Our building envelope

First published 10/29/15

When first looking at how much insulation to put into the house, I
looked to the Passive House certification system. To get Passive House
certification, there is a hard limit on the amount of energy needed to
heat and cool a space of 15 kWh/m² per year, which works out to about
1/10th the amount that a typical house uses. It turns out that this is a
very difficult bar to meet in cold climates, with the amount of
insulation required being simply immense. With a house like ours it
would have required walls roughly 2 feet thick (instead of the 6-8″ that
are typical of current construction). After doing a bit of math, it
didn’t seem like this was an investment that would ever pay off, so
instead we only took inspiration from the Passive House ideals, rather
than attempting to meet them. We still ended up with a wall assembly
roughly 15″ thick, trying to balance cost and efficiency.

Another heuristic that I picked up from our architect is the 60-40-30-20 rule. This suggests putting R-60 insulation in the ceiling, R-40 in above grade walls, R-30 in basement walls, and R-20 underneath the basement slab, which is much closer to what we actually did. I’ve elsewhere heard referred of concepts similar to this, labeled as “the pretty good house“. While a house like this won’t reach that Passive House standard, it will still cut by half or two thirds the amount of energy needed to heat and cool a home.

For those of you familiar with reading building plans, I have put up the final house plan with all building details, here. These seem to not often be shared, but I am not too concerned about someone copying our home; if you would like to borrow any details, please do so.

Starting from the top, we built a cathedral ceiling using a single pitched shed-style roof. We used manufactured trusses with 3′ of depth built in for insulation. Into this space a layer of 3.5″ Roxul batt insulation was laid down covering the entire bottom of the ceiling cavity. Loose cellulose insulation was then blown in above, mostly filling the 3′ space, reaching an estimated R-80 for the ceiling. It was originally specified to be R-60, but there was enough space for more insulation, and cellulose is relatively cheap and easy to install, so a last minute upgrade was done during installation. With the energy modeling that we did after the house was complete, it turns out that the R-60 would have been sufficient, as only a very small proportion of the heat loss would have been through the roof even with R-60. Beneath the rest of the insulation was a standard polyethylene air and vapor barrier, preventing air movement and controlling water movement. The final interior layer is veneer plywood sheets that make for a very attractive ceiling to the room below.

For the above-grade walls, we ended up using a double stud wall assembly. The structure of the house is built with 2×8 lumber, which allowed both for necessary support on the tall wall in the upstairs, as well as to give a deep space for insulation. The bays in this wall were filled with Roxul batt insulation, a mineral wool product. Mineral wool is a quite well regarded insulation, as it is very pest, water, and fire resistant, is easy to install, and holds its shape well, all leading to good long service. On the inside of this 2×8 wall is the polyethylene air/vapor barrier. Inside of that is a 2×4 utility wall, holding all of the electrical, plumbing, and data cabling. This wall is also insulated with Roxul insulation. Finally, a standard layer of sheetrock provides the interior walls. A big advantage of the utility wall is that all of the openings that need to be cut, for outlets, fixtures, etc., don’t need to punch holes in the air barrier, keeping the air tightness higher. Outside of the 2×8 wall, there is a layer of plywood, then 1.5″ of Roxul comfortboard, which is very similar to the batt insulation we used, but a firmer and coming in sheets. By wrapping all the walls in mineral wool, it prevents thermal bridging, where heat would pass through the wood framing (which is a poor insulator compared to the insulation it holds in place). Finally, the exterior has 1×3 wooden strapping to allow for air circulation and water drainage, on which Hardie brand cement board is mounted. Altogether these walls are a nominal R-47 (the actual R-value is lower because of heat that conducts more easily through the wood studs than the insulation).

For the below grade walls, which are about 5-6′ tall around two sides of the walkout basement, the final assembly was a simpler one. From the outside, there was a 2″ thick layer of rigid EPS (expanded polystyrene) foam, with a 4′ wide skirt of the same material buried approximately one foot below the outside ground surface. This skirt reduces the ability of cold winter temperatures from passing through the soil to reach the lower walls and foundation of the house. Essentially, escaping heat is slowed down as it has to go through or around that insulating skirt. Inside of the 2″ of foam, there is a waterproof membrane stuck to an 8″ thick poured concrete wall. Inside of the concrete wall is another layer of 4″ EPS foam, with the polyethylene barrier to the inside. The final layer is the same 2×4 utility wall mentioned above, covered by drywall. This total assembly measures R-40.

