Hoping to install an HRV in our house to improve winter air quality. One floor ranch with a full basement located in Vermont, so cold winters. The upstairs in entirely finished and there is no current duct work. I would plan to install the unit in the basement in an unfinished area with access to a place for a condensate drain. What are the best and least disruptive ways to install the necessary duct work upstairs? Are there flexible ducts I could feed up through the 2x4 framed walls using minimal holes or would need to install the rigid 3 1/4 by 10" ducting?