The Bridge of Sighs built in Venice over the Rio di Palazzo connects the interrogation rooms in one building to the old prison across the river. Legend has that it was supposedly named because the windows on the bridge were the last view a prisoner had of Venice before being imprisoned. It is a beautiful bridge that heard the sighs of much sorrow. 
In construction there is another bridge, sometimes beautiful, that leads to much sorrow and it too involves “sighs” – the involuntary, sad, sorrowful exhale of breath kind that occur while reading one’s energy bill. It also involves another kind of Psi – the expensive, unnecessary kind that comes from having Thermal Bridges.
I mentioned in the last blog posting that I had attended a class in Berkeley given by the fine folks at Passive House Academy on the THERM software program that detects 2D heat transfer in constructions. Bob Ryan taught the class and has also developed a super tool for easily coming up with Psi values (which are the differences between a 1D and 2D calculation of heat flows). Heat flows of a construction change when they encounter the heat flows of a construction going in a different direction, such as a North/South wall intersecting an East/West wall, a roof and wall intersection, or the intersection of materials that have different thermal properties. Psi is the amount of change or difference in how the heat flows at these locations compared to how it flows in a section of the construction where it is not encountering the effects of heat flowing in a different manner.
A thermal bridge is the path for the heat to get across a construction at a different rate than at other, less complicated places (like a bridge gives a car an easier traffic flow across a canyon). Psi is how much easier or harder it is for the heat to flow at that complicated part of the construction. It would be easier for a car to drive across the Grand Canyon on a bridge than to find a path from one rim, across the canyon floor, and up to the other rim! The difference between the two traffic flows would be the Psi.
There can actually be beneficial thermal bridges, called negative thermal bridges, that prevent heat from escaping faster. These thermal bridges can actually make it easier to achieve the PHPP requirements for a project.
Calculating thermal bridges can be difficult. The PHPP only deals with 1D heat flow, Therm measures 2D. Both are needed to calculate the Psi values and for a Passive House construction in the US if the Psi is > .01w/m2K then you have a thermal bridge, so knowing how to calculate them is important. Bob has made that job oh so much easier with his Psi XLS calculator tool. We were able to use it in our THERM class and it walks you through the steps of the information you need to know for all sorts of different scenarios and then does the calculation for you. Psi XLS has pictorial examples of different thermal bridges, details the measurements you need to provide, shows you how to tell what your boundary conditions are, and allows you to adjust for the temperature conditions of your project. Once you enter all the information a miracle® happens and you get your Psi value.
By being able to calculate these thermal Psi values an architect or designer can tell if there will be a heat flow problem. This is important not only for those long sad sighs when you look at your heat bill, but also so that the change in temperature from the heat leaving more quickly in an area does not cause a condensation problem that could lead to mold or decay. A thermal bridge free construction is more comfortable and healthier than one full of thermal bridges – and there are a whole lot less sighs!
Currently the Psi calculator is available with the THERM classes and will soon be available for purchase. I will keep tabs on its launch date and let you know. In the meantime, if you would like Passive House Academy to teach a class to your Passive House group, visit their website and use their handy dandy contact form.