Underneath the basement concrete slab is 4″ inches of rigid foam, giving R-16. A couple other finishing details really help to reduce the basement heating loads. First, the basement concrete slab is actually encased on 3 sides by foam, coming down from the walls, and then with the foam underneath – it essentially looks like a foam ‘bathtub’. This significantly reduces thermal bridging, making the diffusion of heat into the ground or concrete walls much slower.

All the windows in the house have fiberglass frames, are triple paned with argon fill, and have an insulating value of roughly R-5. The windows facing south are all chosen to allow in more of the sun’s heat (called solar heat gain coefficient, SHGC), whereas windows facing all other directions have a low SHGC. The difference between them is that the high gain windows allow in approximately 50% of the sun’s heat, whereas the low SHGC windows let in only about 30%. The interesting thing about passive solar design is that much of the heating load is carried by the heat coming in the windows, with energy modeling suggesting that about 30% of the house’s total heat needs are being provided by the sun shining through the windows.

The final thing that always needs to be mentioned when discussing a building envelope is air-tightness. There used to be ideas floating around that a house needed to ‘breathe’, but these ideas have long since been discredited, and high quality builders now look to build houses as tightly as possible, then actively bring in fresh air with a device like a heat recovery ventilator (HRV). Airtightness is meaured in air changes per hour (ACH), which calculates the total volume of air inside a building, and how many times in one hour that this much air would be replaced when a home is slightly depressurized with a big fan. Old leaky homes may have a number like 10 ACH (or even higher), meaning that the air inside is replaced by outside air about 10 times each hour when depressurized. Passive house rules require .6 ACH. Our house, with the polyethylene barrier mentioned multiple times above, measured at 1.47 when the house was half done, the outer 2×8 wall in place with the air barrier. The house is probably much tighter than that now that the utility wall, sheetrock, paint, and caulking are all in place, but we never quite got around to getting a second measurement after the house was complete.

Another way to get a feeling for how this house is put together would be to watch the time lapse video below of the day a crane was used to put up the walls and set the ceiling trusses in place (video created by our builder Stephane Charette of Bala Structures – he is the one singing as well).

Other building assemblies
I must say that there is a lot of interesting work going on by high-efficiency builders trying out many other ways to build very high efficiency homes that are simpler and cheaper. Even now, less than 2 years after our house was
completed, there are already some things that we could change to
simplify and make cheaper the house that we built. A lot of this sort of work shows up at the Green Building Advisor website, which I have become a frequent reader of. One other example comes from Ecohome, where our LEED certifier Mike Reynolds works. Their recent Edelweiss House is significantly more efficient than our home, and was also cheaper to build, as they did it with only a standard 2×6 wall and lots of exterior insulation. Needless to say, though the house that we have built is a very good one, there are many good ways to put together a high performance building envelope.

Building site and orientation

First published 10/29/15

Forest surrounds the home on all sides, with amazing views of a small valley to the south, and a back bay of the river visible about 150′ to the west of the house. We chose a hilltop location, in large part because of the views it afforded. The other advantages of this location are excellent southern exposure for both passive and active solar, as well as it being much less buggy than the surrounding lowland areas, due to it being drier and windier. That wind is a double-edged sword, however, as we are much more exposed to the whipping winds of storms and winter, which probably increase our heating loads a bit and have also put me in place to watch trees topple over and land within 20′ of the house during a particularly vicious summer storm.

The clearing that we opened was just large enough to build the house and yard, a bit less than half of an acre. On the north, west and south sides of the house, this means that the forest begins only 20′ to 30′ from the house. To the southeast is a grassy yard big enough to cover the septic field, and to the east is a parking area with our solar panels tucked onto the north side of the clearing (to reduce the shading from the trees). Unlike most of the homes that are built along waterfront, we did not clear sight lines towards the water. We have a screened view of the water, which actually makes those glimpses out onto the bay more special.

