1. The Impact of Thermal Conductivity on
Building Enclosure Durability
W
hen designers are deciding which insulation
products to specify for a given project, two common
questions come to mind: “What is the R-value?”
and “What is the permeance?” It would be unfair to take credit
for the discovery of Relativity; however, the answer to these two
questions is, “It’s all relative.” If Einstein hadn’t been so focused
on the speed of light, he would have had the time to establish
the third theory of relativity: R-value relativity. Thermal
insulation’s effective performance and permeance properties
are relative and dependent upon project location (specifically,
temperature and climate). In other words, a building assembly
will perform differently in Miami, Florida, than it would in
Anchorage, Alaska. Therefore, it is essential to understand
the dynamics of building products under regional climatic
conditions in order to increase overall building durability.
Generalizations Are Not Effective
Insulation types should not be treated equally, as there is a
wide range of varying performance characteristics. However,
some codes and standards organizations are pushing for
equal testing on these types of products. Buildings aren’t
constructed in laboratories, so why do semi-irrelevant tests,
which have no bearing on real life in-situ scenarios, persist? If
buildings were built with the quality and precision (including
white gloves and lab coats) accomplished in the laboratory,
building failures wouldn’t exist. Obviously, building failures
do exist, and the industry spends millions upon millions of
dollars annually on building repairs. A better mechanism for
relevant building components and assemblies testing needs to
be in place. Designers should ask manufacturers and product
suppliers for tested data that is relevant for their applications
and locations. It’s understandable that codes and standards are
necessary for marketing value. However, project teams should
consider additional questions and analysis outside the realm of
standardized testing.
In layman’s terms, R-value is defined as a material’s
resistance to heat transfer. One might assume that the
higher the R-value, the lower the thermal conductivity
(k-value)—resulting in higher performance. However, the
answer (again) is that it is relative. Three mechanisms of heat
transfer cumulatively determine the relativity of performance:
conduction, convection and radiation. In general, residential
and commercial insulations use an ASTM Standard test
method to measure heat flow through a given medium. In the
case of insulation, the standardized test to determine heat flow
is ASTM C518, Standard Test Method for Steady-State Thermal
Transmission Properties by Means of the Heat Flow Meter
Apparatus. This standardized approach is an accurate way to
measure and determine the R-value of a product. However,
the mean temperature to report R-value is stated at 75o
F
(24o
C). It doesn’t make sense to insulate buildings from room
temperature, but using this mean temperature of 75o
F (24o
C)
results in higher repeatable accuracy. So, is it the intention
of this test procedure to produce a more accurate result to
the nearest third decimal place or actually provide valuable
and applicable information for designers to increase energy
efficiency and envelope durability? As an industry, a great deal
of knowledge and awareness are required to fully understand
how temperature-dependent thermal conductivity (TDTC)
impacts building and component durability. Hopefully,
information provided in this article will resonate with
designers and direct them to ask the right questions, including
“What is the R-value of a product for my climate zone?”
Researchers, organizations and educational institutions
conduct research to understand how temperature affects
insulation materials’ thermal conductivity. Organizations,
such as the National Roofing Contractors Association
(NRCA), Building Science Consulting Inc. (BSCI), RDH
and the University of Waterloo in Ontario, Canada, all
invest time, money and resources to promote the in-situ
22 JOURNAL OF THE NATIONAL INSTITUTE OF BUILDING SCIENCES – AUGUST 2015
By Rockford Boyer, B. Arch. Sc., BSSO
Special, General
and R-Value Relativity
Building Enclosure Design
2. performance of insulation under realistic
conditions. Research conducted by these
organizations determined that R-values
are, in fact, dynamic, based on the
temperatures with which they interact.
Depending on the insulation type
(whether mineral fiber or foam plastic),
three or four major variables can
affect the R-value performance of the
insulation. Conduction and convection
exhibit a linear relationship between
temperature and R-value, whereas
radiation impacts R-value to the fourth
power. Additionally, blowing agents
used in certain foam plastic insulations
typically are unaccounted for, but can
have an adverse effect on R-value. These
blowing agents have both a condensation
point and a boiling point, meaning they
have the potential to change from one
phase to another phase based on ambient
temperatures. If the temperature is low
enough, condensation occurs in the
blowing agent, and the increased thermal
conductivity of the liquid in the cell
significantly reduces the R-value. “Figures
1, 2 and 3” (see this page, right and below)
illustrate heat transfer mechanisms,
as well as the effects of condensation
in the cells. Moisture present within
insulation also can drastically reduce that
insulation’s efficiency.
