Building on our earlier Passivhaus 101 and Insulation R-Value articles, I’m now going to provide an overview here of the Passive Solar House Design — with the intent being to explain the basic principles and design elements.
To put it simply, passive solar building/house design is based around the idea of maximizing solar heat gain when and where it’s useful (so as to offset heating requirements) and minimizing it when and where it’s a liability (so as to offset cooling requirements).
These aims are achieved primarily through the use of large windows (oriented in such a way as to achieve desired heating/cooling effects at desired times of the year), large amounts of thermal mass and/or insulation, external shading devices or shutters, and internal night-insulation systems (think window quilts). Other system elements utilized include: solar ventilation (solar chimney effect systems); solar water heaters; solar stoves; earth-sheltered walls; and/or attached greenhouses or sunrooms.
Effective passive solar home designs will of course vary widely based on the regions where they are located, and also on the microclimate and the particular geology of the site in question.
Probably the most consistently important factors in effective design, though, are those relating to window placement, size, and orientation — and also those relating to the potential thermal mass meant to absorb the solar energy that passes through these windows, within the well insulated building in question.
Think of a building located in Northern Europe with large southern exposure windows and a lot of dark stone thermal mass that is exposed to the sunlight that comes through these windows even in the depths of winter — coupled with well insulated walls, airtightness, and thick night-curtains — as an example.
In a tropical environment, conversely, the aim would be to minimize heat gain during the hotter parts of the day and/or year — possibly through the use of easily operable external shutters or sun-shades.
An effective analysis of the site is key, in other words, regardless of the region of the world in which you’re located.
Passive Solar Home Design Elements
Solar Orientation — Accounting For Latitude; Insolation Levels, & Sun-Path
We’ll start off our discussion here of passive solar home design elements by focusing on the most important one — building with the solar orientation, insolation levels, and local sun path in mind (so as to minimize heating and cooling energy needs).
When designing a passive solar house or building from the ground up one needs to make especial note of: the position of the sun in the sky at various times of year; the path that the sun takes through the sky during the day (the sun-path); and total + seasonal solar insulation levels (accounting for average expected levels of sunlight, cloudy days, etc.).
With this highly local knowledge, one can then design a building so as to allow for high levels of heat gain when wanted (during the winter, for instance) and also for minimal heat gain when unwanted (perhaps during the summer). The primary means through which this is achieved is the placement and sizing of windows, and also the orientation of the walls in question (often straight walls are not the best answer when looking to maximize solar heat gain). Homes in cold climates are often long along the West-East axis, so as to maximize southern exposures.
Basic rules of thumb (which sometimes have to be modified) are probably known by most of those reading this — exposures facing the equator (southern exposures in the Northern Hemisphere or northern exposures in the Southern Hemisphere) allow for maximum winter sun; eastern exposures allow for morning sun; western exposures for afternoon sun; etc. The sun is lower in the sky during the morning and later afternoon than during midday, so that light can be allowed in while still blocking midday sun — through the use of long roof overhangs, etc. The sun climbs higher in the sky during the summer than during the winter (in the Northern Hemisphere anyways).
Something else to keep in mind, of course, is that trees that drop their leaves during the autumn will allow winter sun through while still blocking summer sun — which is one of the many reasons that effective passive solar home design needs to take the nearby external environment into account.
It’s also worth situating the rooms within the home in question so as to preferentially experience solar exposure based on the time of day that it’s primarily in use (i.e., bedrooms getting morning exposure, etc.).
All of that said, some effective means of designing passive solar buildings are not particularly intuitive. As a result, many of those designing passive solar homes now rely upon software tools such as the US DOE Energy Plus system — which has integrated several decades worth of experience to create a system that’s now very well regarded.
Insulation & Weatherization In Passive Solar Homes
While effective insulation and good weatherization is important in any home (if you don’t want high energy bills, that is), it’s especially important in passive solar homes — as the intent is to rely upon solar heat gain completely, or nearly so. The reality is that poorly done weatherization (creating high rates of air infiltration and loss) is the primary reason for heat loss during winter (and coolness loss during summer).
In other words, if you want to greatly cut down on your energy bills right now, then you can simply find out where air infiltration and loss is occurring and increase the weatherproofing. And watch your energy bills drop. If this is done very effectively and there are good levels of insulation, then it’s possible in many regions to simply open the windows at night or during the day to allow preferred air temperatures in (depending on the season/region) and close them up the rest of the time, without needing separate heating or cooling tech.
In passive solar homes, effective weatherproofing and insulation (and thus near airtightness) is a necessity if primary reliance upon sunlight for climate control is to be possible. This is often combined with some sort of filtered energy recovery ventilation system — whether mechanical or itself passive.
When paired with large amounts of thermal mass, it becomes possible for the heat gain achieved through the day via sunlight to continue warming the air in the house long into the night hours. And perhaps more importantly, high levels of thermal mass absorb “excess” heating during the day — improving the daily temperature gradient so that sunny afternoons aren’t considerably warmer than the nights that follow. All the more reason not to get cheap when it comes to effective levels of thermal mass (as some home builders seem to do).
