As we continue to talk about the comfort of our buildings and their energy efficiency, thermal bridging comes into play. In fact, thermal bridging is the unwritten prompt behind a number of construction trends. Don’t believe me?
Continuous insulation (or “CI”) is the big one, but thermally-broken windows, structural insulated panels and advanced framing techniques all attempt to address thermal bridging. But do you know exactly what it is and how to make sure your building doesn’t have too much of it?
Let’s shine the spotlight on thermal bridging and answer a few key questions you may have:
- What is thermal bridging?
- Where can you find thermal bridges?
- Why is thermal bridging important?
- How are thermal bridges detected?
- How do you prevent thermal bridges?
What is thermal bridging?
As the name thermal bridging implies: ‘therm’ refers to heat, and a ‘bridge’ is a way to connect two things.
Thermal bridging is heat transfer, as it physically travels through a material.
Taking it back to high school science class for a moment, there are several different ways that heat gets transferred:
- Conduction (physical materials in contact with each other)
- Convection (air circulation)
Thermal bridging is conduction in action.
All materials allow heat to travel, like electricity. Copper is very conductive, whereas rubber is an insulator.
Another way to think about it is to use the road as a metaphor. On the molecular level, certain materials (conductors) allow unrestricted temperature movement so heat can travel fast. Fast and Furious fast.
In other materials (insulators), heat travels sluggishly and hits barriers, like the worst traffic jam you can imagine. In our buildings, a traffic jam is the goal!
Where can you find thermal bridges?
Anything that is not specifically designed as insulation can potentially act as a thermal bridge, which means a lot of the structural components in your building are acting as a highway system.
As I mentioned before, there is a parallel to electricity here. Metals are some of the most conductive materials and will transfer heat readily. They are your Autobahn.
In wood construction you’ll still find metal in the form of fasteners, structural steel and cladding attachments. And in commercial (non-combustible) construction the studs themselves will be steel.
Wood is the next most impactful thermal bridging offender. Aside from the obvious (studs), it is also found at rim joists, structural stud packs and at window extension bucks.
Concrete is also a poor insulator (it runs a paltry R-0.16 per inch) though it has very high thermal mass. What does that mean? Once it warms up, it retains heat for a long time.
Massive concrete acts as a thermal bridge when it spans both conditioned and unconditioned spaces, for example: patios, balconies or a podium floor slab.
Why is thermal bridging important? And why should you care?
Thermal bridging reduces the effectiveness of the entire wall system.
In 2018, 88% of single-family and multifamily homes built in the US were wood framed. We design these buildings to maintain a comfortable temperature year-round. Regardless of climate, the typical code minimum assembly involves a wood cavity filled with fiberglass batt insulation.
This basic building material – wood – has an inherent R-value of 1.25 per inch. The adjacent fiberglass batt is 300% more effective at insulating than wood.
So a R-19 wall is a misnomer because the wall area isn’t 100% insulation — it’s also made up of framing members. Each stud is a thermal bridge: allowing a large amount of heat to transfer between the conditioned interior space and exterior.
How are thermal bridges detected?
Most often, you’ll see thermal bridging shown using shots from a thermal imaging camera (FLiR is a well-known brand).
Remember the movie Predator? Predators used thermal imaging to identify heat.
When not used to find alien prey, thermal imagery can also be used to identify hidden areas of moisture damage (from thermal bridging or otherwise).
It’s also calculable, on the theoretical side of course. Here are some tables if you want to nerd out on the effective R-value of your entire wall assembly.
What are the risks?
Aside from overall decrease in wall effectiveness, concentrated areas of thermal bridging can result in cold spots. The initial risk here is fairly benign: occupants will be uncomfortable.
But of greater concern is how these cold spots may act over time. These can contribute to interstitial condensation — wetness in the wall assembly localized at a cold thermal bridge. If left unchecked, these areas are prime candidates for mold and subsequent deterioration of the building materials.
How to prevent thermal bridges
1. Continuous Insulation (CI)
Exactly like it sounds. The concept of CI is to add an uninterrupted, self-contained layer of insulation outboard of the wall sheathing. This is the building method that most directly addresses thermal bridging without altering the standard wall framing, and thankfully is becoming an institutionalized practice.
2. Specialized thermally-broken materials
Once you’ve identified the likely thermal bridges on your project, there are probably a couple niche products out there to address them. Thermally-broken windows, fiberglass or nylon cladding attachments, or even wooden nails can be used to ratchet up the overall effectiveness of the complete assembly.
3. Advanced framing methods
Another forward-thinking concept is to adapt the framing itself. Advanced framing aims to reduce the amount of wood by increasing both the stud width (2×6 or greater) and spacing width (24” OC) and being generally frugal with corner connections and drywall nailers.
4. Double-stud or SIP walls
Like advanced framing, this method takes a big chunk out of the potential thermal bridge material found on a project. Double-stud walls use two separated vertical stud walls connected by full-width top and bottom plates. Structural Insulated Panel Systems (SIPS) use a foam and OSB sandwich to create panelized walls. In both cases, there are still full-width structural members required to make it work.
5. Effective air-sealing
Air-sealing provides a tertiary benefit in that it helps the cavity insulation to perform to its fullest potential. Reducing drafts themselves minimizes the risk of interstitial condensation in the first place.
Thermal Bridging is ever-present in contemporary construction techniques. Whether you are going for passive house standard or just keeping up with current codes, knowing how to identify them and evaluate their impact will make your buildings better.
Think air sealing might be the answer to your thermal bridge woes? Sign up to watch our latest video workshop: The Beginner’s Guide to High-Performance Air Sealing.