Category Archives: Passive Solar

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.

Thermal Mass

In our climate, even on the hottest days of summer, the outdoor nighttime temperature drops below the indoor temperature. Using massive materials inside the insulated envelope, we can take advantage of that diurnal temperature swing to reduce the amplitude of the indoor temperature swings. The mass absorbs heat during the day and radiates it back at night.  If we do a good job of keeping that mass shaded during the summer there’s no need for mechanical cooling and the dog has a nice cool floor to lie on.

Thermal Inertia
Talking about thermal mass in more detail gets a little more complicated. I’m afraid we’re going to need a few definitions:

  • Heat capacity is the ability of a material to absorb heat.
  • Diffusivity is a measure of the speed heat moves thru a material.
  • Effusivity describes the ability of a material to exchange heat with it’s surroundings. It is similar to emissivity (as in low-e or low emissivity windows).

Good materials for thermal storage have high thermal inertia.  They have a high heat capacity, but low diffusivity and effusivity.  Metals don’t work well for thermal storage.  They can take on a lot of heat because they have a high heat capacity, but they can’t store it very long because they also have high diffusivity and effusivity.  Metal heats up quickly, but it gives it right back. Clay is much better for storing heat in a timeframe that’s useful for conditioning houses.  It has a high heat capacity, but low diffusivity and effusivity. It’ll take all day to heat up a cold earthen floor sitting in a room with a warm air temperature, but it will take all night for it to radiate that heat back. That’s what we’re looking for.


Even when the two items are identical in temperature, the metal feels colder. Why? Wood is not a good conductor of heat, so it is slow to absorb the heat from your hand. Metal has higher thermal effusivity, so the heat from your hand flows into the metal quickly – creating the sensation of it feeling cold.

Mathis Instruments

In our climate massive construction is awesome in the summer. The downside is that dense materials like tile, concrete, and compressed earth block also feel cool to the touch during the winter. That’s why European stone castle walls are covered with tapestries.

Mean Radiant Temperature
To derive your Mean Radiant Temperature, look around you and take the temperature of every surface you see. You are exchanging heat with all of those surfaces. Surfaces warmer than you radiate heat to you and all the other colder surfaces. You’re just another room surface exchanging heat with all the others. To be comfortable all the surfaces around you need to be within a few degrees of each other (and you), and in a well insulated house with good windows they will be. However, believe it or not, our skin does not have good temperature sensors. Instead, we have excellent heat flux sensors. All of the surfaces in a room can be exactly the same temperature, and some will still feel colder than others when we touch them. The surfaces that feel colder are the ones with higher effusivity. The castle tapestries have low effusivity so they feel warmer than high effusivity stone.

Radiant Heating
In a typical (minimally insulated and drafty) house, radiant floor heating feels great because the mass is heated up to about 80 deg F. The floor radiates heat up to other surfaces, and brings the mean radiant temperature up so we’re nice and toasty. The problems are:

  1. radiating 80 deg F from the entire floor is a lot of heat.  A house that needs that much heat is wasting a lot of energy, and it should be insulated better.
  2. you lose a lot of ability for a slab to absorb free heat coming in the windows from the sun if the mass has already been heated by radiant tubing.

In an efficient well sealed house, a conventional concrete radiant floor heating slab won’t have to rise above about 73 deg F to meet the heating load (assuming the entire floor is heated). You will wonder if the heat is really on because it won’t feel warm. Even though the floor slab is adding heat to the house and the mean radiant temperature is high enough that we aren’t radiating much heat to the other surfaces, concrete has a relatively high effusivity. It exchanges heat with us pretty easily and feels cool even with a slight temperature difference. In a passive solar house, high effusivity materials located in areas that get direct solar gain will feel tactically warm on sunny days, but those same materials in northern rooms without solar exposure or in southern rooms on cloudy days won’t.

If you use radiant heat, insulate the house well enough that a small area of radiant will heat the entire house. Locate it in northern rooms (especially bathrooms) that can’t be heated by the sun.

