A Home Energy Audit: Mansard Roof and Overhang

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Early this morning we conducted a home energy audit of a small house with a Mansard roof. The main complaint is that the top floor is hotter than the 1st floor in summer and colder in winter. I went into the attic first and there is about 14″ of fiberglass insulation, we measured the house and started the blower door test. Under the new ASHRAE standards the house has an infiltration rate ~25% above the recommendation for this home. I used a smoked stick on the outside of the house and this is what I found:

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The smoke stick shows how the air is being drawn between the floors by the vents in the overhang of the Mansard roof. This was occurring at both the front and the back of the house and has a huge impact on the comfort and efficiency of the home. What happens in the summer is that it feels like the heat has its fingers wrapped around the top floor so its always significantly warmer than the first floor.  In the winter, the heat from the first floor is being removed by the air moving from one overhang to the other and the heat that rises from the first floor is being removed so the 2nd floor never gets really warm.


This is a the design of the joists inside this Mansard Roof; there is nothing to stop the air from passing between the joists into the ceiling of the first floor. We see this problem in homes with overhangs as well, where the continuous air/thermal boundary has been broken and it is like have a big tear in the back of your Patagonia (Or North Face) jacket; it impacts the performance of the jacket.

This is easy and inexpensive to solve:

  • Remove the soffit cover.
  • Seal between the joists with foam board and foam.
  • Install insulation under the overhang (R-30 or R-38) depending on the joist depth.
  • If the soffit cover cannot be sealed add an air barrier such as 1/8″ plywood.
  • Replace the soffit cover.

We pride ourselves on finding issues and solutions to uncomfortable homes, so if you have an issue with a room in your house or the top floor is not the same temperature as the 1st floor, give us a call 202-557-9200.



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