Advanced framing involves both designing for minimal wood waste as well as minimizing heat loss. To this end, advanced framing began with a transition from 2 x 4″ construction on 16″ centers (spacing), to 2 x 6″ construction on 24″ centers, which could be accomplished with little to no increase in total board feet used. This alone increases wall insulation thickness from 3 1/2″ to 5 1/2″ with an increase in rated insulation R-values from R-13 to R-21.
However, that is not the whole picture. The rated value of the insulation does not take into consideration the conductive heat loss of the studs and headers. Field testing and analysis has also shown that blown insulation has far higher practical R-value than batt insulation (from rolls) due to the inevitability of small gaps and cavities around plumbing and electrical. Building science now analyzes the composite R-value of the wall assembly to include the conductive heat loss of the studs, imperfections in the insulation, as well as the additional insulation value of the sheathing and sheetrock.
The wood studs themselves conduct heat, which significantly reduces the composite insulation value of the wall assembly. When we consider all of these variables the composite 2 x 4″ wall value is not R-13, but R-8 or R-9. If the R-13 batt insulation is not installed well, leaving 1/4″ gaps near studs, the R-value can be reduced another 25% to R-6. Similarly, if fiberglass batt insulation is installed well, the composite 6″ wall insulation value is not R-20, but only R-14 and may be as low as R-11. Blown-in insulation is far preferable to batt insulation in wall assemblies because it leaves no air gaps, fills-in around plumbing and electrical without voids and eliminates potential heat loss sources.
Because of the significance of these heat loss variables, additional advanced framing measures have evolved to eliminate them: 1) Two-stud corner assemblies have replaced four-stud corner assemblies; 2) Jack or “trimmer” studs that have traditionally supported door and window headers have been eliminated in favor of steel ties and brackets; 3) Headers, the beam that supports a doorway, are now engineered for a calculated load instead of over-sized by “guesstimate” to be safe. 2 x 4″ studs can be offset within a 8″ wall assembly so that the studs that support the drywall are not continuous with studs that support the outer sheathing. All of these measures work to eliminate conductive heat losses with little additional material costs and minimal increases in labor costs.
The energy savings resulting from these framing techniques are impressive: a 2 x 8″ stud wall with offset 2 x 4″ studs can increase composite wall R-value to an actual R-26. (NOTE: none of the composite figures include window area heat losses). Compare the overall R-value of the standard 2 x 4″ stud wall structure with a composite R-8 to the R-26 that can be achieved with a 8″ double-stud wall. The heat retention is about four times greater in a wall that is only twice as thick. Energy modeling and real-world experience with such features demonstrates that the long-term savings and return on investment on such features is well worth the initial difference in cost.
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