“Thermal bridging” is a term to which we often refer in the metal building industry, but, though fairly straightforward, is a concept many folks still don’t fully understand. Due to the major shift in building envelope performance expectations over the past few years, in addition to the impact of thermal bridging on the overall performance of metal buildings, it is more important than ever to get a handle on this central tenet of air leakage.
Thermal Bridging: The What and The Why
Let’s go over the formal definition of thermal bridging and then I’ll give you my take on it. According to the American Institute of Steel Construction, thermal bridging is defined as “the loss of building energy through thermal conductivity of elements that ‘bridge’ across the insulation of a wall or roof enclosure of a conditioned space when the outside temperature is warmer or colder than the interior space.” In other words, thermal bridging occurs when a more conductive (i.e. something with a poor insulating value) element creates an easy pathway for heat flow across a thermal barrier. The end result? Everyone’s worst nightmare: Heat loss and the potential for condensation.
Thermal bridges tend to be more common in older buildings that are poorly insulated, but we see them occur in new construction, too. Technically thermal bridges can occur anywhere within a wall or roof assembly. In metal building construction, the most common catalysts for thermal bridging are steel structural members, such as rafters, purlins, girts, sheeting as well as any discontinuities within the assembly (e.g. compressed rolls of fiberglass). Thermal bridging can happen in both the roof and wall assemblies, which means that it has a major impact on the overall energy efficiency of a metal building.
How to Resolve It
The most common way to combat thermal bridging in metal building construction is to install thermal spacer blocks within the roof assembly. Most builders in our industry are familiar with thermal blocks, but many may not realize how integral
they are to the performance of the building envelope. Let’s take a closer look at thermal blocks as a tool for preventing thermal bridging.
Thermal Block: The Unsung Hero?
Despite its importance to the thermal performance of a building envelope, a thermal block is an extremely simple product. Usually consisting of a polyisocyanurate or polyurethane foam core, thermal blocks come in a variety of widths and lengths, and are most commonly used in standing seam roofs.
In single layer, double-layer and Filled cavity roof insulation systems, thermal blocks are placed between the metal roof sheet and the purlins. Placement on top of the purlins is key, as this creates a thermal break at the structural members. In single and double-layer wall insulation systems, more often than not, thermal break tape is installed between the wall girt and the wall sheeting. Whether installed in a roof or wall system, the goal of a thermal blocks and thermal break tape is always the same: Isolate the outer shell from the inner steel frames of a metal building.
My Advice for Testing Thermal Performance
When contractors ask what to know before their building gets tested, I tell them this: Many insulation systems that are tested per ASTM 1363 Hot Box Testing require the exact product used in the testing. For example, many Long Tab Banded systems require an R-5 thermal block. This means that, in order to claim the tested U-Value, you must use an R-5 thermal block. It’s important to note that many systems within the ASHRAE 90.1 tables require R-3 minimum thermal blocks and can be confused with other systems privately tested.
Ultimately, thermal blocks create a thermal barrier that would otherwise be absent at the structural members of a building, which are some of the biggest pain points when it comes to reducing heat loss. Remember, though, that thermal blocks alone won’t fix all your air leakage or potential condensation issues. Thermal blocks aren’t a solution in and of themselves; they’re a supplement to a better performing PEMB insulation system.