Passive Solar Heating

During one of our regular lunches, Architect Audrey Van Horne and I discussed the fact that "Solar I" was about to be 25 years old. We also lamented the fact that passive solar design and construction had not caught on, and is incorrectly regarded by many as a failed experiment.

Here is the story of "Solar I", a low budget project which transformed a small non-descript Seattle house into an elegant passive solar structure that has been using less than half the fuel of a traditional structure. - George


  • House  Before


    During the late 70's I became very interested in energy efficient construction and solar heating systems. I was not alone, all sorts of groups and agencies were attempting to respond to the energy crisis and to a host of environmental concerns.

    By the middle of 1980 I had attended a number of seminars regarding passive solar design and construction and heard about a competition for such projects. At that time I was 'George Guttmann General Contractor' and in that capacity attempted to find a client who would want to contract for such a project, no luck. My wife and I then decided to build a passive solar "spec house". The idea was to use such a project to demonstrate how passive solar heating works, develop a reputation in the field and make a few dollars.

    I contacted the Architectural firm of Van Horne and Van Horne and asked them if they would be interested in a "joint venturing" this project. I had worked with John and Audrey Van Horne on several projects before and always liked their work. Audrey Van Horne took on the project and designed the home.

    House  After


    We started with a very small old one bedroom house in the central area of Seattle. We ended up with a two bedroom, two bath home. Western Sun, gave this project the highest honors in its category. It was The Seattle Times "Home of The Month". We had an open house with an estimated 2000 visitors in one weekend. It was on several local TV programs. And now, 20 years later we know that it really works!

    The project was a great success, but mortgage rates in the summer of 1981 were 17% and nothing was selling. We rented out the house at a loss for a number of years, then were able to rent it out at cost. A few years ago our daughter bought the house from us and resides there with her husband and two cats.

  • Yes! - it works great! The annual heating bills for the house have been about $200 per year. That is less than 1/2 of the heating costs for a similar sized home in this area.

    And these heating costs have remained about the same no matter who or how many people have lived in the house, with or without pets, oil or gas heat, renters or owners.

    The house is also light and airy. It is cool in the summer. It has been easy to maintain. It requires no training manual. It is great!

  • Passive solar heating depends upon the design of a structure to capture and store the heat of the sun. It usually depends upon glazing (windows) to capture the sun's rays. It uses mass (e.g. stone, concrete, brick) to absorb the heat and release the heat slowly. Heating and Cooling Diagram

    Passive cooling depends upon the design of a building to shade the structure during the summer month. It depends upon convection currents - warm air rises. The warm air in the house is vented out through high operable windows and in the process draws in cooler air from the basement and/or the outside.

    Such passive solar systems are not new. Pueblo indians used passive solar principles to temper the interior air of their cliff dwellings. They placed their homes on the south side of canyons and made maximum use of the low winter sun angels. They also made maximum use of the shade created by the overhanging cliffs during the summer month. Similar passive solar uses can be found in various mediterranean structures.

  • The original design contained some basic passive solar design principles using "off the shelf" materials. Based upon the experience of the last 20 years, we now know that these basic principles and materials work very well. This and the many other solar projects we have seen and read about have taught us the following:

    • A good quality design is imperative: too much or too little glazing, glazing in the wrong area, too much or too little mass or mass in the wrong area, plus many other considerations are critical. Failure to understand these principles and apply them properly are a guarantee for failure.
    • Good insulation is very important but today's insulation standards may be enough: we used 5.5" batt fiberglass (R19) insulation in the walls; we used 10" fiberglass batts (R30) in the cathedral ceilings.
    • An understanding of the sun's path at various times of the year and in any one particular location is imperative for a good design. For example: the sun's angels are much lower in Seattle than in Tucson, and the house must be designed to capture the sun's rays during the heating season and shade them during the cooling season.
    • A direct and open unobstructed southern exposure is required for heating purposes, yes, the structure must have an unobstructed wintertime southern exposure (unless the house is south of the equator). Some deciduous trees can be used to shade summer sun.
    • Most of the windows must face south, a few can face east and west, very few should face north.
    • Concrete, concrete block, tile and brick work well as a mass to absorb the heat. While absorbing the sunlight such mass prevents the space from overheating (unlike a closed car in a parking lot). When the sun goes down, the mass slowly releases the heat and lengthens the period of solar heating time.
    • An open design helps the heat to circulate.

    Designing for passive solar heating

    by Audrey Van Horne, Architect

    There is a lot of pleasure in living in an a solar house, the solar heating provides a comfortable even heat and often some sunny places to enjoy. It takes a bit of adjusting one's life style; things like - 'don't leave the door open when it is chilly outside; draw the curtain over the glass in the evening when you want to keep the heat in.'

    Planning is required if passive solar heating is what you want. The building needs to collect sun from the south, (because that is where the sun is. ) So building in a location that has access to the south sun is a primary need. There are lots of elements that have to be adjusted to gain the heat - things such as how much window to collect the sun, and an area - a wall or floor (or both) to collect the heat - or get warm when the sun shines on it, and finally a way of moving that warmth to other parts of the house.

    Unless there is circulation of the air throughout the house, the solar space can get very much too warm, and other spaces wont get warm unless there is a way for the warm air to move into the rest of the space. Trapped warm air will make the space much too hot on a sunny day. Warm air will rise - so a two-story circulation path will work the best. And openings must be planned to create the circulation path.

    All this planning is part of the design of the house. And yet, the design must first serve the needs of the occupants or family who lives there. The layout of the spaces is primarily for the function of the family, but it must be adjusted and planned for passive solar heating if solar heating is the goal. The capability to design and integrate the solar heating into the plan for the house is a skill that architects are trained to do.