An area often of particular importance in passive solar building design is the roof/attic — as its the portion of the house that receives by far the highest level of solar exposure. In cold climates, it can make sense to maximize this in some way, and in warm climates it often makes sense to minimize this if possible (dark roofs, white roofs, green roofs, skylights, etc.).
Other Passive Solar Building Elements
Other than these basic design elements (and others not discussed), passive solar buildings often incorporate: solar water heating systems; attached greenhouses or sunrooms; removable sun shades and window quilts; and appropriate landscaping.
With regard to solar water heating systems, the tech is pretty widely known at this point, so I won’t spend too much time on it — I’ll just note, though, that there are both passive solar water heating systems and active solar water heating systems available. The difference in such systems is that passive systems are designed so as to not need pumps, whereas active systems utilize electricity to provide on-demand pumping action. It’s argued by many that passive systems aren’t well suited to regions where temperatures drop below freezing regularly, due to the potential for freeze damage from stagnant water (active systems can heat the water to above freezing to avoid this).
With regard to attached greenhouses or sunrooms, the solution is a widely known one and has long been in use in many parts of the world. Such structures can be very effective at gaining heat during the day, but also at losing it during the night (even with lots of thermal mass incorporated). As a result, it makes sense to open the house to such rooms during sunny days and to keep them closed off during the night and during cloudy days.
In addition to added heat gain, such spaces can of course give one the ability to grow plants year round or out of season. Even in rather cold climates, such structures can actually be kept at a decent temperature passively simply by sinking them into the ground a fair bit, with a large equatorial exposure and high amounts of thermal mass + window quilts keeping the temps above freezing even when they are well below outside.
With regard to removable sun shades, shutters, and window quilts — the idea is a simple one as well: to alter and improve the functionality of the heat-gain windows in question. In other words, to limit heat gain during the summer through the use of external sun shades or shutters, and to limit heat loss at night through the use of window quilts.
Such systems give one the option of altering heat gain and/or heat loss as need be, without having to necessarily open the house/windows up. This is important, as heating/cooling needs vary not simply due to solar exposure but also due to the heat stored in the land. In other words, even when autumn and spring solar insulation levels are roughly equal, heating needs tend to be greater during the spring due to what’s known as thermal lagging (the effect of heat sinks storing and giving off thermal energy — the same way that the thermal mass in a passive solar house does).
With regard to landscaping in passive solar house design, the main things to keep in mind are: the effects of deciduous/evergreen tree differences on heat gain; the effects of plant cover as windbreaks; and the effects of plants as insulation and/or water management (water is very effective as thermal mass, it should be remembered).
Passive Solar Home Materials
While passive solar homes are often constructed similarly to other well insulated buildings — unless they are earth-sheltered designs, that is — and utilize many of the same building materials, they also rely on applications of these materials that are somewhat different than in conventional designs.
In particular, the use of large amounts of building materials that can function as thermal mass is notable — whether stone, concrete, brick, adobe, etc. (Water can also be used as thermal mass in dry climates and/or in greenhouses, but is far less suitable in climates with higher levels of humidity.)
In order for such materials to function effectively as thermal mass, they need to be left bare, and need to experience direct sunlight. Think of the way that dark stonework warms up nicely in the sun even when air temperatures are quite low, and you get the idea.
What this means in practice is that floors and walls that are made of such materials, and intended to function as thermal mass, should be kept bare/empty — so, no rugs, carpeting, large furniture, pictures on the walls, etc. Obviously, this only applies to areas intended as thermal mass. Other parts of the building can be decorated as usual.
Thermal mass needs vary by climate and exposure levels quite a bit, so I’m not going to offer any mathematic formulas here — you’ll have to work out the particular needs of your exact situation. Generally speaking, though, a fair amount of thermal mass is needed to modulate internal temperatures and to store absorbed solar radiation (approximately 5–10 times the surface area of the windows in question).
Something perhaps counter to intuition is that such thermal mass “shouldn’t” be particularly thick — due to the material functioning best with regard to re-radiation when the whole material heats up rather than just one side of it. (Obviously, though, particulars will vary as home designs do.).
Other than thermal mass, passive solar buildings also make extensive use of double- or triple-glazed windows and removable window quilts and/or external shutters/shades. Since the intent is often to maximize solar gain, though, care must be taken to avoid using windows featuring glazing that reduces such gain on relevant windows (i.e. equator-facing exposures).
As a side note to that, it’s worth realizing that skylights are often hard to justify from a passive solar design basis (due to maximizing heat gain at the wrong time of year, and heat loss at the wrong time as well). Attached greenhouses or sunrooms that can be closed off from the main house can make for a somewhat comparable alternative, though, with specific benefits otherwise unavailable as well. Insulated light tubes are another possible option.