PRINCIPLES/THERMAL ENVELOPE
Moisture Transfer
Moisture transport within and through the building envelope must be controlled to prevent moist air
from contacting and condensing on cold elements within the envelope. Condensation, and
subsequent freezing, within the envelope can result in efflorescence on exterior facades, shifting
and failure of exterior cladding, disruption of parapets, and wetting, staining and damage of interior
Moisture moves through walls primarily through diffusion and convection, with convection generally
being associated with much larger rates of moisture transfer. Gravity forces and capillary action
can also be important, particularly at the facade of the building. Moisture diffuses through materials,
or assemblies of materials, at a rate determined by the water vapor pressure difference across the
material and the resistance of the material to water vapor diffusion. Similar to thermal conductivity,
some materials (glass, metal) have a high resistance to water vapor transfer while others (some
paints and insulation materials) have little resistance to diffusion. The moisture transmission
properties of envelope materials must be considered in relation to the insulation properties and the
expected temperature profiles within the wall as discussed in the section Design/Vapor Retarders.
While moisture transfer via diffusion is generally not as significant as the convective transport of
water, it still needs to be accounted for in thermal envelope design.
Convective transport of moisture refers to moisture carried by airflows through the building
envelope. Warm air can carry significant quantities of moisture, and typical air leakage rates result
in moisture transfer rates several orders of magnitude greater than the rate of moisture transported
by diffusion. The rate of convective moisture flow depends on the airflow rate and the moisture
content of the air. While an effective vapor retarder will control diffusion, a continuous air barrier
system is necessary to control convective moisture transfer. In some envelope designs, a single
system can perform effectively as both a vapor retarder and an air barrier.
Thermal Envelope Elements
The flows discussed above are controlled through the design and construction of the building walls,
roof, glazing systems and foundation. A variety of opaque wall systems exist, employing thermal
insulation, air barriers and vapor retarders to control these flows. The manner in which these
elements are best incorporated into walls is the main thrust of these guidelines in the Systems
section. Because of the emphasis on thermal insulation and air leakage defects, these guidelines
concentrate on these opaque sections of the building envelope. In the development of these
guidelines, very little information specific to the thermal performance of commercial building
foundations was found. For many of the wall systems, details on the connection of the wall
insulation and air barrier to the foundation are included.
The other major thermal envelope elements are windows and skylights. These guidelines do not
address glazing system design other than the thermal integrity of the connection of these systems
to the opaque portions of the envelope. The lack of inclusion of fenestration system design does
not at all imply their lack of importance to the energy balance in commercial buildings. Fenestration
systems are major elements in the energy balance of office buildings, and their design is a critical
part of the building design process. The selection of glazing materials, systems and window
treatments such as overhangs and shading devices can have major impacts on building energy
use. Daylighting strategies are available that can improve the environment within the building and
reduce energy use, and fenestration system technology is developing continually to improve
performance. Information on the design of windows and skylights is available in a variety of
sources including the chapter on fenestration in the ASHRAE Handbook of Fundamentals, the
AAMA handbook on skylight design and in Hastings and Crenshaw.
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