December 17, 2012
Sean M. O’Brien, PE, LEED AP
In an ideal world, a continuous layer of R-10 insulation could be installed over the entire surface of a building, resulting in an enclosure with an effective insulating value of R-10. However, such a structure would be both aesthetically unacceptable and impractical to build. Real buildings need thermally conductive framing, wall openings, and various combinations of continuous and interrupted insulation.
The biggest factor affecting insulation performance on a component basis is thermal bridging. A 152-mm (6-in.) thick piece of fiberglass insulation provides an R-value of approximately 19. This same insulation installed between metal studs only provides about R-9.2, according to compliance tables in American Society of Heating, Refrigerating, and Air-conditioning Engineers (ASHRAE) 90.1, Energy Standard for Buildings Except Low-rise Residential Buildings. Some guidelines, such as the California Energy Code, only allow designers to assume R-5.5, taking into account losses through top and bottom stud tracks.
The prevalence of steel-stud construction has led most codes to require continuous insulation in exterior walls. While stud bridging is now widely understood, heat can also bypass insulating components through paths around opening perimeters, via shelf angles in cavity walls, and through metal furring in rainscreen walls. The last possibility is often ignored, as many consider any insulation outboard of steel studs to be ‘continuous insulation.’ However, continuous furring supporting rainscreen cladding reduces cavity insulation effectiveness to a similar degree as steel studs will de-rate fiberglass.
Similar to thermal bridging on a component basis, a more global effect occurs in buildings with high percentages of glazing. Even with high-performance glazing and thermal breaks, a typical window only provides an effective R-value of about 2 or 3—much less than the R-15 of a typical code-compliant opaque wall. In many buildings, curtain walls and windows end up being the primary thermal bridges, increasing overall building heat loss/gain even when highly insulated walls are used. As an example, a building with 40 percent code-compliant glazing needs R-10 continuous insulation added to all opaque walls to produce an overall increase of R-1 on the whole-building scale.
A material’s ability to store heat affects its effective thermal resistance. This is reflected in the codes—requirements for insulation in ‘massive’ walls made of concrete or brick masonry are usually less than those for lightweight/framed wall systems. This is because walls that store and release heat improve energy efficiency by ‘buffering’ the interior from sudden changes outside.
Massive walls also help smooth out daily cycles by storing heat during the day (when building interiors are warmed by solar gains), and releasing it to the interior at night to offset heating loads. The placement of insulation in massive walls is also important, with exterior insulation being preferable as it allows for better energy exchange with the interior environment.
Air leakage can also have a negative impact on insulation performance. If the air within a porous insulation (e.g. fiberglass or mineral wool) is forced to migrate (as occurs in buildings lacking airtight barriers), the material’s effectiveness is greatly reduced as moving air transports heat directly from across the insulation, regardless of its R-value.
Similarly, the building-wide effectiveness of closed-cell/non-air-permeable insulations can be reduced by air leakage through other areas of the enclosure. A relatively small air leak can result in the same heat loss as a far larger area of insulated/opaque wall. The potentially significant impact of air leakage on heat loss/gain in buildings is one of the drivers for the addition of airtight barriers to modern energy codes.
As with most building problems, understanding the physical phenomena behind them is the key to preventing them in the future. This is especially true with insulation—the effectiveness of which only becomes more important as energy costs increase and codes become more stringent.
Sean M. O’Brien, PE, LEED AP, is an associate principal at national engineering firm Simpson Gumpertz & Heger Inc., specializing in building science and building enclosure analysis. He is involved in both investigation/forensic and new design projects. O’Brien is a member of American Society of Heating, Refrigerating, and Air-conditioning Engineers (ASHRAE) and the New York City Building Enclosure Council (BEC-NY) and a frequent speaker on topics ranging from condensation resistance to energy efficiency. He can be reached via e-mail at firstname.lastname@example.org.
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