2) Thermal Performance
Best practice calls for at least two layers of insulation. The bottom
layer can be fastened or adhered, but ideally the top layers should only
be adhered in order to avoid fasteners. Metal fasteners can transfer
a lot of heat energy, creating thermal bridges from the building’s
interior to the outside. Most fasteners have a 2- to 3-inch metal plate
to withstand wind loads. Covering the fasteners with another layer of
insulation reduces a building’s thermal losses.
A second insulation layer also allows board joints to be overlapped
with staggered installation patterns, further reducing thermal loss.
All board joints should be as tight as possible and none larger than
Let’s put some numbers to potential thermal losses. Let’s say we
want an R-value of 20 and find a 3.3-inch-thick polyisocyanurate foam
with an R-value of 20. 4. If we install it in a single layer with fasteners
through the board, we have an approximate 15% loss of R-value. To
meet the insulation goal, we may need two 2-inch layers and the top
layer fully adhered to the bottom.
A question I often get from contractors is how they can eliminate
fasteners in regions with high winds that threaten roof uplift. In this
case, fasteners through all layers may be necessary. However, contractors can still meet the R-value insulation requirement, but they will
need to account for the fastener losses in their design.
Skylights and roof hatches are another component of roof thermal
performance. Codes may specify a smaller R-value for these components compared to the rest of the roof. For example, a code may
require that skylights have only an R- 5. Let’s imagine a roof that
should have an R- 30 value. If it has a large number of skylights, the
resulting thermal loss might be 3–5%. If the roof also has only a single
layer of insulation and mechanical fasteners, the roof’s actual value
THE ROOFPOINT RATING SYSTEM
RoofPoint is a rating system for the environ- mental impacts of roofing. It encompasses
management of energy, materials, water, and
About 65% of the rating system is dedicated
to durability/lifecycle and energy management.
Within the durability/lifecycle section are nine
points: durable insulation, drainage design,
traffic protection, wind uplift resistance, hygro-thermal analysis, construction moisture management, durability enhancement, maintenance
program, and installation quality management.
It is interesting to put some numbers behind
increases in service life and decreases in construction material and construction waste.
A 17-year roof, which is the average for a low-slope roof, can turn with limited additional expense into a 30-year roof. We often think about
buildings in terms of a minimum 60-year lifecycle. At year 60, a building with a 30-year roof
would need its third roof. If the building has a
17-year roof, it would be well into its fourth roof.
The savings – in energy, time, money, materials,
waste and occupant discomfort – really add up.
For more information about the rating
system, visit www.roofpoint.com.
SOME HEAT IS ABSORBED BY THE ROOF
and transferred to the building below
hits the roof
SOLAR REFLECTANCE and THERMAL EMITTANCE
the fraction of solar
energy that is reflected
by the roof
the relative ability of the
roof surface to radiate