This is the second of a two-part column on Thermal Bridges. This part will deal with figuring the Psi value to enter into the Areas sheet of the PHPP for your Thermal Bridge.
In order to figure out what is happening with our Thermal Bridge we need information from our inputs into the PHPP as well as a calculation with a 2D Heat Transfer modeling program such as THERM. Our PHPP inputs will tell us what the U value is of the parts of the construction we are examining as if the heat only flowed in one direction and met no other conditions that would change its direction or rate of flow. The THERM program will use the information we provide about the assembly and go into more detail about how the heat is flowing that will include all the conditions we modeled that would affect the direction and rate of the heat flow then give us a U-Value for the entire area we modeled. We then calculate the difference between these to correct for the 2D heat flow that the PHPP doesn't model and that difference is the Psi value.
In order to have an accurate assessment of our construction it is important to make sure we are comparing apples to apples. While there are construction materials and boundary conditions already in the THERM libraries, they need to accurately reflect what it is that we are using in the PHPP. I found this out the hard way – by creating an obviously positive thermal bridge example for this blog posting that ended up being a negative thermal bridge. Ouch! After some really great review with fellow Certified Passive House Consultant Hayden Robinson of Hayden Robinson Architect, I was back on track and this posting grew a lot more informative. Thanks Hayden!
When you list an assembly in the PHPP you know that you will have to justify the U-values that you use and have the proper air film coefficients. Those exact same U-values and air film coefficients need to be used in the THERM calculation for the most accurate Psi calculation. So how do you know what to use? Let’s start with the PHPP U Values sheet. I have circled the information you need to transfer into THERM.
Here is where you enter new Boundary Conditions into THERM.
Here is where you enter new Materials into THERM.
Now build your construction using the new materials and air films you have modified. You will need to assign U Factor Tags to the outside length to get the report needed for figuring Psi. Once we have modeled the TB in THERM, assigned U Factor tags to the exterior portions of the construction, and calculated the heat flow we can ask for a U Factor report of the total length of our model and it will tell us what the U Value of the total construction and the difference in temperature from the inside of our construction to the outside (the delta T).
We are now ready to figure out the Psi or difference between the rate the heat is flowing through the construction at the linear TB where the heat flow is being affected by the different materials that are adjoining with their dissimilar heat flow properties, and the rate the heat is flowing when it is flowing in one dimension without interruption.
Because this is a simple TB we can easily figure the Psi by multiplying the PHPP's U-value of each assembly that we used to create the construction by the length we modeled for both assemblies in THERM and add them together. We then multiply the U value that THERM gives us for the entire model by the total length of the exterior of the entire model. We subtract the total of the one dimensional heat flows from the two dimensional heat flow and what is left is the Psi value that can be entered into the PHPP Areas worksheet along with the actual length that the TB exists. In this example it is the length of the "Z" dimension into the drawing where the 2 different constructions meet as modeled.
If this were a more complicated situation where the difference in temperature between the inside of the construction and the outside of the construction were different for one of the dimensions it would be a more complicated calculation and the temperature difference would need to be considered in the calculation. Other factors that make for more complicated calculations would be more complex constructions or interactions with the ground. The PHPP manual offers assistance on this as well as the publication “Thermal Bridge-Free Construction” that is available from PHI in German.
One final note – calculating Thermal Bridges are incredibly important and very time consuming. One very thoughtful fellow has developed a Thermal Bridge tool that will make all that calculating so much easier. I had the opportunity to try out the Psi XLS tool that Bob Ryan of the Passive House Academy developed to make figuring out Psi values less complicated. He did an excellent job with it and it will certainly make our jobs easier. That should be on the market soon and when it is I will give it a complete review.