Bathroom mechanical venting
We are building a relatively tight house with a balanced Lunos (through the walls) ventilation system. 2 of the bathrooms are on interior walls so they need additional (unbalance) exhaust fan that will be user controlled and turned on whenever needed. This is being vented to the roof. What mechanisms do we use to prevent cold air entering these exhaust ducts? And also, to minimize thermal bridging through the duct penetrations? thank! Daniel
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My opinion is that you should try to go through the wall, both to avoid a roof penetration and because there are good quality wall caps with dampers available. If possible slope the duct downward to the exit point.
There isn't much you can do about the thermal bridge issue. The damper at the wall cap keeps cold air from moving in, but it will still be cold inside the duct due to conduction through the damper. Insulate the duct well. That's about all you can do.