Passive House detail at 174 Grand St
What looks a bit like a racing stripe running horizontally around the top of the building is actually not decorative; instead it addresses a problem that we, and i guess many others, completely overlooked: a C.M.U. parapet wall is a massive thermal bridge!
What normally happens: The structure of the building is constructed of concrete, and the walls and roof are insulated on the exterior. This is absolutely a good thing, as exterior insulation is far superior to cavity insulation of the interior stud walls. What has been historically overlooked, however, is the fact that by not wrapping insulation all around the 42″ high parapet (NYC building code) we create a huge cooling rip for the building. As you can see in this Therm model (heat transfer modeling software), the upper corner is much cooler than the other interior surfaces and so condensation is more likely to occur here (grey: concrete, yellow: EPS, blue: XPS).
One way to solve this problem is to use a different concrete-based material which has a similar fire rating to CMU but is a much better thermal insulator: A.A.C or Autoclave Aerated Concrete! AAC has an R-value of up to 1.25 per inch, compared to 0.07 (hr*ft2*F / BTU*in). This is quite easy to see in the different isotherms:
Here’s a last close-up shot how CMU and AAC come together. The metal strip you see in the photo is the flashing detail which will terminate the roof membrane later…
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