    The overall design addresses the house plan and the location on the property to gain the sun, and surfaces to collect the heat from window openings located to capture the sun. The flow of air, the location of other windows, the type of heating for the house, the materials used for the storage of heat - or the mass - are all part of the design and plan.

    Good design results in a very livable house that has been laid out to take the best advantage of the sun and the heating potential.

  • The amount of solar heating available on most Seattle winter days is small. Good design is required to absorb and store as much of that heat as possible. Good insulation is required to reduce the loss of that heat.

    The insulation standards we used in this project were 'revolutionary' for 1980. By todays standards they are quite normal: 5.5" batts in the walls, 10" batts in the ceilings, and standard insulated glass.

    How Times Change

    In order to get 5.5" of insulation into the walls, we had to frame with 2x6 studs and then had to find door frames for walls of that extraordinary thickness.

    Although none were available 'off the shelf', Dunn Lumber came to the rescue. I visited their 'door shop' and they came up with custom made door jambs for the 2x6 walls.

    Today, it is the 2x4 sized door jambs that's the odd size, R19 wall insulation the standard and R30 insulation in some ceilings does not meet code requirements. Almost all windows come with insulated glass, windows with uninsulated glass are a special order. Insulation materials come in all types of materials and forms. And contrary to the predictions of many in the housing industry, the prices of these products have come down relative to the costs of other construction components.

  • The "Solar Age" didn't last very long and the support for projects of this type evaporated by the mid 1980's. As a result, there were few attempts to rigorously test the components of these types of homes. However, based upon our observations it appears that some of the components in the home had little value. For example:

    • The water barrel heat storage system at the basement windows and the associated fan system never seemed work. The water in the barrels remained at a relatively constant temperature and didn't respond to solar heating. We ended up removing the barrels after a few years. By contrast, the ceramic floor tile, concrete and concrete block in the living area works like a charm.
    • The Tromb Wall seems to work, but not as well as similar material used in the solarium and the other south facing living spaces.

    The lesson here seems to be that the amount of solar heat gained and stored in our area is enough to make a large impact in areas with direct solar exposure and direct heat storage. However, it seems that the amount of solar heating available in our area is insufficient to overcome the heat loss that occurs when one attempt move the heat from one area to another.

    I have also had the chance to visit and inspect many other solar homes and solar components and have found several other types of systems that tended to fail or result in limited value:

    • Very few of the thousands of solar water heaters installed in the 1980's are still in operation. I am unaware of a single such system in our area that actually works or has been cost effective.
    • Phase Change Salts and similarly expensive heat storage components seem to be an expensive alternative to ceramic tile, stone, brick and concrete.
    • 100% solar heating may be theoretically possible, but in our area and with today's technology it is very complicated and very expensive. based on our experience with "Solar I", the first 50% or so in savings are relatively easy and the costs and benefits are reasonable.

    Solar systems also gained a poor reputation for some wrong reasons. The solar tax credits of the 1980's attracted some 'snake oil' sales people who only cared about the tax savings.

    Solar systems also attracted a large number of amateurs who understood very little about construction and even less about solar systems. Many of their "inventions" performed very poorly.

  • By the time we were selecting the floor tile, our budget indicated a need to "tighten our belts". So I brought a few darker colored floor tile samples to the Van Horne and Van Horne offices. We placed the tiles on a sunny window sill. After a few minutes we removed the tiles from the sill and used some of our sophisticated testing equipment (our hand) to determine which tiles were warmer and retained heat for a longer period of time.

    The winning sample was a 3x6 matt glazed tile that was light brown in color. It was also one of the least expensive samples - what luck!

    And now, 20 years later, this same tile is in excellent condition! A much better condition than that of the bathroom vinyl or carpeted floors.

    • Alternative Building Practices: We used a re-enforced and concrete filled block wall on the interior of the structure, opposite the south wall. This wall is part of the heat storage system, but also acts as a structural shear wall, a common requirements in most new construction. Block construction is a common technique but seldom used for such an application. As such, this type of work can't be done on 'auto pilot', it requires extra planing and organization and a more versatile construction crew. And all that costs more.

    All of this demonstrates that passive solar construction is more expensive than conventional construction. It is more expensive because:

    1. It is unusual and non standard.
    2. It requires more architectural/design work.
    3. It requires some non-standard materials and techniques.

    At a minimum, passive solar construction costs 10% more than conventional construction. But good planing should help keep these costs under control.

  • In a hypothetical $150,000 new construction project, my experience suggests that passive solar design and construction might cost and added $22,500 (15%). And based upon our experience, such a structure should require less than 50% of the heating costs of a conventional home. Assuming a conventional heating budget of $700.00/yr, this suggests an annual savings of $350 or 1.5%/yr of the costs of the passive solar components.

    1/11/01 Some of the data suggests that the energy savings from passive solar heating might be twice as high as the above numbers. I would rather underestimate the benefits and avoid the type of unrealistic and damaging 'hype' that accompanied passive solar information in the early 80's.

    1.5%/yr is a relatively poor return on investment. But a passive solar home has some additional advantages:

    • Passive solar homes are light and airy!
    • Passive solar summer cooling must be part of the design and costs nothing!
    • There are no 'Solar Utility' bills and they will not increase!
    • The Passive Solar components of a home need little if any maintenance!
    • Passive Solar heating does not add greenhouse gases to the environment!

    And passive solar design and construction is not an 'all or nothing' proposition. It can start with the selection of a lot and building site with a good southern orientation. Increasing glazing on the south side, and decreasing glazing on the north side can help. Placing the garage on the cold side of the house and the living spaces on the south side will also help. Each of these and many other small steps can start to transform a house into more energy efficient and pleasant home.