I was on the phone with Marc the other day and he pointed out something that hadn't really occurred to me until he mentioned it: when we hired Ben and Eli, we took on a new job. One of the key things that you will hear from anybody who has any experience building a passive house is that your team has to be on the same page about what you're trying to do, or important things will fall through the cracks, and compromises will be made that make a lot of sense from the viewpoint of one participant in the project, but create a huge problem from the perspective of another participant.
We've made a ton of changes to our house since we first started brainstorming about it with Marc a year and a half ago. A lot of them hit quite recently, when Ben talked us out of our Shogun foundation, and when Eli talked us into flipping the roof. We had to give up some things we'd been looking forward to, but we tried to keep a razor focus on the air barrier and insulation, because together those give us what we really want in this house: a consistently comfortable indoor environment.
But one point that Eli really pushed us hard on was the roof. In order to understand what the controversy was, I need to explain the original roof design that Marc came up with. On the way, let me point out that because Andrea and I are so interested in the details of this process, we never have a conversation with our honored teachers in which we just listen to what they tell us and do it. Consequently, nearly every aspect of the house has our fingerprints on it in some way, and we don't always remember which things we decided to do were "nice to have" and which were crucial. The roof design wound up being one of those things.
The problem with an almost-passive-house roof is that it has to perform consistently in two senses: first, the insulation has to be consistent. We can't have it settling down from the top, bunching up on the bottom. Way back when, when we got our first estimate from Keith at Thermal House on insulating the roof, we were absolutely floored: it was going to cost about twenty thousand dollars. Why so expensive? Because we'd chosen to do a cathedral ceiling, not a sensible flat ceiling.
Flat ceilings are easy to insulate--you can just dump the insulation on top of the ceiling joists and spread it out, or if you want to get fancy, use dense-pack cellulose. Because the ceiling is flat, there is nowhere for the insulation to go. You wind up with an irregularly-sized air gap above the ceiling, since your roof certainly isn't flat, but in most situations it should be pretty easy to get a nice thick blanket up there.
With a cathedral ceiling, you don't have the luxury of flat. Keith solved that problem by using a really expensive foam product that he was confident wasn't going to move. But we can't afford to spend $20k insulating our roof. So Keith and Marc and Andrea and I did quite a bit of negotiating. At the time, we'd been planning to use open-web trusses to support the roof. We were confident we could get trusses that would span the distance from the north to the south wall, and would support the dramatic 7' overhang we wanted on the north. But insulating open-web trusses is really hard, as Andrea explained in her recent post.
What we came up with was that instead of using trusses, we'd use I-joists. Andrea did some research on the Internet, and found a supplier for a 24" deep I-joist that would span the distance we wanted, and additionally would support the cantilevered overhang. Marc and Keith both liked these better, because Keith was confident that he could do a dense-pack cellulose installation without any voids using the I-joists. Marc liked the I-joists because they cause very little thermal bridging: the webbing between the flanges on the I-joist is only 7/16" thick, so even though its R-value is substantially less than that of the roof, there isn't enough of it to create a problem.
At this point I need to talk about roof venting. There are two kinds of roofs: warm and cold. A warm roof is a roof with no air vent. They are popular in applications like ours, because building roof vents is fussy when you can't just vent the eaves into the attic. Typically a warm roof consists of a ceiling, rafters of some sort with insulation packed between, a layer of waterproof sheathing, a layer of insulating board (typically polyisocyanurate), a waterproof layer, and then the roofing material. So it's not a particularly simple roof.
So we got this brainstorm to do a vented roof, even though we're doing a cathedral ceiling. Marc had a clever design. He proposed that we cut a piece of insulation board to the spacing between the webbing of each I-joist. We could then cram this up against the top flanges of two I-joists. This would create an air space between the insulation board and the sheathing that's nailed to the top of the I-joist. Now we have a clear vent running the entire rise of the roof. It's pretty easy to construct, and it'll keep the roof cold.
So that's the roof design we'd been planning on for nearly a year. But then Eli comes along, and he has a number of problems with it. And they are real, serious problems. First, the top flange of the I-joist is going to be really cold on a cold winter's night. The bottom flange is going to be really warm. So the bottom flange is going to expand, and the top flange is going to contract. The roof is supported on both ends, so it might bow in the middle, or the I-joists might warp a bit, but the bottom line is that there's going to be some unwanted movement. Eli felt that this was potentially a big deal; Marc wasn't sure it was, but he didn't claim it definitely wasn't, either. I have no informed opinion on the matter—I have to trust Eli and Marc.
