How Your Home Works

The Basics





House Performance
We expect our homes to provide shelter from the sun, rain, wind and snow, and we
expect them to keep us warm and comfortable. We also expect them to be sturdy and
durable.

A number of factors work together to meet these needs. These include the building
shell, the outside environment, the mechanical system and the occupants themselves.



The Building Envelope
The building envelope is the shell of the house that protects us from the elements; it
comprises the basement walls and floor, the above-grade walls, the roof, the windows
and the doors.

We expect a lot from the envelope: it must provide structural support for the walls and
roof, protect the structure from deterioration, allow for natural lighting of the interior
and serve as a means of getting in and out. Finally, the envelope must separate our
warm and comfortable controlled indoor environment from the weather outside.

To maintain our indoor environment, the envelope must control the flow of heat, air
and moisture between the inside of the home and the outdoors.



The Envelope and Heat Flow


As part of the controlled indoor environment, we add a heating system to overcome
the cold Connecticut winters. We try to build our homes so that we don't heat the  
Outdoors. We try to keep the heat in!

But heat will move wherever there is a difference in temperature. Basically, heat flows
from areas of warmth to areas of cold.

Many people believe that because hot air rises, most heat loss will be through the
ceiling. This is not necessarily so. Heat moves in any direction – up, down or sideways
– but always from a warm spot to a colder one. A heated room over an unheated
garage will lose heat through the floor. Similarly, heat loss can occur through walls in
the basement or crawl space, as well as above the ground. Heat moves to the cold. It's
the job of the envelope to control the flow of heat between the indoors and outdoors.



How Does Heat Flow?
Heat flows in three distinct ways. In a part of the house envelope, such as a wall, heat
can move in any or all of three ways at the same time.

Conduction. Heat can be transferred directly from one part of an object to another
when particles bump into each other. For example, the heat from a cast iron frying pan
is transferred to the handle and eventually to your hand. Some materials conduct heat
better than others. Insulation works by reducing heat flow with tiny pockets of air,
which are relatively poor conductors of heat.
Convection. Heat can also be transferred by the movement of a fluid, such as water,
or air. In an uninsulated wall space, for instance, air picks up heat from the warm wall
and then circulates to the cold wall, where it loses the heat. Some heat is also
transferred by the mixing of warm and cold air.
Radiation. Any object will radiate heat in the same way that the sun radiates heat.
When you stand in front of a cold window, you radiate heat to the window and so you
feel cold, even though the room temperature may be high.


The Envelope and Airflow
Uncontrolled airflow through the envelope can be a major source of heat loss and can
lead to other problems. Since warm air can carry large amounts of water vapour,
airflow is also the main means by which moisture is carried into the envelope.

Under winter conditions, air is forced through the building envelope. Air moving out
carries heat and moisture, while air moving in brings uncomfortable drafts and dry
winter air.

For air to move from one side to the other, there must be a hole in the envelope and a
difference in air pressure between the inside and outside. The difference in air
pressure can be caused by any combination of the following:

Wind effect. When wind blows against the house, it creates a high-pressure area on
the windward side, and air is forced into the house. There is a low-pressure area on
the leeward side (and sometimes other sides) where air is forced out.
Stack effect. In a heated home, the less-dense warm air rises and expands, creating a
higher-pressure area near the top of the house. Air escapes through holes in the
ceiling and cracks around upper-storey windows. The force of the rising air creates
lower pressure near the bottom of the house, and outside air rushes in through cracks
and openings around the lower floors.
Combustion and ventilation effect. Appliances that burn fuels such as wood, oil or
natural gas need air to support combustion and provide the draft in the chimney.
Open chimneys and fireplaces tend to exhaust lots of air. This air is replaced by
outside air drawn in through the envelope. This is why a room often becomes drafty
when there's a fire in the fireplace.
Ventilation fans in the kitchen and bathroom, central vacuum systems, stove-top grills,
clothes dryers and other exhaust fans also cause this effect.



The Envelope and Moisture
Moisture can cause concrete to crumble, wood to rot and paint to peel; it can also
damage plaster and ruin carpets. In its many forms, moisture is a major cause of
damage to building components.

Moisture can be solid, liquid or gas (water vapour). It can originate from outside as
ground water in the soil or as ice, snow, rain, fog and surface run-off, or from inside in
the form of water vapour produced by the people in the house and their activities,
such as washing, cleaning, cooking and using humidifiers.

In its different forms, moisture can move through the envelope in a number of ways:

Gravity. Water running down a roof or condensation running down a window pane
shows how gravity causes water to move downward.
Capillary action. Water can also move sideways or upward. Capillary action depends
on the presence of very narrow spaces, as with lapped siding or porous materials,
such as concrete or soil. (Think of how a paper towel absorbs water.)
Diffusion. Water vapour sometimes moves directly through materials by diffusion,
depending on the difference in water-vapour pressure and the material's resistance to
this pressure.
Air movement. As water vapour, moisture is carried by moving air. This can happen,
for example, where there is air leakage through a crack in the house envelope.
Far more moisture can be carried by airflow through a small hole in the envelope than
by diffusion through building materials.


Condensation


Water vapour becomes a problem when it condenses and becomes liquid. This
happens at 100 percent relative humidity, when the air cannot hold more water vapour.

A typical example is condensation on windows. Air contacting a cold window loses
heat. As it cools, it loses its capacity to hold water vapour, and condensation occurs
on the surface of the window.

Condensation on an extremely cold window appears as frost. Because the interior
surface of a single-glazed window is colder than that of a double-glazed window, a
single-glazed window is more likely to have problems with condensation or frost
buildup, even under conditions of lower humidity. Condensation is more likely to occur
in humid areas of the house, such as the kitchen and bathroom.



The House as a System


Before beginning any retrofit work, it's a good idea to review what's involved and to
understand how your work might affect other aspects of the house. If you think things
through and plan carefully in the early stages of work, the work will meet your
expectations and you will have fewer unpleasant surprises.

A house is made up of components that work together to form an integrated system.
The performance of one component depends on its relationship with other
components in the same system. Your home's ventilation and heating components,
construction materials, their assembly and the behaviour of occupants all interact – a
change to one can affect all others.
Call for further information or to schedule an appointment
Bill Hoffner
Owner
(860) 889-7246
How Your House Works