Passive Cooling. Passive cooling is the idea of cooling without mehancial ventilation. In Minnesota this is less of a concern than passive heating, but still something to take into consideration. Because of the airtightness standards and other building codes, there are some limations to pasive cooling design.

You can have a vaulted ceiling with operable skylights or windows. Another alternative is a thermal chimney, like a three story entryway with operable windows up high.

Exhaust fans that move air from conditioned to unconditioned spaces such as the attic are not allowed by buiding code in Minnesota.

All of our Amaris homes are built with air-to-air exhangers which exchanges and conditions fresh air and can help cool down a well insulated home without air conditioning.

Landscaping is one other strategy that can contribute to passive cooling techniques. Good old fashioned trees are a way to shade a home in the summer.

Yesterday we talked about direct gain passive solar. There are other passive solar design methods such as indirect gain or isolated passive solar. These systems are not as practical and are more expensive to implement. Other systems that incorporate convection air flow from conditioned to uncoditioned spaces are not allowed by many current building or fire codes.  For this reason, we are not going to spend a bunch of time on the passsive systems not likley to be implemented.

Isolated Passive Solar. An isolated passive solar system is a one or two story solar sunroom that is attached to the south facing portion of the home. This method is a very expensive, less efficent way to implement passive solar for a new home, unless you happen to just want a one or two story sunroom. The solar sunroom may be an excellent way to modify an existing structure that is south facing but was not solar designed.

Solar sunrooms are great for growing plants and vegetables. Just like with the direct gain passive solar, the isolated passive solar utilizes thermal mass to store heat as well. In addition to tile or brick, the thermal mass might include a small hot tub, pool or spool.

Windows can be opend on both floors to create the circulation. Hot air rises and will come into the bedrooms. Cold air falls down to the first floor and out the window. In an extreme cold climate like Minnesota, this will only work in the spring and fall.

Passive solar is a big part of a Net Zero Energy home. There are various passive solar systems. Direct gain passive solar is the easiest and cheapest to implement. Direct gain solar is a system where the living space is the solar collector, heat absorber and distribution system. There are two parts to this type design. South facing glass and thermal mass.

South Facing Glass. We recomend the rear of the home be south facing because it natrually has the most amount of windows in most modern home designs. The goal (and requirement) is to get southern exposure into as many of the everyday living spaces. From a design perspective this will take exta planning.

South facing windows need to be screened from (high) summer sun by overhangs, roofs or other methods, but they also need to allow the (low) winter sun fully into the home during heating season. South facing windows are typically taller than standard windows to allow passive solar to reach the full depth of the entire room. Passive solar designs also tend to elongate the home on the east/west access, thereby shorting the north/south access.

As mentioned in other posts, the most used rooms (e.g. family room, kitchem master bedroom, etc) should be south facing. Unused rooms (e.g. dining rooms, spare bedroooms, laundry room, etc) should be located to the north. An open floor plan optimizes passive solar design.

Northern Glass. Because we live in extreme northern climate, north glass should be eliminated if at all possible. If north glass is used, it should be minimised and install high “U” value windows. If budget is an issue, storm windows on north facing glass is a way to accompish the same thing.

East/West Glass. East/west glass should also be minimized too. The “baking” affect is not as great in Minnesota, but east/west glass can cause rooms to overheat in the summer. We like to use transom or other accent windows on the east/west access.

Skylights. Some older passive solar designs used skylights, but it is my belief that skylights should never be used in a direct gain solar design. You will bake the house in the summer defeating the purpose of a true passive design. Also, if you live in the North, you will eventually have a leak.

Thermal Mass. The passive design requires a thermal mass to absorb the heat generated by sunlight and then releases it back into the room at night. Thermal mass includes the floor and wall surfaces covered by certain darker ceramic tile, brick, etc. Becuase the thermal mass absorbs the heat during the day, the room temperature is tempered during the day.

There are a number of calcualtions to consider for thermal mass, most of it boring stuff. The amount of masonry per sqaure foot of south facing glass. The amount of south facing glass as a percentage of square footage and so on. Suffice it to say that although a passive solar design does not need to look different than most construction, it is scientifcally calculated and built; it doesn’t just happen by accident.