Trees were kept strategically so that the house would be well shaded during the summer season, both from the south, but most especially against the hot afternoon sun out of the west. The best way to accomplish this is with deciduous trees, red oak and sugar maple in our case, so that all through the leafy summer season the house stays as deeply in shade as possible, while in the winter the lack of leaves allows the sunlight to stream right in. The existing larger trees at our site helped to determine where we would set the home so as to best take advantage of that shading.

The rule of thumb that I have read and been told multiple places is that one ought to be within 15 degrees of south to take advantage of passive solar heating, but with our site, we were able to square our house perfectly to the sun. This means that at solar noon the sun shines straight into the south windows of the house. This perfect east/west orientation gives a few unexpected benefits. One is that our entire house acts as a sundial; one can simply look at the angle of the shadows cast on the ground as the sun streams in the windows to know the time. Also, on the spring and fall equinox, we get the sunrise and sunset shining all the way across the home through the windows on the east and west sides. I noted this spring that in the downstairs bathroom (at the northeast end of the house), sunsets around the equinox are really the only time that the room gets direct sunlight at all.

Passive solar design principles suggest that a longer east-west axis is used, with windows focused as much as possible to the south side. This allows for the south wall and windows to soak up the winter sun, while minimizing the east and west faces which can heat up excessively in the summer. We doubled down on this logic by making our home a rectangle elongated east to west, with a much taller south face full of windows. At the same time, the north side was banked into a hillside with only a few small windows, as north windows are always in the shade and constantly lose heat through the winter. We broke only one rule of passive solar design by putting a lot of west facing windows, but this was a worthwhile trade-off to capture the views of the river, and we do have trees that provide good summer shade to the west.

With all of the big windows, especially in the upstairs, it certainly has the effect of bringing the outside into our home. The views change with the seasons, with veiled views during the summer and fall due to all the leaves on the trees, with much clearer lines of sight in the winter. This allows us to really soak in the valley and hill to the south, and the river to the west.

In figuring out details about the house, its orientation, and which trees to keep or cut, I spent a lot of time looking at the solar chart shown below. It packs an enormous amount of information into one graph, showing the length of the day, angle of the sun above the horizon and simultaneously its location in the daily east to west motion, and does all of this for every day of the year. With a bit of measurement and calculation, I could imagine how the sun would play across the house throughout the day and across the seasons, and plan for windows, shading, and solar panels.

Accessed from http://www.gaisma.com/en/location/ottawa.html
See their site for a longer explanation of how to read the graph

All in all, we have been very pleased with the siting of our home. I have recently read that there is some doubt as to whether applying passive solar principles is worthwhile in the age of superinsulated homes (see here), but I still wouldn’t change a thing. Even though we did a lot to maximize our solar gains, we at the same time were able to meet all of our aesthetic goals, both inside and out.

 



1 South as measured by the sun is a bit different from magnetic south, and solar noon a bit different from noon on a clock. Solar south is the ‘true’ south, and corresponds to the point where the sun is exactly half-way across its trajectory across the sky and at its highest point. Solar noon is the time at which the sun reaches that point. At our house, this works out to be at about 12:05 pm local time in the winter, and 1:05 pm in the summer. Your match of solar noon to the clock will depend on where you are located within your time zone. 

As a further aside to this footnote, I also need to mention that I had always thought that the ‘real’ time with daylight savings time was during the summer, probably because I enjoyed the brighter and longer afternoons. But actually, it is the winter clock setting where noon on the clock closely matches with the actual middle of the day as based on the movement of the sun.

