What are the characteristics that make a house “energy-efficient”?  According to the United States government there are four main criteria.  First, a thermal envelope that is tightly sealed.  Second, effective insulation (high R-value).  Third, efficient ventilation.  Fourth, lower than average utility bills for heating and cooling that result from the other three factors.  Let’s take a brief look at each of these.

First, what is a thermal envelope?  It’s all the exterior components of the house, and the additional materials that are used to protect the house from the weather outside.  This includes the roof, the framing, the exterior siding, the insulation, the vapor barrier, the windows and the foundation.

Second, let’s look at energy-efficient insulation and R-value (which measures heat transfer).  Most local building codes have a minimum requirement for insulation.  Usually, only the walls and ceilings are required to be insulated.  But an energy-efficient house will have insulation in its floors and foundation, too.  And the R-value will be higher than the building code requires.  For instance, instead of R-11 insulation in the walls, R-30 would be used.

Third, what is efficient ventilation?  It’s a way of controlling the air flow so that the thermal envelope maintains its seal, but moisture does not build up and create mold.  Ventilation is important not only to maintain the integrity of the building materials, but to maintain a healthy indoor environment.  It affects air quality and indoor pollution.  If the house is not properly ventilated, poor air quality can create health hazards.

Fourth, let’s take a look at how the first three items can lower your average utility bills for heating and cooling.  The thermal envelope includes effective weather stripping and insulated windows, as well as energy-efficient siding and roofing materials.  The quality of insulation has a direct effect on how much heat escapes on cold days or how much cool air escapes on hot days.  Controlled ventilation maintains a balance between healthy air flow and the escape of too much hot or cold air.  When all of these elements are working as they should, smaller energy-efficient furnaces and air conditioners can be used.  This translates directly into lower utility bills and less impact on the environment.