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.
It's a good thing we didn't set up a webcam on our building site last autumn, because it would have shown nothing more than snow falling (and now melting). This is because we opted not to put in a foundation before winter. Yes, we had naively hoped to be living in the house by now, but things didn't quite work out that way.
There's an old saying about project management: "Fast, inexpensive, or good -- pick two." Hopefully this means our house will be inexpensive and good! We could have rushed last autumn to get the foundation dug and poured, but it hardly seemed worthwhile to pour a foundation which we'd only worry about all winter. So we decided to kick back and aim for spring, which is almost here.
The good news is that our plans look great! I spent much of the last six months working on our plans in Google SketchUp, a free 3-D modeling application which will surely get its own blog post. This process has allowed me to nip all sorts of problems in the bud, and Ted and I feel very confident about the design.
I uploaded new drawings to the floor plans page. Sorry, they're only 2-D floor plans and not 3-D SketchUp designs, but they reveal the changes we made this winter:
We added a doorway between the main entry and the meditation retreat area. Originally the retreat space was only going to be accessible from outside and from a secret door in the master bedroom, but we decided instead to have a sliding/pivoting bookcase in the main entryway and hide the door behind it.
We removed the ground-floor guest bedroom. This had been a requirement, because we anticipate having guests who won't be able to climb stairs. But it was eating up too much space on the ground floor, at the expense of the kitchen and living room, so we decided to use the meditation room as a ground-floor guest bedroom when needed.
If someone with a disability moves in full-time, they will need to access a bedroom upstairs. We are therefore leaving room for a elevator next to the staircase. I hope we'll never have to install it, but it's good to have the option. And check out the awesome pneumatic elevators we found online!
Removing the ground-floor bedroom allowed us to enlarge the kitchen, living room, and dining room. In the old plans they were mashed into a single room, but now they are quite clearly defined.
We added a mud room! It's next to the kitchen, and I suspect it will be our usual entrance since it's closer to the garage.
There are many more changes and improvements, but I'll save those for the SketchUp post, since they'll be easier to explain in 3-D.
BTW, for the sake of honesty I changed the site name to "Almost Passive House" (from "DIY Passive House"). The house will not be an official Passive House, but we hope it will perform nearly as well. I like to think of it as the green-building version of "Kosher style" or "Be a model, or just look like one!"
Ordinary houses breathe through leaky joints and poor seals, losing heat and wasting energy. But our house won't leak, so we'll use a heat recovery ventilator (HRV) to admit fresh air and expel stale air, transferring heat from one stream to the other.