With the cost of lumber increasing while the quality of framing lumber decreases and its availability fluctuates, alternative wall assemblies are gaining acceptance by those concerned about both energy efficiency and construction cost. One of the constraints to wider adoption of advanced, high performance wall assemblies is general confusion among consumers, developers, owners, designers, builders and material suppliers around the metrics used to compare the thermal performance of one assembly versus another.
- R-value – R-value measures the thermal resistance to conductive heat transfer of a material. In the real world, R-value varies with the temperature levels on each side of a material, although it is typically treated as a constant value. The higher the R-value, the better the thermal resistance to conductive heat transfer. For example, at 30° F, R-13 fiberglass batt has an R-value of about 13.6 and at 70° it has an R-value of about 12.9. R-16 EPS at 30° has an R-value of about 16.4 and at 70° it has an R-value of about 15.6. R-20 XPS at 30° has an R-value of about 24.2 and at 70° it has an R-value of about 23.
- Nominal R-value – is the rated/labelled R-value of an insulation product and so given the way in which the term R-value is used, might be considered the same value as R-value.
It is important to keep in mind that R-values and nominal R-values do not include the effectiveness of a wall assembly that includes other wall assembly components, interfaces and adequacy of installation. Surface heat transfers such as the transfer of heat through the wood or steel studs or the transfer of heat through screws and other types of fasteners is not included in either simple R-value or nominal R-value calculations.
In the real world of the built environment, there a many other factors that affect the simple and nominal R-values of a material other than the material itself such as temperature, moisture content, density, the release of the gas used to manufacture foams, air pressure, the age of the material itself as well as errors in the installation of the materials. The cumulative effect of these other factors can be quite large, but their individual and collective impacts on the insulating ability of different wall assemblies have not been fully characterized and so there really is no choice but to consider the R-value of a building component a constant when comparing them in decisions about wall assembly make-up.
R-value is the reciprocal of U-value as noted below.
- Required R-value – is the minimum code-acceptable resistance to conductive heat transfer in a wall assembly. The required R-value calculation must be done using the R-values of each individual component within the wall assembly the same way in which U-value (note definition below) is calculated.
- Center-of-Cavity R-value – This is the estimation of R-value for a portion of the wall that includes just the insulation in the wall cavity, if there is any.
- Clear-wall R-value – This is the estimation of R-value for a flat exterior wall area containing only insulation and framing materials. Clear means it does not include any windows, corners or interfaces to any other part of the exterior envelope such as roofs, foundation or other walls.
- Equivalent R-value – A high mass wall such as a concrete wall can have a significantly higher R-value than its measured R-value due to its ability to store heat. Because high mass walls will warm during the day when exposed to regular sunshine, they will release that warmth into the interior of a structure during the course of the night when the outside temperature cools. Equivalent R-value is a short term R-value that is true only when considered from the perspective of one day or a few days of temperature fluctuations.
- Performance R-value – – is used to compare the insulating capabilities of one system to another by determining what one type of construction would need to be used to equal the R-value of another type of construction method.
- U-value – U-value measures the speed (rate) of thermal transmission (thermal conductance) per degree of temperature through a wall assembly. U-values do include surface heat transfer and are therefore more reflective of the actual insulating capability of a wall assembly. The lower the U-value, the better the insulating properties in a wall.
- Effective R-value – This is the total conductive resistance provided by all of the components within a wall assembly. It is the same as the Required R-value and U-value.
- Required U-value – is the maximum code- acceptable heat transmittance for a wall assembly.
- In-Service R-value – This is the same thing as the Effective R-value, the Required R-value and the U-value.
- Whole wall R-value – This is the estimation of R-value for the entire opaque wall including the thermal performance of not only the clear wall area but also with typical envelope interface details such as wall to wall connections (corners), wall to roof connections, wall to floor, wall to door and wall to window connections. Whole wall R-value takes into consideration the thermal bridging impact of the amount of framing in the wall compared to the amount of insulation as well as the effectiveness of the sealing of the enclosure at reducing air infiltration.
Interface details can create a huge difference between the clear wall R-value and the whole wall R-value because they exacerbate the issue of thermal shorts. For some conventional wall assemblies, the whole wall R-value can be as much as 40% less than what is measured for the clear wall R-value. One consequence of this can be an over or under estimation of the size of the required HVAC equipment. Poor interface details may also cause excessive moisture condensation within a wall assembly.
It is possible to affect actual whole wall R-values by many different means including altering some interface details in the design and construction of the building envelope. What is important is that a consistent and accurate metric for comparison be used when designing, building and promoting the cost/benefit tradeoff of certain wall assemblies. Whole wall R-value would seem to be the most accurate.