I'm designing a new house for myself, building in Raleigh, NC. I plan to use ICF, I’ve wanted to do this for the past 10 yrs. Walls will be ICF, and will spray foam roof and planning geothermal system. Love the benefits of ICF, including the energy efficiency. I'm looking at Windows, I prefer the look of Alum clad windows. I will probably go with Eagle, now owned by Anderson. Kolbe and Loewen are nice, but they are coming in about 40% more. My question is, with the efficiency of building envelope, should I be looking at Triple pane or other alternatives for a more efficient window? Aesthetics are very important to me. My design is European/French Chateau and plan to make everything look old, for that old world charm look. I'm already looking to cut some cost, as I'm already over my original planned budget. I have a lot of windows, and many radius top casements. so I'm looking at spending about $50k or so on windows and French doors. Or is this climate not as demanding, as say the Northeast where i currently live and have built several houses. I guess my real questions is with all I’m doing to create an airtight, efficient house. Do i need to step up on the windows? trying to find some balance here.

Posted By mbtracy on 06 Sep 2013 02:54 PM
I'm designing a new house for myself, building in Raleigh, NC. I plan to use ICF, I’ve wanted to do this for the past 10 yrs. Walls will be ICF, and will spray foam roof and planning geothermal system. Love the benefits of ICF, including the energy efficiency. I'm looking at Windows, I prefer the look of Alum clad windows.

Your ICF walls are going to be around a R-23 value. Your windows and doors are going to be the "weak link" in the wall structure R Value.

Raleigh is classified as a subtropical humid climate. You are also in a mild hurricane zone area. These two factors would make me lean towards a window that does NOT have wood on the outside, even aluminum clad windows. I would lean towards fiberglass or uPVC windows. Being in a hurricane/high wind zone, I would also lean towards a window that is rated for high pressures. DP60 ratings is where I would lean towards.

I would get triple pane because a double pane is around R-2/R-3 while triple pane windows move towards R-6/R-8 realm.

Are you going to incorporate passive solar into the homes design?

RESFEN is a software tool available for free that was put together to answer your question quantitatively. I ran a few cases for Rayleigh NC for a one-story masonry house of 2400 sq. ft. with 80 sq. ft. of windows on each of 4 sides. Comparison includes double pane versus triple pane and low versus high solar gain, all labeled insulating windows. Yearly costs for heating plus cooling are included, assuming a gas furnace and electric A/C. Shading of solar gain was assumed "typical" in the menu selection.

Window type, U-value, SHGC, Leakage rate, Yearly utility
INS 2SS, 0.26, 0.31, 0.3, $487 (double-pane, low SHGC)
INS 2PY, 0.29, 0.56, 0.3, $476 (double-pane, high SHGC)
INS 3SS, 0.18, 0.26, 0.3, $467 (triple-pane, low SHGC)
INS 3PY, 0.18, 0.40, 0.3, $455 (triple-pane, high SHGC)

For this windows selection, high solar gain windows, which generally cost the same or slightly more than low solar gain, appear to save costs and be worth specifying assuming any extra cost is modest. Triple-pane windows versus double pane widows of the same solar gain save only about $20 per year in utility costs, so in this mild climate are probably not worth the significant extra investment usually required. There might also be a slight comfort factor advantage to the triple-pane windows, but shades or curtains might be another approach to addressing comfort.

To make this analysis more accurate for your house, you can download RESFEN and put in values for your house and for the windows that you are considering. You can specify the exact window characteristics for the windows under consideration.

Other important factors to consider with most triple pane windows:

* dB 47 (triple pane are much quieter than double pane windows)
* DP 70 (design pressure)  (A DP 50 can withstand a Level 5 Hurricane)
* U-Value of 0.13
* Air Leakage = 0.01 (basically completely air tight)

As far as RESFEN goes, it does have some issues. If you plug in an AI rating of 0.30 vs 0.01 (triple pane), it doesn't properly adjust for the air leakage. Even when you do 600 sqft of glazing, it will take maybe $5 off the total heating and cooling costs for the year. It's a proven fact that air leakage in a home is a huge penalty in heating and cooling costs. RESFEN doesn't accurately take that into account. There is no way that 600 square feet of glazing with AI readings of 0.30 vs. 0.01 will only penalize you $5 per year in energy costs.

