Andrea promised a while back that I'd explain how the foundation works, but I haven't done that yet. We went through a fairly long and painful process to get to the foundation we have now. I think it was worth it—the current foundation looks very cool, and I think it will function well. So let's take a tour.
What you see to the right is the pier foundation as viewed from the west side, looking east down toward the road. You can just see the roof of the car behind the foundation of the garage. That's the sort of U-shaped bit of concrete to the far right, which I will not be talking about, since it's pretty much just a normal foundation.
The horizontal bar of concrete that's closest to the camera is a grade beam. It's mostly resting on footings that have been pinned to ledge, although there's a point in the center where the ledge comes so high that there's just grade beam on ledge, with no special footing underneath. What you can't see from this angle is that the ledge drops off pretty steeply on the other side of the grade beam.
In the center, you can see a wall heading east from the grade beam. This is a shear wall—it's there to prevent racking in the east-west direction. It's solidly on ledge all the way down. The big dark grey wall that's further down the hill is another shear wall that's to prevent racking in the north-south direction.
Racking is a situation where there is a differential force on a wall that tends to try to bring it out of square. The force is called a shear force. Shear walls are shaped so as to resist the shear force. Shear force in earthquake country comes from the ground moving, while the inertia of the house tends to want it to stay in the same place. So the foundation accelerates in the direction of the shear force, and the top of the wall has to move to catch up to it. If you don't have a good shear wall, the wall can fail.
A house that's on piers has limited shear strength, because the piers are just big sticks poking up out of the ground. The footing might look like it will have some shear strength, and indeed the rebar that connects the footing to the pier, combined with the rebar in the pier, does give it some shear strength. But many homeowners in the San Francisco Marina learned the hard way that a house standing on piers with no shear wall is likely to collapse.
But we don't get earthquakes in Vermont, right? Well, not as often as California, but it's best to be prepared. What we do get, though, is high winds. With winds, the ground wants to stay in place, and house wants to move, which will tend to make the piers fall over. We have a pretty tall wall to the south. So we need enough shear strength to accommodate that wind force without failing. So our structural engineer, Ben, specified these shear walls.
The other advantage of the shear walls is, I hope, entirely psychological: there are two piers that are not directly pinned to ledge. I worry a little bit that these piers might somehow fail—they might start to wander, or we might have gotten the drainage and cover wrong, and they might move due to ice expansion. I don't think this is a real risk—we examined this issue carefully and concluded that the footings were secure. But if for some reason we blew it, the shear walls give us a huge additional safety factor that will allow us to correct any problem that should arise. Even though this is extremely unlikely, the fact that they are there gives me a lot of peace of mind.
Referring back to the picture at the top again, you can see a slot in the north end of the north-south shear wall, and a notch in the south end. These are to accommodate the laminated veneer lumber (LVL) beams that will be running east-west along the tops of the piers. So let's talk about wood for a minute.
Anywhere where there is horizontal contact between the foundation and the concrete, we will have pressure-treated lumber. So the grade beam at the top will have a 2x (two-by, not two layers) of PT on top of it. The shear walls will have 2x or 4x, depending. There are notches on the east side of the grade beam on the north and south ends to accommodate the LVL beams; each of these notches will have first a layer of PT, and then the LVL beam will rest on top of that. The LVL beams will be attached to the piers with metal brackets.
Ultimately, when all the foundation wood is in, we will effectively have a sill plate to rest the floor box on. In places, it will look just like a traditional sill plate. In places, the top of the plate will be an LVL beam.
What ties this foundation together is the floor box. The bottom of the floor box is a structural membrane made of zip sheathing nailed to the bottom of the floor joists according to a nailing schedule provided by Ben. This will create a box that is very resistant to deforming as a result of differential forces. This is what allows the shear walls, which are not otherwise connected to the piers, to keep the piers stable.
By the way, I should point out that while it might sound like I know what I'm talking about here, I'm relaying to you a layperson's understanding of how this all works, not a structural engineer's. Hopefully what I'm saying here will be of some benefit in terms of telling an interested reader what sorts of things to look out for, but it's no substitute for actually hiring a structural engineer to analyze your specific project. If you're flying your house the way we are, you definitely want expert help.
