Tag Archives: Chapter 4 – Design

Passive Solar Design

This is the second article in a series originally written for New Life Journal.

By: Clarke Snell

Let’s not beat around the bush. In this day and age, heating and cooling our houses amounts to spending a lot of money to create a lot of pollution. That’s because most of the energy we use for this purpose comes from burning fossil fuels. What’s worse, as a society our response to skyrocketing oil and gas prices has been to keep making the skies dirtier. The weird thing about this whole scenario is that everything we’re burning is just stored solar energy.

Here’s the process: Plants turn sunlight into energy which is turned into living tissue. Animals eat the plants. Plants and animals die. Wait several hundred million years. Drill deep wells and dig big holes to access resultant oil, gas, and coal. Transport all over the planet and burn copiously until supply begins to get scarce. Fight wars and panic until lights go out and heat goes off.

I don’t know, wouldn’t it make better business sense to skip the “middle man” and go directly to the source, i.e. the sun? Duh. The technique is called passive solar design: the conscious manipulation of the sun’s direct energy to affect the temperature inside a building. It is clean burning, runs for free after installation, has no moving parts, comes with a lifetime guarantee, isn’t susceptible to power outages or unexpected supply shortages, requires no special maintenance, and can be accomplished by simply rearranging the materials used in a conventional modern house at little or no extra expense.

Though its most effective real world implementation is a beautiful dance between science and art, the concept behind passive solar design is elegantly simple: if you want heat, let the sun in; if you want cool, don’t let the sun in.

Our loving star has made the process so much easier by methodically changing its path through the sky throughout the year. In our region, the winter sun rises to the southeast, stays low in the sky to the south, and sets to the southwest. The summer sun rises to northeast, stays high in the sky most of the day, and sets to the northwest. This is an amazing stroke of luck because it means the sun is low in the sky when it’s cold outside and high in the sky when it’s hot outside. Low sun is easy to let into a building, while high sun tends to be blocked by the roof and other protrusions of the building itself. Perfect!

With this basic observation under our belts, we’re ready to realize a passive solar masterpiece. First, we need to find the right place to build. In our region, that means a site that will give us unobstructed access to the low southern winter sun. Some trees or other obstructions to the east and especially the west would be great to block the hot rising and setting summer sun. (A ridge or evergreens to the north might block some winter winds, but wind is very site specific so we’d have to spend some time on site to make that call.)

Next, we’ll design our building to let in a lot of winter sun and block a lot of summer sun. Building shape is the most basic parameter. In our area, the best shape is longer on the east-west axis, creating more wall surface on the south and less on the east and west.

The main avenue for sun to enter the building will be through glass. From a heating point of view, only south-facing glass will create a net solar heat gain, so other glass should be minimized. However, north, east, and west glass are an important part of our natural ventilation cooling and daylighting strategies. This is where the delicate interplay of science and art comes in, in other words we’ll find beautiful compromises.

The heating equation, in any case, is straightforward, we simply have to carefully match the square footage of our southern glass windows and doors to the amount of “thermal mass” we place in the building. Thermal mass simply means something that stores heat, so technically everything is a thermal mass. Dense heavy materials usually store heat well. Water, concrete, stone, and earth are good examples. A great place to put mass in a building is in a concrete or earthen floor. Sun flows in through glass covered openings and is stored in the mass of the floor. The mass sucks up heat, thus preventing the house from overheating during the day, then slowly releases the heat after the sun goes down keeping the house warm at night. The trick is creating the right balance. Science to the rescue! We have everything from rule of thumb glass to mass ratios to computer assisted thermal modeling at our disposal.

Next, we’ll need to design our roof overhangs and other protuberances so that they follow our mantra: block sun when it’s hot, let in sun when its cold. The poster child for this is the southern trellis covered with deciduous vines (grapes and hops are two options for you vintners and brewers out there). Thick leaf cover that blocks the sun in spring and summer dies back in fall and winter to let the sun through. Since we know where the cooperative sun will be in the sky at any time of year, roof and window overhangs can be sized to interact with the sun exactly as we like.  We’ll add covered patios on the east and west, again to block low hot sun, and one on the north to create an outdoor room that will be shaded all summer long.

