HOME HEAT LOSS, HOW AND WHERE
Escaping warm air and intruding cold air through the building envelope
Heat Loss by Inadequate or Defective Insulation
Heat Loss by Air Movement
Infiltration of Outside Air in Winter
Air movement associated with heat loss through your home is caused by three main forces:
the stack effect, mechanical systems and wind. The most prevalent cause is by the stack
effect. Simply put, warm air rises when surrounded by cold air, i.e. the heated air in your
home rises when surrounded by the cold outside air. As the illustrations below points out,
this rising heated air escapes through the top of the building envelope thus creating low
pressure in the lower levels of your home and drawing in cold air.
Heat energy escapes from your home primarily in two ways, through air leaks and
by inadequate or defective insulation. First lets look at heat loss by air movement.
Very leaky houses are uncomfortable and have high energy bills, so finding and curing
infiltration problems is a high priority for weatherization operations. The rate of air
infiltration in a home depends on many factors, the most important being the size and
location of holes in the thermal envelope and the difference in temperature between inside
and outside. Wind and leaks in duct systems can also have a major effect on infiltration,
but these effects are not usually as constant over the heating season as is stack effect
infiltration, which is at its worst on coldest days. Note that in the middle of the heated
envelope there is a neutral pressure zone where neither infiltration nor exfiltration occurs
due to stack effect. This explains why caulking and weather stripping in mid-envelope tends
to save less energy than careful attention to the bottom and top of the envelope, where
these natural driving forces are greater.
Before adding insulation to your attic it is beneficial to air seal beforehand. This is because
its much easier to locate and correct these leak points before adding insulation. The
existing insulation in your attic should be evaluated in terms of its current effectiveness.
Rolled and fiberglass batt insulation performance can be significantly reduced if incorrectly
installed which is common. Also common is displaced insulation due to remodeling projects,
upgraded electrical fixtures and even animal infestation. These potential issues should be
resolved before adding additional attic insulation.
Side wall insulation is not nearly as accessible and open to evaluation as other areas of the
building envelope, but is visible with infrared. Common defects are caused by settlement
and in the case of many older homes built before the 1960's, no sidewall insulation at all.
Conductive heat loss through uninsulated side walls can represent a significant heating
and cooling load, and its usually cost effective to add insulation.
What is a building envelope?
The exterior of your home is also called the "envelope" or shell. (See the orange line
showing the envelope in the diagram.) The insulation, outer walls, ceiling, doors, windows,
and floors all work together to control airflow in and out of the structure, repel moisture, and
prevent heat from being lost or gained inside your home. A high-performance envelope
helps maintain a consistent temperature even under extremely hot or cold conditions. The
goal of Home Sealing is to improve the home envelope to make homes more comfortable
and energy efficient.
Thermal Resistance Dual Effect
Every measure taken to improve the building's thermal resistance to heat loss also results
in reduced heat gain during the warm summer months, thus improving year-round thermal
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