Passive House software (PHPP, WUFI) works better but it is not free.

Another glitch is that when one inputs a lower U-Value and all other variables remain the same, the cooling costs INCREASE. How is that possible? All variables are the same; SHGC, AI, and the only change is that the U-Value of the windows goes from 0.28 to 0.13 and RESFEN penalizes you and shows a higher cost to cool!

RESFEN will also penalize you when it comes to running ductless mini-splits as the software is not advanced enough to recognize the heating costs of a gas furnace with duct work vs. an electric ductless mini split.

When modeling homes I was told that RESFEN is antiquated and that to properly do a Passive Home design one has to use a modernized software like PHPP. Whether or not that was a sales pitch by the company, I am not sure but free isn't always the best or most accurate.

Do you recommend that i have someone run the #'s on my house? are there companies that do this? I love the idea of ICF, spray foam, geothermal, etc. but obviously, like most people, i have a budget. So sure, triple pane might be nice, along with other higher efficiency features, but i have to draw the line somewhere.
I would assume that once you get to a certain point in efficiency, the cost for each extra level gets higher and higher. I'm also struggling with the fact that most houses, even custom houses in Raleigh area, are built cheap! So even though i plan to GC my project and do alot of the custom work myself, my house is going to cost substantially more than my comps. Frankly, its pretty rare to even see decent windows... most are garbage, and look at a 5-10yr old house, and they are falling apart.

I'm very much into architecture and design, but i'm a bit newer to this energy efficiency side of things. Even the geothermal was not in my original plan, but after a few recommended it and showed me the benefits, i'm now sold. and i love the idea of not having the ugly condensers sitting outside.

Posted By mbtracy on 07 Sep 2013 09:27 PM
Do you recommend that i have someone run the #'s on my house? are there companies that do this? I love the idea of ICF, spray foam, geothermal, etc. but obviously, like most people, i have a budget. So sure, triple pane might be nice, along with other higher efficiency features, but i have to draw the line somewhere.

Frankly, its pretty rare to even see decent windows... most are garbage, and look at a 5-10yr old house, and they are falling apart.

I'm very much into architecture and design, but i'm a bit newer to this energy efficiency side of things. Even the geothermal was not in my original plan, but after a few recommended it and showed me the benefits, i'm now sold. and i love the idea of not having the ugly condensers sitting outside.

It depends on what you are going after. If your primary goal is energy efficiency, then yes.

Geothermal is great when it is correctly installed and when it works properly. It can become a nightmare if you had a bad install or if parts start to fail. It is quite expensive so ROI is a long time coming. Ductless mini-splits can provide heating/cooling and are less expensive and more reliable than geothermal. You will have to crunch the numbers and see for yourself.

I agree, 90% of homes built are poorly built and the main goal is interior visual but the home is usually an energy pig.

I recommend getting in touch with someone who has the better passive house programs and see what they come up with. Is your design a passive design?

I would ditch the geo. It is great, however the return is low and gets lower as you increase r value and tightness of your home. Pay off on my r28/foam/triple pane home in high heating climate zone 6 of geo vs a 98% high eff. gas furnace was about 30 years including good rebates from local utilities and the government tax credit. However I happen to live in an area where gas prices are very low and electric is quite a bit higher. Compared to a cooling dominate zone, the numbers would probably be less, and mini splits may be a good option. IF that is the case, the cost between mini splits and geo will be significant savings.

