ing them from the insulation and resulting in a
simulated R-value that was essentially identical
to the design intention (Figure 3).
Our study showed that the continuity of the
first inch is critical for the efficiency of the spray
foam insulation performance. By simply pulling
the studs in-board, even by a small amount, to
allow a percentage of the insulation to be uninterrupted, the assembly R-value can be increased
by about 70%. In the event that the studs are
required to support exterior sheathing, it should
be possible to fasten the sheathing using discontinuous shims or spacers so that once again, the
majority of the insulation in that outer 1-in layer
remains continuous. It’s important to remember that other factors—such as the continuity
of the slab through the insulation, or window
openings—will often decrease the thermal performance from our ideal conditions. But these
too can be improved through careful detailing.
The ultimate lesson is that small changes in the
design that permit as little as an inch of continuous insulation can lead to dramatic improvement in overall performance.
CASE STUDY: RAINSCREEN SUPPORTS
Rainscreens are a common exterior cladding
The increasing significance
of thermal bridging in lab
continued from page 13
the actual R-value of many façades is approximately 40 to 70% less than the design-intended
R-value, so our findings suggest far greater
significance than was originally anticipated. As
the amount of insulation we specify continues
to increase, the conductive losses resulting from
thermal bridging will continue to grow as a
percentage of the building’s total energy load.
Adding more insulation will have a diminishing
return as the heat flow through the envelope is
increasingly determined by the thermal bridges.
Our research suggests that we are near a point
now where the key to improving thermal performance lies in better detailing rather than
increasing insulation thickness. A simple illustration of this is featured in the renovation case
studies of three existing masonry façades.
Spray-applied insulation is once again
gaining popularity particularly because of its
ability to fill unseen voids and often provide
an integral vapor barrier. In the northeast,
it’s a particularly popular technology for
renovating existing un-insulated masonry
façades. Conventional details often call for
metal studs to support interior gypsum board,
and spray foam is installed between the studs
following manufacturers’ recommendations.
Unfortunately, this typical installation creates
discontinuities at 16-in or 24-in center spacing.
While the web of the steel stud is quite slender,
they are highly effective heat-transfer devices
because of the conductivity of the material and
the flanges which provide significant contact
area to collect and disperse heat.
Thermal images of three existing building’s
renovations revealed dramatically different
results. The first case, Building 1, had applied
3-in of insulation, Building 2 employed 2-in
of insulation and Building 3 had used 4-in
(Figure 2). While hand calculations of the
thermal resistance would show the façade with
the least insulation to be the poorest performer
and the one with the most insulation to be the
best, the thermal images revealed a different
story. Building 1 had placed the steel studs flush
against the exterior construction, resulting in
an R-value that was 54% less than the calculated R-value. Building 2 pulled back the studs
1-in, allowing for half of the applied insulation
to be continuous and decreasing the R-value by
only 16%. As a result, Building 2 was observed
to have a higher R-value than Building 1,
despite having less insulation. Building 3 took
Figure 2: Thermal images and details of the three cases studied.
Figure 3: Simulations of the three spray foam cases