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Helpful? 5

Study Shows That Expensive Windows Yield Meager Energy Returns

An engineer investigating ways to optimize the design of net-zero-energy homes concludes that inexpensive triple-glazed windows are good enough

Posted on Sep 14 2012 by Martin Holladay, GBA Advisor

An architectural cliché from the 1970s — the passive solar home with large expanses of south-facing glass — is making a comeback. In recent years, we’ve seen North American designers of PassivhausA residential building construction standard requiring very low levels of air leakage, very high levels of insulation, and windows with a very low U-factor. Developed in the early 1990s by Bo Adamson and Wolfgang Feist, the standard is now promoted by the Passivhaus Institut in Darmstadt, Germany. To meet the standard, a home must have an infiltration rate no greater than 0.60 AC/H @ 50 pascals, a maximum annual heating energy use of 15 kWh per square meter (4,755 Btu per square foot), a maximum annual cooling energy use of 15 kWh per square meter (1.39 kWh per square foot), and maximum source energy use for all purposes of 120 kWh per square meter (11.1 kWh per square foot). The standard recommends, but does not require, a maximum design heating load of 10 W per square meter and windows with a maximum U-factor of 0.14. The Passivhaus standard was developed for buildings in central and northern Europe; efforts are underway to clarify the best techniques to achieve the standard for buildings in hot climates. buildings increase the area of south-facing glass to levels rarely seen since the Carter administration.

What’s the explanation for all this south-facing glass? We’re told that there’s no other way for designers to meet the energy limit for space heating required by the Passivhaus standard: namely, a maximum of 15 kWh per square meter per year.

Struggling to meet this goal, many Passivhaus designers have found that the typical triple-glazed windows sold in North America have U-factors that aren’t quite low enough (or SHGCs that aren't quite high enough) for their designs to meet the standard. Because of this, these designers often end up specifying very expensive triple-glazed windows from Germany or Austria.

What about cost-effectiveness?

As I have often noted, these Herculean efforts to meet the Passivhaus standard pay no attention to cost-effectiveness. Even when designers find it necessary to invest in measures that are much more expensive than a photovoltaic(PV) Generation of electricity directly from sunlight. A photovoltaic cell has no moving parts; electrons are energized by sunlight and result in current flow. array, they plow ahead because they have to meet the numbers dictated by the PHPP software.

These investments in very expensive building materials are probably a waste of money. An excellent paper by Gary Proskiw, “Identifying Affordable Net Zero Energy Housing Solutions,” looks into the cost-effectiveness of large expanses of south-facing glazing as well as the cost-effectiveness of low-U-factor windows. Proskiw, a mechanical engineer from Winnipeg, Manitoba, who specializes in residential energy issues, concludes that heroic window measures don’t pay worthwhile dividends.

Proskiw’s analysis and conclusions are fascinating and thought-provoking, and I believe that most designers of low-energy homes will want to read Proskiw's paper. (I'd like to credit GBAGreenBuildingAdvisor.com reader Sasha Harpe for alerting me to the existence of the report.)

Proskiw’s paper focuses on ways to optimize the design of a net-zero-energy (NZE) house. While the paper mostly focuses on Canadian climates, it considers one U.S. home (the net-zero-energy house built by Habitat for Humanity in Wheat Ridge, Colorado) in its analysis.

The value of extra south-facing glazing

Proskiw asked an interesting question: should the designer of a superinsulated home add extra south-facing windows “to increase solar gains and reduce the space heating load”? He tackled the question by analyzing the cost of this measure and then comparing the cost to the energy benefit.

For the purpose of his analysis, he considered an 1,800-square-foot net-zero-energy house located in Winnipeg, Manitoba. He assumed that the base-case house had R-44 exterior walls and south-facing glazing with an area equal to 6% of the floor area. According to his analysis, the cost to build an R-44 wall is $170 per square meter ($15.80 per square foot). What if the designer chose to add another south-facing window — one measuring 1 square meter (10.76 square feet)?

Proskiw estimated that a triple-glazed, low-e, argonInert (chemically stable) gas, which, because of its low thermal conductivity, is often used as gas fill between the panes of energy-efficient windows. -filled fixed window measuring one square meter costs $488. Proskiw estimated that the net cost of adding this window to the house would equal the cost of the window minus the cost of the displaced wall area; he calculated the incremental cost this way: $488 - $170 = $318. (Proskiw knows that this calculation method underestimates the cost of adding a window, since it ignores the costs associated with framing the rough opening, the cost of the header, and the cost of trimming the opening; however, if these costs are included, Proskiw's argument is only strengthened.)

The base-case house had a modeled energy consumption of 1,462 kWh per year. Adding the extra window resulted in a modeled energy consumption of 1,443 kWh per year. In other words, the extra window saved only 19 kWh per year, which Proskiw valued at $1.90. He calculated that the payback period for this measure is 167 years. “Given that the life expectancy of an insulated glazing unit (IGU) is about 25 years, it is clear that inclusion of the extra 1 square meter of south‐facing window area can never be economically justified,” Proskiw wrote. “From a design perspective, these results indicate that increasing the amount of window area in a NZE house, as an energy saving measure, has to be examined extremely carefully since it is unlikely to be economic relative to other options.”

According to energy expert Marc Rosenbaum, adding a south-facing window measuring 1 square meter (gross area) on one of his Massachusetts house designs (a house from the Eliakim's Way development on Martha's Vineyard) would save 120 kWh a year (worth about $12 a year, according to Proskiw's method). If the window could be installed for an incremental cost of $318 — (the actual incremental cost is likely to be higher) — the simple payback period would be 26 years.

What about investing in really good windows?

Proskiw also compared two different types of triple-glazed window:

  • The less expensive option ($360 per square meter) was a “relatively conventional triple-glazed unit with an insulated spacer.” This window did not include argon gas or any low-e coatings.
  • The more expensive option ($488 per square meter) was a “triple-glazed unit with one low-e coating, two argon fills, and an insulated spacer.” The incremental cost for this window was: $488 - $360 = $128.

Proskiw assumed that the window measured one square meter and faced south. The energy savings attributable to the glazing upgrade was 8 kWh per year, which Proskiw valued at $0.80. The upgraded glazing had a simple payback period of 160 years.

Expensive glazing doesn’t make economic sense

Proskiw wrote, “The reason the two window upgrades fared so poorly, from an economic perspective, is that the space heating load in a NZE house is very small compared to any other type of house. By adding window area or upgrading window performance, the space heating load is reduced but it is already so small that there is little opportunity for further savings.”

He went on to note, “The preceding discussion used the incremental analysis of costs and
benefits to illustrate the economics of adding glazed area and of upgrading windows in a Net Zero Energy House. Although it used single examples, the process could be easily used for other windows in other houses. A more rigorous analysis, using a wider range of windows, houses, locations, etc., would yield similar results in most cases.”

What’s the lesson for designers of superinsulated homes? “Since windows and their upgrade options are so expensive, the investment would often be better spent on improving the energy performance of some other conservation or renewable energy option,” Proskiw concludes.

Inexpensive triple-glazed windows are fine

One way to summarize Proskiw’s findings: builders of superinsulated homes in cold climates should choose affordable (usually triple-glazed) windows rather than exotic windows with extremely low U-factors.