Eagle makes good windows, I have installed quite a few of them. Their newer windows are better yet. However they are limited in important options such as glazing, compared to Canadian fiberglass companies and even Kolbe that you mentioned. They also do not have a 1 3/8" triple pane window, rather they cram another piece of glass into a dual pane sash, resulting in very poor triple pane ratings. (unless they changed this recently). Kolbe for example can do any type of glass under the sun, and have the 1 3/8" triple pane glass. I have the Ultra EP in my home. The choice came down between Kolbe and Inline fiberglass windows. Either would have been a great choice, but various reasons lead me to land on the Kolbes.

Much like is stated above, energy wise- you will most likely not see a return on triple panes. Numbers Lbear are quoting are not your typical triples, but rather super high performing windows. Alum clad triples will have a u of about .17 at best, and high solar gain is around .21-.23 depending. Even comparing .13 u values to .29 and .17 to .29 is dollars a year. But they have other advantages, not sure if they would all be truly seen or appreciated in NC vs a high heating area.

I love the idea of ICF, spray foam, geothermal, etc.

All that and triple panes? In Raleigh? How cold does it get there?

I wasn't really considering a passive design. Perhaps i should read up on it, but from what i know, I’m not really interested in altering the aesthetics of my house with overhangs, changing window locations, etc. and my house design is integrated into the hill to provide function/design....

I was under the impression that geothermal was comparable to typical systems with the state/federal rebates? Is that not the case?

My primary goal is not energy efficiency. My primary goal is a strong, architecturally appealing home. I am after a "period style" home, so it will look old, like it was built in Europe 100 yrs ago. ICF brings strength, energy efficiency, and so many other advantages, that to be its a no brainer. Spray foaming the roof just makes sense with ICF, and i like the benefits of spray foam. Used it in my last house.

lzerarc - why did you go with triple pain? Are the benefits of triple pain mainly for the northern, colder climates?

ICFHybrid - Raleigh does not get very cold. maybe 20's or teens a few times a winter, but only for very short periods. Heat and humidity will be the problem, mostly cooling cost.

Posted By mbtracy on 08 Sep 2013 11:47 AM
I wasn't really considering a passive design. Perhaps i should read up on it, but from what i know, I’m not really interested in altering the aesthetics of my house with overhangs, changing window locations, etc. and my house design is integrated into the hill to provide function/design....

I was under the impression that geothermal was comparable to typical systems with the state/federal rebates? Is that not the case?

My primary goal is not energy efficiency. My primary goal is a strong, architecturally appealing home.

If the home is already designed, then forget about redesigning it for a passive house. Leave it be.

If the home is facing due south, just be careful about southern and western glass exposure because it can heat up the home quite a bit during summer. Lower SHGC windows ( < 0.30) will help prevent that.

You made the statement that your goal is a strong and architecturally appealing home. Energy efficiency is the secondary goal, which is fine. If that is the case then double pane windows might be OK for you. Also remember, not all triple panes are made the same, there are good, better and best when it comes to triple panes. Some manufacturers will attempt to stuff 3 panes of glass into a frame that was designed for 2 panes of glass. The spacing between panes is also important.

Posted By Lbear on 07 Sep 2013 07:42 PM
If you plug in an AI rating of 0.30 vs 0.01 (triple pane), it doesn't properly adjust for the air leakage. Even when you do 600 sqft of glazing, it will take maybe $5 off the total heating and cooling costs for the year. It's a proven fact that air leakage in a home is a huge penalty in heating and cooling costs. RESFEN doesn't accurately take that into account. There is no way that 600 square feet of glazing with AI readings of 0.30 vs. 0.01 will only penalize you $5 per year in energy costs.

jonr previously provided to you a calculated result for the cost of a leakage rate of 0.3 cfm/min, which is the standard spec for maximum leakage for windows sold in North America. I cannot remember his result without looking it up, but it was a very small cost savings for taking a window from 0.3 cfm/ft^2 to 0.01 cfm/ft^2. You traditionally refer to windows with the higher spec as "leaky," but the numbers do not bear you out.

