Windows

To PHPP and beyond

Submitted by Andrea on July 17, 2011

Our building site was relatively quiet last week. Concrete is curing, and our electrician set up the main panel and meter in anticipation of CVPS turning on the electricity this week. Ted and I also spoke with several solar installers to see about getting some PV panels at the roof ridge and also a solar hot water system. More on that as it unfolds.

The biggest news is that we recently partnered with Efficiency Vermont to pursue Passivhaus certification [follow the link to read their "About Us" page]. The cool part is that our house will be part of a research project to evaluate the suitability of Passivhaus construction for Vermont. They'll install monitoring equipment in our house and closely study its performance.

Peter Schneider, Efficiency Vermont's Passivhaus consultant, was particularly interested in studying our house because it has several unusual features: a pier foundation and partial shading. Vermont's abundance of sloping, ledgy lots makes pier foundation a tempting solution, and of course trees are rampant hereabouts. So hopefully we'll provide useful data for would-be Passivhausers in North America.

Peter was on vacation last week, so he hasn't gotten farther than the first few rounds of PHPP tweaking, but Marc helped pick up the slack. This will all probably change this week, and I'm probably jinxing things just by typing this, but so far it looks like we can pull off Passivhaus performance with the following general specs:

  • 11-7/8″ I-joist floor deck (16 oc), stuffed with dense-pack cellulose and with 4″ of polyiso underneath.
  • 9.5″ I-joist wall framing (24 oc) filled with dense-pack cellulose and with 4″ of exterior polyiso.
  • 24" roof joists, filled with dense-pack cellulose.
  • Schuco SI-82+ windows, which we ordered this week from European Architectural Supply in Lincoln, MA. The windows are PH-certified and made from uPVC. Yes yes, PVC is evil, but this is unplasticized PVC which is apparently a bit less evil. It's made without phthalates and can be recycled, at least in Europe. But hopefully the windows won't need recycling for a long long time.
  • Climatop Max and Climatop Ultra-N glass. The glass offered by Schuco is pretty darn impressive. For the south windows we upgraded to Climatop Max, which has a SHGC of 0.6, but for the rest of the house we went with the Climatop Ultra-N, which has an SHGC of 0.5. All the glass has a Ug of 0.105 (which PHPP callously rounds up to 0.11).
  • We haven't decided for sure on the HRV yet, but we'll probably either do the Zehnder ComfoAir 350 or the Paul by Zehnder Novus 300. The latter adds about $1,400 to the already formidable cost, but the efficiency is 93% as opposed to the ComfoAir's 84%, which would win us quite a bit within PHPP. Another knob to turn would be to add more polyiso under the floor or use larger I-joists — we'll hopefully do the cost-benefit analysis this week and reach a verdict.

It seems like the biggest advantage in our design is the ludicrously simple house shape. We're basically building a shoebox with a shed roof, which means there aren't many corners or thermal bridges undermining our envelope. Marc, Ben, and Eli already minimized thermal bridging before we decided to go for Passivhaus certification, so we're picking up a lot of PHPP points without having to change our plans.

We're waiting on a few more details, though, including some THERM data Peter is confirming with PHIUS. Hopefully that won't kick us back out of the ballpark, but as I said we still have some knobs left to turn.

If you could design your dream window, what would it be?

Submitted by Andrea on April 24, 2011

Marvin Windows is currently running magazine ads asking well-known designers this question and showing pictures of the pretty windows they came up with:

Marvin Windows magazine ad

I really wish they'd ask a few American green-building gurus this question, because I'm sure they'd get an earful:

  • Whole-window U-values lower than 0.17, and glass U-values around 0.10 (the window in the Marvin ad has a U-value of 0.30, which is totally lame)
  • Solar heat gain coefficient (SHGC) greater than 0.50
  • Visible transmittance (VT) greater than 0.50
  • Tilt-turn operation with inward swing
  • Geeky details I won't bother explaining, such as 4/16/4/16/4mm triple-glazing and a plastic warm edge spacer of Psi=0,038

In Europe, dozens of manufacturers make windows like that, but in North America hardly anyone does. The best North American window vendors are making a reasonable attempt, but they all fail on at least one of Ted's and my requirements:

  • Serious Windows' highest-performing windows are casement, not in-swing, which means they open outwards with a little crank at the base. This is mechanically inefficient, and it also limits the size of windows they can sell. We have some gloriously big south-facing windows in our dining area, and we don't want to make them smaller. Also, their best SHGC is 0.42, which is not as good as the standard Passivhaus windows from Europe.
  • Canadian companies Accurate-Dorwin and Thermotech Fiberglass didn't sell in-swing windows last time I checked. My main complaint about out-swing windows is that the screen is inside the window, which I simply don't like. And yes, I know about retractable window screens, but I'm still not jazzed about opening a heavy triple-paned window with a crank.
  • Inline Fiberglass makes a tilt-turn frame, but they still aren't big enough for our front windows. They offered to sell us a modified doorframe full of glass, but c'mon! Also, their best windows use Serious glass, which has slightly lower SHGC and VT than the European glass.
  • I could list a few more North American vendors, but they are mostly using Inline frames and Serious glass, whose shortcomings I listed above. And you'll notice I didn't mention the big-name American manufacturers -- as far as I know, Marvin, Pella, and Andersen aren't even trying to build Passivhaus-worthy windows.