“From an energy perspective and based on the incremental costs and energy savings, window selection should be based solely on the need to control condensation,” Proskiw wrote. In most cases, that means that you should choose glazing with a warm-edge spacer. “Further, the window area should be limited to that necessary to meet the functional and aesthetic needs of the building. As such, south‐facing glazing area should be restricted to 6% [of the conditioned floor area] (to control overheating) and total window area should also be limited to that required for functional and aesthetic considerations. On a broader level, these results indicate that our long‐held belief in the merits and value of passive solar energy as a key component of Net Zero Energy House design need to be carefully re‐examined and likely challenged.”

Simplicity is a virtue

Proskiw’s paper includes many other nuggets of wisdom that are worthy of attention from designers of superinsulated homes. For example, he noted that low levels of air leakage are easier to achieve if the building’s envelope has a simple shape. Proskiw advised, “Draw out complicated details. If you can’t draw it, you probably can’t build it.”

Proskiw is a big fan of drainwater heat recovery devices (for example, the GFX and the Power-Pipe), which he called “extremely robust and reliable devices.”

When it comes to space heating and water heating equipment, Proskiw advocates simplicity. “One of the most common problems (both observed and reported) with Net Zero Energy Housing has been the complexity of the mechanical systems (space heating, domestic hot water heating, ventilation and cooling). While there may be a temptation to use every thermodynamic opportunity to maximize performance, the reality is that complex mechanical systems almost always prove to be problematic, expensive and far too unreliable. Perhaps the most trouble-prone example has been seasonal heat storage systems which attempt to capture and store large amounts of energy between seasons. While technically feasible, such systems are usually extremely expensive, produce nominal savings and may require the homeowners to adopt a full‐time repairman as a live‐in family member.”

Solar thermal systems and buyers' remorse

Proskiw went on, “The need to simplify mechanical systems was arguably the most consistent comment offered by designers during the interview phase of this project. For example, one designer had used a solar thermal system in conjunction with a GWHR [gray water heat recovery] system and a desuperheater on a heat pumpHeating and cooling system in which specialized refrigerant fluid in a sealed system is alternately evaporated and condensed, changing its state from liquid to vapor by altering its pressure; this phase change allows heat to be transferred into or out of the house. See air-source heat pump and ground-source heat pump. — three separate technologies to heat water. Overall, he found the solar thermal system to be leak‐prone and not as effective as originally hoped. In retrospect, he felt that it would have been preferable to use additional photovoltaic capacity in place of the solar thermal system since most of the DHW [domestic hot water] heating was provided by the geothermal and GWHR systems. Perhaps the issue of mechanical system complexity was best captured up by one NZEH designer who summed up his approach: ‘Just say no!’”

Among the lessons learned: “Some of the surveyed designers who had used solar thermal systems for DHW heating stated that given the problems with reliability and performance, they would have replaced the system with a larger PVPhotovoltaics. Generation of electricity directly from sunlight. A photovoltaic (PV) cell has no moving parts; electrons are energized by sunlight and result in current flow. array and a (relatively) conventional electric DHW heater.”

An interesting postcript

In January 2013, I received copies of an e-mail exchange between Peter Amerongen, the developer of the Riverdale Net Zero project in Edmonton, and Gary Proskiw.

Amerongen wrote (in part), “I think Gary is underestimating the benefits of additional south glazing area. ... That is a small quibble. The bigger issue is that if you limit conservation to benefits that are less expensive than PV, you will never reach net zero in our part of the world.” (Peter Amerongen's full comments can be read below; they are posted as Comment #62.)

Gary Proskiw responded, “Let me try to explain the problem with windows. Imagine a somewhat conventional house with a gross monthly heating load as shown by the red line in the graph below:

“Also shown are the monthly base loads (averaging around 500 kWh/month). The difference between the gross space heating load and the base loads is the heating load which has to be supplied by the heating system. For the conventional house, this means that the heating season will extend for about nine months per year - as shown by the range ‘A.’

“For a net-zero-energy house, the gross monthly heating load is much smaller, as shown by the green line. This means that the heating season for the net-zero house will last about four months per year, as shown by the range ‘B.’

“Now, when additional window area is added to the house there is greater opportunity for passive gains. However, they are only of value if the house can actually utilize the gains. In the conventional house, that means the windows could theoretically reduce the heating load over a nine month period. But in the net-zero-energy, the ‘window’ during which these benefits can be realized is less than one-half that of the conventional house. Further, the months in which the net-zero house could theoretically use these additional solar gains occurs during the period of the year when solar gains are at a minimum.

“In other words, adding a square meter of window area to a net-zero house will produce much less benefit than adding that same square meter of window area to a conventional house. Of course, the same holds true for conservation (adding one more batt of insulation to a net-zero house will produce less benefit than if that batt were added to a conventional house). The difference is that one square meter of window area costs about ten times as much as (say) an equivalent amount of wall area, so the consequences of excess glazing area are economically more problematic than excess insulation.

“Basically, here's the problem. When one square meter of window area is added to a house, two things happen: the gross space heating load is increased (since the window has a lower R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. than the wall area which it replaced) and the passive solar gains are increased. Unfortunately, the increased space heating load is present 24/7 during the heating season. The extra passive gains are also available, but can only be utilized during a portion of the heating season (unless overheating is allowed to occur). As the house becomes more energy efficient, that ‘portion’ becomes increasingly smaller. To illustrate, the percentage of the solar gains which were actually usable by the house in the two cases below was 33% for the conventional house and 28% by the net-zero house, as calculated by HOT2000.

“One can then make the argument for thermal storage to facilitate greater utilization of passive gains; however that is usually a very expensive proposition which produces nominal benefits.”

[Editor's note: to read Peter Amerongen's response to these comments by Gary Proskiw, see Comment #93, posted below.]

Martin Holladay’s previous blog: “Who Deserves the Prize for the Greenest Home in the U.S.?”

Click here to follow Martin Holladay on Twitter.


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1.
Fri, 09/14/2012 - 07:34

Extremely important
by Dan Kolbert

Helpful? 1

The Pretty Good House team has been stumbling toward similar conclusions - simple shapes, simple systems, simple windows, etc. Perhaps obvious but hard to resist the pull of the new and shiny.

Also, ironically, as window performance keeps improving we find clients and designers keep wanting to make them bigger, resulting in a "net zero" improvement in overall wall R-value.


2.
Fri, 09/14/2012 - 08:39

Response to Dan Kolbert
by Martin Holladay, GBA Advisor

Helpful? 0

Dan,
As you know, I'm a fan of the Pretty Good House movement. I'm also a fan of the approach taken by Carter Scott, who has perfected the $250,000 net-zero-energy home in Massachusetts.

Yesterday, many news outlets reported on the accomplishments of a team of designers and builders who completed a net-zero-energy house on the campus of the National Institute of Standards and Technology (NIST) in Washington, DC. One news outlet --ABC's Channel 7 -- even reported that the house was built "at a surprisingly affordable price ... about $800,000."

News flash: you can do it for a lot less than $800,000.


