What is a thermal bridge and why is it so important to break it?
Calculating heat flow through building enclosures is much more complex than calculating it through individual materials, particularly when highly conductive components such as steel or concrete are in play. The R-value written on your insulation indicates what it is capable of, how well it actually performs is up to the designer and builder.
When one section of a wall conducts heat quicker than another it is referred to a thermal bridge. A thermal bridge can greatly reduce the overall R-value of a wall. Thermal breaks are the solution. It is easy to look at the R-value rating of a material and assume that our entire wall assembly matches it uniformly. However, when factors of framing materials and any discontinuities in the wall structure are considered in thermal bridging, the R-value becomes a variable number and cannot be premeasured
Wood, metal and concrete in exterior walls act as thermal bridges and they conduct more heat than the surrounding insulation. Installing R19 batts of insulation with R5 wood studs on either side results in a wall that performs closer to R13 or even lower, depending on the amount of framing. The true or 'effective' R-value of a wall is further impacted by the quality and amount of windows you install.
This isn't only an issue of individual heat loss and higher bills, it is also about the overall sustainability of our communities and utilization of the collective resources.
The growing popularity of metal studs
Metal studs are becoming more popular in residential construction but do not perform at a consistent and effective level. For interior walls metal studs are satisfactory and have little impact on the overall thermal bridging system of a building. However the energy performance of buildings using metal studs on exterior walls seriously decrease when thermal bridging is ignored.
Metal conducts heat so rapidly that there is little to no point to insulating both sides. It is estimated that metal studs with batts in the cavities can reduces the overall performance of that insulation by 60 to 70%, or more.
According to Building Science Corporation, "The R-value of 6 inch deep steel studs installed at 16 inch centers with cavity insulation of R-21 is reduced to R-7.4, a value only 35% of the nominal". -
If putting heat conduits on either side of batt insulation reduces performance by half, the investment is essentially doubled to achieve a favourable R-value. In theory and in practice it is becoming a more advised choice to invest budgets in other thermal bridging alternatives. Proloft becomes an affordable choice when working with the total building equation.
Building codes and thermal bridges
Thermal imaging provides a good perspective of heat movement. The darker colours represent cooler temperatures so that wall studs and screws within the wall radiating cold into the home is visible, highlighting the importance of a seamless blanket of insulation as a thermal break.
It is not only sensible to address thermal bridging in construction, it is becoming law. Recent revisions to building codes have increased the thermal requirements of building enclosures in many regions, including requiring a thermal break. Contrary to common assumptions the insulation values in building codes are not about promoting the most efficient and cost effective wall systems, they exist to protect end users by ensuring there is a minimum level of quality performance.
If breaking thermal bridges has now made it into code as a 'must do' practice, then the effects must be significant. Thermal bridges not only affect a building's energy performance; they also have health and safety benefits. Durability implications causing cold spots will increase the risk of condensation. Beyond heat loss and comfort, air quality and structural integrity can then be affected in terms of mold, mildew and rot leading to potential health and safety violations.
Thermal bridges that cause significant heat loss are not limited to just studs in a wall. They include discontinuities in the insulation, corner junctions, badly installed insulation (convective thermal bridges), concrete slab junctions at walls and balconies, service openings (structural thermal bridges), and 'penetration' thermal bridges such as metal I-beams that pass through wall assemblies.