What is an R-Value?
R-value is a measure of apparent thermal conductivity, and thus describes the rate that heat
energy is transferred through a material or assembly, regardless of its original source.
The SI unit for R-value is kelvin square meters per watt.
What is a LTTR and how is it Different from an R-Value?
In short, the LTTR provides a technically supported, more descriptive measure of the long-term
thermal resistance of polyiso insulation—15-year time-weighted average.
In 2003 the polyiso industry introduced the concept of long-term thermal resistance, or LTTR.
LTTR is a 15-year, time-weighted average of the foam's R-value.
It represents the actual R-value that the polyiso insulation will provide while in servic and enables building owners and designers to predict exactly how much insulating value will be provided and how much energy will be saved over the life of the roof.
LTTR represents the most advanced scientific method to describe the Long-Term Thermal Resistance of foam insulation products using blowing agents other than air,
including polyiso, polyurethane and extruded polystyrene.
This method is based on accelerated aging by conditioning thin slices of foam insulation at a particular temperature for a specified number of days. The method is based on
consensus standards in both the United States and Canada and provides a 15-year time-weighted average LTTR.
Using techniques in ASTM C13031, CAN/ULC S7702 predicts an R-value that has been shown to be equivalent to the average performance of a permeably faced
foam insulation product over 15 years. In Canada, this is used as the design R-value.
What is the Difference Between Closed Cell and Open Cell Insulation?
Generally speaking, closed cell foams consist of trapped gas bubbles formed during the foam's expansion and cure. These gas bubbles consist of the blowing agent and are permanently locked into place during the curing of the foam.
Open cell foams; however, are quite different in nature. The blowing agent gas is not trapped by the forming cells and instead is released to the atmosphere during foam expansion and curing. The foam cells have "holes" in their walls, enabling them to interlock and interconnect. The spaces within the cells are filled with atmospheric air, much like a sponge.
How does the Insulating Value of Polyisocyanurate Compare to Fiberglass?
The standard R-value test designed by the American Society for Testing and Materials (ASTM) does not account for moisture or air movement which can dramatically lower the R-Value of fiberous and open-cell insulating materials.
Testing these materials in non real-world conditions leads to a favorable bias towards fiberous insulations (fiberglass, rock wool and cellulose fiber). Very little input went into the test for solid insulations, such as polyisocyanurate, extruded polystyrene, expanded polystyrene or urethane foam which tend to have higher and more stable R-values in real-world conditions.
The three most common methods for measuring R- Value are ASTM C 177, ASTM C 518, and ASTM C 976, the publications list their sources of error and variance as follows:
ASTM C 117
, 5.7: The thermal transmission properties of a specimen of material have the potential to be affected due to the following factors: (a) composition of the material (b) moisture or other environmental conditions (c) time or temperature exposure (d) thickness (e) temperature difference across the specimen (f) mean temperature.
ASTM C 518
, 4.3: The thermal transmission properties of specimens of a given material or product may vary due to variability of the composition of the material; be affected by moisture or other conditions; change with time; change with mean temperature and temperature difference; and depend upon the prior thermal history.
ASTM C 976
, Note 6: Discrepancies are especially likely for nonuniform specimens with high conductance surface elements connected to thermal bridges when measured resistances, R, are obtained under still air conditions and the standardized surface resistances are typical of high wind velocities. The user is cautioned to be aware of such possible discrepancies.
Measuring R-Value without taking into account the conditions of water vapor (moisture levels) and airflow leads to a more favorable result for insulating materials that are permeable to air and water. Moisture can mat down fiberglass batt and blown insulation, decreasing its performance as an insulator and reducing it's effective R-Value. The Data Sheet
for fiberglass insulation even states that "Commercial roof/ceiling thermal applications require that the building envelope block the movement of air from the outdoor environment to the conditioned space" and failure to do so could lead to "loss of thermal control, discomfort of the building occupants and frozen pipes".
These conditions make it difficult to compare R-values between the industry standard of fiberglass to insulation such as closed cell polyisocyanurate rigid foam board. Effectiveness of these different breeds of insulation has too many external variables to be reduced to a comparable number.
Radiant energy has also been ignored in the standard test for R-Value. Adding a foil face radiant barrier to your insulation can increase the real-world R-Value by around 3-R depending on the thickness and type of insulation material.
How does a Radiant Barrier Work?
Radiation is the movement of heat rays across air spaces from a warm object to a cooler object such as the heat we feel from a wood stove. Insulation in an attic will give off heat radiant heat to the cold attic space in winter and to the living space in the summer.
Regular insulation will not stop radiant heat loss, instead it must be reflected with a radiant barrier.
What is the "R" Value of a Radiant Barrier?
Radiant barriers do not technically have R-Values, this is because they do not "absorb" heat like R-Value rated products. A simple comparison is to imagine standing in the shade of a tree on a hot sunny day. The shade has no R-Value, but it cools you down the heat by blocking the transfer of radiant heat flow from the sun.
While the barrier itself has no R-value, it is possible to calculate an R-value for a specific reflective insulation installation. Using a foil-faced polyisocyanurate board typically increases the effectiveness of an insulation system installed properly with a 3/4" dead airspace by about an R3.
How Much R-Value do I Need?