Green Building 101

The Dryer Conundrum

Submitted by Ted on August 12, 2011

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.

Insulating the floor of a Passivhaus on piers...

Submitted by Ted on August 6, 2011

The Passivhaus Institut has a really cool book that covers various Passivhaus building techniques in excruciating detail, including sectional drawings and descriptions of materials. Andrea put it on her Christmas wish list the year before last, so I got her a copy. It has dozens of different foundation details for different sorts of building environments.

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.

What makes it passive?

Submitted by Andrea on April 8, 2011

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.).

The term "passive house" is easily confused with another green-building term: passive solar. According to the the most recent editor of the Wikipedia article on passive solar building design,

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.

[ For more information on Passive Houses, see What is a Passive House? ]