    Ted and I are therefore likely to order windows from Europe. If money were no object we'd want something like Optiwin's Passivhaus-certified three-wood window, but sadly we are on a budget, and ever-plummeting dollar doesn't help. But the high-end American windows I listed above aren't cheap either, and there's enough variety and competition in Europe, particularly from the former Eastern bloc, that we can get something good for an acceptable price.

    We haven't made our final decision yet, but we're likely either to get German windows manufactured in Slovakia or Polish-made windows. We rejected some Lithuanian windows, not because we have anything against Baltic states but simply because the importer is based in DC and we want to work with someone more local. If we're handing someone a five-figure check just to place the order, we want to be able to drive over and hassle them from time to time.

    We're still waiting on a couple more estimates, but once we make our decision I'll post all sorts of titillating window specs and diagrams for your perusal.

    [Added on 2011-10-03: It's possible I've been a little too hard on North American glass manufacturers, since there's apparently some difference in how they test glass performance in Europe, but I stand by what I said about tilt-turn vs. casement operation.]

What makes it passive?

Submitted by Andrea on April 8, 2011

I was recently asked a simple but excellent question: What makes it passive?

The word "passive" turns up a lot in green building, and it can refer to several different things. When I say we're building an almost passive house, I'm referring to the Passivhaus building approach that was standardized in Europe and inspired by energy-efficient building methods pioneered in North America. The Passive House Institute US site summarizes:

A "passive" house achieves overall energy savings of 60-70% and 90% of space heating without applying expensive "active" technologies like photovoltaics or solar thermal hot water systems. Energy losses are minimized, and gains are maximized. Superinsulation and air-tight construction minimize losses.

Passivhaus certification is somewhat easier to attain in Europe than in North America, mostly because of their relatively moderate climate, but also because you can buy much whizzier building products over there (see my post on European windows).

After a considerable amount of waffling, Ted and I decided not to go for full Passivhaus certification, but we're still planning to use as many passive house techniques as we can (superinsulation, avoiding thermal bridges, sealing the house extremely tightly, using mechanical fresh-air systems, etc.).

The term "passive house" is easily confused with another green-building term: passive solar. According to the the most recent editor of the Wikipedia article on passive solar building design,

In passive solar building design, windows, walls, and floors are made to collect, store, and distribute solar energy in the form of heat in the winter (Passive Solar Heating) and reject solar heat in the summer (Passive Solar Cooling). This is called "passive" solar design (or climatic design) because, unlike "active" ( solar heating, photovoltaic, etc.) solar systems, passive solar systems do not involve the use of mechanical or electrical devices, fans, pumps, etc.

Passive solar home design was undoubtedly discovered by cave dwellers who noticed that south-facing caves were more comfortable year-round than caves facing other directions (cave dwellers in the southern hemisphere would have chosen north-facing caves). This is because the sun is angled low in winter and high in summer, meaning that winter light and heat will penetrate deeply into a south-facing cave, and summer sunlight will be blocked by the cave overhang. Furthermore, a cave with a solid earth floor retains winter heat gains even after sunset, because earth floors have a high thermal mass which absorbs heat during the day and then slowly releases it at night.

The cliff-dwellings at Mesa Verde in southwest Colorado are the textbook example of passive solar building. The dwellings face south and are protected from the hot summer sun by a gigantic overhang, but during the winter they are bathed in light.

The advent of mechanical heating and cooling systems made it easier for builders to ignore passive solar techniques. The problem got worse when people started building houses with ginormous windows, often facing a nice view in a direction other than south. Ted's parents' house has a great room with floor-to-ceiling windows facing a lovely view toward the west. Every afternoon the room is flooded with light, which brings welcome solar gains in winter (they can turn off their heater for much of the day) but way too much heat during the summer.

It is much easier to achieve Passivhaus certification if you maximize solar gains with clever window placement, thereby reducing the need for mechanical heating. Our building site isn't perfect for passive solar since we have quite a few trees blocking the sun toward the south, but it's not too bad, particularly since most of those trees will lose their leaves every autumn.

To get the maximum bang for our passive solar buck, I used SketchUp to simulate the solar shading at different times of year. I entered our latitude and longitude, and then I told SketchUp to show me what shadows will form on different dates (including the date of this blog post). Our house has big windows facing south, so they'll be our primary source for solar gain, and I tweaked the length of the roof overhang so it will admit plenty of sun in winter without allowing too much unwanted summer heat:

We're being careful to order windows with a high solar heat gain coefficient (SHCG), which means that the glass won't filter out too much of the warm sunlight. Again, refer to my future post on windows for more about SHGC.

[ For more information on Passive Houses, see What is a Passive House? ]