Thermal Bridging Research: Investigating Insulation Thickness for Renovations

November 12, 2014

One of our many in-house research efforts is a large, collaborative project. We investigated into the Thermal Performance of Façades through the AIA Upjohn Research Initiative.

Thermal bridging in building construction occurs when thermally conductive materials penetrate through the insulation creating areas of significantly reduced resistance to heat transfer. These thermal bridges are most often caused by structural elements that are used to transfer loads from the building envelope back to the building superstructure. Though design professionals generally understand that thermal bridging is a concern, few can quantify the extent of its impact on building performance.

Small changes in designs can still lead to dramatic improvements in performance. With careful detailing and attention to the issues of thermal bridging, the design and construction industry can improve the performance of our building envelopes.

Today we’re sharing our findings regarding insulation thickness for renovations.

Spray applied insulation is once again gaining popularity, largely because of its ability to fill unseen voids and provide an integral vapor barrier. In the Northeast, it is a particularly popular technology for renovating existing, uninsulated masonry and cast in place concrete facades. Conventional details often call for metal studs to support interior gypsum board and these studs live in the same space as the insulation, creating discontinuities at 16” or 24” center spacing. While the web of the steel studs is quite slender, they are highly effective heat transfer devices because of the conductivity of the material and the flanges, which provide significant contact area to collect and disperse heat.

Thermal images of the renovation of three separate, existing buildings revealed dramatically different results. The first case had applied 3” of insulation, the second employed just 2” of insulation, and the third used 3.5”. While hand calculations of the thermal resistance would show the façade with the least insulation to be the weakest performer and the one with the most insulation to be the best, the thermal images revealed a different story. The 3” of insulation included steel studs flush against the exterior construction, resulting in an R-value that was 55% less than the calculated R-value. The second building pulled the studs back by 1”, allowing for half of the applied insulation to be continuous and decreasing the R-value by only 15%. Consequently, that façade was observed to have a higher R-value than one with the studs penetrating through to the exterior, despite having less insulation. The third façade took the studs back even farther, completely separating them from the insulation and, as a result, the simulated R-value was nearly identical to the measured values.

Our study showed that the continuity of the first inch is critical to the efficiency of the spray foam insulation performance. By simply pulling the studs in-board by a small amount, to allow a percentage of the insulation to be uninterrupted, the assembly R-value can be increased by about 70%. In the event that the studs are required to support exterior sheathing, it should be possible to fasten the sheathing using discontinuous and non-conductive shims or spacers so that, once again, the majority of the insulation in that outer 1” layer remains continuous. Nevertheless, small changes in the design can still lead to a dramatic improvement in performance.