Passive Solar Performance

Years ago, I purchased a used book outlining 1970’s era passive solar design. It was a good introduction. However, when we started researching the design for this build, we quickly discovered that many of the ideas formerly held as “truisms” of passive design, just weren’t correct scientifically.  There are now plenty of precise reference materials that can point you in the right direction.
Basically, if you want to take advantage of the sun’s free energy to provide 25-60% of your home’s heating requirements, the basic formula is “Passive Solar Performance = Super-Insulation + triple-pane windows + thermal mass + minimal air leakage” will work for you. 
Insulation:  From my perspective there is no sense in designing a passive solar house unless it is “super-insulated”.  To me, this means walls that are at least R40.  In Canada or other northern climates, R50 or more is a better bet.  By comparison, the Ontario building code requires only R20 walls!! After researching how best to make a super-insulated wall, we settled on the “deep-wall” or double-walled approach, where two 2×4 walls are constructed a few inches apart, creating a cavity that is then filled with dense-packed cellulose insulation (3.5 lbs per cubic foot).  For more information on this wall system, see, a blog created by Conrad Norbert.  Conrad’s blog is amazing and contains an incredible amount of detail.  We used this wonderful resource in developing our own design.
Windows:  For best solar performance, you want to have a ratio of glazing area to floor area of about 12%  (see the CMHC book “Tap the Sun” or Dan Chiras’s book “Solar House: Passive Heating and Cooling”).  That means the overall area of your windows must be equal to about 12% of the area of your floor, for maximum solar heating.  You need not worry about over-heating so long as you calculate your window over-hangs (eaves, awnings, etc) correctly.  Again, this is laid out really well in Tap the Sun.  Essentially, you can download sun-angle data by latitude, and use this data to determine the depth of eave or window over-hang required to shade your window on the longest day of the year. The experts agree that you should buy the best windows you can afford, and generally that triple paned is required.  With the correct glass installed for the south windows, and the best coatings applied for all other directions, triple-paned windows will provide the best solar heat gain and the best insulation value.  Remember, that is a lot of glass, so a better window will make the house much more comfortable when the sun is not shining.
Thermal Mass: Once you exceed a glazing to floor space ratio of about 8%, then you need to add thermal mass in the form of bricks, concrete, doubled drywall, etc., in order to store the sun’s heat for release after dark and to eliminate over heating.  Apprarently, a 4″ concrete pad is the most effective means to do this.  I’ll leave it to the experts to explain how and why this works.
Air Leakage:  the house needs to be TIGHT.  Leaky houses let cold air in and hot air out, and can play havoc with the performance of a super-insulated design.  It is recommended to aim for R2000 levels of air leakage (1.5 exchanges per hour) or less.  I think the passive house standard is something like 0.5 exchanges per hour.  You can gain a great deal of performance by making the house envelope as tight as possible – I recommend reading the blog for details in this regard.  And remember, if the house is that tight, you need a good HRV!
This entry was posted in Design, Planning and tagged . Bookmark the permalink.

Leave a Reply

Fill in your details below or click an icon to log in: Logo

You are commenting using your account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )


Connecting to %s