Fixing thermal bridging in masonry foundation walls

Thermal bridges and where they occur

With all the effort to design and install more energy-efficient masonry walls, more focus is being placed on transitions to address thermal bridges which could impact or even negate increased insulating efforts. Many buildings designed to be more efficient see a disconnect or performance gap between how they were intended to perform and their actual performance.2

A thermal bridge is a location where a more conductive material interrupts a plan of thermal resistivity, reducing the overall thermal resistance of an assembly and possibly leading to enough thermal loss to create condensation. This can lead to mould, mildew, or structural damage.

Thermal bridges occur all over a building in transitions, terminations, and penetrations. Foundation to wall transitions, balcony projections, window and door openings, and roof parapets can all serve as thermal bridges. Often, they can be solved with extra diligence, such as tracing the insulation layer around the entire building envelope and circling areas of discontinuity. They can also be rectified with materials like single-barrel screw masonry anchors or a compressed mineral insulation at window heads and floor lines (also addressing fire performance), or gun foam sealant in small gaps and cracks.

Aside from the masonry anchors, these are not terribly specific to masonry veneer walls. However, while identifying areas where thermal bridging can occur is a relatively straightforward process; some areas were harder to solve until recently, as they involved junctions where masonry loads are applied to the foundation below.

This particular thermal bridge location, where a masonry veneer wall bears onto the masonry ledge at the foundation, creates a linear thermal bridge which continuously as opposed to intermittently. Image courtesy Owens Corning

Addressing the thermal bridge under masonry veneer

This particular thermal bridge location, where a masonry veneer wall bears onto the ledge at the foundation, creates a linear thermal bridge. This type of thermal bridge occurs continuously across an area as opposed to intermittently, such as with fasteners placed throughout the ci. There are generally three approaches to this dilemma (Figure 1).

The first would be to not address the thermal bridge at all. While over-insulating elsewhere could be acceptable in some jurisdictions, this solution is a likely barrier to the aforementioned net-zero building or voluntary higher standards like Passive House, which will require extra effort to address these bridges.

The next solution attempts to address the issue by introducing additional insulation, overlapping the area to maintain the temperature at the location where insulation could not be placed. While this solution is demonstrated to address thermal performance, it may come at the cost of additional insulation, additional foundation depth or wall height, and the risk of exposure or damage.

However, what if a certain type of insulation had enough compressive strength to help address this thermal bridge? A handful of materials have been made to address this without the sometimes-extreme insulation overlap, including composite and homogenous materials. Cellular glass presents an interesting approach to solving thermal bridging challenges in masonry construction.

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