Passive solar is a great way to heat any home, lowering energy bills and providing some heating capacity for any power outage disaster. If one is fortunate enough to have solar heating, they can be sure that they will always have at least some heat, even in the coldest of weather.
Of course, most passive solar homes are designed as such from the outset. Because of that, most people think they can’t take advantage of solar heating. Yet, almost any home is capable of having at least some passive solar heating, with some modifications. The trick is in understanding how passive solar works and then looking at the home to see what can be done to make it more solar friendly.
A passive solar system consists of only five basic components. These are:
- Aperture or collector. This is a fancy name for the windows that the sunlight comes through. In the northern hemisphere, these windows have to be south-facing and can’t be covered by foliage. The closer to directly south that the windows face, the better, with 30 degrees off of south being the maximum possible angle.
- Absorber. The absorber is what the sunlight hits when it comes through the aperture, converting that light to heat. To be effective, the absorber must be a dark color and made of a material that will readily transfer the created heat into the thermal mass.
- Thermal mass. Once the absorber converts the light to heat, that heat needs to be stored somewhere. In a passive solar system, this is usually concrete or rock. The absorber is physically connected directly to the thermal mass, allowing it readily to transfer the heat.
- Distribution. A passive solar system absorbs heat during the day and releases it at night. This is accomplished by a combination of radiation and convection. The heat naturally radiates from the thermal mass to the surrounding air, warming it. Then, convection causes that warmed air to rise, heating other parts of the home. In some cases, ceiling fans may need to be used to assist with the convection process.
- Control. The control limits the ability of the sunlight to hit the absorber in the summertime, while allowing it to hit it during the winter. This is usually the roof overhang. In winter, the sun is lower on the horizon, allowing it to shine through the windows. In the summer, it is more directly overhead, where the control blocks it.
In most cases, the absorber is the floor covering and the thermal mass is the floor. This works great if the floor covering is black stone or tile, over a concrete slab floor. In homes that are designed to be solar homes, the part of the concrete slab that is working as the thermal mass is usually thicker than a typical four-inch-thick slab.
Interior walls also can be used as absorbers and thermal mass, if the windows and those walls are placed in such a way as to allow the sun to hit the wall. At times, that is accomplished through upper windows, with a cathedral ceiling. A standard drywall interior wall won’t work for a thermal mass; instead a masonry wall is needed.
As the sunlight comes through the aperture, it strikes the absorber, where its energy is converted to heat. Some of that heat radiates into the room, keeping it warm. The rest is transferred into the thermal mass, which stores it. As long as the ambient room temperature is higher than the temperature of the thermal mass, it will continue to absorb heat. However, once the sun goes down and the ambient room temperature begins to drop, the heat stored in the thermal mass begins to radiate into the room.
Since heat rises, natural convection helps to spread that heat around. In a two-story solar home, the south side usually has a cathedral ceiling, with a loft or a balcony for access to the second-story rooms. The rising heat can then help to keep the bedrooms warm.
In the summer, the sun is higher off the horizon, so the control is able to prevent it from entering the home through the aperture. While a roof overhang is a common control, some homes use shades or canopies over the windows.
Converting a Non-Solar House to Passive Solar
Now that we have an understanding of how a passive solar home works, let’s see how we can apply that to any home. The first thing that we have to understand is that all homes have some passive solar effect happening, whether we want them to or not. South-facing windows allow sunlight to enter, striking any available surface, where it is converted to heat.
The problem with most homes is that this process is haphazard, making it ineffective. There aren’t enough windows, the material used for the absorber is not efficient at converting the light to heat and there isn’t a thermal mass to store that heat for night use.
Many of those problems can be fixed fairly easy. The first issue is increasing the amount of windows on the south-facing side of the house, so that there is more sunlight entering the home. Generally speaking, the more window area you have, the better.
Many homes are built on a concrete slab, without a basement. That concrete slab can become the thermal mass, even though it isn’t as thick as the slab usually used for a thermal mass. By changing the floor covering to something that works better as an absorber, the amount of heat created can be increased. Ideally, the floor covering will be black, but any dark color will work.
The floor covering used as the absorber also needs to be something that is hard and does not make a good insulator. Stone or ceramic tile are excellent, while linoleum is poor and carpeting is horrible. The best possible material is black slate flagstones. Not only do they work well as an absorber, but they will also thicken the thermal mass, giving it the ability to store more heat.
You have to be careful putting slate flagstones over an existing slab. The flagstones will raise the floor about two inches. So, you end up with a trip hazard at the entry to the room. This can be mitigated somewhat by putting a bevel on the stones at the entryway, so that any feet which strike the edge will be directed upward onto the flat of the floor, rather being caught on the lip.
The other potential problem with putting flagstones over an existing cement slab is in kitchens. The existing cabinets have a toe kick at the bottom. If you don’t remove the cabinets before installing the flagstones, you’ll eliminate a large part of the toe kick. If you do remove them, you can reinstall them after installing the new flooring. This will keep the counter height the same, along with maintaining the toe kick. The only thing lost will be some of the space between the countertop and the upper cabinets.
If there is an interior wall that is located so that it will be hit by the sunlight from the new windows, it can be removed and replaced with a masonry wall, using an appropriate material on the wall to be an absorber. Not only does this increase the effectiveness of the passive solar heat, but it looks good, too.
Another excellent way of adding passive solar to an existing home is to build on a sunroom. Any sunroom is almost by definition a perfect passive solar system. If the rear of the home is facing south, remove the sliding glass patio door and attach the sunroom there. Be sure to build it with a good thermal mass in the floor. You can also add thermal mass to the outside of the home. A ceiling fan will help distribute heat from the sunroom into the rest of the house.
Although the remodeling necessary to convert a standard home to passive solar is somewhat extensive and also somewhat expensive, it can be accomplished in stages, spreading out the cost. The money saved on your monthly energy bill will help fund the expansion of your passive solar system.
While passive solar will keep a home comfortable, please keep in mind that you won’t wake up to a toasty warm home if that’s all you are doing. As the night progresses, more and more of the heat that is absorbed into the thermal mass is dissipated, lowering its efficiency. By morning, the heat has already been dissipated and the home may seem chilly. However, it won’t be as chilly as if you hadn’t had any heat at all.
Most people use passive solar together with more active heating means. This provides a good balance for all situations. Once the passive solar system has been installed, it costs nothing to operate, so any heat generated by it reduces your monthly energy usage.