Finally, we’ll work with the surrounding landscape to heighten our design. In tandem with our patios, we’ll add shade trees, especially to the west and north. Plants not only create shade, but evaporative cooling which is the natural technology mimicked by your refrigerator and clanking, polluting window A/C or HVAC unit. We’ll also create a focus to the south, perhaps placing an outdoor kitchen under the trellis with a kitchen garden in front of it. We’ll place doors and windows that encourage cross-ventilation and allow effortless transitions to outdoor rooms. Don’t forget that in our climate a little tweaking back and forth between sun and shade makes the outside comfortable for most of the year. Outdoor rooms are inexpensive access to the mansion of nature. Of course, we’ll also design a unified insulation strategy that includes measures to slow convective, conductive, and radiant heat loss through the building, but that’s another story.

Ta-da! A passive solar masterpiece that will supply a baseline of heat and cool at the right time of year which can then be enhanced to create the specific indoor environment of your choosing. Though you may not get the picture from this frantic overview, none of these design features need to control the look or feel of the building. Passive solar is flexible if you are. It’s a pivotal design concept, not an architectural gestalt.

Disagreement abounds even on some of the basics. For example, some people feel that our climate is too wet to allow for natural ventilation as a cooling strategy because open windows plus humidity can result in mold. In the end (here’s where you refer back to that lovingly pawed copy of my column from last month that’s taped to the fridge), the right approach to passive solar is going to have to match the specifics of who you are with the specifics of the place your house will sit.

I will however be unequivocal about one thing: you are going to heat and cool your house with solar energy one way or another. The only question is if you want it free and clean or expensive and dirty. This may sound like a laughably obvious choice, but a cursory glance at any cityscape or subdivision will show that the sun is presently laughing at us, not with us.


Employee lounge at the Sleep Inn in the beautiful strip mall section of Urbana

Jeff and I are at the Passive House training. Things are going well. Cool geeks, incredibly stringent performance standard. Today, we were looking at an assembly that included an R-40 wall (12 inch TJI studs with cellulose, OSB on both sides, then another insulated 2×4 stud wall), R-75 subslab insulation, and R-10 perimeter insulation. In this configuration, an uninsulated bottom plate created a thermal bridge that wasted 8% of the available heating load for the entire building under the Passive House standard. Yikes!

Optiwin makes the nicest windows I've seen. This is a window at a house we toured today. You are looking at a great sill flashing pan. Incredibly sturdy and designed with a counterflashing corner that goes up under the trim. This will be covered with a brown aluminum flashing piece to match the window. All of this can be yours for $100/sf.

Anyway, it’s not all hard work. For fun Jeff is testing the pH of Coca-Cola in the hotel. (That’s pH… not PH for Passive House. Get with the program!) According to Jeff this was necessitated because they have been planning on stepping up to kegs for Kombucha. The keg guy thought the acid levels in Kombucha would corrode the kegs. They’re made for Coke, so if that shit has a similar pH to Buchi, then the kegs will survive.

Internal Nauhaus Institute memo: By the way, Scobie didn’t make the label. They’re going all high brow looking for she-she crowd or somethin’. Scobie Lives!!

Anyway, here are the Coke study results:

High tech Coke testing contraption
Coke pH leveled out after the demons were excorcised
Coke pH leveled out after the demons were excorcised
Just another mad building scientist
Just another mad building scientist

— Clarke

Passive House Criteria

Certification Requirements:

  • Max space heating and cooling energy < 1.4 kWh/sf/year (4777 btu/sf/year)
  • Max primary energy usage < 18.6 kWh/sf/year (63,500 btu/sf/year)
  • Air tightness < 0.6 air changes per hour @ 50 Pa

The energy calculation uses these values:

  • Indoor temperature: 68 deg F
  • Internal heat gains (from lights, people, cooking, etc.): 0.7 Btu/hr/sf
  • Occupancy: 377 sf/person (other values between 215 and 538 sf/person may be used with an explanation)
  • Domestic hot water use: 6.6 gal/person/day
  • Domestic hot water temperature: 140 deg F (120 is standard in the US)
  • Domesitic cold water temperature: 50 deg F
  • Average air flow rate: 12-18 CFM/person

Passivhaus certified windows must meet these standards:

  • U value < 0.14 (R-7)
  • SHGC > 0.5

This presentation is a good overview: