.
Brad, I can't give you a comparison since my UPVC windows are the only European tilt/turns I've ever seen in person. I do have one sash that had some separation at the miter joint and the manufacturer is sending a replacement, but other than that, everything is fantastic. Yes, they are white, but my exterior trim is white and my wife and I like the crisp look, both interior and exterior. If price were the same, sure I would have opted for a wood/aluminum clad window, but these were the best bang for the buck I found.
Agreed, Bob. The few major United States players that do offer triple glazed units, offer units that don't even come close to the performance and quality of most German and Canadian triple glazed units. It would be nice if a United States based manufacturer would step up and start building to the quality and performance level of these imported windows. There would certainly be a market for them and they'd surely be able to compete with these imported windows too.
Thanks, Adam. I'll have look at both UPVC and fiberglass options again. Like you said, they probably provide the best bang for your buck.
On my build I used Marvin Integrity fiberglass windows with some fixed and some casement styles. All of my East, West and (very limited) North facing glass windows were ordered triple pane. Also the fact that those directions are the ones the road noise will come from influenced my decisions tad bit. But it really didn't seem to add that much to the cost. I did it mostly for comfort and secondly energy efficiency. I also think that I like to build ahead of the curb. I bet in ten years they become a kind of standard practice.
Joe-did your triple pane Marvin windows have three panes of glass or two panes with a film in the middle? How big are the spaces between the panes or how thick is the glass panel in all?
Thanks, Joe. I actually have Marvin Integrity Ultrex Windows in my current home and while they've been a decent performing window, they were part of the 2001 batch of windows that Marvin produced that had exterior cladding delimitation. So, for the last 15 years I've been dealing with cladding that has been literally bubbling and flaking off the window. It's been a complete nightmare. In contrast, I also have a 3-panel, 8' tall patio door from Marvin that has aluminum cladding that looks as good as the day it was installed 15 years ago. Consequently, whether warranted or not, I've resolved that my next home will have aluminum clad windows.
So, this experience also factors in to my window decision making process. Ideally, I'd choose aluminum clad, triple glazed windows, but I have a hard time justifying paying twice as much for them over either triple glazed UPVC or double glazed aluminum (see ballpark cost for my new windows below). And despite preferring a window with a wood interior, I'm still open to UPVC. However, I do worry about what a UPVC window will look like in 15 years. I mean, how "green" is it to have to have to replace your windows because the cladding didn't hold up?
Marvin Aluminum 34K
Intus Aluminum 62K
Intus UPVC 31K
Anyway, I have a pretty good idea which window most of the people who come to this forum would choose (UPVC), but I would be interested in hearing about their experiences, thoughts and rationale.
I have to say that I just chose triple pane windows for comfort. Mostly, when the house feels less drafty, I have been able to keep it at 55 degrees or 60 degrees and not mind, but with the windows I had, I often felt cold when it was 72 and bought space heaters and sweaters and electric mattress pads and throws and down comforters in the Winter and still shivered in my bed at night while I spent $3000 last year on oil, plus, extra electricity. For the Summer, last year, I spent $600 on electricity cranking up air conditioners, and tossed and often tossed and turned feeling too hot to sleep. I know people say the triple pane don't pay for themselves, but I would say that, the older people get, the colder they get and the more they turn up the heat. Plus, there isn't gas on my street, so my mind thinks oil could double or triple in the next five years and the windows would have paid for themselves if it does.
My windows needed to be changed, because some of them fell down when I didn't know it and I couldn't sit next to them at all in the Winter. If my windows didn't keep falling down, I would have gotten window inserts instead, because I found some that said they had a better U factor than some double pane double glazed windows, but my triple pane U factor beat theirs by 12 points.