Why Is The Thermal Envelope So Important?
When people are thinking about building a new house they generally work with an architect with a clear opinion on the style, size and rooms that will be a priority. But the most important issue every homeowner and architect should consider when building a new house is the thermal envelope and how the house will work as a system to limit the use of energy and water while also minimizing wasteful and toxic materials. This is an environmentally sound objective as well as a financial and comfort goal.
The first thing homeowners should consider when they are building or remodeling a house is the thermal envelope or shell. This part of the house must be structurally sound and protect the house from the elements including wind, water, storms, and sun. The envelope includes the foundation, walls, windows/doors, and roof. This exterior armor is the most important factor in maintaining heating and cooling in the house and preventing mold and mildew.
The materials used for the envelope of the house must be climate appropriate, structurally sound, and aesthetically pleasing, all working together. The various parts of the envelope must stop or slow the flow of air, water, and heat, while still allowing a way for water that gets into the structure to dry out.
Thermal insulation, house energy efficiency is important in all parts of the world but particularly in colder climates.
One of the best examples of thoughtful use of appropriate materials was with a house I reviewed several years ago, The New England Farmhouse, while working on Prefabulous + Almost Off the Grid. A very energy minded architectural firm, ZeroEnergy Design were the green architects on this project in Concord. MA. Because the house needed to be sited with the front of the house facing north, small windows were installed in that area to avoid heat loss. Larger windows were placed in the rear of the house to get the best advantage of solar gain. Often architects will want to be consistent in size with all the windows throughout the house - but the strategy used in this house worked best for optimizing solar gain and minimizing heat loss.
The house was built with cutting edge technology although fitting perfectly in this historical neighborhood. Windows on the northern front of the houses are small.
There is more glazing on the rear of the house for passive solar gain and excellent daylighting.
Larger windows were used on the southern side of the house to flood the area with sun.
Windows must of course be appropriate for the location. Triple pane windows, for example, are more often used in a cold climate to prevent the transfer of wind and cold air from entering the house. A double pane window will generally work just fine in a warmer or moderate climate. Windows have become increasingly efficient, beautiful and in some cases the most efficient glazing is becoming thinner. Windows are rated by U- values, with the lower numbers, the more efficient. Higher levels of insulation are generally used in the northern, colder areas and lower levels work well in warmer, more southern areas.
A similar dynamic works for insulation levels, specified by R-values. This number describes the ability of the insulation to resist heat traveling through it. Unlike U-values, for R-values, the higher the number the more efficient the material.
Air leaks in a house’s envelope can be found during construction by using a blower door test , which can expose areas of air infiltration. Finding these leaks will prevent drafts and leaks coming into the house. Leaks can then be sealed with tape, spray foam, rubber gaskets, and caulking. A good web site to find out more information about energy efficiency and the thermal envelope is at the National Renewable Energy Laboratory.