Another design and construction issue relevant to thermal insulation is the need to avoid convection
around and through the insulation material. As discussed in Brand, if the insulation is applied or
ends up in a position that allows air to circulate around it or behind it, convection currents will be set
up that can reduce the insulating value of the insulation system as a whole. For example, if an air
space exists between a masonry backup wall and a layer of cavity insulation in a heating situation,
the air will be warm and will tend to rise within the cavity. As it rises, cold air from outside the
insulation will be drawn into the air space and be warmed. The effectiveness of the insulation
system will be quite poor, and heat transfer through the wall will be well above that determined
based on material R-values alone.
While some envelope designs intentionally include air spaces that serve an insulating role, such as
with radiant barriers, these air cavities need to be well-sealed and carefully designed and installed
for optimal performance. Undesirable air spaces that lead to convection around and through
insulation occur when a design incorporates ill-considered air spaces into the envelope and when
the insulation is attached to an irregular surface. These spaces also arise when the insulation is
repositioned over time due to air pressures or forces arising from differential movements of
envelope elements. Irregularities in exterior walls, especially masonry backup walls, make
insulation attachment a critical issue for the avoidance of air spaces. In cavity walls the insulation
attachment system should be able to accommodate surface irregularities and hold the insulation
tight to the air barrier. The attachment of rigid insulation boards by adhesive alone is more
problematic because some adhesives do not have the tensile strength to hold the boards close to
an irregular surface. Also, some insulation materials are too stiff to conform to these irregularities.
Attachment of insulation with dabs of adhesive can make the situation worse due to air spaces
between the dabs.
ASHRAE Handbook of Fundamentals, American Society of Heating, Refrigerating and Air-Conditioning
Engineers, Inc., Atlanta, 1989.
Brand, R., Architectural Details for Insulated Buildings, Van Nostrand Reinhold, Toronto, 1990.
ICAA, Insulation Application in Commercial Construction, Insulation Contractors Association of America,
SPI, Contractor/Applicator Handbook. Spray Applied Polyurethane Foam Perimeter Insulation, The Society of
the Plastics Industry, Inc. Polyurethane Foam Contractors Division, Washington, DC, 1991.