3.
Fri, 09/14/2012 - 11:03

How apropos!
by Randy George

Helpful? 0

I actually logged on to the GBA site this morning to do a little more research into Intus windows which, along with Marvin's triple-glazed and Thermotech, we are having our contractor get estimates for as we get ready to build our new house. I've shared Dan's Pretty Good House piece with our contractor and have used the ideas contained in that article as our guiding principle for this project. But the recent appearance of Intus on the scene has me intrigued. It appears that they offer a window with a 7.7 R value for less money than the other two manufacturers I mentioned. The premise of this piece is that the European windows are too expensive. But if they're not so pricey, why not do it? And who wouldn't want tilt and turn? The big problem is the 12 week lead time. Any thoughts, folks?


4.
Fri, 09/14/2012 - 11:20

Edited Fri, 09/14/2012 - 15:59.

Response to Randy George
by Martin Holladay, GBA Advisor

Helpful? 0

Randy,
U.S. builders looking for European glazing specs at the lowest possible price have all turned to Intus. The only downside: their affordable windows have vinyl frames.

However, Intus windows are certainly cheaper than any other European triple-glazed window (as far as I know). Of course, if you want to follow Proskiw's philosophy -- and if you have no objection to vinyl windows -- you might as well buy a triple-glazed vinyl window from Harvey Industries or Paradigm. (Those are vinyl windows made in the U.S.). That's the low-cost approach taken by Carter Scott.


5.
Fri, 09/14/2012 - 11:44

overheating?
by Paul Eldrenkamp

Helpful? 1

My initial impression regarding the first round of homes built to the Passive House standard in New England is that overheating is a bigger problem than anticipated, not necessarily mid-summer when south glass is less of an issue, but in the shoulder months. If it turns out that the extra south-facing glass adds to the cost of cooling, or that builders will need to include some sort of exterior shading, Proskiw's argument might become stronger still.


6.
Fri, 09/14/2012 - 11:48

Edited Fri, 09/14/2012 - 11:53.

Response to Paul Eldrenkamp
by Martin Holladay, GBA Advisor

Helpful? 0

Paul,
"My initial impression regarding the first round of homes built to the Passive House standard in New England is that overheating is a bigger problem than anticipated..."

It's 1982 all over again... and the latest generation of designers is learning the same lessons that those of us with gray hair learned in the '80s.

September and March are the months to watch. So if it's sunny right now, you'll get a good idea of how your house is performing.


7.
Fri, 09/14/2012 - 15:24

Response to Martin Holladay
by Jack Barnes

Helpful? 0

It is worth mentioning that the Intus products use U-PVC, while the windows used by Carter Scott are the ordinary plasticized vinyl variety. Theoretically the unplasticized Intus windows will be longer lasting and not adversely affected by ultraviolet light. Low cost is great until you have a window that won't open on a 90 degree day!


8.
Fri, 09/14/2012 - 15:45

Response to Jack Barnes
by Martin Holladay, GBA Advisor

Helpful? 0

Jack,
There's a lot of nonsense bandied about in the vinyl window industry, and the much-ballyhooed advantages of "uPVC" over "PVC" is an example of nonsense. Vinyl is vinyl.

Of course, there are differences in the quality of vinyl lineals, but these days, these differences in quality have more to do with the thickness of the vinyl than the composition of the plastic.

Here's what Alex Wilson discovered when he researched the topic: "Plasticizers are not used in any American vinyl window extrusions and never have been. I do not believe there is any difference between UPVC and the PVC used in American windows. I asked two contacts in the vinyl industry about this a few months ago, and both were quite adamant about that. Phthalate plasticizers are widely used in flexible vinyl, including vinyl flooring, wire sheathing, and shower curtains--where it can account for 40-50% of the total weight I think. Vinyl window extrusions and vinyl pipe do not require these plasticizers."


9.
Sat, 09/15/2012 - 02:29

bad assumptions? 1. in the
by mike eliason

Helpful? 0

bad assumptions?

1. in the last quote we have from intus (before we were asked to be the WA rep) a 1m^2 fixed window w/ glazing with a Ug=0.088 was $289 shipped. this is nearly half what was calc'd in the study.

2. the payback calculation is from winnipeg, canada, which has a meager 10,500 HDDs per annum. most of the inhabited US is half of that AND gets more insolation, so the payback is probably going to be significantly faster.

3. in the higher-end design world, it's not uncommon to get quotes for poorly-performing double pane wood windows that are the approx. same cost as phenomenal european PH windows. in many of these instances, performance isn't considered at all beyond code - and with the utilization of the PH windows would make a fairly significant dent in energy bills, with no net cost (or in some cases, a net reduction in cost). in these instances, simple payback isn't even in the picture ('we want the best looking wood windows'). to me, this is an area where the study falls flat - not all houses shooting for NZE are shooting for lowest first costs. and in addition, some that shoot for NZE just do it poorly - e.g. the zHomes out here in issaquah).

4. can't the determination be made that ANY window (especially those on N/E/W orientations) can never recoup their costs in savings without some R-15+ superwindows?

the intus quote was actually cheaper than US windows of comparable (but still worse) performance by nearly a factor of 1.7. i think it's still something that needs to be better researched (reference the CEN v NFRC conversation). but we're also gravitating towards similar paths - compact form, utilization of GFX, etc.


10.
Sat, 09/15/2012 - 05:28

Response to Mike Eliason
by Martin Holladay, GBA Advisor

Helpful? 0

Mike,
Your examples tend to confirm Gary's analysis and recommendations.

Like other Passivhaus designers who are aiming for affordability, you have gravitated to fixed Intus windows. This decision is consistent with the advice given in the subhead of this article: namely, that "inexpensive triple-glazed windows are good enough."

Let's face it -- not everyone is happy with a fixed vinyl window. Fixed vinyl windows are only an option if you include operable windows elsewhere.

Most custom home clients want something better than a fixed vinyl window. That's why GBA guest blogger Roger Normand has been struggling with his decision on whether or not to spend $70,000 for Unilux windows (at an average cost of $134 a square foot) for his house in Maine.

We can all play with the numbers to prove any point we want. If you assume that the price of a fixed vinyl window is representative of the average window cost of a Passivhaus home, you can prove your point. But that's a bottom-rung window for a Passivhaus. If I choose the Unilux price, it's easy for me to prove that the investment is uneconomic in terms of energy payback.

Here's the bottom line: very expensive windows are hard to justify from an energy-savings perspective. Moreover, large expanses of south-facing glass are likely to cause problems with glare and overheating, and aren't worth the extra cost compared to a south-facing wall with fewer windows.

Finally -- I'm glad for the growing consensus on the need for efficient homes to include a gray water heat recovery device and to have a compact shape. All we need to do is get more architects on board...


11.
Sat, 09/15/2012 - 07:03

Intus aluminum also affordable?
by Randy George

Helpful? 0

We're still waiting for a final quote, but the word we've gotten from our contractor is that Intus aluminum windows are still less than Marvin triple-glazed. We should find out soon. All other things being equal, this would make Intus a sensible choice. The problem is that all other things are not equal. 12 week lead times and delays in dealing with warranty issues or mistakes on the order have me worried! Does anyone have any experience with Intus that would allay my fears?


12.
Sat, 09/15/2012 - 08:38

As a builder I know of only
by aj builder, Upstate NY Zone 6a

Helpful? -1

As a builder I know of only one way to assure non-stock materials get to a site on time and correct. Purchase far ahead of time, take delivery, verify order and then warehouse till needed.

When I as a builder am allowed to control my order timing there are less problems.