I recommend the book highly even if you don't actually wind up building what's in it, because the drawings are really helpful for understanding how to avoid thermal bridges, how to detail the airtight seals between floors, walls and ceiling, and also for ideas about what sort of material to use. I searched it carefully for details that would work for our foundation, but it didn't cover the case we originally designed: a house on a frostwall foundation with no basement. It had some drawings that were very helpful for thinking about how to detail the foundation, and when Marc and Andrea and I were brainstorming about how to build the actual foundation, that detail was very helpful in figuring out what to do (although Marc might argue that it led to me being obsessed with details that weren't all that important).
What the book does not cover at all, however, is how to do a floor when your house is on a pier foundation. Both Marc and Peter were a bit concerned about how that was going to work, but it went pretty well in the PHPP model. Normally in a slab foundation, you'd lay down a really thick layer of expanded polystyrene foam insulation (EPS). This would isolate the interior of the house from the ground. Typically the ground under the house will be warmer than ambient, though, so the EPS doesn't have to do as much work as our floor has to do to keep the house warm.
So we are going with a fairly thick floor—11 7/8" thick, with 4" of polyisocyanurate rigid foam insulation. The floor joists will be I-joists, to minimize thermal bridging. The insulation between the floor joists will be dense-packed cellulose. One really nice thing about this is that the floor will have a lot less foam in it than a typical floor—only 4", rather than the typical 8" or more of styrofoam insulation below the slab that you'd see in a Passivhaus.
An additional complication is that normally to get a good air barrier on the slab of a Passivhaus, you'd have a polyethylene membrane under the slab. This would then connect to the wall air barrier with some kind of sticky tape or expanding foam tape. We don't have that option with the floor box, because there's no place to put the polyethylene membrane.
Instead, the bottom of the box will be sheathed with zip sheathing. Zip sheathing provides an excellent air barrier. The edges of each piece of zip sheathing will be taped together. Remember, this tape is on the bottom of the sheathing. The bottom of the sheathing will be resting on the LVL beam or on the pressure-treated sill plate. This means that the sheathing has to be taped before it's nailed to the plate or to the beam.
In order to accomplish this, Eli's team is going to build the floor box in sections, upside down. They are going to tape the seams on the bottom of each section before flipping that section. When the time comes to install the sections, they will (handwaving, Eli, help!) to seal the joins between the sections.
The joint between the floor-bottom sheathing and the outer wall sheathing will be sealed with a gasket or caulk, as shown below. I'm not sure what sort of gasket to use if we go that route. We'd talked about using iso-bloco tape to seal the edge, but that stuff is very expensive. Another option would be to use EPDM gaskets. I don't know how much the EPDM gaskets cost—maybe they're just as expensive—but I suspect they are cheaper. It may also be that caulk is a good option, although I've heard arguments to the contrary.
Today's lesson: if you can do something today, do it today, because tomorrow your excavation guy may have time to do a bunch of work, and you'd like to fit into his schedule when he has the time. The problem with being your own general contractor is that it's your job to keep track of these issues. But in our case, Eli has been acting a little bit general-contractorly, and so I forgot that I'm still supposed to be keeping track of the schedule.
Really, when we decided to build the house ourselves, we weren't planning to pull in a lot of subcontractors, so in theory it was all going to be a lot easier, and we'd just call people when we needed them. When we pulled Eli in, that changed, because we decided to have him do a really substantial amount of work to get us under roof quickly. The problem with accelerated timeframes is that you really need to stay on top of the schedule or you'll wind up in a jam.
Today we wound up in a jam. Eli had suggested that I work on sealing the below-grade part of the garage foundation, but it was a little informal, and we were going to do it this week, and then it turned out that our electrical contractor's son, Sachary, had some time and could use a few bucks, so we wound up hiring him to do it. They were going to be on the site to set up the temporary electrical panel, so it sounded like a good plan.
