Saturday, February 18, 2017

Design - Maximizing Passive Solar Gain (Cont'd) - Building Shape, Location and Orientation, Room Arrangement and Protected Entry

A  previous post presented an overview for using solar gain to condition living space. It ended with a list of design considerations for passive solar.  This post focuses on three items at the top of the list -- the location of the building, the room arrangement within the building and a protected entry to the building.

As I pointed out in the previous post, my understanding of passive solar comes from many sources over the years.  However, as a blueprint for discussion, I am following the format in Mazria's comprehensive book, The Passive Solar Energy Book, Complete Guide to Passive Solar Home, Greenhouse and Building Design, and then using our passive solar project as an expression of the principles covered.  Our project depends more on solar gain from the summer sun than the winter sun but it also incorporates most, if not all, of the design considerations for solar gain in winter.

Building Location
According to many sources, 90% of the useful solar gain during the winter months occurs between 9:00 a.m. and 3:00 p.m. standard time.  Consequently, it is mandatory that the building not be shaded by other buildings or trees during these hours.  I see debates in the literature as to whether deciduous trees to the south of the building are acceptable.  Some say that their branches are bare enough during winter to
Location with no trees and plenty of room on the south;
overhang for shading windows from the summer sun the
underside of which appears to be designed for reflecting
 the winter sun into the windows
allow sufficient solar gain and their leaves prevent unwanted solar gain in the summer. Others say that the shade during the summer is valid but the trunks and bare limbs block too much sunshine in winter and therefore it is better to use overhangs to block the summer sun and forego trees altogether. These same folks are quick to agree that trees and trellises are perfect for shading east and west windows from the summer sun.

If there is a choice, especially if the site is smallish, the building is best located at the north side. The further the building is away from the south property line the less likely future development will lead to buildings and trees that interfere with solar gain. And skewing the building to the north provides more sunny area in the winter for coming and going.  (Anyone who has had an entrance facing north can appreciate this advice.  Managing snow and ice on the steps and sidewalks with no help from the sun is a hassle.)

Building Shape and Orientation
A building that is rectilinear (elongated) in the east-west axis has the greatest potential for collecting more winter sunshine per square foot of living space.  According to one author, the potential for low-angle wintertime solar radiation received by the south side of a rectilinear building is three times that received on the east and west sides.

It may come as a surprise that the rectilinear design also minimizes cooling requirements during the summer.  The east and west sides of the building collectively receive more summer heat than the south side but heat gain is reduced by having short east and west sides.  Even though the south side is longer and riddled with windows, the sun angle in summer is so high that, in the absence of overhangs, it shines on the glass at such as steep angle that most of the radiation is reflected instead of absorbed.  

The amount of rectilinearity varies according to climate.  For harsh climates -- both cold and hot -- it is better if the building's rectilinearity is more compact so as to limit the amount of envelope exposed to the environment.  
Clerestories facilitate a wider house;  these appear to be
inoperable for ventilation (are they inaccessible for cleaning
and thermal shading as well? )

As to the north-south width of the building, Mazria recommends a width of 2 to 2 1/2 times the height of the windows for a single story building which means about 14 - 18 feet wide for a direct gain system and 15 - 20 feet wide for an indirect gain system (see previous post for definitions).   These dimensions amount to a house that is only one room deep.  When clerestory windows are included in the design, the house becomes two rooms deep.  But clerestories can be problematic unless they are accessible inside for cleaning and manipulation of thermal shades and operable for ventilation. In the absence of clerestories (or skylights, which can cause overheating in summer), the width of the house can still be increased and the darker and cooler areas at the back can be used in intentional ways that bring us to the next design consideration -- room arrangement.

Room Arrangement
Downloadable diagram taken from Mazria's book
(click on the image to enlarge it)
The back spaces in the building will be cooler and darker than the front spaces regardless of passive solar design -- direct gain, indirect gain and isolated gain. Therefore, active living spaces like kitchen, dining room, living room and perhaps a bedroom should occupy the front area and spaces for bedroom(s), storage, passageways and stairs should lie to the back.  The arrangement becomes less constrained with the addition of clerestories that send solar radiation further back into the building.

Protected Entry
The concept of a "protected entry" can mean at least a couple of things.  First, it can mean placing the entry in a protected area that ideally faces south and is shielded from north and west winds (northern hemisphere).   It can also mean that the entry opens into an airlock whereby exterior air -- cold air in winter and hot air in summer -- is moderated by the semi-conditioned air in the airlock before an interior door is opened and it enters the living space.

Our Project As An Exhibit
Our building is nestled into a 15 degree south facing slope that allows earth sheltering
The clerestory windows are 3 x 5 ft each
(Click on the drawing to enlarge it)
without major re-contouring of the grounds behind the house.  The living area is rectilinear in shape by a factor of 2:1 for most of its length, which is about twice that recommended by Mazria for a single story. However, it has a continuous wall of clerestories to provide solar gain for the additional width.  By virtue of a partial second story and a catwalk, there is easy access to the clerestories. 

Protection from the summer sun comes from overhangs above the windows.  Trees are absent from the front of the house, to assure that both the solar collector for the AGS system and the windows remain unshaded all summer. For the first story, overhangs will merely be an extension of the shed roof.  Above the clerestory windows on
Trees are absent in front of our house; the cedar trees
to the west and northwest will eventually provide shelter 
from the west winds and will shade the building 
from the late afternoon sun
the second floor, the overhangs will be stick-built add-ons that not only shade the windows in summer but will, as Mazria suggests, reflect the low winter sun through the windows in winter.  

The area immediately north of our house fits Mazria's prescription but of little advantage for us. The roof has a low pitch to the north that minimizes the amount of ground that is shaded all winter and earth berming raises the grade.  The two together mean that only 13' outward from the wall is shaded by the house on the shortest day of the year when the sun angle is about 30 degrees from horizontal. This outcome was not part of our planning because, except for maintaining native landscaping, activity north of the house will be minimal year-round.  We were interested in berming as high as possible for the AGS system and pitching the roof low to maximize headroom at the back of the second story.  And the umbrella for the AGS system, extending 20' out from the house, is what keeps the soil warm rather than maximum exposure to sunlight.

Our room arrangement was flexible because solar radiation will
Click on drawing to enlarge for details
deep into all areas of the house through the clerestories, providing heat and light, and because the master bedroom is partially one tier deep. The AGS system will not contribute light but it does distribute heat evenly throughout the house all winter long. The kitchen, dining room and one bedroom are in the first tier of rooms. The the living room, one bedroom with a walk-in closet, the stairs and the bathrooms are in the second tier. Major storage occurs along the back wall in space that exists mainly as part of the AGS system.  (For clarification on AGS, click on the "Featured Post" in the left column above.)

The entry faces south and, by being in the southeast corner of the second tier, is sheltered from the north and west winds by the first tier.  It opens into a sizable airlock as does the door from the attached garage.  In addition, the entire building is protected by a shelter belt of fast-growing eastern red cedars growing west and northwest of the house that will eventually deflect the prevailing winter winds.

The third post on passive solar will deal with the windows, the thermal mass, interior and exterior colors and supplemental heat sources.