Discovered an error on 9/9/2013, and corrected as follows:
Just to have an indepedent confirmation of jonr's result, I have computed the heating/cooling cost for North Carolina to replace a generic 41" x 59" window that leaks at 0.3 cfm/ft^2 with one where the leakage rate has been reduced to 0.01 cfm/ft^2. So 0.3 cfm/ft^2 = 5.5 m^3/(hr*m^2), so for the 41" x 59" window, this is a maximum leakage at the 75 Pa measurement condition of 8.56 m^3/hr.

The leakage at this pressure differential must be corrected to "natural" convection conditions, just like for blower door tests conducted at 50 Pa. In the case of blower door tests, a rule of thumb is divide the measured value at 50 Pa by a factor of 20 to get the natural infiltration rate. Since the pressure for windows tests is 75 Pa rather than 50 Pa, assume a pressure dependencd for leakage rate of (P1/P2)^0.8, and the correction to natural leakage is to divide by 27.7 rather than 20. So the window leakage under natural conditions is 0.31 m^3/hr.

The sea level density of air at 72F (295K) is 1.19 kg/m^3, so the mass leakage rate for one window is 0.369 kg/hr. Use the specific heat of air to get into a form to compute the heat loss, so 1.009 kJ/(kg*K) times 0.369 kg/hr gives 0.372 kJ/(hr*K). Now we have the energy losses all set to compute for any climate, and just multiply this value by the heating degree hours, which are days x 24, and a heating system efficiency to get the annual energy to reheat the leaked air.

So for Rayleigh, NC, the heating degree days are 3457 degF days, and the cooling days are 1417 degF days, which combine to 64987 hrs annually. (Think of this as just a heating problem rather than heating plus cooling for simplification.)

So the annual energy content for the leakage rate per window is 24170 kJ (22910 Btu), and for an energy content of natural gas of 38000 kJ/m^3 (~1020 Btu/ft^2), burned at 98% efficiency, the natural gas requirement would be 0.649 m^3 (22.9 ft^3), or 0.229 CCF. At a cost of $1.00 per CCF (varies regionally) then the cost to reheat the infiltration air for one window is $0.23 annually.

You mentioned that RESFEN gave a value of $5 per year for this same change in leakage rate for 600 ft^2 of glazing. The one window above is 16.8 ft^2, so the analysis above would give $8.21 for 600 ft^2. So unless there is an error in the above analysis, then the $5 penalty looks reasonable. You always seem convinced that a 0.3 cfm/hr window would result in huge heat losses. Please share your calculated results so that we all can learn.

Some caveats to the window leakage analysis are as follows:
1. Measured leakage rates for windows are rarely given. The analysis is based on the window spec., not a measured value.
2. These specified leakage rate is only for the window inside the frame, and does not include leakage between the outer frame of the window and the wall. This leakage around the outside of the frame might be substantial, but does not vary with the selection of a single, double, or triple pane window, so is not useful in window selection.

From everything I read, low air infiltration is critical in a passive house, as it is for any energy efficient home build. Why are the building science experts running around sealing every nook and cranny? Windows with AI Ratings of 0.30 would not be used in a modern day passive house build.

David Paulus, PhD., P.E. stated, "Air infiltration becomes a greater percentage of total energy loss as the window’s U-factor becomes lower, because the energy loss through heat transfer is reduced."

I was told by experts that RESFEN is inaccurate and has major calculating flaws. I experienced a lot of them myself when running calculations. For example when one inputs a lower window U-Value and all other variables remain the same, the cooling costs INCREASE. How is that possible? All variables are the same; SHGC, AI, and the only change is that the U-Value of the windows goes from 0.28 to 0.13 and RESFEN penalizes you and shows a higher cost to cool!

It is a fact that as long as all other variable remain the same, lower U-Values on windows are always better. However, RESFEN disagrees with that and penalizes you for lower U-Values by increasing costs to cool. That is a programming error inherit in RESFEN, one of many.

