green building begins with energy efficiency of the building shell

Fall 2008
Living Spaces
Homes and Landscapes of Western Massachusetts and Northern Connecticut


This article, the second in a series describing green/sustainable building, focuses on energy efficiency as it relates to reducing the heating and cooling requirements of your home.  Separating the interior of the home from the outdoor environment is critical to energy efficiency of a green home.  Energy travels through the exterior shell (walls, ceilings, and floors) in two predominant ways.  Heat energy is conducted through the shell and air is exchanged between the inside and outdoors.  Isolating the home is accomplished by reducing heat flow using insulation and by preventing air from exchanging between the interior and exterior by air sealing.  By reducing energy lost through the outer shell of the building, heating and cooling bills will decrease. 

Insulation and Air Sealing

Conduction (heat flow through a material) is reduced by insulation in the walls, floors, and ceilings.  High quality windows and doors also reduce conduction, although the windows and doors conduct significantly more heat than the wall areas.

Air motion from inside to outside affects efficiency in two ways.  Air moving through insulation significantly degrades insulation performance and the energy used to condition (heat or cool) the air is lost when it escapes.  The conditioned air escapes through penetrations/cracks in the shell, such as electrical outlets and ceiling lights (especially recessed lights even if they are "airtight").  Leaks can also occur around windows and doors.  Chimneys and duct chases are common large leak areas.  Sealing these penetrations with spray foam or other materials reduces the uncontrolled leakage between a home interior and exterior by filling the holes that allow the air to move. 

With all insulation, the quality of the installation is critical to its performance.  Some insulation types are better at reducing air infiltration.  Fiberglass typically allows the most air to travel through it.  It is difficult to cut fiberglass insulation to fit around obstacles.  Blown-in cellulose insulation is typically better at reducing air infiltration.  When blown in, it tends to fill the wall cavities much more tightly and more easily conforms to obstacles like pipes and electrical boxes.  When filled at a sufficient density, settling is not a problem.  Cellulose insulation uses recycled paper and thus reduces waste.  Spray insulation foams can also effectively insulate a house and prevent air leaks.  These foams are very effective air sealing insulations.

The strategy for achieving improvements will vary for new homes and remodeled homes.  New homes allow much more flexibility, while existing homes have more restrictions.

New Homes

In a new home, planning additional insulation is typically the most cost effective energy upgrade.  Super-insulation techniques and very high quality windows can result in a new house that is much more efficient than the energy code requires.  This strategy allows the use of standard high quality mechanical systems and is typically more cost effective than using more costly mechanical systems (solar or geothermal) to achieve similar energy use reductions.  Super insulation may involve building a double 2x4 exterior walls or adding foam insulation to the outside of a standard 2x6 wall.  Triple-glazed windows would typically be specified.  If the energy demand of the home is significantly reduced with super-insulation, the mechanical system choices become less important because fuel use will be very low.

Remodeling

In a remodeling situation, the opportunities are more limited and complicated.  The scope of the project will dictate what strategies can be used.  While remodeling, it is usually not possible to remove all finished interior surfaces within the house to add insulation and air seal the shell.   Typically the amount of insulation within an existing wall is limited.  If none exists, insulation can be blown-in from the exterior or interior.  If a minimal amount of insulation exists in the walls, blowing-in more is difficult or impossible.  However, if the house is gutted on the inside, many improvements can be made.  If the house is re-sided, foam insulation can be added to the exterior.  Two inches of exterior foam is ideal, but technical details are tricky and cost effective solutions are still being developed.  Once the insulation and air sealing are maximized in an existing home, then upgrading the mechanical systems with either higher efficiency equipment, geothermal, or solar should be considered.  These systems will be discussed in future articles.

Reducing energy consumption is a central component of green building.  Both insulation and air sealing need to be addressed to achieve significant energy use reductions and to move a house into a greener direction.

Free Energy Audits

Massachusetts electric and gas utility companies all offer free energy audits of existing homes and will subsidize insulation and air sealing improvements.  An excellent first step for an existing home is to schedule an audit to identify the best insulation upgrades.