Window Installation
If you have ever observed how a typical window is installed on a new building, you probably noticed that the window unit is nailed to the face of the buildings sheathing with plastic or metal strips that come factory attached to the perimeter of the window frame. These are called “nail flanges,” and the utilization of these to install a window unit is a very common practice that most builders will utilize as it is industry standard and acceptable per code. Unfortunately, this is industry standard, and code acceptable technique puts a serious wrench in the window, wall and overall building performance.
“Fenestrations” which is just architectural jargon for building openings are the weakest and most expensive point in a buildings wall assembly. Windows and doors, even if you spring for the high-performance stuff, will typically generate about a third of the overall wall’s thermal resistance. A good window can be marketed upwards of an R9 or R10. The higher the R, the greater the thermal resistance. While a typical wall assembly can easily achieve an R30. That said, a fenestration, or a window is the wall’s weak point. Thus, it’s a no brainer that we want to make sure to do everything possible to get the most performance out of that window unit, and when most builders are installing windows, utilizing nail flanges to the outside of a wall, that kills performance.
In the Passive House world, we utilize computer simulations to model how a window will perform in a wall assembly given the walls build up and window ratings. Emu, a local Passive House consultant has gone through the tough work of modeling a few window installation techniques and demonstrated how the assemblies perform using color striations called Isotherms.
The goal is to have smooth and consistent Isotherms. In this diagram, the top left image shows a nail flange window, set to the exterior of the wall. Here, we can clearly see scrunched, wavy and inconsistent isotherms. This essentially cuts the windows advertised performance rating in half. So, if you bought an R9 window, your builder just made it an R4 thanks to the installation.
The bottom right image shows a window set in the center of the wall with an insulated frame. As you can see, we have smooth, straight and consistent isotherms. Utilizing this installation technique, you’re going to get every bit of the advertised R9 and more!
It’s these types if details that make all the difference in a building’s overall performance. Your architect may specify nice high performing windows for your project, but if your builder is not well versed in high performance installations, it could be a waste of money. At BONSAI we are well versed in designing and building high performance structures. These specifications and installation techniques will not only save on energy losses and thus money on utility bills but will keep your interior environment comfortable and healthy. Reach out if you’d like to discuss how we can help you achieve total comfort and efficiency on your next build!
