With the new standards for the federal Energy Star Qualified Homes program issued earlier this year, it's likely that the term "thermal bridging" will increasingly make its way into your conversations with homebuilders and framing contractors.
The new–and tougher–qualification standards, which go into effect on a limited scale for all homes permitted on or after January 1, 2011, (and fully so a year later) will include "buttoning up the thermal enclosure," according to Sam Rashkin, the program's national director for the U.S. Environmental Protection Agency. "It takes it to a mature building science level."
For builders, this means creating a thermally superior building envelope that mitigates thermal bridges–the points along a wall or roof section that conduct heat and cold through the structure, such as along wall stud lengths.
Wood is less of a thermal bridge than other residential framing materials, but it's far worse than insulation. Thus, the more studs and trusses you have, the more opportunity there is for thermal transfer through the structure, either directly through the wood components or through gaps in the insulation installed between and around them. Those thermal bridges, in turn, increase the strain on the home's heating and cooling system, jacking up the energy bill.
"The ideal is to insulate without gaps, voids, or compression," says Rashkin. And, naturally, do it without sacrificing structural integrity or code compliance.
Wall assemblies are particularly vulnerable to thermal bridging. Heat transmission through walls is slightly greater than through high-performance windows, and much greater than through roof structures.
The new Energy Star standards will require a verified Grade 1 envelope, which cuts the home's energy factor (or thermal resistance) in half, to about 12%. To achieve that level of efficiency, the standard explicitly specifies minimum insulation values for walls, floor, and attic/roof structures per the 2009 International Energy Conservation Code. But the practical result also cuts down on framing.
Builders can create more insulation opportunities by employing advanced framing practices such as 2x6 wall studs spaced 24 inches on-center, two-stud corners, single top plates, and fewer cripple studs under framed openings. Applying a layer of rigid foam insulation between conventionally built wall framing members and the exterior cladding material, a Grade 2 assembly, can achieve the same insulation values and meet the Energy Star standard.
Both methods and their insulating values will be verified by a home energy rater authorized by Energy Star (a similar inspection process to achieve LEED for Homes or NAHB Green certification), but remember: These are considered to be the baseline.
The gold standard would be to employ structural insulated panels or insulated concrete forms, which all but eliminate thermal bridging for walls and, in the case of SIPs, roofs. "No exposed framing is ideal," Rashkin says.
For dealers serving the nearly 6,000 builders nationwide who participate in the Qualified Homes program (which accounts for more than 150,000 new homes a year), the upgraded thermal envelope standards within Energy Star's 2011 requirements might require a shift in inventory, perhaps from sawn framing members to engineered and factory-built components that span longer and wider, as well as more insulation products.
It's also an opportunity for dealers to develop expertise about the new standards and position themselves to respond to demand, both as an LBM supplier and as an installer for all of the product categories affected by the requirements, including windows and doors.
For more information and documents related to the 2011 requirements, go to www.energystar.gov/index.cfm?c=bldrs_lenders_raters.nh_2011_comments.
–Rich Binsacca is a contributing editor to ProSales.