Insulating Wall Corners and Ganged Framing
Corners are often weak links in the "chain mail" of air-thermal barrier with which we strive to armor buildings. Framing must intersect here, be fastened and extend most of the way from inside to out. Framing members always have lower R-values than the insulation that typically rests between them, or possibly, "skins" them. A typical 6" nominal stud, for example has a relatively stable R-value of about 5.5 over the -30 F to 110 F temperature range of interest, versus the nominal R-value of 19 or more added to the wall assembly. Conventional cavity insulation is interrupted at every framing member but at least twice over at corners. Space for cavity insulation of any type is limited in corners. While this is probably of less interest in more temperate climates, it's an issue for those seeking high performance and long-term sustainability in northern climates.
Metal framing, heavy structural or light gauge steel studs, is a thermal bridge, a heat sink in fact that has essentially no R-value. Corners with steel framing especially need to be thermally disconnected from the outside environment.
Framing members will be ganged or doubled up at corners so as to create narrow airways that run from the back of interior walls to back of exterior sheathing. In many building systems, this is as good as directly in-to-out air leaks.
These concerns are not limited to 90 degree corners of exterior walls. They are also found where interior walls intersect exterior walls and at roof corners. They are relevant wherever members in a structural system must be ganged and fastened. As an extreme example, consider the cell intersections of cells of a dome structure, with their struts, fastening plates and even the metal fasteners. One looks for ways to mitigate the thermal effects of structural necessities.
As noted above, these include thermal bridging and unintended airways. Changes in framing details will change customary trade sequencing, as when box corners must be insulated before sheathing. Many framers prefer to stand-up walls squared and fully sheathed. Such details may have to be done by carpenters or laborers who may not, without training, fully appreciate the values of doing them well.
Any measures against thermal bridging through framing should take into account the possibility of a double vapor barrier that would trap moisture inside wall assemblies. Sub-assemblies of any kind are subject to vapor drive, which as we are fond of saying, always wins. If condensation happens, assemblies must have a way to dry either to inside or out.
Box corners - These are probably the least cost change that a builder can make. They present only one stud edge to weather on each side of the corner. They add only one ganged joint. They provide a cavity for insulation to the outside. The insulation would best be rigid board, which affords temperature stable R-value and precise dimension tolerance without trimming. That last bit about dimensional tolerance will be an easy sell to carpenters, who will value "crisp and tight."
Air sealing - Ganged framing always joins in crisp, tight lines on plan details. In the field, ganged framing through which light never passes are airways to reckon with. Framing lumber is far from perfectly rectangular in section, as every nail banger knows. Ganged framing at corners, and frankly, everywhere else, should be caulked unless some other part of the wall assembly will be the air barrier.(NOT house wrap, too many unavoidable workmanship issues) Even then, we have seen many instances where, say, spray applied membrane does not provide a fully continuous air barrier. A bead of caulk applied to the exterior side before sheathing or inside before wall finish should be in the construction sequence.
Skinning - This is a technique that provides an insulating thermal break over framing that takes many forms. A home that is wrapped in 1"of rigid insulation before the siding is installed is one example of skinning to reduce losses at framing corners. An extreme example is structural insulated panels (SIPS) installed over outside of usually heavy framing like ultra-thick sheathing. One can also cover framing inside or out with insulation board of various thickness depending on the degree of performance sought. High performance of all kinds of skinning depends on thorough and effective sealing of joints at faces and edges of the skinning material. For example, effective sealing of SIPS in our experience has often required a review of shop drawings to devise sealing appropriate of the manufacturers' edge detail. Another extreme, but highly effective skinning detail is monolithic, spray-applied, exterior foam as part of cavity wall construction. Exterior cavity wall insulation is common in heavy commercial construction and rare in residences.
Double wall construction - This is a premium performance approach to expanding the cavity available for insulating and reduce losses at corners - at a premium cost. It is discussed elsewhere in the Insulation-Guide.
Advanced framing techniques - for new construction advanced framing is a methodology that provides the same level of structural performance while reducing the amount of framing materials used. Reduced framing means fewer thermal bridges and less ganged framing which harbors air leaks. While some building inspectors and code jurisdictions still balk at the technique it has been gaining acceptance across the country. Check with your local codes before you start building using advanced framing techniques. Advanced framing is not a fix for existing construction however.
In this infrared image the bright line indicated by the arrow is where there is an interior wall intersecting the exterior wall and the corner bay was not properly insulated.
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