PRINCIPLES/THERMAL ENVELOPE
2.2 THERMAL ENVELOPE PERFORMANCE
The previous section described the various performance requirements that must be considered in
the design and construction of the building envelope. This section concentrates on the thermal
performance requirements of building envelopes, i.e., the control of heat, air and moisture transfer
between the inside and outside of a building. Discussions of these flows exist elsewhere (ASHRAE,
Brand, Hutcheon), and this section presents only a brief overview.
Heat Transfer
Heat is transferred by three mechanisms: conduction, radiation and convection. The rate of
conductive heat flow through an envelope element is determined by its thermal conductivity, the
temperature difference across it and the thickness and area of the element. The rate of conductive
heat transfer through an element is described by its U-value, the rate of heat transfer divided by the
temperature difference and the area, or the R-value, the inverse of the U-value. Given the same
temperature difference across a 2.5 cm (1 inch) thick piece of steel (low R-value) and a 2.5 cm
piece of insulation (high R-value), heat will be conducted through the steel at a much higher rate.
Controlling conductive heat flow across a building envelope involves increasing its R-value. This
can be done through the use of materials with low thermal conductivities and by increasing the
thickness of envelope materials, specifically the insulation.
Insulation levels are generally chosen based on an analysis of the severity of climate and the
material costs balanced against future energy costs. However, specifying a certain insulation level
for a building only applies to the insulated portions of the building between structural elements and
only if the insulation is properly installed. Such structural elements, and other penetrations of the
insulation system by elements with significantly higher values of thermal conductivity than the
insulation, are often described as thermal bridges. Installation problems include the occurrence of
gaps and voids in the insulation that increase the heat transfer rate through the envelope. One of
the major points of these guidelines is that the actual insulating value of a wall can be quite different
from the design value due to thermal bridging of the insulation, other discontinuities in the insulation
system design or poor installation. In order to effectively control heat conduction, the envelope
must be insulated continuously, with minimal interruptions by structural elements and other
penetrations.
Heat transfer by radiation is primarily a glazing system issue, though it does occur within the
envelope. Radiative heat transfer through the glazed portions of the building envelope is a complex
issue involving interior heating and cooling loads and daylighting strategies. Significant amounts of
energy can be transferred through radiation, making glazing system design very important to the
energy balance of a building. Although glazing is a very important thermal envelope performance
issue, these guidelines only address glazing systems in relation to the maintenance of airtightness
and thermal insulation integrity at the connection of the glazing system to opaque portions of the
envelope.
Convection is heat flow carried by the bulk movement of air between two locations at different
thermal conditions, and can be a significant factor within the envelope, either intentionally or
unintentionally. Air can circulate within even very small spaces, resulting in significant heat flows.
While properly designed air spaces can be part of a thermally effective building envelope, it is
otherwise undesirable to have gaps between envelope components, particularly between the
insulation material and adjoining elements. Convective heat transfer is also associated with air
leakage through the building envelope.
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