Theory Versus Real World
Although lab tests can help determine
the individual characteristics of a
product, these characteristics can change
drastically when installed or tested as
JOURNAL OF THE NATIONAL INSTITUTE OF BUILDING SCIENCES – AUGUST 2015 23
Figure 1: BSCI tested the apparent R-value on four polyisocyanurate manufacturers’ samples,
including a sample of high-density mineral wool roofing material. LTTR stands for long-term
thermal resistance; NRCA represents that organization’s testing results.
Figure 2: NRCA examined R-value minimum and maximum ranges for polyisocyanurate
samples.
Figure 3: The University of Waterloo tests illustrate the R-value of four common insulation materials at varying temperatures and relative humidities.
Continued on page 24
R-Value vs. Temperature & Humidity
3. 24 JOURNAL OF THE NATIONAL INSTITUTE OF BUILDING SCIENCES – AUGUST 2015
a system. In turning lab test theories
into understandable or real project
situations, a well-known hygrothermal
modeling software program, called
WUFI®
-ORNL/IBP,[1]
now can assess the
combined heat and moisture transfer
in building components based on
building type and local interior and
exterior environments. In WUFI-ORNL/
IBP, a user generates personalized
assembly constructions from a wide
range of default materials, climates
and conditions to predict their in-situ
performance.
Sometimes, though, data associated
with generic or default materials can
be inaccurate, assumed, estimated
or missing completely. This is
the case with one of the generic
polyisocyanurate materials located
in the WUFI-ORNL/IBP database for
North America (see “Figures 4 and
5,” above). “Figure 4” references the
generic default material located in the
North American database, whereas
“Figure 5” is derived from out-of-scope
testing and resembles actual in-situ
temperatures. The two graphs show a
discrepancy between the temperature
versus R-value for the default value,
as well as the actual tested value for
a similar insulation material type.
The objective of this comparison is
not to discredit the software program,
but to educate users on encouraging
manufacturers to input actual test data
into the software database. Product
manufacturers must take responsibility
in providing accurate hygrothermal
data to building professionals to ensure
designs are as realistic as possible.
Refer to “Figure 6” (see below) for
a realistic thermal prediction of an
ASHRAE-compliant wall with the use of
three common continuous insulation
materials.
Thermal Conductivity and
Building Durability
How can R-value affect the durability of
a building assembly? Primarily, it does
so based on the condensation potential
of the condensing plane (e.g., in the
case of “Figure 6,” the sheathing board).
A higher potential for condensation
can occur when low temperatures, high
humidity and low-permeable materials
are present within a building assembly.
Air leakage, which ex-filtrates
from the interior to exterior, also
has a negative impact on interstitial
condensation. This is especially true
if non-permeable components are
used. Predicted maximum temperature
swings for extruded polystyrene
and mineral wool insulation types
are 7o
F (46o
F and 39o
F), whereas
polyisocyanurate insulation has a
maximum fluctuation of 11o
F (43o
F
and 32o
F).
When calculating the potential
condensation hours for a given
time period, utilize temperature-
Figure 4: The WUFI-ORNL/IBP 5.3 database predicted the R-value of polyisocyanurate.
Generic Polyiso R-Value at Range of Exterior Temperatures
Generic Polyiso R-Value at Range of Exterior Temperatures
Exterior Temperature °F
R-ValueperInch
Exterior Temperature °F
R-ValueperInch
Figure 5: BSCI tested and imported polyisocyanurate roof material into WUFI-ORN/IBP to
predict annual energy heat loss. Note the contradicting information from WUFI versus third-party
tested data. Underestimation of the heat loss and condensation hours can be detrimental to
building assemblies.
Figure 6: This code-compliant wall is modeled for the Chicago climate. Note: Insulation depicted
is comprised of mineral wool R7.5 continuous sheathing board and R14 batt.
WUFI Predicted R-Value
3rd-Party Tested R-Value
1” Air Cavity R7.5 Comfortboard IS Moisture Control Layer
Interior Gypsum Board
3/4” Sheathing Board
R14 Comfortbatt
Vapor Control Layer
Brick
