When comparing the overall performance of insulation systems, it is important to consider how the insulation interacts within the whole wall assembly. USDOE supported research(1) lists the important parameters to consider in a whole wall assembly comparison as follows:
2. Thermal Mass
3. Air Tightness
4. Moisture Tolerance
Interestingly enough, standard accepted test protocols used in North America for determining the thermal performance of insulation materials focus only on R-Value achieved in laboratory conditions. In addition to the other parameters above, standard laboratory tests do not consider several real world parameters that also affect insulation performance such as:
* Thermal bridging
* Extreme temperatures
* Installation deficiencies
* Radiative heat losses
* Aging effects on thermal performance
Depending on the type of installation, these parameters may have a significant impact on the real world performance of the insulation system. This translates directly to either an increase or decrease in energy consumption.
To date, a standardized laboratory test has not been developed to measure insulation performance in a whole wall assembly, taking into account many key parameters that affect insulation performance and sustainability.
As a result, innovative insulation systems have been at a great disadvantage in laboratory test situations that only consider conductive (and to some extent convective) resistance in ideal conditions.
In addition to the effects on energy efficiency, failure to consider all key parameters that affect insulation performance may lead to the oversizing of critical heating and cooling mechanical systems in both commercial and residential applications. This may result in premature aging of mechanical equipment from not operating within peak efficiency parameters. In addition innovative insulating systems may be excluded from installations where superior energy efficiency may have been realized.
In the end, are we interested in a building envelope that tests well in a laboratory environment, only to under-deliver after installation….or a building envelope that performs in the real world, reducing energy consumption and enhancing overall thermal comfort?
(1) Home Energy Magazine Online “Wall R-Values That Tell It Like It Is”, Jeffrey Christianson (ORNL) and Jan Kosny (University of Tennessee)
Thermal Bridge: A component or a system of components in a building envelope through which heat is transferred at a substantially higher rate than through the surrounding envelope area.
Thermal bridging through structural framing members in the building envelope can be a major contributor to heat loss/gain. Studies funded by ASHRAE, the California Energy Commission, and Building Science Corporation found that the average framing factor in a US residential home is on the order of 25%(1,2). As a result, if cavity insulations are utilized alone, approximately 25% of the structure (not including doors & wondows) is under insulated. Research conducted by Oak Ridge National Laboratories(1) revealed that thermal bridging accounts for approximately 10 – 15% of residential energy usage in the United States, which is typically not included in building load analyses used in the sizing of critical mechanical equipment.
Further studies by ORNL(3) indicated that the R-value of what we think is an R-19 fiberglass wall system can be reduced to as little as R-14 when proper framing factor and interface details are considered in a wall assembly test such as the ASTM C1363.
(1) “How the Same Wall Can Have Several Different R-Values: Relations Between Amount of Framing and Overall Thermal Performance in Wood and Steel-Framed Walls”, Jan Kosny, David Yarbrough, Phillop Childs, Syed Azam Mohiuddin
(2) John Straube and Jonathan Smegal “Building America Special Research Project: High -R Walls Case Study Analysis Reserch Report” -0903, Building Science Corporation, March 11, 2009
(3)Energy Desing Update, vol. 19, no 9, September 1999 “How Thermal Shorts and Insulation Flaws Can Degrade an “R-19″ Stud Wall to a Measly “R-11″
The long term mechanical and thermal stability of an insulation is an important consideration that is often overlooked. The thermal performance of some conventional insulations are drastically reduced over time due to sagging, settling, or off-gassing. In addition, mechanical breakdown may occur when exposed to wet/dry or freeze/thaw cycling under adverse conditions.
Thermal Drift: Reduction in measured “R-Value” of an insulation as it ages. This is directly related to a product’s chemical stability. Primarily a concern with foam products containing CFC’s.
IMPACT: Causes a reduction in the insulation’s “R-Value” over time as insulating gasses are released and replaced with air.
Physical Stability: The ability of an insulating material to maintain its shape, rigidity, and strength over time.
IMPACT: Breaches in insulation due to physical degradation, settling, sagging, or installation shortcomings can result in significant heat loss from the building envelope, through thermal short circuiting and air leakage.
The proper performance of any insulation system is directly dependent on the product being installed according to manufacturer’s recommendations. However, some insulation systems also rely on contractors following standard building practices and nominal framing patterns when constructing the building envelope. Studies(1) have indicated that between 30% and 40% of residential structures do not follow nominal framing patterns. This will have an impact on not only the framing factor, but will also increase the chance of installation deficiencies with cavity insulations such as fiberglass since custom cutting and fitting will be required. This will potentially introduce air/moisture infiltration and thermal short-circuiting.
Although it seems insignificant, installation deficiencies can lead to a major decline in thermal performance of a building envelope. Studies completed by ORNL(2) concluded that the performance of what we think is an R-19 fiberglass wall can be further degraded from R-13.6 to R-11 when common installation factors (interface details, piping, wiring, etc) are considered.
(1) “How the Same Wall Can Have Several Different R-Values: Relations Betyween Amount of Framing and Overall Thermal Performance in Wood and Steel-Framed Walls”, Jan Kosny, David Yarbrough, Phillop Childs, Syed Azam Mohiuddin(2)Energy Design Update, vol. 19, no 9, September 1999 “How Thermal Shorts and Insulation Flaws Can Degrade an “R-19″ Stud Wall to a Measly “R-11″
Air infiltration can account for as much as 30 to 35% of heat loss / gain in a building envelope(1,2). Most conventional insulations offer very little resistance to air infiltration, relying on other building materials such as drywall, poly, and sheathing to combat air leakage. The integrity of these air barrier systems is often compromised over time through a combination of installation deficiencies, and normal household activities (e.g. screws, naills, etc.). As a result, air can be transferred through the building envelope, resulting in convective heat transfer.
Often times, this air contains moisture which can condense within the building cavity, deteriorating the building structure and leading to potential mold and mildew problems. In addition, the performance of most conventional insulations may be compromised if they become wet since water is a good thermal conductor.
Water Absorption: A process by which water permeates into a substance.
IMPACT: Water is not a very good insulator. As a result, water absorption accelerates the rate of heat transfer via conduction through an insulation material. In addition, water absorption may compromise the physical stability of the insulation, resulting in swelling or sagging.
Air Infiltration: The amount of air leaking into or out of a building mainly through cracks in walls, windows, and doors.
IMPACT: Insulations may perform at a fraction of their nominal thermal resistance if air infiltration occurs. Air infiltration can account for 30 – 35% or more of a buiding envelope’s cooling and heating costs and can contribute to problems with moisture, noise, dust, pollutants, rodents and insects(2)
(1) NAIMA Pub. No. BI 480 6/99 “The Facts About Insulation and Air Infiltration”
(2) Technology Fact Sheet “Air Sealing – Seal Air Leaks and Save Energy”, USDOE