Building Design and Construction Handbook - Merritt - Ventech!

13.22 SECTION THIRTEEN
13.3.7 Thermal Insulation
A substantial reduction in heating and cooling loads can be made by the judicious
use of thermal insulation in wall and roof construction. Addition of insulation results
in an increase in thermal resistance R, or a reduction in the coefficient of heat
transfer U of the walls and roof.
Any material with high resistance to flow of heat is called insulation. Many
kinds of insulation materials are used in building construction. See Art. 12.3.
Note that the maximum overall conductance U encountered in building construction
is 1.5 Btu/(hr)(ft 2 )(�F). This would occur with a sheet-metal wall. The metal
has, for practical purposes, no resistance to heat flow. The U value of 1.5 is due
entirely to the resistance of the inside and outside air films. Most types of construction
have U factors considerably less than 1.5.
The minimum U factor generally found in standard construction with 2 in of
insulation is about 0.10.
Since the U factor for single glass is 1.13, it can be seen that windows are a
large source of heat gain, or heat loss, compared with the rest of the structure. For
double glass, the U factor is 0.45. For further comparison, the conductivity k of
most commercial insulations varies from about 0.24 to about 0.34.
13.3.8 Convection
Heating by natural convection is very common, because air very easily transfers
heat in this manner. As air becomes warmer, it becomes less dense and rises. As
it leaves the proximity of the heating surface, other cooler air moves in to replace
the rising volume of heated air. As the warm air rises, it comes in contact with
cooler materials, such as walls, glass, and ceilings. It becomes cooler and heavier
and, under the influence of gravity, begins to fall. Hence, a circulatory motion of
air is established, and heat transfer occurs.
When a heating device called a convector operates in a cool space, heat from
the convector is transmitted to the cooler walls and ceiling by convection. The
convection process will continue as long as the walls or ceiling are colder and the
temperature difference is maintained.
Heating of building interiors is usually accomplished with convectors with hot
water or steam as the heating medium. The heating element usually consists of a
steel or copper pipe with closely spaced steel or aluminum fins. The convector is
mounted at floor level against an exterior wall. The fins are used to greatly increase
the area of the heating surface. As cool room air near the floor comes in contact
with the hot surfaces of the convector, the air quickly becomes very warm and rises
rapidly along the cold wall surface above the convector. Additional cold air at floor
level then moves into the convector to replace the heated air. In this manner, the
entire room will become heated. This process is called heating by natural convection.
13.3.9 Radiation
The most common form of heat transfer is by radiation. All materials and substances
radiate energy and absorb radiation energy.
The sun is a huge radiator and the earth is heated by this immense source of
radiated energy, which is often called solar energy (sunshine). Solar-collector de-