Passive Solar Design

Posted August 29, 2008

Passive solar design is about the orientation of a building and the placement of windows and mass in order to capture the heat from the sun and then to store it in the mass of the building.

One of the first things to consider when designing a house is the orientation to the sun.  The long axis of the house should be oriented directly east-west, with one of the long sides of the house pointing south.  The south wall should then be designed so that the majority of the windows in the house are on that side and that there is a minimum of windows on the north side.  One of the errors that many people make when doing a passive solar design is to have too many south facing windows.  In the average house you want no more than 15% of the floor space as windows on the south side of the building.  If you have more windows then you are prone to overheating, particularly in the fall and spring. When specifying materials you need to look for a window that has as high a Solar Heat Gain Coefficient (SHGC) as possible if you are in a northerly climate. The SHGC is a number between 0 and 1 that is the portion of the solar heat that is allowed through the window. A window that allowed all the heat through would have a value of 1. Most windows have a value of between 0.3 and 0.6. In addition you want to design overhangs that will shade the windows in the summer and let in as much light as possible during the winter (I will cover this more in depth in a later post)

In addition to the windows you want to have thermal mass in the building.  Thermal mass acts like a flywheel for heat.  If the sun shines on it, or the air around it is warm, it will store some of the heat, and once the air temperature drops, it will release some of that stored heat to the air. Once again, however, you can have too much of a good thing.  If you have too much thermal mass, it can never get above room temperature and will not contribute to heating the house, as in order for a thermal mass to be effective, it has to reach a temperature that is greater than the desired air temperature in the house.  This is because heat will always travel from a hot object to a cool object.  If the thermal mass is at the desired air temperature, it will only transfer heat to the air when the air temperature is below the desired temperature.  Also, with concrete, the most common thermal mass, the active zone is only about 4 inches deep, so there is little to no advantage having a 10 inch thermal mass.  A thermal mass will be most effective if it is directly exposed to the sun and is dark in color, as dark colors absorb more heat and light colors reflect the heat.

If the thermal mass is in an exterior wall, it is best if you can insulate on the exterior of the wall and leave the interior exposed to the air.  If you insulate the thermal mass on the inside of the house, such as in an Insulated Concrete Form (ICF) the heat will have to travel through the insulation first before it can heat the concrete, resulting in a less effective transfer of heat.  In my house I used 4 inches of Roxul Drainboard for the insulation. It has an insulating factor of R4.3 per inch, which is comparable to foam, and it has the advantages of being cheaper, being a more environmentally friendly product and providing a drainage plane for the walls. The only drawback I could see is that it has to be protected from the weather (I used concrete board).

Posted under Design

5 Comments so far

  1. Donna Connell June 14, 2010 11:34 am

    We are building a straw bale home in Madoc, ON. (30 min. N or Belleville) The house is oriented with long axis E-W and a south facing exterior wall of about 29 ft long.We are planning to put 6 foot wide French doors with two 1 ft sidelights on either side in the middle of the south wall. We would like to put two more windows on either side of the doors but are not certain about optimal size/total window area for passive solar design.

    I know that the recommendation is to have the south facing windows about 15% of the floor space.

    Is that TOTAL floor space of the house (main floor) or just the linear floor space along the south facing wall?

    The part of the design that is different is that there are two 6 ft. long walls that are angled at about 45 degrees from the south wall to the east and west walls respectively. We want to put windows in those walls as well, but are not sure how big they should be to optimize solar gain without too much heat in summer or loss in winter.
    Do you have any suggestions?
    Thank you very much,


  2. Ward Edwards June 14, 2010 7:52 pm

    The percentage of window to floor space is the square footage of the window to the square footage of the floor the windows are located on.

    In a super insulated house such as a straw bale, you will need less windows than you would with a typical house. We have a super insulated home and we have around 10% of the floor space as windows. About 8% is in on the south facing windows and the remaining 2% is on the east and west side. With this configuration, on a bright sunny day in the winter, the heat will not come on during the day, and the house will warm up by a couple of degrees. If you are willing to have a wider temperature swing, you could have a larger amount of windows and not use any supplemental heat at night, but the house will drop by a few degrees during the night. Some of the things you will have to take into consideration is that you will need a significant thermal mass to reduce the temperature swings (we have a 3.5″ concrete slab), but the plaster on the straw bales should give you quite a bit of mass. The main thing to consider with insulation is to minimize air infiltration since this is where most of the heat loss occurs. The render on the strawbale will help a lot, but be careful on the installation of the windows and the joint between the walls and the rafters. Also make sure you have shading on the windows during the summer, either overhangs or trees.

    The two angled walls will still make some contribution, depending on the size of the windows, but the concern here is that the west window will gain more heat during the summer. This could be mitigated somewhat by planting a bushy deciduous tree (such as an apple) in front of the west window to shade it during the summer.

    The other thing to remember is that in this area the magnetic declination is about 13 degrees off of true south. Make sure you take this into account when siting the building. This won’t affect the south wall much, but can make quite a difference on the angled walls.

    I hope this helps answer your questions

  3. Karyn December 5, 2010 4:59 am

    I am located 15kms north of Kingston, Ontario, and am trying to determine some important factors in planning my passive solar home. I wondered if you could direct me to a link where I can determine the recommended roof overhang depth and angle for my area, the actual arc of the sun for our area by season and the thermal mass to glazing calculator. I am not far from Madoc, so I am happy to see that the magnetic declination off true south is 13 percent here, but in which direction of true south (13degrees to the east or west of true south?
    Thanks for your advice, Karyn

  4. Ward Edwards December 5, 2010 7:54 pm

    Hi Karyn

    There is another page on my site that shows how to calculate the necessary overhangs. If you want to calculate the sun position for a certain time of day, google “calculate sun path” and a number of calculators are available. The magnetic declination for Madoc has true south as 13 degrees west of magnetic south. A good way to determine true south is to take a sighting on the North Star, as it is true north, and go 180 degrees from that. You can also put a stick in the ground at your sight on a sunny day, and when the sun’s shadow is the longest, it points 180 degrees from true south.

  5. troy davies February 13, 2011 7:02 pm

    Hi this message is for Karyn. Karyn I live in Kingston, I am a carpenter and have worked on many different types of alternative building projects all over the world. I am very interested in passive solar design. I would like to get your contact information to come see your house or even help out.



  1. Building Green September 3, 2008 11:13 am

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