William Robinson, Reichel, Claudette Hanks, Baker, Eugene, Evans, Audrey A., Picou, Stephen | 1/10/2007 1:45:58 AM
R-value is standardized measure of a material’s resistance to heat flow — the higher the R-value, the greater the insulating ability. However, the actual R-value of a wall or ceiling can be a lot less than the R-value of the insulation material in it, depending on the installation. For the Energy Star label, homes must be inspected at the open-wall stage (or tested with an infrared camera) to receive full credit for proper insulation installation.
Whole-wall R-value is a concept that takes into account the gaps in insulation (at windows, doors, studs, voids, etc.). Although homeowners may not be able to determine their whole-wall R-values easily, the concept is helpful in comparing the actual energy performance of options. Insulation should surround all conditioned space (except a slab-on-grade in the Deep South) with as few gaps as possible.
Recommended Insulation Material R-values: Recommended R-values are based upon cost analysis of insulating areas of the building envelope in relation to the benefits in a climate region. As energy costs rise, so do the R-value recommendations. Recommended insulation levels for steel may not perform as well as recommended levels in wood-frame buildings, but the cost of achieving the same performance would outweigh the benefit. Also note that the R-value measurement standard assumes no air leakage through the insulation. Air currents in standard density fibrous batts and loose fill materials reduce their insulating value.
Installation: Compressing (squeezing) insulation erodes its R-value and should be avoided. Batt and roll insulation should be slit and trimmed to fit around wiring, electrical boxes, etc. If using paper-faced batts, staple the paper on the ends, not inside, the studs. Insulation should be trimmed to fit into voids around rough openings, chimneys, etc. Even a 2% gap makes a difference in the rating of a home, and the compression of insulation can make it almost useless.
Continuous coverage to reduce thermal bridging: Non-insulation materials, even wood framing, create a thermal bridge, allowing greater heat flow through the wall. Materials that are good conductors of heat (such as metal framing) can substantially erode the effectiveness of insulation. Building systems that reduce thermal bridging, provide a continuous “thermal blanket” surrounding the conditioned space and therefore preserve a higher “whole-wall R-value” include: using insulating foam sheathing in addition to wall cavity insulation, OVE (24 inches on center), SIPS and ISPS (foam core panels), ICF (foam forms and concrete) and AAC (insulating concrete).