Previous owner installed HRV to deal with radon. It was ineffective, since shutting it down had only couple 10th effect. I have installed radon mitigation system and knocked it down from 11 to 1.4.

So now I have a lovely Honeywell HR200b1005 200cfm unit collecting dust.

My plan is to keep it in the basement and instead run stale air ducts to kitchen (1st floor) and top of stairs of 2nd floor with fresh air ports running to three bedrooms on 2nd floor distributed by duct I put in insulated conditioned kneewall that connects them.

I realize it will not be perfectly designed system, but house is not exactly well sealed (’55 gut rehab of ’38 Cape) and until I get siding redone and superinsulated, this should do.

Question: for running inside of conditioned space in the house, should I be running insulated 6″ flex duct or is there another option?

Yes, I do know metal ducts are the best, but I am running in spaces where I do need the flexibility.