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.).
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.
My current enormous task is to design a complete framing plan. I am literally mapping out exactly where every single stud and beam will go, which I hope will save us a lot of effort when we're working with actual lumber. I'm keeping a running list of questions to ask Marc and our structural engineer, just to make sure we're not doing anything too stupid.
Obviously our primary goal here is structural soundness, but my driving obsession is to Avoid Thermal Bridges. This is one of the central tenets of Passivhaus construction, so I thought I'd tear myself away from SketchUp for a few minutes and explain what this actually means.
To paraphrase Homes for a Changing Climate, thermal bridges are the path of least resistance for heat to flow out from a house. They occur when an element in the house has higher heat conductivity than the surrounding materials. For example, a balcony slab that isn't thermally isolated from an interior concrete floor can suck the heat right out of the house.
The most common thermal bridge in a wood-frame house might be the wall studs themselves. In a 2x6 wall, studs extend through the thickness of the wall. The inside of a stud wall is normally covered by drywall sheets on the inside of the house and cladding outside the house. In the diagram at the right, you can see that the wall is full of fiberglass insulation, except for where the studs are. So the insulated parts of the wall will have an R-value of, say, R-19, but the studs themselves are only about R-6, meaning that much more heat will escape through the studs than through the insulation batts.
We plan to address this in several ways. One is to raise the wall's overall R-value by putting additional rigid foam insulation outside the stud assembly, beneath the exterior cladding. Another is to use as few studs as we can get away with. To accomplish this we are using Optimum Value Engineering, which does all sorts of clever tricks to minimize the amount of lumber used in construction.
So in a nutshell that's what I've been doing, trying to design our house frame with as few thermal bridges as possible. It's a little trickier than it sounds, at least for a construction neophyte like me.
Incidentally, here's a peek at the framing plan so far. It's missing most of the windows and, notably, a roof, but you get the idea.
Sigh... every time I think I have some detail of our house planned, the rug gets pulled out from under me. This time it's the foundation plan, which had been nicely settled since September.
The problem is that we're on a sloping lot, but it doesn't slope steeply enough to have a walk-out basement/garage. So we decided to do a slab on top of a 4-foot frost wall. We were going to follow one of the standard approaches, but Ted lost sleep thinking about where the dew point was going to hit inside of the wall (condensation + studs + tightly-sealed house = mold), so we needed to tweak things.
Marc suggested a material he'd recently heard about called FoamGlas. It's a fairly nifty product: a strong, insulating block with good compressive strength. Just the thing to provide the thermal break in our foundation, which meant the dew point would hit inside the exterior rigid foam insulation where there's no risk of mold. Here's what it was going to look like [click image for larger version]:
Both Marc and our structural engineer approved, so I felt good about the plan.
Ted, however, was a little skeptical, since it's a fairly new product in the US. We called the technical contact at Pittsburgh Corning today to ask some questions, and it turns out that FoamGlas has a cousin in Europe called Perinsul which is designed for our exact purpose and has a much longer track record. Perinsul has higher compressive strength than FoamGlas, and it's also pre-coated with an impervious seal. We could have sealed the FoamGlas ourselves with some asphaltic mastic, but the lower strength and the lack of a history for our application make it a non-starter for Ted.
So now we're back to the drawing board (i.e. Google searches & SketchUp). Pittsburgh Corning is willing to import some Perinsul for us from Belgium, and I asked them for a price quote, but it seems absurd to have foundation blocks shipped overseas (even lightweight ones). The only upside is that it might convince them to start manufacturing these babies in the US.
We asked Marc for some alternatives, and he proposed autoclaved aerated concrete (AAC). AAC is swell stuff (we were familiar with it in Arizona under the name E-Crete), but it's not manufactured in the Northeast. I spoke to a helpful sales rep today who will send me a quote from a factory in Florida, but it's also not an ideal solution.
The General Plastics site was hilarious, BTW. It had an interactive product finder with choices like "Is this an application where you are trying to keep something hot (but under 250 degrees F)?" or "Is this an application where you are trying to keep something cold (+40 degrees F to -40 degrees F)?" And that's just from the section on Thermal Insulation — there are other question trees as well. It's like a "Choose Your Own Adventure" for evil scientists!
Planning this house has been a crazy amount of work for me, but fortunately I can rest once all the specs and plans are finished. Oh, wait, maybe not...
For a while I've been meaning to post my report card from 7th grade wood shop. That's the class where you learn how to use tools and build stuff -- we followed a series of steps to make a wooden locomotive.
I spoke to a window vendor today who has a lot of experience with Passive House projects. He pointed out that some of our windows are not sized very efficiently. The problem isn't the big windows -- it's actually the small ones that have a high frame-to-glass ratio (the glass has a better U-value than the frame).
With that in mind, I played with resizing the front clerestory windows, making them larger. I removed one of the front windows to keep the glazing numbers from getting too high, and I spaced them 14" apart so they won't look like they're squished in the corner. That will also make them much easier to install.
But Ted is not fully convinced, so we need to run it by Camilo (our architect). Instead of sending the pix in an e-mail I figured I'd just post them here.
Basement rim joist areas; holes cut for plumbing traps under tubs and showers; cracks between finish flooring and baseboards; utility chases that hide pipes or ducts; plumbing vent pipe penetrations; kitchen soffits above wall cabinets; fireplace surrounds; recessed can light penetrations; poorly weatherstripped attic access hatches; and cracks between partition top plates and drywall.