Just completed exterior renovation on a 200+ colonial. Removed everything from outside air sealed, installed roxul, plywood, taped joints, typar, 1 inch foam board, 3/4" strapping, vertical grain clapboards, etc... I have an ell that the radon level is now slightly elevated because it is so tight. It is basically 20 X 30 and attaches to the main house via a large opening. It has the kitchen and living room in it. It is a slab which I did not do so not sure how leaky it is. I want to install a single HRV and was looking at twinfresh comfo, Mica 60, Mica 150 style through wall units.