Another issue is that RESFEN, like NFRC ratings, are not accurately recorded. The best possible AI Rating for a window by the NFRC is 0.1, as they will not round to 0. So a window that has an AI Rating of 0.03 it will still receive a value of 0.1 from NFRC. Even windows that achieve LESS than 0.01 in tests would still receive a rating of 0.1 from the NFRC. At least the AAMA records down to 0.01

On a 2,000 sqft home with 600 sqft of glazing. I input a U-Value of 1.00 for all the windows. (yes, that's not a typo, a single pane of glass of a U-Value of 1.00). I then left all variables the same except changing the U-Value from 1.00 to 0.25. RESFEN claimed that changing 600 sqft of windows from a U-Value of 1.00 to a 0.25 would only decrease my cooling bill by $10 per year. That's RESFEN for you...

great post Lee.

Lbear: you skirted the question as usual. "I read that" or "I was told by experts" does not quite answer the question.

Hate on RESFEN all you want, but at the end of the day its a free, governmental program. Do you honestly expect it to be perfect?! Its like asking Google Sketchup to be Autodesk Revit. Its good for the basic, code min. stuff that most builders and energy raters are using to show basic compliance of code min. homes and how it 'could' effect loads. I would never expect it to be used to be a determining factor in a super performing home. (however try cranking your SHG value down instead of your u for cooling loads.......)

mb- yes, I feel triple pane pays for itselt better in heating climates, but even running numbers it was still a long payback. However the cost differences between dual and triple was about $2k for me, so it made more sense to just install them now since you never know in 5 years if that payback changes a lot more. Plus there are a couple big solar collectors with seating near by, so on a cold day the triple pane is a lot nicer to sit beside. These windows (made up of 4 mulled) have a average u of .19 and SHG of .42.

Posted By lzerarc on 09 Sep 2013 09:00 AM

Hate on RESFEN all you want, but at the end of the day its a free, governmental program. Do you honestly expect it to be perfect?!

That's probably why it is so seriously flawed, it's a government program.

I'm not looking for perfection from a free program but when the program tells me that I will only save $11 a year on cooling costs going from a single pane of glass with a U-Value of 1.00 to double pane glass with a U-Value of 0.25 on 600sqft of glazing. There is something seriously wrong with the way it computes data.

Lbear-

Forget RESFEN for now. Let us focus on how much the energy leak from a standard spec window amounts to. That is, what is the energy savings and cost saving for substituting a window with a leakage spec of 0.01 cfm/hr for a window with a leakage spec of 0.3 cfm/hr. We all know that the lower leakage would be better, so do not bore us with that statement again. We all know that the lower leakage windows cost more. So please provide the tradeoff with actual numbers for the energy savings, which is the only meaningful information to make a decision on whether or not to pay more for the lower leakage window.

Feel free to use any and all models or computational approaches that you would like.

what is the point of Resfen if its required for a building permit, yet its info is not accurate? Are the other, more accurate programs easy to use, or do they require extensive training?

I don't think RESFEN is required for a building permit. REScheck is required, but that is a completely different program for computing the weighted area * thermal resistance (R-value) for the overall house. It is easy to use.

Lbear-

You have suggested high performance triple-pane windows for Rayleigh NC. While you are calculating energy costs for leakage as we discussed above, why don't you also compute the heat losses through a 41"x59" (generic size) window for Rayleight NC for windows with U-values of 0.3, 0.2, and 0.1. I have provided the heating and cooling degree days above for Rayleigh, which may be combined into a single number.

I get annual costs of about $6, $4, and $2 per window assuming natural gas at $1.00 per CCF and 98% thermal efficiency, and treating A/C as the same cost per Btu as heat. Thus, from strictly a cost savings point of view, you want to limit how much extra you spend for high-performance windows.

Shading to avoid adding to the summer heat load might be as important as U-value, although the original poster seems more interested in the aesthetics than anything else, not wanting to use overhangs etc. to improve the performance and longevity of the buildiing.

Posted By Lee Dodge on 09 Sep 2013 01:31 PM

We all know that the lower leakage windows cost more.

Why is the above an absolute statement? I've priced out a lot of windows and some lower leakage windows were priced LESS than the higher leakage windows.

A faulty premise results in a faulty analysis and conclusion.