SPRAY FOAM HAS AN “EFFECTIVE
R-VALUE” THAT IS HIGHER THAN
ITS TESTED R-VALUE
No, this is not correct.
Your spray foam insulation does not have a higher “effective
R-value” than its tested R-value. Stop saying that it does – this is
illegal, and gives you and the industry a bad reputation.
R-value is R-value; it is derived and documented based on a tested
value, K-Factor, and does not magically have a higher resistance
than the value listed on its TDS just because you think it is better.
Let me explain.
K-Factor is a physical property measured using ASTM C-518 and
is important when it comes to optimizing a building’s thermal
performance and meeting energy code. K-Factor is a material’s
thermal conductivity; measured as the amount of heat that passes
through a thickness of material per unit area in one hour, if the
temperature difference between the hot and cold side of the
material is one degree. So the lower the K-Factor, the better the
thermal performance. To put units to it, it is the number of BTUs
that would pass through one inch of a spray foam material per
square foot, in one hour, if there was a one-degree Fahrenheit
difference between the hot side and the cold side of the foam
While K-Factor is the measured value, most of the construction
industry works with and refers to R-value.
R-value is thermal resistance and is the inverse, or opposite, of
thermal conductivity (K-factor), so the higher the R-value, the
better the thermal performance. R-value is the amount of heat
that is resisted and does NOT pass through a thickness of material
per unit area in one hour, if the temperature difference between
the hot and cold side of the material is one degree.
On the other hand, when used appropriately, open-cell foam
wins the economic analysis every time. Both types of foam are air
barriers and when installed at equivalent R-values (i.e. different
thicknesses), closed-cell foam can easily cost twice as much as
open-cell foam, because the cost per R of closed-cell foam is
double that of open-cell foam, with no thermal performance
So, because open-cell foam is more cost-effective when installed
at equivalent R-values, its payback timeframe is much quicker.
This means open-cell foam is typically the better choice for interior
applications that don’t require the additional physical properties
of closed-cell foam; most often this includes interior building
envelope applications, like wall cavities and attics, in climate zones
1 through 4.
Next, an attendee comes by and says, “The guys in the booth over
there said 3" of their closed cell foam has ‘effective R-value’ of
R- 38, what’s the effective R-value of your spray foam?”
When it comes to the building industry and all of this thermal
performance business, the problem is that K-Factor and R-value only
measure one form of thermal transmission, conduction, or heat transfer
by touch; these values do not address convection (heat transfer by air
movement) or radiation (heat waves).
So, R-value alone does not tell the whole story.
R-value of an insulation can be derated, based on poor application, this is
where effective R-value comes into play. All insulations are designed to
be installed so that they are in contact with an air barrier on all six sides
of the insulation. So, if fiberglass or cellulose is not installed properly, not
uniform and has gaps and voids, it can be derated. Meaning its effective
R-value would only be a fraction of its tested value.
This is a little different for spray foam insulation, because the insulation
is an air barrier. In a stud-wall cavity application, the insulation does
not have to contact the inner face of the cavity, because the insulation
is already on the same plane as, and is already in contact with, the air
barrier because the insulation and the air barrier are the same material.
By providing an air barrier solution, spray foam can reduce a structure’s
convection heat transfer load and provide a thermal benefit that is not
captured by R-value.
Additionally, spray foam insulation can also have an impact on radiant
heat transfer and the associated radiant loads.
Have you wondered why a traditional, vented attic temperature could
easily be over 120°F when it is only 90°F outside? This is primarily due to
heat radiating off of the superheated roof deck to the inside of the attic.