Wednesday, July 22, 2015

Window Installation

With all of the engineering and design that goes into a house like this, why would window installation be any different? EVERYTHING in a passive house is modeled, examined, and optimized thermodynamically. For now we will be talking about the thermodynamic analysis of the Passive House Windows and how this translates to the positioning of the windows within the wall system.

A typical new construction window comes with a nailing flange that is part of the frame of the window. This requires the window to be nailed directly to the wall, resulting in the window being flush with the outer edge of the exterior wall.

Passive House windows don't come with the nailing flange, this allows flexibility in location of installation within the wall system. The fact is, in a thick wall (12 inches or more, further details on wall thickness will be addressed in a later post) you typically don't want to install the window on the outside of your wall. You want to install the window in the middle of the assembly, or more specifically, as close to the middle of the R-Value of the wall as possible  (if your wall assembly includes more than one type of insulation, this may not by the exact middle).

This detail is from a 475 Blog Post that goes into quite a bit of detail regarding installation of air barriers to windows to achieve Passive House Level air tightness.

So in an effort to not bore your with thermodynamic details - here is why we want to keep the window in the middle of the wall:

It is more efficient - this simple detail can reduce your heating demand by as much as 22%

It is more comfortable - it helps keep all of the surface temperatures of the house within 6 degrees of each other, eliminating all of the cold drafts

It is more healthy - With such consistent temperatures, there is little opportunity for condensation, which means there is little opportunity for mold growth.

I found a fantastic page on Therm Analysis. The photo below shows an example window thermal analysis.

Thanks to ECO Houses of Vermont for the above detail!

Basically a Therm Analysis identifies the temperature gradient though a solid material. The rainbow colors shows how heat is flowing though the material. Looking at the photo's above; The image to the left has the window installed to the outside of the wall assembly. The isotherms are disrupted by the location of the window. This will result in colder spots where the frame of the window meets the wall.

The center image shows the window installed in the middle of the wall assembly. The isotherms seem to be pretty aligned.

The far right image also shows the window installed in the middle of the wall. In this case the window frame has also been wrapped in insulation. This added insulation further decreases the disruption of the isotherms. However the ability to add the extra insulation is a function of the size of the window frame.

Pretty amazing how just moving the window to the middle of the wall has a measurable impact on energy efficiency, and surface temperature two key elements for Passive House efficiency and comfort.

In my next post I am going to get into a little bit more on Therm analysis and Thermal Bridges.


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