Passive Solar Design

Basic Principles

That the sun provides heat is hardly a new idea, and structures since antiquity have often faced south to use the sun's energy. The idea of passive-solar design took off in the 1970s following the "oil shocks" that drove up the price of fossil fuels. Passive-solar design where everything just sits there is to be contrasted with active-solar design where pumps, fans, and other mechanical devices move heat around.

Early passive-solar houses were enamored of the idea of free heat and focused on proper siting, but they often gave little thought to insulation, windows, or other issues and as a result did not perform all that well. For us, passive solar means not only designing the house to utilize the sun's energy but also being extremely well insulated and airtight, using the proper windows, and using solar panels to generate most of our electricity. It's a package deal where everything has to work together.

Passive solar design
The basic principles of passive-solar design are well known.
  • Most windows are on the south side.
  • In winter, with the sun low in the southern sky, solar energy floods through the windows and is absorbed by the floor and walls.
  • Extra mass in the floor and walls – thermal mass – holds that heat and releases it overnight.
  • Wide eaves and overhangs keep the house well shaded in summer when the sun is high in the sky.
But as with many things, the devil is in the details.
Siting

In theory, a passive-solar house should face due south. In practice, constraints of our building site didn't allow that, and the house faces about 15° east of south. This turns out to be blessing. It takes several hours, until early afternoon, for the house to warm up from solar heating. By facing a little to the east, we capture more morning sun and warm up a little faster than if we had faced due south. We pay a very slight penalty in terms of the total solar energy captured, but it's worth it in terms of comfort. We didn't plan this or anticipate it, but it's worthy of consideration in designing other passive-solar houses.

Heating and Cooling without an HVAC System: Lessons Learned

Passive-solar construction does involve some extra costs, but savings elsewhere can more than make up the difference. In particular, we have no HVAC system – meaning heating, ventilation, and air conditioning. We have no furnace or air conditioner in a climate where the yearly temperature swings between 25°F and 110°F. That also means there's no ductwork threading through the house. How the house performs is discussed on the Thermal Performance page, but the simple answer is "very well."

We do have two sources of supplemental heat for cold rainy periods in the winter when we get no solar heating. Downstairs we have a natural gas fireplace, and upstairs there are two electric baseboard heaters. Neither provides instant heat – nothing in a passive solar house happens quickly – but they do the job nicely. In hindsight, the natural gas fireplace was perhaps not a good choice. It uses quite a bit more energy than we had anticipated – see the Energy Use page for details. At a minimum we should have looked for a gas fireplace with a higher efficiency. In addition, heat distribution from the fireplace is very uneven through the large living/dining/kitchen area even with a ceiling fan to move the warm air around. It's too warm near the fireplace and heat gets trapped up near the vaulted ceiling while the kitchen is still chilly. Rather than the fireplace, adding a couple more solar panels and using two or three electric baseboard heaters downstairs would have provided more efficient and effective heating and had us very close to net-zero energy use. The fireplace gives a cozy feel, but it's not a great source of supplemental heating.

Speaking of electric heat, conventional wisdom is that natural gas heat is environmentally superior. That's true if the electricity is coming from a fossil-fuel-burning power plant. The power plant is only about 35% efficient at converting fuel energy into electric energy, so 65% of the fuel's energy has been wasted before the electricity gets to your electric heater. With natural gas, all of the fuel's energy reaches your house, and a modern furnace is about 90% efficient at turning that energy into heat inside your house while only 10% is wasted in the exhaust gases going up the vent. Natural gas is the winner by a large margin in this comparison. But the situation is entirely changed if you generate your own electricity with solar panels. Now there are no waste-heat losses from burning fossil fuels, and electric heaters are 100% efficient at turning electric energy into heat. So electric heat becomes the environmentally superior alternative. Similarly, an electric clothes drier, which we have, becomes superior to a gas clothes drier. The moral of this tale is to size the solar system appropriately and use electricity for everything.

We do have one other source of supplemental heat that – in hindsight – was clearly a mistake: We installed a radiant floor heating downstairs in the master bedroom and in the library. The floor is heated by circulating hot water from an on-demand gas water heater through tubes in the concrete floor. The large thermal mass of the concrete floor means the response time is extremely slow – 10 to 12 hours to see much effect. We installed this, at considerable expense, out of concerns that passive solar heating and super-good insulation still wouldn't be sufficient in the winter. We were wrong. We've never used the radiant heating in the library. We've used it in the bedroom maybe half a dozen times in 12 years, but this is another situation where a small electric baseboard heater in the bedroom would have worked much better. Trust that the sun will get the job done most of the time, and supplemental electric heat with solar panels is the cheapest and most effective way to have a backup plan.

Summer cooling is more challenging than winter heating because there are no sources of supplemental cooling. It is what it is. Our cooling is to open the house at night when the temperature outside temperature drops below the inside temperature. Because of the very low humidity where we live, the temperature usually drops to around 60°F even if the daytime high had been 100°F. In addition, we have two upstairs skylights that can be opened. Since hot air rises, hot air venting out of the skylights tends to gradually pull cooler air in through the open downstairs windows. This mostly works very well – see the Thermal Performance page for more details. Ceiling fans and a few other smaller fans are important in the summer to have some air circulation during the afternoon and early evening. With low humidity and air circulation, indoor temperatures in the 75-78°F range are very comfortable.

