The key aspect of thermal insulation system performance is maintaining continuity over the entire
building envelope. This involves placing and attaching the insulation so that there are no gaps
between insulation elements, and between the insulation and its substrate. Thermal bridges must
be avoided, and the insulation must remain in position over time. BIA Technical Note 21A
discusses insulation of cavity walls, covering topics of materials and their properties, and points out
two general criteria for cavity insulation. First, the insulation must allow the cavity to perform its
function of providing a barrier to rain penetration and allow moisture to drain back to the outdoors.
Also, its insulating properties must not be degraded by moisture in the cavity. Two other important
issues regarding insulated cavity walls are the manner in which the insulation is attached and the
position of the insulation, inside or outside the inner wythe.
The debate on whether to place insulation within the cavity or on the inside of the inner masonry
wythe has been going on for decades. Both alternatives have advantages and disadvantages as
discussed below. An advantage of interior insulation is that the insulation (and often the vapor
retarder and air barrier) can be installed from the floors after the masonry work is complete. The
installation can then be easily inspected and any defects repaired. One disadvantage of interior
insulation is that the entire building envelope, and perhaps elements of the structural frame, are
outside of the insulation and subjected to the full range of outdoor temperature fluctuations. This
exposure increases the associated dimensional changes and places more severe requirements on
materials. Also, the insulation (and again often the vapor retarder and air barrier) are not
continuous over the building envelope but are interrupted by floor slabs, beams, columns and
partition walls. These interruptions act as thermal bridges and require very careful attention in order
to maintain the continuity of the air barrier system. Finally, when services such as electrical are
installed they can end up being cut into the insulation and the air barrier.
Interior insulation often involves friction-fit batts installed between furring strips or studs. If this
approach is used, the batt must fill the entire space to restrict any airflow, since airflow through or
around the insulation will severely degrade its effectiveness. To this end, the spacing between the
furring or studs must be kept uniform so that the batts are held securely. The insulation must be
continuous over the entire interior surface, with no gaps at the floor or ceiling. If there is a dropped
ceiling, the insulation must be carried past the ceiling to the slab above.
Cavity insulation also has advantages and disadvantages. On the plus side, the insulation can be
applied over the entire backup wall, uninterrupted by floors, beams, columns and other elements,
greatly reducing thermal bridging. The structural frame and the inner wythe are now separated
from the outdoors by the insulation, providing a more stable temperature environment. The concern
about electrical services, chases, ducts, etc. penetrating the insulation, vapor retarder and air
barrier are eliminated. One disadvantage of cavity insulation is that since the insulation and
masonry go up together, it is more difficult to inspect the work and repair any defects. The
installation must be applied from a staging, and weather conditions can interfere with construction
and affect the quality of the work. Also, the insulation must be worked around the veneer ties in a
manner that does not compromise the insulation system effectiveness. Care is required in
developing the flashing and insulation details so that they do not interfere with each other.