The only thing is, Wayne, our excavation guy, was going to be on site to bury the electrical some more once the service was in. And since he was on site, he wanted to do some more work, because burying the electrical was a pretty minor job, and there's a ton of work to get done on the site. Wayne is one of those wise, experienced excavation guys who's dug a lot of foundations and really knows what he's doing, and doesn't mind offering his advice on how to do the earthwork right. As a consequence, he's in high demand, and has had a busy summer, despite the economic situation. But he's tried to do right by us by squeezing us in here and there, and he's succeeded.
The only trouble today was that we hadn't really thought about how it was going to make sense for Wayne to do some of the earthwork while he was on the site (actually I think Logan was doing most of the actual earth moving—Wayne was just there to make sure we were all clear on what the plan was). So we hadn't put in the low-voltage carrying pipe, and we hadn't put in the water supply pipe for the garage. We could have done those things any time in the past two and a half weeks—we just didn't think to do it, because I was hammering out a water channel further uphill, and Andrea managed to catch a nasty flu.
To add an extra complication, when we'd talked about how many sleeves to put through the garage wall, we had a miscommunication. We needed a 3" sleeve for the electrical, and a 2" sleeve for the low-voltage, and we'd talked about putting in another electrical conduit for the solar grid-tie (which I will explain in a separate post). We'd also talked about putting in a 2" sleeve for a water supply line for a hose bib on the garage, and maybe a utility sink in the garage. But then we figured out that we didn't need the sleeve for the grid-tie, because there was enough room in the 3" sleeve. So we deleted that, and in the process managed to lose track of the 2" water sleeve.
(BTW, for those who don't know, a sleeve is just a piece of pipe you run through the foundation wall before you pour the concrete. When the time comes to run services through the concrete, you have a convenient pipe in the concrete wall, and you can either join conduit to it, or just run a smaller pipe through it.)
So today, we needed to cut a hole in the concrete using a wet core drill. And we needed to run a 4" sleeve from the house to that hole, and through the hole. Fortunately Eli came by, and he knows everybody in town (I think literally), and his friend Dan, who is a plumber, just happened to be driving down Western Avenue when Eli called him, and Dan came to the rescue. Eli got one of his guys to rent a core drill, bring a bucket of water for the wet part, and a generator to power it. Once the hole was cut, Dan ran the pipe through it and up the hill to the place where it's going to enter the house. Andy, our electrical sub, ran the low-voltage conduit.
Anyway, I think that's what happened—by the time I got to the site at 6:30 this afternoon, all the work had been completed, and I didn't see who actually did it. Here's the sleeve sticking through the wall into the garage:
I was there to finish cutting a drainage channel through a bit of hard rock up at the top of the foundation. I've been working on this a couple of hours a day, maybe three or four days a week, for the past three weeks. Here's what it looks like:
I've actually removed a substantial amount of rock, but it looks pretty unimpressive. It'll be covered up in sand tomorrow. Sigh.
It is with great pleasure that I unveil our newly-poured piers. Foundation, I dub thee Nerdhenge.
Since the piers will be extremely visible, we paid extra to add some pigment to the concrete mix. The color was called "Onyx," a name apparently chosen by someone who either never saw (A) the actual product or (B) onyx. But we're satisfied — it has an attractive bluish cast compared to unadorned concrete, and it will hopefully give our house a certain klassy je ne sais quoi.
The photo above shows the lower three rows of piers, and the photo below shows the view from the top. This picture shows the grade beam, which Ted can explain in a geekier post.
But seriously, our foundation design looks like Eastern European public art:
That top section is a grade beam, and the long/wide sections are pinned to ledge. In fact, nearly every one of the footings will be pinned to ledge. (For you flatlanders out there, ledge is the rocky substrate frequently lurking below the topsoil in hilly regions like New England.)
So far they've formed most of the footings, and the plan is to pour the footings early next week and then pressure-wash the whole area to clear away the organic material (AKA dirt). The piers will get formed and poured after that.
Ted will surely post more about the hows and whys of our foundation, and we'll put up pictures as well.