Inspiration for our home

First published 9/17/15

Building a highly efficient ‘green’ home is something that I thought about for many years before it actually happened. I never formally studied architecture or building science, but I dabbled in researching the topic for a decade. I remember that I was absolutely inspired when I first came across some of the designs for highly efficient homes from the 70’s, especially some of the passive solar designs of that time. There were terms like Trombe walls, usage of large water tanks for thermal mass, ‘earthships’ with greenhouses inside the home, and more. There was a great deal of experimentation going on in building innovative and green homes for the future, with the hope of drastically reducing the amount of energy that it takes to both build and run a home. This experimentation really was necessary, because as I read further, I came across the critiques of all of the things that didn’t work, causing things such as mold and massive overheating in the summer. While there were a lot of interesting ideas here, clearly I was going to need further inspiration elsewhere. And I did go on to find further work on passive solar design done much more recently, that has distilled out some of the best design principles to take advantage of that free energy source, the sun.

More recently, I came across Passive House, another green building design philosophy that focused almost exclusively on reducing the amount of energy used in a building (Passivhaus in its original German). By focusing on energy reduction, the building envelope becomes the prime target. Massive amounts of insulation, compact shapes with a minimum of surface area, triple-paned windows, high airtightness, these are the things that allow heating (and air conditioning) loads to go way down, and as I read in multiple places, a house that can be heated by only a hair dryer. As for electrical loads, there are now efficient appliances and mechanical systems that, in conjunction with a well built shell, bring a certified passive house down to as little as 10% of the energy use of a typical home.

The third major strand that we needed to bring together for our project was renewable energy, so that we could build a home that was off of the grid. Unfortunately (or perhaps fortunately depending on your perspective), the property that we fell in love with was very far off the beaten track, so far off that it would have been prohibitively expensive to bring in power lines. It was both going to be cheaper, and much more interesting for me, to build a home that was completely off of the grid. Today is a very exciting time for renewable energy, with solar panels dropping precipitously in price, new types of batteries just becoming available that are more powerful and reliable as well as less expensive than those that came before. I am not alone in thinking that renewable energy is the future, and it is quite a ride to see that future arriving and to be a part of it.

Finally, there was the architectural style to consider. It is possible to build an efficient home in any style that allows for a relatively compact building shape, and I was drawn in particular to some of the contemporary styles. I have seen a certain style of home described in some places as “contemporary mountain” that have stylistic elements that we drew from, including a single pitched shed roof, deep overhangs, use of lots of larger dimension wood, and a close alignment to natural surroundings. My impression is that this style is currently most popular in the Pacific Northwest. I’d say that the single home that provided the greatest inspiration for style came from Nils Finne of Finne Architects, and a home that he built on the shore of Lake Superior.

An introduction to the problem of living sustainably

When thinking about solving the problems of sustainability, or any other complex global issue for that matter, it is easy to feel overwhelmed, even helpless. The problems are so large that one wonders whether one person can even have an impact. Don’t despair, there is much that each of us can do. I recommend that you focus on things that you are passionate about, those that you can stick with over time, and those that can make the biggest impact. Don’t tie yourself up in knots of guilt, or make changes to your life that are going to make you miserable, as that isn’t going to be productive. What we really need to do is to rally the support of whole societies, and one of the ways of doing that is to show naysayers that with sustainability you can ‘have your cake and eat it too’. This doesn’t mean that we can all live in mansions and drive massive gas guzzling cars, but we could all have homes that are wonderful to live in with readily available transport to get everywhere we need to go. We also need to accept that moving humanity to a more sustainable trajectory takes time, with the results taking years or even decades. I personally am putting together a 15 year sustainability plan for my family (to be linked once written up more fully).

Just as we must admit that it will be a long road, we are also all at different places upon that path. Someone who is just thinking about sustainability for the first time might be able to dramatically reduce their personal footprint by making those changes that constitute the ‘low hanging fruit’. For someone who has already taken many steps to reduce their own impact, their goal may instead be to convince others to improve their own practices, be it friends and family, or the businesses and government that provide us with our goods and services. People also have different means to act. If you are a renter who works long hours just to make ends meet, it may be harder to make major changes to your behavior than for someone with more time and resources at their disposal. The important thing is that each of us who cares about sustainability and the future of our world acts, and does what they can.

