The Science Behind Green Building
Sustainable building takes most of us back to the classroom for a refresher on the physical properties of energy, air, and water. Green building practices, as well the selection of the appropriate building materials, revolve around a few basic principles of science. Science is what sustainable building relies on, principles that once understood can guide you every step of the way, including:
- A house is a system of interrelated parts.
- Energy loses some of its potential each time it is converted from one form to another, which explains why passive solar heat is much more efficient than electric heat.
- Form follows function when it comes to design, meaning that construction should be tailored to the environment in which the house is built.
- Air leaks in the building envelope represent a significant loss of energy and open the door to moisture damage inside wall and ceiling cavities.
- Controlling the movement of heat, air, and moisture involves every part of the building and everyone on the building team.
Building Science - Heat & Water
The science behind green building just makes sense. Careful planning and detailing at the start of a project will enrich and make the project much simpler. Builders spend just months building a home, but if it’s done correctly, the final product should last for generations to come.
Heat flow can flow in any direction (in or out), though it always flows from hot to cold. When we apply this to buildings, and the basic physics that always applies, heat is transferred in any of three main ways:
Conduction: the flow of heat through solid materials due to a temperature difference across the material. Think of a frying pan. Would you rather pick up a hot cast iron pan or a wood handled one? We all knew the answer to that one when we were 2 or 3 years old. Wood is a better insulator than iron. Insulation is a better insulator than wood.
Convection: occurs when gases and liquids are able to move and carry heat with them. Think of a cold draft on your feet on a winter’s night. Hot air rises and cold air is heavier so it falls. When you have both at the same time it creates a draft.
Radiation: occurs when heat is transferred from one surface to another without contact (conduction) or air movement (convection). An object that possesses more heat energy will radiate the heat through space to an object that is colder as we mentioned above. On a hot summer day would you rather stand in the sunshine or in the shade? The air temperature is the same but the experience of heat is a result of the radiation from the sun.
Water movement is just as important as heat flow. Water comes in three forms:
In all forms, it is a major enemy of the building world. A large part of the construction process focuses on keeping moisture out of a building. Rain and snow work hard to find their way under the shingles and siding. It can wreak havoc causing rotting, mold and other damage. It is key to prevent moisture from entering the building in order to create a healthy project that will stand the test of time.
There are many important building methods that will help prevent water infiltration. While water always flows downhill, through capillary action, water can also travel up through porous materials causing additional problems.
Note: Builders should always put felt paper on the roof from the eaves to the ridge with the upper course overlapping the lower course. The same holds true for building wrap under the siding. If it is applied incorrectly, moisture will seep through and rot the sheathing.
Moisture flow directly affects our thermal comfort. Moisture can exist as a gas, liquid, or solid. Water vapor, the gaseous form, is always present both on the inside and outside of house. Though it is commonly referred to as relative humidity, being the relative amount of water vapor in the air, at any one time, water vapor has a correlation with the air temperature, and thus our comfort. Warmer air holds more moisture, and colder air holds less. We know about dew in grass after a cool night. Moisture turns from vapor to liquid at the dew point, a temperature at which air can no longer hold the moisture.
Water (like heat) always flows from areas of higher concentration to areas of lower concentration. If there is moist, wet soil on one side of a foundation wall, it will try to find a way through the concrete foundation wall and into the dry, warm basement. It is very important to correctly waterproof foundations and install footing drains to move moisture away from the building. Damaged to foundation is difficult and expensive to repair, and can cause a whole host of other house problems.
Water can be sucked or pulled through many building materials. It can actually travel uphill and cause unseen rot under siding or under shingles. Builders have to think like water and carefully seal the smallest of gaps in a structure. Proper flashing and detailing is key to help prevent this problem.
If air can get into a wall cavity, so can moisture. As building occupants, our daily activities such as showering, cooking and even breathing create a lot of moisture. A plastic vapor barrier applied to the studs under the drywall helps prevent mold from growing in the wall. It is important to put a system in place to catch this moisture and controlling its movement. Good quality exhaust fans and making sure that the dryer vent is correctly installed are both important steps. Better yet, eliminate the cavity, or the space between the studs, that holds insulation that allows air flow. Spray cellulose or spray urethane foam do this job well.
Green Building Best Practices
- Look for designs that use energy as close to its sources as possible to minimize conversion losses: passive solar heat, for examples, over electrical resistance heating.
- Consider heat transmissions in all its forms - convection, conduction, and radiation - in selecting building materials and building practices.
- Create an effective air barrier and make sure everyone on the build team understand their role in maintaining it through various stages of construction.
- Include some form of mechanical ventilation in the house and strive for a "pressure neutral" interior as minimum requirement.
- Make sure construction details are able to handle water movement via gravity, diffusion, and capillary action.
- In areas of high noise, consider window upgrades, sound barriers, and alternatives to conventional stick framing.