13.
Sat, 09/15/2012 - 08:42

Intus, Paradigm
by Dan Kolbert

Helpful? 0

I know Chris Corson here in Maine has used Intus on several projects with no significant complaints. And our PGH host, Steve Konstantino of Maine Green Building Supply, has been repping them - you could contact him and ask how that has gone.

Martin, I haven't looked at Paradigm triples in a while but the last time I did I was underwhelmed by the frames. I should check them out again - while not a big fan of vinyl windows they have been my go to window for vinyl replacement double-glazed for reasons of locale, quality and price for quite a while.


14.
Sat, 09/15/2012 - 09:08

Edited Sat, 09/15/2012 - 09:10.

"It's 1982 all over again...
by Greg Smith

Helpful? 0

"It's 1982 all over again... and the latest generation of designers is learning the same lessons that those of us with gray hair learned in the '80s."

"One way to summarize Proskiw’s findings: builders of superinsulated homes in cold climates should choose affordable (usually triple-glazed) windows rather than exotic windows with extremely low U-factors.

“From an energy perspective and based on the incremental costs and energy savings, window selection should be based solely on the need to control condensation,” Proskiw wrote. In most cases, that means that you should choose glazing with a warm-edge spacer."

I wonder.......

Clear triple pane (center-glass) - U.31, SHGC .70, interior glass temp 52.6F at Delta T of 70F

Since triple pane windows may require upgraded hardware, and the additional glass pane can contribute to potential seal failure why can't glass and window designers could come up with a method of fabricating windows that result in triple pane performance in a double pane window?

Wait...maybe there is an option (new invention apparently)!

Option #1 HSHG LowE surface 3 - U.26, SHGC .69, interior glass temp 55.1F at Delta T of 70F

Option #2 HSGC LowE surface 3 - U .28, SHGC .70, interior glass temp 54.3F at Deltas T of 70F.

Both options 1/2" 90% argon filled airspace.

I wonder if the dual pane with clear triple pane performance will ever catch on?

Regards,

Oberon


15.
Sat, 09/15/2012 - 10:00

Response to Oberon / Greg Smith
by Martin Holladay, GBA Advisor

Helpful? 0

Oberon,
Gary Proskiw's choice of clear triple glazing to illustrate an inexpensive triple-glazed window was rather idiosyncratic -- it's actually a fairly rare type of triple glazing, and I don't think it's actually any cheaper than triple glazing with one or two low-e coatings or with argon.

You may well be right that double-glazed windows with low-e and argon are perfectly adequate for cold-climate homes -- especially if there are warm-edge spacers that address condensation. All designers have to do is the math -- to decide whether the cost of the upgrade from double to triple glazing yields enough energy savings in their climate to make sense.


16.
Sat, 09/15/2012 - 10:13

Marvin tri-pane?
by Jesse Lizer

Helpful? 0

Randy
you keep mentioning Marvin tri-pane. Are you seriously considering the Marvin version or the new Integrity tri-pane? While the Integrity gives you a single option since it is new to the market, the Marvin version is very expensive and hardly any more efficient then their cheaper little brother the Integrity in a dual pane fiberglass frame. IMO, your money if far better spent on tri pane options other then Marvin premiums (if that is infact what you are pricing). Especially when you compare Marvin to Intus...you are comparing a high quality, architecturally driven wood window with a vinyl window with impressive ratings at a decent price. I too went down the road in estimating until deciding I just do not like the vinyl look in any way, and I also do not like white windows.
In my findings in zone 6, a high solar gaining dual pane window still seems to be the ticket on the south. Switching to tripane has payoffs far exceeding the life of the window. I have also given no value to the "comfort" of tripane and I have never found the draft some speak of off of my current dual pane double hungs (Pella).
Many or most on this forum will probably disagree with my dual pane assessment as tripane is becoming the new standard for the entire house. Cost effective measure for my personal glass package selections have been properly sized and shaded dual pane on the south, limited to no windows on the east and west, tripane, and a low amount of tripane on the north (views).


17.
Sat, 09/15/2012 - 10:23

Marvin tri-pane?
by Jesse Lizer

Helpful? 0

Randy
you keep mentioning Marvin tri-pane. Are you seriously considering the Marvin version or the new Integrity tri-pane? While the Integrity gives you a single option since it is new to the market, the Marvin version is very expensive and hardly any more efficient then their cheaper little brother the Integrity in a dual pane fiberglass frame. IMO, your money if far better spent on tri pane options other then Marvin premiums (if that is infact what you are pricing). Especially when you compare Marvin to Intus...you are comparing a high quality, architecturally driven wood window with a vinyl window with impressive ratings at a decent price. I too went down the road in estimating until deciding I just do not like the vinyl look in any way, and I also do not like white windows.
In my findings in zone 6, a high solar gaining dual pane window still seems to be the ticket on the south. Switching to tripane has payoffs far exceeding the life of the window. I have also given no value to the "comfort" of tripane and I have never found the draft some speak of off of my current dual pane double hungs (Pella).
Many or most on this forum will probably disagree with my dual pane assessment as tripane is becoming the new standard for the entire house. Cost effective measure for my personal glass package selections have been properly sized and shaded dual pane on the south, limited to no windows on the east and west, tripane, and a low amount of tripane on the north (views).


18.
Sat, 09/15/2012 - 10:53

Edited Sat, 09/15/2012 - 16:13.

Warm-edge spacers
by Garth Sproule 7B

Helpful? 0

Are today's warm edge spacers durable enough for cold or very cold climate Zones? I see that Cardinal Glass still uses stainless steel spacers on most or even all of it's IGUs. I assume that the reason has something to do with the robustness of common warm-edge spacers. Nothing worse than losing the seal after only a few years of service.


19.
Sat, 09/15/2012 - 15:05

Edited Sat, 09/15/2012 - 15:08.

Martin,My reply was intended
by Greg Smith

Helpful? 0

Martin,

My reply was intended to inject a little ironic humor into the thread by channeling your comment about being back in 1982 with the suggestion of a clear triple pane....I need to work on my presentation.

Anyway, as I have previously commented that (IMO) NA glass packages and windows not being equal to Euro energy performance standards is due primarily to demand, along that sameline, I might also suggest that there is definitely a fairly prevalent opinion among at least some US glass and window fabricators, that doesn't disagree with Mr. Proskiw's findings - with the exception that offering a clear glass triple pane would ever be a good idea.

Some people might even question where the need for superior energy performance, based on cost versus benefit analysis, ends, and bragging rights begins.


20.
Sat, 09/15/2012 - 15:17

Edited Sat, 09/15/2012 - 15:20.

Garth,Cardinal publishes the
by Greg Smith

Helpful? 0

Garth,

Cardinal publishes the durability statistics of their IG units. Since 1993 with the introduction of the stainless spacer system, they claim a 20 year field failure rate of less than .2% percent - based on something like 400,000,000 units in the field.

Although the energy performance of the stainless spacer does trail the best non-metallic systems by a couple of degrees at the edge, Cardinal has stated that they consider long term durabilty as the primary consideration when fabricating their IG units.


21.
Sat, 09/15/2012 - 16:37

Does this make sense?
by Garth Sproule 7B

Helpful? 0

In Proskiw's report, he states the following

"By adding window area, or upgrading window performance, the space heating load is reduced
but it is already so small that there is little opportunity for further savings. Had these two upgrades
been applied to a conventional house, with a much larger space heating load, the energy savings would have been significantly larger and the economics much more favourable."

