R-value at various temperatures and conditions
Infiltration and exfiltration issues
Radiant reflectivity
Thermal mass
Non-insulated portion of the structure
Window efficiency and
Connection details
R-Value is HIGHLY over-rated as a measure of insulation efficiency. In a typical residential structure, mid-wall R-Value for the structure is arguably the least important issue in the energy efficiency of the home. Of at least as much interest are:
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R-value at various temperatures and conditions |
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Infiltration and exfiltration issues |
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Radiant reflectivity |
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Thermal mass |
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Non-insulated portion of the structure |
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Window efficiency and |
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Connection details |
Did you know that R-Value testing is done at 72 degrees Fahrenheit with no infiltration/exfiltration, humidity at 40% or lower and with a small temperature change for a short duration? This test, which is the standard R-Value test was designed when the only insulation material being evaluated was fiberglass. It was developed by the Fiberglass industry, so it's hardly surprising that it would favor them. When the conditions of the test are varied fiberglass doesn't do well. For example, at 20 degrees F with 50% humidity, fiberglass is R-0. EPs acually gets higher R-values as the temperatures decrease, and humidity does not affect it at all. The test is an unreliable guide to efficiency.
Imagine how effective insulation is when doors and windows are left open- infiltration and exfiltration issues are similar. Air and moisture flow through the structure greatly reduce the energy efficiency of the home. SIPs address this issue better than conventional stick or steel frame construction. Blower door tests indicate that SIPs are 20% tighter than very well built stick frame homes and as much as 40% tighter than most conventional construction.
If you've ever used a space blanket, you've seen how effective reflective radiant sheeting can be for insulating. Any material which keeps radiant energy from converting to condutive energy is considered good as a radiant barrier. Stick frame, steel studs and masonry are all exceptionally bad at this and SIPs are good at it.
Some materials are slow to change temperature- they have inertia to temperature change. Air and metal are very bad at this and non-metallic solids are good at it. SIPs are excellent insulators where thermal mass is a factor.
A standard stick or steel frame wall has studs every 16", which translates to about 20% to 25% of the actual surface area- there is no insulation where the framing is so the less framing, the higher the insulation efficiency. According to Oak Ridge National Labs, this one issue reduces the efficiency of a wood stud wall by 33% and a steel stud wall by as much as 55%. SIPs rate a 7% loss of efficiency.
SIPs are generally tighter at the window connections, but the quality of the window (R-value and low E) is often the most under-rated issue in energy efficiency. If you look at thermographic images of highly energy-efficient homes the loss through windows is striking.
Connection details at the corners, wall top and bottom and at openings are another weak spot for energy efficiency in conventional construction which is well-handled in SIPs.
As you can see- the real issues of energy efficiency are ignored when R-value is stressed.
In addition, thermal mass can have significant effect on thermal efficiency. Thermal mass describes two effects, the heat sink or storage of heat, and the inertia of materials to heat change.
The March, 2001 Rural Builder Magazine included an excellent article regarding Thermal Mass, written by Rob Pickett of Southland Log Homes. The article "Thermal Mass- Beyond R-value", deals with the limitations of R-value as a predictor of energy efficiency, and is a useful in seeing why, in spite of Building Department focus on energy efficiency, energy problems still confront us. According to the article:
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"R-value measures how readily heat moves by a process called 'conduction' from the heated side of a material to the cold side under steady-state conditions..."Thermal mass "..is the result of the dynamic process of a building components' "heat capacity," modulating heat flow over the course of a day. For example, on a summer afternoon with a 90 degree F outside temperature and cooler inside temperature, the conduction heat flow moves toward the inside. At night, when the air temperature drops to 50 degrees, and inside temperature may be 75 degrees, the driving force for heat flow reverses back toward the outside." |
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"The heat capacity of a building's walls- such as logs, masonry, or concrete- accounts for variable energy needs over daily and annual cycles, an occurrence which engineers, standards and codes refer to as the thermal mass effect." Among the benefits of building with thermal mass vs. insulated stick frame are: Consistent thermal value without the "thermal bridges" inherent in light frame construction, Consistent volume compared to insulation voids, compression and aging, More consistent interior temperatures." |
Thermal mass is an under-valued factor in energy efficiency. as are infiltration/exfiltration, and radiant deflection. The under-valuing of these factors, to the benefit of R-value has a great deal to do with why increased energy efficiency awareness in the general public (and in Building Jurisdictions) has not resulted in the energy efficient homes that were anticipated. This is not really surprising, given that most energy efficiency studies are carried on by the vested interests in insulation, and "more is better" is a typical approach to R-value. Structural Insulated Panels are an excellent energy efficiency alternative because they are efficient in all four areas. As a result, SIPs significantly out-perform their R-value.