What are the "cleanest" materials for a fresh-air intake duct?
I need to run about 8 feet of 3" duct for fresh air intake directly into a soundproof, airtight music room that was built with zero/low-VOC materials. Air exhaust will be via another 3" duct with fan. As the ductwork needs to include a few twists and turns for sound attenuation, it must either be flexible duct or include a few elbows (I may also insert some sort of diffuser box/transfer duct, if necessary). For the intake side, I'm considering class 0, flexible aluminum duct, and am also interested in the newer HDPE products. Can you comment on either/both, and do you have any other recommendations for the best possible indoor air quality (i.e., no VOCs, PVC, fungicides, fibers or other toxins/irritants)? Also, what types of filters or filter materials are the greenest (for the intake duct)?
Sounds like your best option would be to go with metal rigid ductwork (26 Gauge) and seal all your connections with a low-VOC, low-odor mastic (like RCD or equivalent mastic). That would be best for air flow and has high recycled content (and recyclable).
You can insulate the exterior (if desired) with a radiant barrier bubble wrap or recycled cotton blanket materials to avoid fiberglass exterior duct wrap.
I like your idea of a diffuser box over the multiple elbows (to reduce friction losses). I fabricated and used a similar system in the past and it worked well.
For more information:
Read Daniel Glickman's Q&A, "Should we replace our PVC-lined flexible heating ducts with another type of ductwork?" to learn more about choosing ductwork for a healthy home.