Concentrate high heat capacity materials in the south rooms where they will do the most good. Use low-medium effusivity materials to store heat. Assuming no radiant heat, a north bath or kitchen would be better off with low effusivity wood or cork floors and wood countertops, but the same room located on the south would benefit from medium effusivity concrete countertops and tile floors.  Likewise, soft earthen plasters will feel warmer than hard venetian lime plasters, and soft lime and gypsum plasters will feel warmer than harder cement based plasters.

This spreadsheet shows the effusivity and interface temperature (how warm the surface feels when you touch it) for a few typical materials. I assumed an 85 degree hand surface temperature and all other surfaces at 70 degrees, but you can go to the spreadsheet and change those values as you see fit: Google Docs | Thermal Effusivity.

Furnace Free in Vermont

In 1998 Marc Rosenbaum was working on a 22 unit cohousing development in Harland, VT. Amory Lovins told the client that she should build a passive solar house without any backup heat. Marc didn’t buy it, and they went back and forth discussing how it could be done. Then Marc published their correspondence at It’s definitely worth reading.

:: The Furnace-free House in Vermont

Low Mass Sunspace

A low mass sunspace is meant to serve as a heater, not a greenhouse for plants or a comfortable place for humans.

William A. Shurcliff:

It is hard to think of any other system that supplies so much heat at such low cost…

One could shorten the warm-up time of the enclosure and increase
the amount of heat delivered to the rooms by making the enclosure
virtually massless–by greatly reducing its dynamic thermal capacity.

This can be done by spreading a 2-inch-thick layer of lightweight
insulation on the floor and north wall of the enclosure and then
installing a thin black sheet over the insulation. Then, practically
no heat is delivered to the massive components of floor or wall;
practically all of the heat is promptly transferred to the air.

And since the thermal capacity of the 100 or 200 lb. of air in
the room is equal to that of one fourth as great a mass of water
(about 25 to 50 lb. of water), the air will heat up very rapidly.
I estimate that its temperature will rise about 40 F. degrees in
about two minutes, after the sun comes out from behind a heavy cloud cover.

At the end of the day, little heat will be “left on base” in the
collector floor or north wall and, accordingly, the enclosure will
cool off very rapidly.

New Inventions in Low Cost Solar Heating–
100 Daring Schemes Tried and Untried
Brick House Publishing, 1979

This works well with airflow between the sunspace and living space
during the day and no airflow at night.

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:

Greenhouse vs Solar Heating for a House


For many homeowners, building an attached solar greenhouse is very appealing. They believe that they can extend their garden’s growing season while reducing their home heating bills. Unfortunately, there is a contradiction between the use of a greenhouse to grow plants and the use of it as a solar collector for heating the house.

• To provide heat for a home, a solar collector needs to be able to collect heat in excess of what plants can tolerate.

• Much of the heat that enters into a greenhouse is used for evaporating water from the soil and from plant leaves, resulting in little storage of heat for home use.

• A home heat collector should be sealed to minimize the amount of heat loss. Greenhouses, however, require some ventilation to maintain adequate levels of carbon dioxide for plant respiration and to prevent moisture build-up that favors plant diseases.

Greenhouse management practices also can affect heat storage. For example, a full greenhouse stores heat better than an empty one. However, almost half of the solar energy is used to evaporate water from leaf and soil surfaces and cannot be stored for future use. Solar heat can be complemented with heat from compost as described in the ATTRA publication Compost Heated Greenhouses. Besides adding some heat to the greenhouse, increased carbon dioxide in the greenhouse atmosphere, coming from the decomposition activities of the microorganisms in the compost, can increase the efficiency of plant production.

Because of the concentrated air use by plants, greenhouses require approximately two air exchanges per minute.

Shading provided by mature trees is not recommended. Older books on solar greenhouse design argue that deciduous trees can provide shade in the summer but allow for plenty of sunlight to enter through the glazing in the winter after the leaves are gone. However, more recent literature notes that a mature, well-formed deciduous tree will screen more than 40% of the winter sunlight passing through its branches, even when it has no leaves.