# Payback between double- and triple-glazed windows?

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I have designed a 4,800 sqft home in NH (Zone 6) with “REMOTE” insulation building assemblies. R-40 walls, R-60 ceiling and R-20 sub slab. The windows were to be Eagle triple glazed U-.265. My client has asked me what the pay back is between using Eagle triple pane versus double glazed U-.31. For the entire house the cost difference is \$2,000 and the efficiency difference is 16%. This seems like a good value to me especially in a house that is deeply insulated. Anyone have suggestions or answers to his question “what is the payback?”

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### Replies

1. GBA Editor
| | #1

Harry,
The answer to your question will always be specific to an individual building at a particular site. It depends on the house specifications, house shape, and house orientation, as well as the climate, the local energy costs, and your assumptions about energy cost inflation.

To perform the calculation, you need to run your proposed design through an energy modeling program. You'll need two iterations: one with the proposed double-glazed windows, and one with the triple-glazed windows. Comparing the two iterations, you'll learn how many BTUs you'll save each year with the triple-glazed windows.

Many people have done such calculations. Using simple payback assumptions in New England, I've seen calculations that show that the triple-glazed windows aren't worth it. Others reach the opposite conclusion, but they probably assume higher rates of energy inflation than the designers who conclude that triple glazing isn't worth it.

Here's my bottom line: forget payback. Choose triple-glazed windows because they will be more comfortable and they will lower the energy bills for the life of the windows. If you can afford them, install them -- the incremental cost is much lower than the cost of replacing double-glazed windows for triple-glazed at some future time.

2. | | #2

Thanks, Martin

i unfortunately don't have access to an energy modeling program. I need to get one as I compute these calculations with a calculator and much paper! But still, my client is building a 1.2 million dollar lake house on Sunapee and so while to me its a moot point, to him its not. This is probably why he can afford such a house and I will never (should never say never!) achieve such wealth.

My sense is that given the funds invested to achieve super insulation (labor and material to do so) the significance of the window selection is greater than with a less deeply insulated structure. And to some extent using the inferior windows sacrifices some of the expense made on so many of the other components of the structure. As you know, an efficient structure is hard to achieve and dozens of details and components need to be thoughtfully handled to get there.

Thanks,

Harry

3. GBA Editor
| | #3

Harry,
If you are designing a superinsulated REMOTE house with a budget of \$1,200,000, you absolutely need to use an energy modeling program to optimize your design. Just winging it is false economy.

If you're the designer, and you don't know how to run an energy modeling program, it's time to get some professional help. You need to hire an energy consultant.

4. | | #4

Not building the cost of energy modeling into a new home is throwing money away. Not putting solar hot water on the roof is also throwing money away. I'm sure some people can afford that. Few of my clients can. I use a simple and free online energy calculator to ballpark heat loss tradeoffs:
http://www.builditsolar.com/References/Calculators/HeatLoss/HeatLoss.htm

5. | | #5

Robert,

Thanks for the link to the free online energy calculator. I just checked it out and it seems easy to use.
hgs

6. | | #6

I will underline Martin's sentiments. Hire someone who knows this stuff.

7. | | #7

Harry
Is there any reason why you are choosing Eagle? (I am assuming Eagle, as in the Anderson Company). I might recommend looking into some Canadian companies such as Fibertec, Inline, Thermotech, etc. All will have better infiltration and tightness ratings, more glass options, fiberglass (which to many is a better product compared to aluminum clad wood), and will most likely come in around the same price, possibly lower.
Download and run RESFEN, a free easy window payback software. I am guessing it will show your payback will be quite long (as in longer then the life of the window), but as others said, talk to him about comfort of the glass. This might be a very valid point for him, especially if he has cold air blowing off of the lake onto a large expanse of glass. But I would think for someone going with that budget and energy efficient considerations, \$2000 triple pane is a complete no brainer. Infact I am amazed he is even questioning it.

8. | | #8

Jesse

Thanks for your response. Eagle was chosen for reasons of durability, selection and service. The Eagles, should they ever need any service, can be serviced by the local lumber yard, LaValley Building supply. I know the extruded alluminum windows are not egual to poltruded fiberglass or even vinyl windows in thermal performance, but I have used them on dozens of projects and they have weathered superbly.

I've investigated the Canadian windows on line and have been impressed by their performance characteristics, (I even got a quote from Inline) but since I have yet to actually see one and service is a concern, I've been reluctant to spec them. Have you used these windows and found their offerings (in terms of size and type) to be good and do they have records of good product service. You hope never to be in that place but its always a concern I consider strongly.

hgs

9. | | #9

Is it really 4,800 sf?

10. GBA Editor
| | #10

Dan,
The budget to build the 4,800 sq. ft. house is \$1,200,000 -- or \$250 / sq. ft. Sounds possible.

11. | | #11

whoa. i recently saw a \$1.2M house w/ vinyl windows. you can easily hit Passivhaus for less than \$200/sf, get the nice wood PHI-certified triple pane windows. and still have enough left over for all the higher end finishes.

12. | | #12

Dan,

The first floor footprint is 1,750 sqft.; the second is 1,350 sqft and the basement is unfinished. The overall higher cost of this project, is due to elevated site costs. A comprehensive storm water management plan was designed for this site's relatively steep slope. All rain water is collected by gutters with gutter toppers and infiltrated sub surface behind a network of retaining walls and linked terraces with rain gardens mitigating surface runoff. Pervious paving, many stone steps. a professional landscape planting plan, extreme protection measures for old growth trees, working around areas of unaltered space, septic system and a new garage all have added to the cost. This project's costs are high but the inherent environmental stewardship that is built in will protect a sensitive shoreline and in the esteem of my client is a value worth the investment.

hgs

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