Green Building is as much, if not more, about the process of how a home is designed and built as it is the products that go into the building. The successful blend of products that go together are dependent on the design and construction practices to achieve maximum success. To fully understand the benefits of a healthy and comfortable structure, it is helpful to understand how a building is meant to function.
We take many aspects of our homes for granted. At its root function, our homes should keep us comfortable, dry, and not jeopardize our health. They are meant to be structurally sound, withstand temperature changes, keep the weather out, and be visually and aesthetically pleasing. Understanding the science of how our homes function will provide a foundation to explaining the benefits of a green designed and constructed home.
The last several decades have seen a growing awareness and demand for more insulation in our homes. Increased insulation is intended to keep us comfortable while reducing energy consumption. Heat, or energy, is transferred by three methods – conduction, convection, and radiation – and always flows from warm to cold. Conduction is the transfer of energy via contact between a warmer and a colder object. Energy is transferred in a home as heat is absorbed into the building materials and transferred through a wall or ceiling assembly. For example, in the winter heat is transferred from the wall covering, or sheetrock, to the framing members to the building skin and is ultimately lost to the colder exterior through the contact of all those materials. Placing a layer of exterior rigid insulation will help to eliminate that conductive heat loss.
Convection is the energy transfer through moving air or liquid. As air is cooled it becomes denser and sinks; and, as air is warmed it becomes more buoyant and rises. On a winter night, the air in front of window is cooled and starts to sink causing a draft, which in turn, causes us to feel cold. So, let’s go back to the wall described earlier. The warmer air from the interior is drawn to the colder exterior (energy flows from warm to cold), and if there any holes in the wall or ceiling assembly it will pass through as it seeks the colder areas. Energy loss by moving air can be more dramatic than conduction if there are many holes or penetrations in the building. Insulation placed in the cavities seeks to slow down that leaking warmer air. Lately, a heavy emphasis has been placed on making buildings tighter to eliminate energy loss. Products have been developed that help to stop air movement all the while better insulating the home.
The last form of energy transfer, radiation, is the transfer from a warmer object to a colder object through space. Think of the sun and all its energy. Even with the extreme distance to earth we derive a good deal of warmth and comfort from that energy at certain times of the year; and other times some discomfort. As the energy is transferred through the building by conduction and/or convection it is ultimately radiated into the cold area on the exterior. Also, as we sit in front of a window on a winter night, our body is warmer than the surround surfaces or nearby window and we also lose some heat to those surfaces.
All these forms of energy transfer have positive and negative consequences, and a green built home, or a high performance home, seeks to create a healthy and comfortable balance with effective use of all three. One of the outcomes of a thermally balanced structure is how the moisture is handled. The outcome of bulk water (rain, condensation) can easily be seen in a home through mildew, mold, and ultimately rotted and deteriorated structural members. What is not as easily seen is the outcome of moisture movement as vapor. Our homes can become humid environments. On that same winter evening, the relative humidity in a home could easily be 75%, or even higher, if we are cooking, showering, and doing laundry. The physical properties of moisture movement tell us that moisture, like energy, moves from areas of warmer temperatures to areas of colder temperatures and from areas of higher concentration to areas of lower concentration. Given those principles it becomes clear that the moisture in our homes will move into the assembly towards the colder surfaces or areas. Along with that, if there are holes or penetrations in the assembly, the movement of air not only accelerates the loss of energy, it also accelerates the movement of vapor into that assembly.
As discussed earlier, warmer air is more buoyant, has more space between its molecules, and thus holds more moisture. When it is cooled, and that space is lost, the moisture condenses, similar to squeezing a wet sponge. So back to that winter night: we could have the omnipresent vapor turning into the unwanted bulk moisture in our assemblies as it hits the unseen, cold surface.
The opposite occurs in our area in the summer where the exterior environment is more moist (higher humidity) and warmer than the air conditioned interior environment. Summers on Long Island can be very humid and warm, thus promoting the movement of moisture in the opposite direction thus leading to condensation on the unseen backside of interior surfaces.
Fortunately, designers and builders can utilize different products and processes that can mitigate that movement of both energy and moisture to keep the interior environment healthier and more comfortable. Knowing and using the right mix of materials within an assembly will allow that assembly to dry itself out by the principle of vapor diffusion, or the ability of moisture to move through materials. Materials have ratings called perm ratings that show how easily moisture can move through the material. The goal is to have only one low perm material in the assembly and more permeable materials on either side of that low perm material so the assembly can dry itself out in both directions.
It may seem like a rather daunting task to get all these parts aligned correctly, but that is what a green or high performance building is all about. Buildings are built tighter and better insulated to better manage the energy and moisture consequences of us living in them. Throughout the entire process, materials are carefully selected to maintain a healthy indoor environment while creating higher levels of comfort. The construction processes reinforce, and many times enhance, the benefits of those selected materials.