Assuming the same delta t, wouldn't the energy savings using identical windows be the same no matter what type of wall or building they are installed in? What am I missing here?


22.
Sat, 09/15/2012 - 20:25

Just an observation
by Darryl in Winnipeg

Helpful? 0

The discussion is focussed on windows of course, but another somewhat interesting observation from the linked report is that none of Proskiw's NZE home scenario's included subslab insulation. I've always had the impression that was a pretty critical part of a containing the thermal envelope.


23.
Sun, 09/16/2012 - 13:19

Edited Sun, 09/16/2012 - 13:23.

Passivhaus and affordability
by Adam Cohen

Helpful? 0

Martin,
I have watched with interest, your coverage of Passivhaus and finally need to chime in.

First, for those of us who have been building low energy Passive design for 30 years, the over glazing issue is well known. I build in Virginia, where if you do not have adequate shading in the shoulder seasons, you will cook! So the contention that Passivhaus designers are relearning this lesson, needs to be appended to say "some have to relearn this". This is a very simple thing to fix and those of us in the movement with more experience have been working hard to mentor the folks who have not been through the crucible of the Carter days. This will not be an issue long.

Second regarding affordability, there is a much more nuanced discussions to be made here. First, please understand i come from a region (SW Virginia) that has inexpensive energy, where the average person has very little interest in Climate Change, and a good percentage of folks are actually card carrying anti-science Foxers, who challenge even the concept of climate change. And lastly, if you discuss interior comfort and interior air quality, you just get blank stares.

So I have had to develop my Passivhaus business with one selling point - affordability, in fact the hurdle that I have to jump is Cost Equity. This means the monthly cost of ENERGY + MORTGAGE is equal for a Passivhaus! As you can imagine, this has been a tough nut to crack with our mild temps and low energy bills. As an example for an 1800 sq ft house this allows for a cost premium to maintain cost equity of only about 20K. So one then has to look at the entire building as a system to be able to make the balance work. Passivhaus ihas a very good tool for doing this in the PHPP.

Windows are critical in this whole equation. We have been using Passivhaus Certified Klearwall Windows (Irish made and cost competitive with the Intus product). But as you can imagine this is only a small piece of the puzzle. Everything has to be looked at holistically to maintain the delicate cost equity balance.

It took 2 1/2 years of trial and error to come up with a system that works, but it does! We are now delivering affordable (defined as under 300K) homes for cost parity with std. construction. (in the $135/SF cost range for a basic home).

The nice thing is that we are seeing the results. Homes are performing within predicted temperature and energy parameters (total energy less than 15kwh / day for a 2000 sq ft home) and we are not alone in our experience. Colleagues from all over the country are reporting similar results in terms of cost and performance.

So the way I look at it is that the early Passivhaus pioneers in the US are now "tunneling through the cost barriers" as well as fine tuning the PHPP for the US climate. With the market expanding, soon there will be US made products (including windows) that will meet the PH requirements and cost will come down as availability goes up. So I see a bright future for this methodology for producing low energy buildings cost effectively.

Lastly I want to say that by no means is Passivhaus the only way to achieve the result of affordable low energy structures. There are many means to this end! All the discussion and sharing of ideas and best practices is fantastic in moving the field forward. As a veteran who pinned many hopes on the advances in the 70's and 80's and was let down by the lack of follow through, by deepest hope is that this time it sticks. As colleagues who are striving to move this country's building industry forward I hope that we we can all work together, share results and new ideas and not argue too much about who's methods are best, after all (I think) we all want results and realizing how small a group is working on this we need to stick together and all grow from each other's experiences.

Well that's my 2 cents, take it with another $2.50 and have a cup of Joe!~


24.
Sun, 09/16/2012 - 16:16

Response to Greg Smith
by Martin Holladay, GBA Advisor

Helpful? 0

Greg,
I agree with you comments. Especially on the south side of a house, clear triple glazing is worth considering -- especially if it is cheaper than any other type of triple glazing (as logically it should be).

And I agree with you concerning designers who are all puffed up about their U-0.11 windows installed as in-betweenies -- because they are so much better than their neighbor's U-0.15 windows installed as outies. This is the realm of bragging rights, not logic.


25.
Sun, 09/16/2012 - 16:32

Edited Tue, 09/18/2012 - 19:49.

Response to Adam Cohen
by Martin Holladay, GBA Advisor

Helpful? 1

Adam,
I'm glad to hear that you have been designing suprinsulated passive solar buildings for 30 years. And I agree, many designers like you all over the country have been refining their designs and coming up with simpler solutions in an example of convergent evolution. I agree with you that not everyone is making the mistake of overglazing their south facades.

And if you are able to deliver Passivhaus buildings for the same cost as conventional construction, then I congratulate you. (By the way, I invite you to submit a guest blog to GBA, providing more details on how you achieved that laudable goal.)

I am nevertheless unconvinced that the Passivhaus route is worth traveling. I think that the net-zero approach makes more sense: find out how many kWh per year you can get from an investment of $1,000 in PV (currently, the answer is about 320 kWh in the Northeast), and use that as a benchmark to gauge the logic of all or your energy improvement measures. If you're thinking of investing an extra $10,000 in windows with fancy glazing, the investment ought to save you at least 3,200 kWh per year in the Northeast, or it just isn't worth it.

I've never heard a Passivhaus proponent come up with a logical reason why designers shouldn't follow the net-zero design path. I still see plenty of Passivhaus designers led astray as they seek the holy grail of 15 kWh per square meter per year. Consider this quote from a guest blog by Roger Normand, who has hired two experts I respect highly, Marc Rosenbaum and Chris Briley, to shepherd him through the Passivhaus design process. Normand wrote, "the Ultratherm 3 windows as currently specified fail to achieve the Passive House standard without additional substantial increases in insulation. The results of Marc Rosenbaum, our energy consultant, were shocking, given the UltraTherm 3 product literature. The reason is that the sash and frame members of the Ultratherm windows are thicker and have a lower SHGC than the Schüco. The Passive House heat load ceiling is 4.75 KBTU/sf/yr. The Schüco windows comfortably met the goal at 4.40 KBTU.sf/yr. Without any other building envelope changes, the UltraTherm 3 came in at 6.35 KBTU/sf/yr. "

So these triple-glazed Unilux windows from Germany still weren't good enough, and Normand's team went back to the drawing board, and kept looking for better windows. This doesn't make economic sense.


26.
Sun, 09/16/2012 - 17:00

Comment # 19
by Garth Sproule 7B

Helpful? 0

Martin
The way I read Greg's comment # 19, is that he thinks that a clear triple is not a good idea. He has shown in other posts (comment #14) that it is rather easy to meet or exceed the performance of clear triples with double pane low e options.

I would also appreciate if you could take a stab at my question (comment #21)

Thanks
Garth


27.
Sun, 09/16/2012 - 18:56

Response to Garth Sproule
by Martin Holladay, GBA Advisor

Helpful? 0

Garth,
I agree with you (and disagree with Gary Proskiw): assuming that two adjacent houses have the same indoor and outdoor temperatures, and the same solar exposure, a south-facing window of size X will gather exactly the same number of kWh of heat annually in both houses -- even if one house is a net-zero house and the other house is a house that barely meets code.