The details to follow about the scope of what must be done are daunting, so I want to mention just a few promising trends. Though we are currently using too much land and releasing too many greenhouse gases, there are technologies coming available that will help to solve many of the problems that earlier technologies have caused. For instance, in the realm of energy, wind and solar are now the cheapest form of energy generation in some places, and both are growing exponentially while starting to displace fossil fuel use. New agricultural technology, such as ‘precision farming’, increases yields while reducing inputs and pollution. Technology can and will do some of the heavy lifting for us, but we still need a culture that will adopt the best of technologies and practices as quickly as possible.

Where are we now? Where do we need to get to?

To understand the basic numbers of sustainability, it helps to describe them at the level of the individual – you, or any person living a modern lifestyle in a rich country. The easiest way to do this is to start with the total amounts of emissions, energy and land use, and then divide that by the number of people (I’ve done a version of this for my own family’s energy use here). This is then the average amount that is used on behalf of each person in a society. Roughly one third of that energy is personal consumption, from building and heating our homes, to driving our cars, to our food, clothes, and electronics. Another third is each person’s portion of the energy used by businesses and organizations that provide us with goods and services – a part of the energy to keep the lights on at your hospital or power a factory is being used on your behalf. Finally, everything that governments do is (at least in theory) on behalf of its citizens, so of all of the energy used to maintain roads or armies or the IRS, a chunk of that is for each and every one of us. Once we know what we are using, we can then compare those numbers with the estimates that ecologists and other scientists can give us about what sorts of levels are actually sustainable. The gap between the status quo and the sustainable level shows us the work we need to do. There are three things that I want you to consider, total energy use, greenhouse gas emissions, and land use (we’ll leave aside other resources such as water for the time-being).

Total energy use isn’t actually something that we need to worry about for its own sake. If we had infinite clean energy, every person could use as much as they want. However, we don’t live in this magical world, and there are greenhouse gas, pollution, and land use costs to all the energy that we use. Tracking energy use is relatively straightforward to do and is highly correlated to greenhouse gases and land use, there are also good records for energy use. In the US, the total consumption of energy per capita is about 230 kilowatt hours (kWh) per day. To put that in perspective, the typical house consumes about 30 kWh a day. Using energy much more wisely and efficiently could allow us, over time, to reduce this total by a factor of 3 or 4 times, down to perhaps 60 kWh per person per day. For a very in-depth dive into energy use both at a personal and national level, see this very informative video by Saul Griffith.

Greenhouse gas production is tightly linked to total energy use, especially considering how much of our energy currently comes from fossil fuels. In 2017, the American per capita production of CO2e (carbon dioxide equivalents) is about 16 tons. The overall global average is 4 tons. The 2015 Paris Climate Accord, agreed upon by virtually every nation in the world, seeks to limit global warming to no more than 2 degrees Celsius. To accomplish this requires that we reduce global per capita emissions down to less than 2 tons CO2e per person. This means that we need to figure out how to reduce our emissions in rich countries down to 1/8, or 12%, of their current level. There is an enormous amount of work to do here. The single most comprehensive examination that I’ve seen of how the world could do this is through a Project Drawdown, which outlines all of the things that could bring greenhouse gas levels down to sustainable levels.

In terms of land use, we need to have space for ourselves and to grow our agricultural and timber products, while at the same time leaving room for all of the non-human species that we share the planet with. With the human population closing in on 8 billion, there are only 5 acres per person of total land area. Humanity has now pushed into just about every nook and cranny of the planet, so we need to be good stewards. Of all that land, about 1/3 is uninhabitable desert, mountain and glacier, 1/3 is agricultural, 1/4 is forest, leaving 1/10 for everything else. Urban areas use about 1/100 of all land. Humanity is already using almost all of the prime territory for agriculture, and there is very little frontier left to grow into, especially since we want to preserve what natural spaces we have left. On top of that the world’s population is still growing, expected to reach 10 billion or more by the end of the century. Put all together, we need to reduce our impacts so that we can provide for the needs of each person on less than 2 acres of land, an area the size of two football fields. This area needs to provide all of each person’s food, as well as many of the other products that they use, wood, paper, leather, cotton, and so on. Optimally we should be cutting in half the amount of land that we are using to provide for each person’s needs.