Have you ever had a friend for dinner, perhaps had some fava beans and a nice Chianti, sat by a window and noticed a draft? You might have a thermal bridge! Even if you have really great insulated glass in your windows there is still a difference in the way heat is transferring between materials of your window and the materials of the wall it is installed in, so you need to eliminate any thermal bridges that could occur between the two.
According to the Passive House Institute, “Windows (glazing and frames, combined) should have U-factors not exceeding 0.80 W/(m²K) (0.14 Btu/h/ft²/°F), with solar heat-gain coefficients around 50%.” In order to calculate the U value of the window we need to do a 2D calculation with a program such as THERM to determine what the heat is really doing when it interacts between the window unit and the wall it is being installed into. Remember, the PHPP only looks at the heat flowing in 1 direction – from the warm side to the cooler side. It does not consider that the heat can also be re-directed toward adjacent materials. The THERM calculations will provide the missing bits of information – the Psi of the spacer interacting with the frame and the Psi of the window unit interacting with the wall it is being installed into.
There has been a lot of talk about just how to calculate what the Psi spacer or Psi g value is. Part of that confusion comes down to the NFRC, the window certification group in the US using a different set of criteria for their calculations. As a result we don’t have a ready to plug in number available for us to use. In Europe the manufacturers are tuned into PassivHaus and the requirements of the PHPP and quite a few are PHI certified, so selecting their windows automatically provides you with the numbers you need. So what do we do with non-PHI certified windows?
Since PHI requires the windows ratings to meet the EN ISO 10077-1 for glazing and EN ISO 10077-2 for frames, I started looking into what those are. What I found was those are expensive to read! A bit more digging and I found that there is a table in EN ISO 10077-1 with generic type information that can be used. An example of this would be if you have a wood frame and non-coated glass your Psi g will be X and if it is low e glass the Psi g will be Y. If you want to actually calculate the exact Psi g you could use the calculation method found in EN ISO 10077-2. Since I didn’t have access to the standards to read, I contacted PHI and asked them what should be done. I specifically asked about the Psi Spacer in the PHPP now being referred to as Psi Glazing Edge which could be confused with the NFRC method of calculating the glazing edge. Benjamin Krick provided me with this answer:
“PHI calculates Uframe and PSIglass values according to ISO 1077-2 (but with the real intersection of glass and frame). That seems not similar to the calculation you have mentioned [reference to my mention of the NFRC glazing edge calculation]. But only this method is valid for PHPP [ISO 10077-2].
In the first step you calculate Uf by using a calibration panel (lambda = 0,035 W/(mk)) instead of the glass.
In the second step the panel is replaced by the original glass geometry and a simplified spacer.
PSIg is then calculated by subtracting the heat flow through the glass and the frame from the heat flow of the whole model.”
Testing has shown that spacers can be simplified in calculations to appear as 2 boxes instead of their normal shape. For an example of this method, please refer to “appendix A1 The Two Box Model” in the report Laustsen, J. B., S. Svendsen, WIS Database. Data Submission Procedure for Databases on Spacer Profiles, Edge Constructions and Window Frame Profiles, version 1.0, Technical University of Denmark, Dep. Of Civil Engineering, Lyngby (Dk), March 2004
Before working your calculations, be sure to adjust the boundary conditions in THERM to equal your project temperatures!
So here are the steps Benjamin outlined in simplified pictures:
For the Psi Installation value, I will refer to the wonderful graphics work of Bronwyn Barry that she created for her presentation “A Tale of Two Rating Systems”, she shows us the critical calculation for determining the U of the window installed as well as a corresponding detail of where those measurements are found.
So let’s look at that calculation one step at a time.
Ug = U glazing found in PHPP WinType, THERM calc, or Manufacturer info
Ag = Area of glazing measured from the interior of the frame edge
Uf = U of Frame found in PHPP WinType, THERM calc, or Manufacturer info
Af = Area of frame measured from the edge of glass to outer edge of the wall.
Psi of Spacer (Psi g) is found by the previous example in this report
Lg = Length of Glazing and that is the entire boundary measure of the glazing.
Psi Install the next example (1d calcs subtracted from 2d calcs)
Lf = Length of frame and that is the entire exterior measure of the frame
In this diagram made by Bronwyn Barry you will see that the entire window assembly as installed has been calculated in THERM. The U of the assembly multiplied by the Length of the assembly is then multiplied by the difference in temperature between the inside and outside of the building. This same calculation, the ULdT, is done for the separate components of the glazing unit and the wall assembly. The 1D ULdT calcs of the Glazing unit and wall assembly are added together and subtracted from the 2d ULdT calculation of the entire assembly to reach the Psi of the install. This Psi value is then multiplied by the entire circumference of the wall opening.
The Psi Install and Psi glazing edge values are then entered into the WinType sheet and you are ready to move onto the shading worksheet.
Next up in our series - Shading.