## Thermal Bridging

At Gregory Duncan Architect we use THERM 6.3 software—a 2-D building heat-transfer modeling tool developed by Lawrence Berkeley National Laboratory—to analyze thermal bridges.

Thermal bridges are areas in a building envelope that have higher than normal heat loss. The color infrared analysis in THERM pictured above shows a thermal bridge at the footing of a heated basement. Adding a small strip of insulation at the base of the foundation wall eliminates the thermal bridge. In this example we reduced the heat loss from a U-Factor of 0.41 W/m²K to 0.30 W/m²K by adding a 100 mm wide by 450 mm high strip of Foamglas insulation.

The next step is to calculate the U-Values for the foundation wall and the floor slab separately.

Here you can see that the concrete foundation at the right provides almost no resistance to heat loss, while the Foamglas insulation at the left prevents most of the heat transfer from the basement to the ground. The U-Value per THERM is 0.196 W/m²K.

Here a 100 mm layer of Foamglas over the existing concrete slab provides a U-Value of 0.382 W/m²K.

Then we input the U-Factor of the footing detail and the U-Values of the wall and floor into a Psi-Value calculator to determine ?, a linear heat-loss value. In this case we get 0.005 W/mK.

Using PHPP energy-modeling software we can then calculate the difference in annual heat demand for the two footing details. Multiply ? by the length of the thermal bridge to get the heat loss in watts per degree temperature difference between inside and outside. In our example, this is 0.005 W/mK * 60 m = 0.3 W/K. From PHPP we get the SI equivalent for heating degree days for heat loss through the ground for our particular climate, in this case New Haven, Connecticut. This value is 28 kKh/a, or 28,000 degrees Celsius * hours annually. So 28 kKh/a * 0.3 W/K = 8.4 kWh/a. So this detail results in extra energy use of about 8 kWh per year more than if the detail were completely thermal-bridge free. And the detail without the extra insulation results in a Psi-Value of 0.184 W/mK. Doing the math again: 0.184 W/mK * 60 m * 28 kKh/a = 309.1 kWh/a. So the extra insulation saves 309.1 kWh – 8.4 kWh = about 300 kWh per year. If heat is provided by a heat pump with a coefficient of performance (COP) of 3, that means it takes 100 kWh of electricity to make up for the extra heat loss every year. That’s the amount of energy consumed by a 100-watt incandescent lightbulb left on for 1000 hours.

So, is it worth it to add the extra strip of insulation? Depends on the incremental cost of installing it and how much value we place on reducing the energy use of the building. Assuming \$0.20/kWh, 5% discount rate, period of 30 years, and 2% electricity inflation means that an investment of \$392 is cost neutral, that is, Net Present Value = \$0. Now, the homeowner can determine if spending more than that amount is worth it for the non-monetary benefits of reduced energy use.

Thermal-bridge analysis should be integrated into the architectural design workflow so that all major construction details can be analyzed with increased productivity.