Polyurethane Foam Insulation
An opinion paper
Craig DeWitt, Ph.D.,
spray-in-place foam insulation products such as those based on a
polyurethane formulation have several beneficial aspects over other forms
of insulation. Spray foam insulation currently costs more than
alternative insulation products, but this additional up-front cost can be
overcome when the other benefits of spray foam are utilized and realized.
These aspects include benefits associated with increased
structural/strength properties, enhanced thermal insulation capabilities,
and reduced air infiltration properties.
Clemson University has
been researching the use of spray foam as an enhanced attachment system
for roofing. This research centers on how to retrofit or construct
buildings to be more resistant to hurricane and other high wind events.
Clemson’s research shows that spray foam can significantly improve
the attachment of roof sheathing to trusses and rafters, similar to the
way construction adhesives help bond a floor system together. In a
retrofit case, foam can be sprayed on one or both sides of the sheathing
rafter intersection from inside the finished roof. In new construction,
spray foam can be applied to the entire roof system. The spray foam makes
a significantly stronger roof than either nails or screws alone. More
information on this research is available from Clemson Universities Civil
Engineering Department, or the 113 Calhoun St. Project in Charleston, SC.
and air benefits:
A second aspect of
spray foam is the enhanced thermal insulation characteristics. The stated
R-Value, or thermal resistance value, of insulation is measured under
laboratory conditions. Real-life in-use R-Values are quite different. An
R-13 rated insulation batt installed improperly
may only provide R-9. Whole wall R-values may be even less because of
voids, wood, headers, etc. in the wall. Spray foam can provide a higher
whole-wall R-value because of its ability to better fill wall cavities
around electrical, plumbing, and other obstructions within the wall. The
Oak Ridge National Lab has tested several whole-wall R-values for various
wall insulation combinations. Some of their results have been published
in publications such as energy Design Update, and should be available on
their web site soon.
in Moisture & Energy Issues
The R-value of an
insulation system also depends upon the lack of air movement through the
insulation. Most insulation products use entrapped air as a barrier to
heat transfer. Therefore, to get a high R-value, air cannot move within
or through the insulation.
In a whole-house
situation, part of the energy use is in infiltration air. Air flow
retarder products such as house wraps were develop to reduce the amount
of infiltration air. These air barriers help reduce infiltration as well
as air movement trough the insulation.
Typical loose fill or batt insulation works well if installed correctly,
and if installed in conjunction with an air barrier. Good installation is
difficult to do, however. The insulation is often packed too tightly or
too loosely, cut too short or too long, gapped around plumbing and
wiring, or left out because of access problems.
Spray foams claim a
couple of benefits. First, they fill gaps and voids better. Second, they
perform well as air flow retarders. The result is a higher in-the-wall
R-value. Infiltration is also reduced, so that component of a
building’s energy use is reduced. Both of these benefits result in
raising the “effective” R-value of spray foam when compared to typically- installed loose fill or batt insulation.
Spray foam products
must still be sprayed correctly, and dense-pack blown cellulose can make
some of the same claims. Spray foam is also self-supporting, which
enables its use on the underside of roofs and floors.
underside of a roof rather than a ceiling creates many other benefits as
well. Historically, we ventilated roofs in an attempt to prevent moisture
problem and reduce heat build-up.
Current research shows
that much of the moisture in attics comes from damp basements or crawl
spaces, as well as from the living space. Research also shows that if we
address cawl space, basement, attic, and living
space moisture, we do not need to ventilate an attic. In fact, by
ventilating an attic, we can often make a moisture problem worse.
Attic moisture problems
are a result of moisture condensing on cold roof surfaces. Adding more
vents causes the attic to be cooler, especially at night, which causes
more condensation to form on the underside of the cold roof. Cutting a
hole in the roof causes a bigger hole in the top of our
“chimney”, which makes the “chimney” draw better,
pulling even more moisture upward. I have not seen any attic moisture
problems solved by adding attic ventilation, with the exception of ice
damming. (Ice damming is a “warm” attic phenomenon, and can
better be addressed by reducing the amount of heat leaking into the
attic.) Unfortunately, the building codes haven’t kept up.
Ventilation to reduce
summer heat build up in an attic has also been
challenged recently by research done at the Florida Solar Research Center
and the Building Research Council in Illinois. Much of the heat in an
attic is from radiant heat transfer. The hot sheathing radiates heat to
the ceiling or other objects in the attic. To cool an attic, outside air
is vented through attic or insulation is added to the ceiling to prevent
the attic heat from warming the living space. Research has shown that the
ventilation rate would have to be quite large to make much difference in
an attic temperature. In the summer, the best you could possibly achieve
was outside temperature. Wit a very large fan
using lots of energy, you might get close to outside temperatures. In
winter, this would result in a colder attic as well.
Ceilings are usually
insulated because of the case of piling up chap insulation. Recessed
lights, outside walls, sloped or try ceilings and knee walls all create a
non-uniform thermal “cap” on the building and result in voids
in the insulation. The real-life R-value of an insulated ceiling is very
often less than the claimed R-value.
Ducts are often located
in the attic, which exposes the coolest/warmest air in the house to the
hottest/coldest environment in the house (depending upon the season).
This does not create a very energy-efficient situation, As much as 10% of
the heat or AC can be lost by placing ducts in an unconditioned attic.
From an energy
standpoint, ducts and air handlers should be located within the
conditioned space. This reduces heat transfer to the outside, and reduces
some concern of duct leakage. Recently, building researchers proposed
making crawl spaces into unvented, conditioned plenums, which is now
accepted by code. More recently, building researchers proposed making
attics into conditioned space by eliminating ventilation and insulating
the underside of the roof rather than the ceiling. As a building
researcher, I fully support both concepts.
A roof system insulated
with spray foam reduces energy several ways. Energy loss from ducts
located in the attic is essentially eliminated. The top of the building
is much tighter resulting in less infiltration and exfiltration,
so excess moisture isn’t pulled into the attic. Infiltration
through the ceiling is also reduced. In addition, the attic temperature
is lower, which further reduces energy loads.