Andrea and I started sleeping in the house two nights ago, so some things that I've been noticing about the heating profile of the house are starting to become clear.
When we finally got the HRV running a few weeks ago, I was disappointed to notice that we don't have the kind of evenness of temperature between the upper and lower stories of the house that we were hoping for. My first panicked theory about this was that the HRV wasn't working. Panicked, because the tubes are all nice and snug behind walls now, so if we got it wrong, it's going to be a real pain to fix. The good news is that I don't think the problem is the HRV, although I'm contemplating one tweak to the vent layout which we could do without any wall surgery.
When we talked with Peter Schneider about overheating, he reported his experience with some of the houses that Efficiency Vermont has designed up in Charlotte, Vermont. These all have lots of south-facing windows, and Peter hasn't seen problems with overheating in the summer. Based on Peter's reports, we were kind of hoping to get away without doing one of the features we've got on the plan: active shading on the south-facing windows.
If you look at the drawing of the house at the top of the blog page, which is a view of the south face, you'll see that the lower windows have what looks like louvers on them. These are part of the solar shading plan—the idea is to install them in spring and keep them around until fall, and then take them off and stash them for the winter. These louvers will prevent high-angle light from making it through the window in the summer and heating the interior space. By installing them on the outside, we minimize heat gain inside—hopefully most of the heat that is generated when sunlight is absorbed by these shades will re-radiate into the outside air, rather than heating the inside of the house.
Why is Peter seeing different results? I don't know, but I have a theory. I think the windows in the houses in Charlotte may not have the same solar heat gain coefficient as ours, and may reflect more high-incidence light, while letting low-incidence light in, so that they gain more heat in the winter than they do in the summer, even if the same amount of light is hitting them.
The way our house is set up, we have a heat pump indoor unit on the wall downstairs, right in the center of the house. We have nothing upstairs. So the reason I was worried that the HRV wasn't working is that one possible interpretation of the data is that we're cooling the house adequately, but the HRV isn't doing its job in redistributing the heat evenly throughout the house. If that's the case, it's kind of a big problem.
But having just spent the morning sitting down next to the windows with the heat pump off, I have a different theory. For most of the morning, it was nice and cool downstairs, but as more sun came in and the day went on, it started to get hot, just as it is upstairs in the afternoon. In other words, the heat coming in from the windows is heating the upstairs and downstairs evenly; the reason that it feels hotter upstairs is because the cooling effect of the heat pump completely counteracts the heating effect of the windows. I've confirmed this by turning the heat pump back on.
So this gives me some real confidence that when we get around to building and installing the louvers, we will stop experiencing overheating in the south part of the house. I'm still a little tempted to add one more supply vent on the south side of the house upstairs, but that's something I'm hoping to have a chance to debate with the guys at Zehnder. If it needs to be done, it's a really easy fix, because I can do it up in the utility loft, which doesn't have a finished floor.
My employer BuildingGreen recently celebrated the overlap of National Poetry Month and National Architecture Week with a sustainable design haiku contest. I am not normally someone who writes poetry, but I quickly discovered that writing haiku was a great way to blow off years of accumulated steam from trying to build a Passive House. So I dropped everything and immediately started tweeting a string of cathartic haikus.
Many of my little poems require some basic knowledge of green building, so I am turning this into a teaching opportunity by annotating my wee œuvre below. Let the learning begin!
Heat recovery ventilation
A punch in the nose
To the next one who tells me
"A house has to breathe."
Whenever I tell people we're building an extremely tight house, someone always pipes up, "Well, a house has to breathe." Yes, and that's why we're installing a ducted heat recovery ventilator (HRV). An HRV is a a fresh-air system with pipes to the outside, and it has a heat exchanger that transfers most of the heat between the two streams.
Our HRV is 84% efficient, which means that in cold weather it will transfer 84% of the heat from the outgoing stale airstream into the incoming fresh airstream. Compare this to a leaky house, which gets fresh air and expels stale air through holes in the building envelope, losing oodles of heat in the process.
Lately when people tell me a house has to breathe, I tell them that a human also has to breathe but we do it with lungs and a respiratory system rather than by punching holes all over our body. For some reason, this metaphor really makes an impression.
Wood certification wars
It's hard to believe, but there is still a lot of unsustainable logging going on these days. Siding and decking are particularly bad, since it often comes from old-growth cedar and hemlock forests in British Columbia. It is therefore important to look for sustainably-forested wood, and the two main certification groups are FSC (Forest Stewardship Council) and SFI (Sustainable Forestry Initiative). FSC was created by environmental groups whereas SFI was originally backed by the wood products industry, and even though SFI has distanced itself from the logging industry, critics still say it is less rigorous than FSC.
Things get ugly between FSC and SFI when LEED gets factored in. LEED is the U.S. Green Building Council's rating system for buildings, and they currently give points only for FSC wood, not SFI. SFI representatives grumble that this is hurting the domestic lumber industry, but... well, if you're really interested you can read all about the "Wood Wars" at BuildingGreen.
You will now understand my next haiku:
Can't decide between
FSC and SFI?
Just build out of rocks.
Thermal bridges are a major avenue for heat loss in a building envelope. They occur when material crosses through the building envelope, creating a direct link between the heated interior and the cold outdoors and allowing heat to escape the building envelope. A classic thermal bridge is the shared concrete slab underneath a house and its attached patio; heat from inside the house travels outside through the concrete slab, and the furnace has to work harder to replace all the lost heat. You're basically paying to heat the outdoors. The reverse happens during the summer, when outdoor heat travels through the concrete into the air-conditioned house.
Squinting and roasting
As the western light shines in.
But look at the view!