Although this works where we live, air conditioning is likely necessary for a passive-solar house in a location where it doesn't cool off at night or where the humidity is high.

Another mistake we made, although much less costly than the radiant floor heating, was installing a whole house fan. This is a fan mounted in the upstairs ceiling that blows air up into the attic space and, in the process, pulls in air through downstairs windows. We thought this would help with cooling the house overnight, but it doesn't. For one thing, we may have undersized it. For another, our attic vents are not the usual under-eave vents but, for fire reasons, vents between the concrete roof tiles. I'm suspicious that the possible flow rate through the vents is not adequate to handle the fan. But more important is simply that the house cools off just fine without a whole house fan on nights when the outdoor temperature drops. And a fan drawing in warm air would be of no benefit on the few nights a year that it doesn't cool off.

Another heating/cooling option: Anyone building a passive-solar house in a climate where they are going to need air conditioning should look into a geothermal heat pump (also called a ground source heat pump). A heat pump provides both supplemental winter heat and summer air conditioning. They are expensive, but they are also extremely energy efficient and could be powered with a small expansion of a solar panel system. Definitely worthy of consideration.

Spring and Fall

Illustrations of passive-solar design always focus on getting solar energy in during the winter and keeping it out during the summer. They never seem to mention spring and fall. The sun has dropped low enough in the sky that we start to get some initial solar heating in about mid-September. We're still dealing with 95-100°F days in mid-September! It usually stays pretty warm right on through October. On the flip side, the sun has moved high enough by mid-March that we've pretty much lost solar heating. Yet the overnight temperature can drop into the 30s well on into April.

Spring is easier to deal with – it simply requires an increased use of the supplemental heat in the mornings. You can see on the Energy Use page that our natural gas use for the fireplace is skewed toward spring. But with no supplemental cooling, just overnight ventilation, the house can get somewhat overly warm in the fall. We do have shades on the upstairs windows, and anyone planning a passive-solar house should keep that in mind, but the large downstairs windows that were sized for mid-winter solar heating don't accommodate shades or blinds.

Lighting

Proper lighting can minimize energy use and, at the same time, greatly improve the comfort and efficiency of being in the space. Most people work and function best with natural lighting, and the large south-facing windows of a passive-solar house provide huge quantities of natural light. Most passive-solar houses have their long axis east-west, so by avoiding unnecessary doors you can allow light from the south windows to fill the house. Two other features add to our natural daylighting:

  • Two skylights upstairs and two in the downstairs great room.
  • Six tubular skylights (ours are made by Solatube) that funnel light from the roof through a reflecting tube to translucent panels in the ceiling. Downstairs we have one in the laundry room, one in our only hall, and one in the guest bathroom on the north side of the house. We also have one in the stairwell, one in the upstairs bathroom, and one in Sally's studio. These bring an amazing amount of light into what would otherwise be a fairly dark space. We cannot recommend these too highly.
As a result, we simply don't use or need electric lights during the day. This saves energy and it makes the living spaces very pleasant.

Nearly all of our interior lighting was initially compact fluorescent lighting and a few incandescent spotlights. That has now nearly all been changed over to LED lighting. Any new construction should now be using all LED lighting – for general purpose, for spotlights, and for reading lights. The light quality is excellent, the energy use is a tiny fraction of what incandescent lights used, and the reliability is better than compact fluorescents. I think we've had only one LED bulb fail.

The "white" light from incandescent lights is concentrated at the red-yellow end of the visible spectrum. That's simply a function of the filament temperature. You're not aware of this – your brain compensates – unless you have a real white light source to compare it to. "Warm white" fluorescent lights are intended to produce a similar yellowish light that we perceive as warm. In contrast, an LED light can balance the red, green, and blue parts of the spectrum to produce light that ranges from red enhanced to "natural daylight" to blue enhanced. This is called the "color temperature" of the light. An incandescent bulb filament has a temperature of about 2700 K (kelvins, the metric unit of absolute temperature), and LED bulbs designed to mimic this are labeled as having a color temperature of 2700 K.

But why settle for a yellowish cast to the light now that we have options? A 2700 K or 2900 K LED works well for general lighting in a bedroom or family room, but most people find they prefer a color temperature of 4000 K to 5000 K for kitchens, offices, work areas, and reading. This comes much closer to matching natural daylight, and it makes the colors of things in your house seem more natural. A true "daylight" color temperature of 5000 K or 5500 K (that's roughly the temperature of the sun) works well for some people, but others find that it starts to seem too bluish and harsh. Experiment! Don't keep buying "warm white" simply because that's what you've always had. A color temperature of around 4000 K seems to kind of a sweet spot for many uses.

Windows

Windows are the bane of passive-solar design. You need large windows on the south side to capture the sun's energy. At the same time, windows are the primary source of heat loss from a well insulated house. See the windows page for everything you ever wanted to know about the thermal performance of windows.

© 2021 Randy Knight