I'll leave it to Greg Smith to tell us when, if ever, he would advise anyone to install a window with clear triple glazing.


28.
Sun, 09/16/2012 - 21:54

Integrity vs. Marvin
by Randy George

Helpful? 0

Jesse,
We have been considering Marvin's main line, mostly because you can get different coatings depending on the orientation of the windows. Integrity's tri-pane are only available with 272/180 coatings. But if I knew how to calculate the payback on that difference maybe I would find that the Marvins don't make sense! I appreciate the comment. We're going to take a closer look at Integrity.


29.
Sun, 09/16/2012 - 22:25

Edited Sun, 09/16/2012 - 22:55.

Passive House Symposium in Boston - 27 October
by Kristen Simmons

Helpful? 0

Adam Cohen, Chris Corson and others will share their experience at Passive House New England's Fall Symposium on Saturday 27 October. This year's theme is "Getting It Built", with a focus on real world solutions to the technical challenges of designing and constructing low load buildings, as well as how to find (or create) clients for whom to build them. There will be plenty of time in between sessions for discussion and debate. Martin, it would be great if you would come down for the day.

The full schedule and registration information will be posted in the next couple of days at www.passivehousenewengland.org. If you sign up for the newsletter, you'll get an announcement when registration is open. PHNE is volunteer-led and run organization and a 501(c)3.

Kristen Simmons, AIA
President, Passive House New England

Taking off that that hat and speaking as a practitioner who works on institutional, multifamily, and small residential low load buildings... window size, as well as quantity, has a great impact on total glazing cost, whatever window is being specified. I'm currently working on a deep energy retrofit to the passive house standard, and I'm maintaining most of the existing openings, which are typical size for a 100 year old house. With the same total glazing area, but only 50% of the openings, the cost savings would have been great. I trust that this is partly how Matt and Alan of GO Logic were able to build that red house in the picture at such a low cost. Of course, a good wall will always be cheaper and perform better than any window, but try convincing a client that windows are bad.

As for the "invest in PV rather than the envelope" argument, I can buy it for single family residential, maybe. But if one starts to think about net zero communities, net zero towns, net zero cities, shouldn't the goal be to focus on maximizing energy efficiency in new construction, where it is possible? To provide (possibly) surplus local energy production that can help to offset demands at a historic building that never will be able to approach net zero? Or a factory, or a 100 unit multifamily building? The question, to my mind, should really be where does the line get drawn for net zero? Net zero single family houses are not going to get us very far.

FYI, Matt and Alan will be speaking at the ABX Conference (November) in Boston as part of the NESEA High Performance Homes Track. They gave a great presentation earlier this year at Building Energy 12, and I think that this next one will be as good if not better.


30.
Sun, 09/16/2012 - 23:32

"I'll leave it to Greg Smith
by Greg Smith

Helpful? 0

"I'll leave it to Greg Smith to tell us when, if ever, he would advise anyone to install a window with clear triple glazing."

Martin,

My apologies for not being clear on my post. I do tend to "think-outloud" too much when I am writing and it can make my posts a bit convoluted at times. There's a really good reason that I'm not a journalist - I would probably starve.

Trust me that I have the utmost respect for people who can write clear and concise sentences.

Honestly I can't think of any time that I would recommend a clear triple simply because a dual pane with either high or low solar gain coating (depending on application) will outperform the clear triple everytime.


31.
Mon, 09/17/2012 - 05:28

Response to Kristen Simmons
by Martin Holladay, GBA Advisor

Helpful? 0

Kristen,
Reducing the number of window openings while making each window larger does, indeed, reduce the overall cost of the windows while improving the energy performance of the building. The main risk with this approach is reducing the livability of the building because of glare or overheating -- or because the approach results in the elimination of a small charming window that improves the daylighting or the "feel" of the interior space.

As I've written before, using the cost of PV as a benchmark to help a designer refine the design of a very efficient building doesn't necessarily require the designer to install PV on the building's roof. It's just an approach that helps the designer to gauge the relative cost-effectiveness of different options or energy-performance measures.


32.
Mon, 09/17/2012 - 10:33

Response to Garth Sproule and Martin Holladay
by Philip Drader

Helpful? 0

"Assuming the same delta t, wouldn't the energy savings using identical windows be the same no matter what type of wall or building they are installed in? What am I missing here?"
I have an answer for comments #21, #26, and #27.

The answer is: "useable solar gains", "gain-load ratio", or "solar utilization factor".

Essentially, in a poorly insulated home the heat losses are large, and the combination of internal heat gains and solar gains make only a small dent in meeting those losses (the remainder is made up by the furnace). In that same month, the well-insulated, tight home has much smaller losses, such that the internal heat gains and solar gains together not only meet those losses, but exceed them. In that case, the home heats up to the point where the occupants might open a window just to cool things down - hence losing all that solar energy that made it's way in through the window glazing.

In that second case, the one where the windows needed to be opened to cool the home down, that additional 1m^2 of glazing was extraneous, and it's impact will be very minimal on the annual energy balance. That same 1m^2 window in a very leaky, poorly insulated home will likely make full use of the solar energy provided.

It is worth noting that increasing allowable temperature swing and increasing thermal mass both increase the solar utilization factor.

I agree with Gary Proskiw. If you would like the specific calculations for determining the solar utilization factor, I could add them in the comments, though it is quite likely that most people looking at them will decide that it seems like a lot of work to go through to figure out windows (and they'd be right). If anyone is interested (and has read this far), just ask.


33.
Mon, 09/17/2012 - 10:55

Edited Tue, 09/18/2012 - 10:24.

Response to Philip Drader
by Martin Holladay, GBA Advisor

Helpful? 0

Philip,
I agree with your analysis (although I must add that your analysis does not contradict my analysis). Whether or not your point is the same one that Gary Proskiw was trying to make is a little unclear.

I think we can all agree that the heat gains through the south-facing window measuring 1 square meter will be the same regardless of the amount of insulation in the walls and ceiling.

Your point is that if a house has too much south glazing, the occupants will have to open up their windows and let out the heat on sunny days to avoid discomfort. You are absolutely right, and that's why Gary advised limiting the area of south-facing glazing to no more than 6% of the floor area of the house.

If Gary was trying to make the same point you did -- namely, beware of overglazing -- when he wrote the confusing paragraph in question, then I think he could have phrased it better. (This is what he wrote: “The reason the two window upgrades fared so poorly, from an economic perspective, is that the space heating load in a NZE house is very small compared to any other type of house. By adding window area or upgrading window performance, the space heating load is reduced but it is already so small that there is little opportunity for further savings.”)


34.
Mon, 09/17/2012 - 11:56

turns out I was unclear - not an overglazing point
by Philip Drader

Helpful? 0

Hi Martin,

To use the original example, Proskiw modelled a savings of 19kWh per year with the window in the NZE home. Even accounting for the worst SHGC that he could have used (base on the description of his window, HOT2000 gives 0.3921), that window in Winnipeg would have let in 523.56kWh/yr (ignoring the off-angle incidence factor of around 0.93), and lost at most 106.46kWh/yr in heat (based on an Rvalue of 3.71, which is again the worst that HOT2000 will give for that description of window). The difference between the two, 417kWh/yr, is the theoretical benefit of this window in that climate.

If nobody lived in the building (generating internal heat gains), and if this was the only window (hence no other solar gains are reducing the required heating load), then the window would likely be reducing the furnace usage by close to 417kWh. Savings would be clos to 417kWh if the house was big and large and poorly insulated as even the gains in july and aug could be used as the outside temp is below 21C, and smaller savings would be expected (possibly MUCH smaller) if that same window was installed in a very airtight, super-insulated shed that had a 60W lightbulb that is always turned on.

In the second scenario of the shed, you can probably instinctively see where the presence of that window might not do much to reduce the furnace fuel consumption. In the larger home, with larger losses, is it also easy to instinctively see where the window could be quite useful at providing additional heat and hence reducing the furnace fuel consumption.

It is a lot trickier to understand the effect when the homes are larger, but the science behind it is still solid. So for the big home with big losses, the potential benefit of the window is 417kWh/yr, but in the NZE home that already has a number of windows, the benefit of that exact same window in the exact same orientation and shading is only 19kWh/yr.

I hope this was informative? I can try with different examples if this wasn't as clear as I tried to make it.


35.
Mon, 09/17/2012 - 13:16

PH vs. NZ
by ben graham

Helpful? 0

I think the premise of the article is clear and Martin makes good, logical arguments, but I don't understand why PH and NZ proponents argue over who's best when they different goals. PH being reducing heat load to really low levels and NZ (based on what I read here) to get to NZ in most economical way, typically using PV to offset energy use. I know PH wants to be affordable, but I haven't heard any PH practitioner saying it the cheapest way to go.
Why would you say forget PH because NZ can get you there cheaper when NZ isn't going to the same place? Or is this just an exercise in chest pounding?


36.
Mon, 09/17/2012 - 13:41

Response to Ben Graham
by Martin Holladay, GBA Advisor

Helpful? 0

Ben,
Fair enough. As long as a Passivhaus designer says, "We really want to reduce the heating energy load to 15 kWh per square meter per year, regardless of the cost," then that's fine -- and it's up to the client to decide whether they want to pay what it takes to get there.

However (as I have documented elsewhere), LOTS of Passivhaus designers are claiming that their approach is "the most cost-effective way" to build a low-energy house. It's not.

And I agree with you that designers of net-zero-energy homes have a different goal from Passivhaus designers. Now I'll shut up, because I don't want to engage in chest-pounding; that wasn't my intent.

My intent was to highly Gary Proskiw's paper, which I found thought-provoking and useful.


37.
Tue, 09/18/2012 - 01:17

PV costs in the US and NZEB
by David Coote

Helpful? 0

We're working towards getting our 80 year old California Bungalow style home in Melbourne, Australia to Near Zero Energy status. So far we've increased the ceiling insulation to about R4 (Australian), put R1.5 insulation under the polished baltic pine floorboards (look nice but with gaps you could literally feel a draught through), added a Solar Hot Water system, 1.5kW (peak) PV's and installed an efficient wood heater we fuel mostly with local arborist waste with some wood from my sister's farm etc.

I was surprised by Martin's comments about the return from PV's/$1K in the US NE which I've quoted below. PV's in Melbourne for reasonable quality panels are down to about $2/watt installed. A kW of reasonably efficient panels would give you around 1200kWh/annum in practice. The Aussie and US dollars are close enough to parity at the moment so this works out at 600kWh/$1K. Insolation in the US NE would be less than Melbourne but I think the panels must also be quite a bit more expensive.

"I am nevertheless unconvinced that the Passivhaus route is worth traveling. I think that the net-zero approach makes more sense: find out how many kWh per year you can get from an investment of $1,000 in PV (currently, the answer is about 320 kWh in the Northeast), and use that as a benchmark to gauge the logic of all or your energy improvement measures. If you're thinking of investing an extra $10,000 in windows with fancy glazing, the investment ought to save you at least 3,200 kWh per year in the Northeast, or it just isn't worth it."


38.
Tue, 09/18/2012 - 04:23

Edited Tue, 09/18/2012 - 04:30.

Response to David Coote
by Martin Holladay, GBA Advisor

Helpful? 0

David,
I was using $3.50/watt for the installed cost, and the insolation levels of Syracuse, NY (which isn't as sunny as Australia).

You're right, of course: if the installed cost of PV is lower and the climate is sunnier than in my example, then the return on an investment in PV is even better.


39.
Tue, 09/18/2012 - 09:57

Response to Philip Drader
by Garth Sproule 7B

Helpful? 0

Thank you for providing an excellent explanation of how Proskiw's model could have resulted in an energy gain of only 19kWh/yr when adding 1 sq m of south facing glazing. Well done. The addition of a significant amount of mass, would likely increase the amount of modeled energy gained by quite a lot. I agree with Martin, that it is not clear from the report that Proskiw himself fully understands this. He does go on to describe the limitations of HOT2000 in modeling passive solar. Maybe this is where improvements need to be made first?


40.
Tue, 09/18/2012 - 16:13

Edited Tue, 09/18/2012 - 16:19.

Adam Cohen, 7th Annual North American Passive House Conference
by Michael Knezovich

Helpful? 0

This is a great discussion--if you're interested in continuing it in person, as Kristen Simmons mentioned earlier in the thread, Adam will be in new England this fall.

If you're out West, Adam--and Kristen--will also be at the PHIUS/PHAUS 7th Annual North American Passive House Conference in Denver (www.passivehouse.us/phc2012). You can sign up for Adam's Pre-conference workshop on September 27, and attend his and Kristen's sessions during the main conference, runs Sept 28-29.

And--speaking of windows--we'll be introducing the new PHIUS Certified Product Performance Data program. Hope to see you in Denver--by the current count of exhibitors and registered attendees, it'll be the biggest passive house event yet, but there are still slots.

Mike Knezovich
PHIUS


41.
Wed, 09/19/2012 - 11:52

Edited Wed, 01/02/2013 - 14:39.

Response to Mike Knezovick
by Martin Holladay, GBA Advisor

Helpful? 0

Mike,
Thanks for the invitation to Denver. Thanks also to Kristen Simmons, who invited me to the October 27 Passivhaus Symposium in Boston.

I don't think I can make it to Denver, but I'll do my best to attend the event in Boston.

[Postscript: To read my report on the October 27 symposium in Boston, see Passivhaus Practitioners Share Their Success Stories.]


42.
Wed, 09/19/2012 - 20:02

Triple glazing vs. solar gain
by Mark Heizer

Helpful? 0

If the windows are fixed (or other than casement), need to consider visible light transmission and solar gain. VLT drops dramatically for most advanced triple glazed units. A double glazing low-e unit and a low-e storm could have much better VLT and solar gain that overcomes minor shortcomings in thermal performance for substantially less cost. And it wouldn't look any worse than the 4 megawindows in the photo.


43.
Thu, 09/20/2012 - 00:39

Edited Thu, 09/20/2012 - 00:55.

An architectural cliché from the 1970s?
by David Butler

Helpful? 0

Martin, your lead-in paragraph caught me by surprise. Passive solar design isn't a relic of the 70''s. It was a niche then and (unfortunately) is still a niche.

A fair percentage of homes I work on (including my previous home) are passive solar. I do a lot of work for Debbie Rucker Coleman's clients. (For those who don't know Debbie, she's an architect, long-time passive solar advocate.) Even on homes that aren't specifically designed as passive solar, I always look for opportunities to move the design in the ps direction.

Architects (especially those not familiar with passive solar design principals) seem to have a love affair with glass. Homes with lots of glass appeal to many home buyers, but for reasons other than solar gain. I sometimes find myself pushing back against high glazing ratios, even on the south facade. Too much of a good thing. And as you mentioned, gotta watch those shoulder months. So I don't see passive solar as adding cost to a project but rather a design strategy that starts with lot selection.

On the other hand, Gary seems to be presenting passive solar as an afterthought when he asks if adding a window would be cost effective. That's the wrong question. If you start with a good design, it's the over-the-top envelope specs that become difficult to justify. Ultimately, the appropriate mix usually comes down to climate, energy costs, PV incentives, etc.

I do agree with Gary that it's not worth paying a huge premium (e.g., sourcing from Europe) to get triple-pane high gain glass. Instead, I specify dual-pane high-gain windows for south facade on homes spec'ed for triple-pane. Cardinal's LoE-180 combined with LoE-i81 has a center-glass u--factor of 0.22 with SHGC of 0.59 (see http://bit.ly/RzxxJX, scroll to bottom; whole unit values will be slightly less impressive). These new glazing products are a game-changer for passive solar designers. It may require a few phone calls to find a supplier willing to accommodate a split order, but it's usually not much of a cost hit.


44.
Thu, 09/20/2012 - 00:49

on simplicty and pragmatism
by David Butler

Helpful? 0

BTW, I couldn't agree more with the last 3 paragraphs of your article, as well as your reply to Adam regarding using per-kWh cost of PV as a baseline for efficiency improvements (comment #25).


45.
Thu, 09/20/2012 - 08:37

Edited Thu, 09/20/2012 - 08:45.

Response to David Butler
by Martin Holladay, GBA Advisor

Helpful? 0

David,
Thanks for your comments; I agree with most of what your wrote.

One clarification: I never wrote that passive solar homes were an architectural cliché of the 1970s -- much less that they were a "relic of the 70s," as you put it.

I wrote that "the passive solar home with large expanses of south-facing glass" was an architectural cliché of the 1970s.

Needless to say, a carefully designed passive solar home should not have too much south-facing glass. I think that the two Passivhaus buildings that are shown in the photos used for this article -- both built within the past 8 years -- probably have too much south-facing glass.

Many of us who were building passive-solar homes in the 1970s made this mistake. I made the mistake myself. In March and September, these homes get much too hot.

After a wave of such failed homes, the smart designers started to realize that they didn't need as much south-facing glass. Passive solar homes continued to be built in the 1980s, 1990s, and 2000s, of course. The best such homes paid close attention to the area of the south-facing glazing.

One more point: you wrote that "Gary seems to be presenting passive solar as an afterthought when he asks if adding a window would be cost-effective." Believe it or not, Passivhaus designers regularly use the "let's add another south-facing window" technique -- or, in some cases, the "let's make our south-facing windows just a little bit bigger" technique -- as they desperately try to hit the Passivhaus goal of 15 kWh per square meter per year. That's how they end up with the designs shown in these photos.

Here's how GBA blogger Roger Normand described the process in his EdgewaterHaus blog: "So how did we get to the 4.74 KBTU/sq. ft./year? ... To improve the overall building thermal performance, our architect, Chris Briley, added more south-facing glass and reduced the roof overhang above these windows to eliminate any winter shading. Chris assures us that the vegetative trellis he has designed along the south facade will preclude overheating in the summer. ... Marc Rosenbaum then added 10 inches of cellulose insulation, raising the ceiling R-value from R-76 to — gasp — R-118."


46.
Thu, 09/20/2012 - 16:03

Response to Martin
by David Butler

Helpful? 0

you wrote:
> I think that the two Passivhaus buildings that are shown in the photos used for this article -- both built within the past 8 years -- probably have too much south-facing glass.

It appears that the designer was trying to make up for a low SHGC, which is a real problem with triple-pane. If so, this obviously makes no sense because the additional conducted loads would be larger than had they used more appropriately sized double-pane windows with high-gain glass with low u-factor. Not to mention the huge savings in first cost. This sort of short-sightedness is typical when the solution space is artificially constrained. If PHPP is handling these impacts correctly, then we're left to blame the designer.

As for my critique of your lead-in, I saw the "cliché" comment as showing disrespect for properly done passive solar design. At least your subsequent comments demonstrate that we agree on this.

I realize the article was about the fallacy of adding glass to achieve PH thresholds, but the following quote from Gary's article begs to be refuted...

"On a broader level, these results indicate that our long‐held belief in the merits and value of passive solar energy as a key component of Net Zero Energy House design need to be carefully re‐examined and likely challenged."

My position is that passive solar design should always be a key component of NZE design. What needs to be examined and challenged are designers who don't know what they're doing.


47.
Thu, 09/20/2012 - 16:18

Edited Thu, 09/20/2012 - 16:20.

Response to David Butler
by Martin Holladay, GBA Advisor

Helpful? 0

David,
I'm not sure why you think that Katrin Klingenberg and GO Logic chose low-SHGCi glazingi for their south-facing glazing. The Thermotech windows that Katrin chose for the south side have 0.51 SHGC glazing.

GO Logic specified German windows with triple-pane, argoni-filled glazing. The south-facing glazing has a SHGC of 0.60, according to a video on the GO Logic web site:
http://www.gologichomes.com/featured-projects/passive-house.html


48.
Thu, 09/20/2012 - 16:25

One more thing, David
by Martin Holladay, GBA Advisor

Helpful? 0

David,
Concerning Gary Proskiw's point -- "these results indicate that our long‐held belief in the merits and value of passive solar energy as a key component of Net Zero Energy House design need to be carefully re‐examined and likely challenged" -- here are the questions that designers need to ask themselves:

1. Does the south facade of the design under consideration have more glazing area than needed for normal daylighting?

2. If so, are the thermal benefits of the hoped-for solar gain high enough to justify the cost of the extra glazing area?


49.
Thu, 09/20/2012 - 18:11

Response to Martin
by David Butler

Helpful? 0

I was speculating as to why those windows are so large. Given the specs you posted, yeah, I definitely agree those windows would likely backfire, especially given the low heat loads of these homes. Point taken.

On the other point, the two questions you pose for designers perpetuate the idea of passive solar as a stop-gap measure, rather than a "from the ground up" design approach.


50.
Thu, 09/20/2012 - 21:02

If the issue is overheating, why have high SHGC?
by Bill Burke

Helpful? 0

I'm a bit confused. If you build a well-insulated, well air-sealed building envelope your need for solar gain is greatly reduced. We all seem to be in agreement on that. Homes with too much glass tend to overheat, especially during shoulder seasons when the glass is more difficult to shade. Obviously there's a need to calculate this out, but wouldn't it make sense to drop the SHGC of the windows to reduce gain during the shoulder months? I'm not advocating for incredibly low SHGC, but with a good enclosure and moderate amounts of glass with a SHGC of about 0.40, shouldn't you get the modest solar gain needed while reducing the problem of shoulder season overheating?
I should point out that I'm located in California where heating needs are relatively low in most locations. Am I missing something?
Bill

[Editor's note: A response to this comment, and several other posted comments, can be found on Page 2.]


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