The point here is that west-facing windows become a problem during the summer as the sun begins to set, filling the room with unwanted heat and glare. Sadly, most builders ignore passive solar principles when siting a house, instead placing windows toward the best view.
An age-old question
Of all the haikus I posted on Twitter, this one got the most retweets:
No one ever asked
When they built the Taj Mahal
"What about payback?"
Questions about payback are a common gripe among green builders. I ranted about it last year, but I'd like to add that conventional construction is often cheaper than green building because the costs have been externalized. For example, when we rely too much on carbon-heavy energy, we're shoving the costs onto the people who will be hit hardest by climate change. Or when we use materials with a toxic manufacturing process, we are saddling those workers and communities with the long-term cost.
Ted and I have not always made perfect decisions while building this house, but we sincerely tried to bear most of the cost burden ourselves. It made our house more expensive than I would have liked, but my only real regrets are the times when we cheaped out at someone else's expense.
Do you really need
That geothermal heat pump,
Or would caulk suffice?
A lot of people think the best way to improve their home's energy performance is to add fancy equipment like solar panels or a ground-source heat pump. But you can get a lot more bang for the buck simply by improving your thermal envelope. After that, go ahead and install some eco bling. You might not need that ground-source heat pump anymore, but if you install solar panels you'll be able to generate a much higher percentage of the energy you use.
If you want to build a Passive House, you first have to estimate the energy use in a ludicrously detailed spreadsheet called PHPP (Passive House Planning Package). PHPP is incredibly comprehensive and has to be filled out and tweaked by a highly-trained professional. But it lacks at least one key field:
Where do you input
"Milligrams of Valium"
One of the problems with building a tightly sealed house is that a lot of things we take for granted in a regular house suddenly become difficult when your main ventilation system runs at under 100cfm. A dryer typically blows 150-200 cfm when it's running. This means that it's going to be sucking cold air in through the HRV. On a really cold day, this could cause serious trouble for the HRV—the exchange plate could frost over. But more than that, it's (ironically) blowing warm air out of the house, while at the same time sucking cold air into the house. Again, on a cold day, really not what you want.
Range hoods cause similar trouble—they want to push air out of the house at >100cfm, and the air they are pushing out is generally warm air from the conditioned airspace, which must be replaced with cold air from outside. This seems like a minor issue until you consider that, aside from insulation, one of the main reasons that a Passivhaus has such a low energy budget is that you aren't heating large quantities of outside air as it leaks in through your drafty building envelope. So when you turn on these vents, your undersized Passivhaus heating system may be unable to keep up.
An additional complication is that if you have any appliances in the house that burn any sort of fuel, you are going to be creating a relative vacuum outside of the those appliances, and that might draw combustion products into the interior airspace that ought to be going up a chimney. We already had to tearfully let go of my 30,000 BTU wok ring dreams (actually, Andrea was remarkably dry-eyed) because of combustion products that couldn't be readily ventilated. No gas stove either. But externally ventilated gas heaters are very popular in tight homes, because they can be very efficient. Marc had a wood stove in his house in New Hampshire (although that wasn't a Passivhaus). Anything like this is going to be a potential hazard if you have exhaust fans running separately from your HRV.
Fortunately, we already gave up on a gas heater and decided to go with a heat pump instead. So we don't have to worry about that. But lots of exhaust vents are still something we have to avoid.
What a lot of Passivhaus people do is to set up drying rooms in their houses. This isn't a bad idea—it can be as low-tech as an indoor clothesline, or as high-tech as an enclosed space with a dehumidifier and/or a heater, plus some kind of exhaust fan that exhausts into the living space. We don't really want to dedicate a special room to this task, but we could certainly set up drying racks in the utility room and the mudroom on laundry days, and I suspect we will.
However, on a practical level, there will be times when we will want a dryer, either because we are drying more clothes than we have space for, or we are in a hurry, or whatever. Plus, for resale purposes, not having a dryer is kind of a non-starter. So I did a little research, which I thought I would share here.
The cheapest product I could find is an LG condensing dryer. This works the same way a regular electric dryer does: there's a heating element that heats the clothes to drive the moisture out, and a vent. Where it differs is that instead of leading outside, the vent leads into a condenser system which condenses the moisture out of the air, filters out the lint (sort of, according to some reviewers), and dumps it down the drain.
This would certainly work, and work well, but there are two problems with it. First, it turns out that it consumes more energy than a plain old electric dryer. When you count the cost of heating the replacement air, it's probably a wash, but this is definitely not a win. The second problem is that it cools the condenser with cold water from your tap, which it dumps down the drain. I get the impression that it's not a lot of water, but there are still some problems. Some people love this device, and some hate it. The ones who hate it often talk about problems they've had with leakage and pump failures. I suspect that the Rube Goldberg nature of the condenser has something to do with this.
So I did a little more research, with the help of Green Building Advisor. Actually, a lot of what I learned came from reading GBA, and I recommend this article highly if you want to drill a little deeper than the presentation I'm offering here. GBA talked about a Bosch system called Ecologixx that's called a "heat pump dryer."
When I was reading the Amazon product page for the LG dryer, I just assumed that it had a fan and a dehumidifier, which seemed like it ought to be more efficient than a heating element, but GBA cured me of that presumption. However, the Bosch Ecologixx series of dryer products do in fact work pretty much the way I had hoped the LG would. Most of these products don't seem to be available in the U.S., but the Bosch Axxis dryer is available, and it's actually pretty reasonably priced—about $200 more than the LG. Not everybody loves it, but it looks like a win in theory.
The bottom line is that I feel pretty good about not putting in an outside vent for the dryer. I don't love that this means we have to get rid of our Kenmore dryer, which has been a friend to us for many years, but I suspect that Freecycle will help us to find a good home for it.
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.
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.
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.