Sustainability Begins With Design
“Building Green” is often touted in architectural magazines and online articles as an important consideration for the eco-conscious consumer when planning a new house. Once you get beyond the glossy imagery showing beautiful houses with locally sourced materials and green roofs, you may be left asking yourself “what makes a house design truly sustainable and what are the benefits of designing and building in this manner for the environment, my family and my pocketbook”?
When executed successfully, a sustainable house will result in overall energy savings, improved occupant comfort and health, and a preservation of natural resources. By integrating proper strategies for daylighting and natural ventilation, and harnessing natural sources for heating and cooling, significantly less electricity, natural gas and other natural resources will be required to light, heat and cool your house. Simultaneously the same strategies will result in improved occupant comfort and health. Studies have shown that improved occupant access to natural daylight in buildings results in reduced stress levels, healthier sleep cycles, improved affects on the immune system, and improvements to occupant mood and psychological wellbeing (Source). Similarly, natural ventilation has been shown to not only increase occupant comfort, but has also been attributed to benefits to immune system functionality and respiratory health (Source).
In many ways the most effective and longest lasting strategies for incorporating sustainable principles into your house begins with the way your house sits on your property, the overall size of your house, the layout and design of the house and its interior spaces, and the house’s utilization of naturally available resources such as the sun and wind. These are the building blocks of all sustainable design, and need to be considered thoughtfully before more complex strategies such as Solar Panel arrays and Geothermal heating/cooling are considered.
The first step in any residential design always begins with an analysis of the site and its natural attributes such as sun and wind exposure, existing trees and vegetation, and views. In most climates (including the Pacific Northwest), it is ideal to orient structures along the east/west axis in order to maximize southern glass exposure. This allows the maximum amount of daylight and heat from the sun to enter the house during the cold winter months when the sun angles are low. It is also necessary to incorporate sufficient shading to shield the southern glass area from the hot summer sun by incorporating deep overhangs, and other shading devices which become useful as the sun moves to higher angles in hotter summer months. It is also important to note the prevailing wind direction. Ideally the largest surface areas of the exterior walls are oriented toward cooling summer breezes so the house can utilize natural ventilation to remain at a comfortable temperature throughout the warm summer months. Natural features such as hills (topography) and trees can shade and modify wind and ventilation of a house.
Once an appropriate site location and orientation of the house has been selected, the design team will begin select a home’s overall massing, roof forms, materiality and a breakdown of solid walls and glass areas. These decisions not only have an important impact on the exterior and interior spaces, but they also present an opportunity to react to the environmental characteristics of the site. Since the control of the interior temperature of a home has a major impact on both the comfort of occupants and the cost of energy usage, it is important to consider ways in which the design of the house itself can help to moderate temperatures. Traditionally, passive heating and cooling strategies have been used to achieve more stable interior temperatures with better air quality. It is important to consider the window design and orientation, appropriately designed overhangs and sunshades, the incorporation of thermal mass (such as a concrete floor) and strategies for encouraging natural ventilation.
Window orientation and size have a large effect on solar heat gain and therefore the indoor comfort levels of residents. While solar heat gain is a negative attribute during the summer it is usually positive in the winter. The size of overhangs and sun shading devices should be carefully calculated based on the exact geographical location of the site to be short enough to allow the sun’s heat to penetrate window openings during the winter, but long enough to cut off the sun during the summer in order to shade the glass and the interior spaces. While this strategy is fairly straight forward during the summer and winter, it is more difficult to consider shading requirements in the spring and fall since the sun is at the same angle during both seasons but additional solar gain is often desired in March although not in September.
The orientation of the predominant glazing areas in a house are also important to consider early on in the design process. In the northern hemisphere, northern windows lose heat while southern facing windows allow heat gain into the interior throughout the day. In climates with large temperature swings between day and night, it is beneficial to locate large glazing areas facing east to pick up the warm morning sun to compensate for the cold night temperatures. In warmer climates, western windows are the most prone to overheating, and skylights will have the largest heat gain compared to vertically oriented windows and doors.
Aside from the thermal benefits of moderating or increasing the sun exposure to interior spaces, window and skylights provide an opportunity to introduce various types of natural light in the house. Utilizing natural daylight can drastically reduce the dependence on electricity for lighting and it has been shown to increase occupant’s comfort, productivity and happiness. Indirect light tends to be the most pleasing to occupants as it produces less glare and heat gain. This can be achieved through non-south facing windows or by using light shelves on south-facing windows to reflect light off of the ceilings.
Another element that can aid in both heating and cooling months is the integration of thermal mass into the house design. Thermal mass refers to materials with a high resistance to changes in temperature such as concrete, masonry or water. Objects with high thermal mass absorb and retain heat, slowing the rate at which the sun heats a space and the rate at which a space loses heat when the sun is gone. Thermal mass is best used in climates with large swings in temperature between day and night, and is not effective (actually detrimental) when used in locations with constant hot or cold climates.
Of the many natural ventilation methods typically utilized in a residence, encouraging cross-ventilation is probably the most well known. Typically this is achieved by placing operable windows on the windward side of the house, as well as operable windows on the opposite side of the house which help to draw the cooling breezes through the house introducing fresh air to the interiors and allowing hotter interior air to escape. Placing low windows on the side of the house with a cooler (northern) orientation can aid the intake of cooling ventilation in hotter parts of the day.
A ventilation method that works well even when there is very little air movement outside is called Stack Ventilation which utilizes the “stack effect” which relies on the principle that hot air rises due to its relative buoyancy creating lower pressure in the lower areas of the house which causes cold air to be sucked in through lower spaces. For Stack Ventilation to work in a residence you need plenty of low operable windows combined with high operable clerestory windows, ventilation stacks or wind scoops on or above the roof.
Night purging is a method of passive ventilation that works especially well in climates with significant temperature differentials between night and day. Typically night purging is achieved by keeping the windows closed during the day to maintain relatively cool temperatures during hot days, and opening all of the windows during the night to allow cool air to flush out the hot air that has built up throughout the day. Night purging works especially well when paired with thermal mass such as an exposed concrete floor, since the mass will remain at lower temperatures for longer periods than other materials and can help to bring down the average temperature in the house.
As insulation and glazing technology has drastically improved over the years, reliance upon passive cooling and heating strategies has become less critical to moderating the temperature of a house. A well insulated modern envelope has the ability of maintaining a constant temperature, and window and door technology has improved drastically with the introduction of double and even triple-glazed units, gas fills between panes, and Low-E films with heat and UV blocking properties. Super insulated homes combined with low air infiltration (R60 roofs, R-40 walls for example) reduce both heat gain and loss in modern homes.
The first and simplest strategies we bring to the design of every home is a combination of reducing house size, simple passive heating and cooling strategies, and good orientation on the land, along with a well insulated and sealed house that is designed in response to the local climate and site specific characteristics. Part II of this series will discuss the potential “active” or more complex mechanical-electrical systems that can be used to supplement a “sustainable house”
Josh Meharry, RA. LEED AP.
Rhodes Architecture + Light