The bottom line is that this got Eli to thinking about some other advantages of trusses over I-joists. One of the big problems we'd left somewhat unsolved with the I-joists was how to construct the overhangs. Remember that there are four overhangs. We'd been planning to stick the I-joists out over the north and south overhangs, and didn't really have a solution for the east and west overhangs. Eli didn't like the idea of sticking the I-joists out like that, because it was going to potentially expose the webbing in the I-joists to moisture on the ends, and possibly wick moisture into the roof. He wasn't entirely clear on how big a problem this was, but it was definitely a concern.
Also, whereas the I-joists are only strong if the webbing and the flanges are preserved, trusses can be engineered so that what sticks out past the envelope is thinner, and yet still provides enough support. We'd been trying to figure out how to avoid having the overhangs be two feet thick, and were talking about cutting off the bottom flange and attaching something to the webbing higher up. Eli didn't like that idea from an engineering perspective.
A final advantage of trusses is that they are open, which means you can stick things into them. This gave us a way to do the rake overhangs—the overhangs on the east and west sides of the house. With the I-joists, we had no answer for the rake overhang other than nailing a box to the side of the roof. This could probably have been made to work, but making it structurally sound would have been quite involved.
I heard about Eli's counter-proposal on roof structure while we were up at the site seeing what had been done, I think on Monday. We'd been talking about using trusses for a while, but on Monday we came to the conclusion that we really couldn't use the I-joist roof as originally designed, and we were starting to go down the path of figuring out how to insulate the trusses. We also started thinking about going back to a warm roof, because the cold roof was starting to look like it would be pretty difficult to build with trusses.
I should mention that in the background we'd been hearing about another scheme that Ben, the structural engineer, was thinking about, that involved a two-layer roof. I didn't really like the sound of that, because it sounded like a lot more work, so I hadn't made the effort at this point to find out precisely what he was talking about. I had visions of having to insulate two sets of cavities. Not a happy thought.
On Tuesday, Marc called Andrea to weigh in on the rash of changes we were suddenly making. I was sitting in my chair minding my own business, working on something or other, and suddenly Andrea thrust the phone at me and said, "You talk to Marc." So I wound up explaining the whole truss thing to Marc, and Marc of course was asking his usual intelligent questions and making comments, and I was starting to feel pretty sheepish about where we'd gotten with Eli.
I mentioned some harebrained schemes for mitigating the problems with trusses, and Marc was pretty patient with me, but he reminded me of what we'd been trying to accomplish with the trusses, and the complexity of getting a good air barrier with trusses, and so forth, and by the end of the conversation I was back to believing that I-joists were the right thing, and that we just had to figure out how to make them work. Marc also explained why warm roofs have insulation on top. In a warm roof, you have warm, moist air below, a piece of wood, and then a moisture/vapor barrier. This means moisture can condense on the wood, and has nowhere to go. Venting the roof provides a place for the moisture to go. Without it, the wood could rot.
Andrea reported the outcome of this conversation to Eli via email. A while later Eli called me and set up a Skype session so that he could draw stuff in his CAD package while I watched. What he drew was a roof that used I-joists for structure, but then had a venting system on top of it. The venting system is essentially a second roof, with ribbing that mostly runs parallel to the I-joists.
The eaves are cantilevered on the ribbing, but the ribbing is also used to form ventilation channels. The corner overhangs are done by installing a rib at each corner at a 45-degree angle to the I-joists. The rake overhangs are done by installing ribs at 90 degrees to the I-joists. Throughout the rake and corner overhangs, ribbing is also installed parallel to the I-joists to form vent channels, and vent holes are drilled in all the ribbing that is not parallel to the I-joists, so that even the corner and rake overhangs are still vented.
Of course, this roof has a problem as well. Because it's so nicely vented, if you get a wind across the roof, it can form a relative vacuum at one of the vents. This vacuum can then suck rain or snow into the vented part of the roof, which falls into that roof. If the roof isn't waterproof, the wood gets wet. So Eli is proposing to put down some kind of waterproofing.
One thing that just occurred to me while I was typing this, though, is that now we essentially have a warm roof with wind channels above it, because the bottom layer of sheathing has a moisture barrier on it. So I need to try to understand whether that aspect of this roof really makes sense. But at least from the perspective of supporting the eaves, this roof is the best design we've seen so far, and I'm feeling pretty good about being able to iron out the details.
But now perhaps you can see what an interesting job the prospective owner-builder faces when trying to get a passive house built with the help of a lot of really talented and experienced people. I feel very lucky, but Marc is right in saying that this is a significant job.
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...
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.