Steve, I like the idea of insulated ductwork but I think flex duct is a bad way to go about it. Flex duct is one of the most restrictive ductwork options and a quick search on sites like greenbuildingadvisor shows that it is generally a bad idea. It allows for bad shortcuts such as not installing it stretched to its full stretch, tight 90, 180, or even full U-turns in the flex, bad trunk setups. I've seen other recommendations where rigid ductwork should be used and flex only allowed in the final 10 feet of a run.
It's a frictional loss issue that is the problem even if it is installed properly. The best way to use flex duct but not encounter the issue is to respect maximum duct lengths and be sure that the ductwork is upsized to the appropriate level to be sure that the airflow is correct. The issue with that is unless your flex runs are relatively short, you'll need to make very large diameter runs and on top of that you'll need to factor in the thickness for the insulation and how you plan to fit that flex into the space you are putting it. Once you get to a certain size of flex duct, you might even have R8 flex but you've now got so much surface area of the flex that you've got new problems.
I like this duct grade PVC idea since it is rigid and won't be leaky or have high frictional losses but I don't know what to expect in terms of how to best insulate it. My head says to create a sealed box around it out of rigid foam(beadboard or XPS are cheap) or drywall since it is cheap and stuff it full of cellulose.
As a personal note, if my attic had flex duct in it when it was built(it doesn't), it would probably be R4 so I'd be inclined to replace it with the R8 and since my attic is loose-fill I'd probably baffle around it with drywall or wood and then just loose-fill it to R60. So basically the ductwork would run in a channel that is 2 feet wide and covered 2 feet above the ductwork with cellulose. That is the way you can easily insulate that ductwork if you have the room. In Minnesota, the idea of putting ductwork in an attic is ludicrously stupid because we have winters where we usually hit -20f once or twice and if the ductwork has 130 degree air going through it, we would have a 150 degree difference in the ducts versus the outside. Which is more than the 90 degree with the rest of the houses insulated components. R8 is a hysterical in such a circumstance.
Since I think flex duct is a bad idea and rigid metal doesn't seem appropriate in an attic. How about ductboard? Take the ductboard and then add additional insulation around that, which should be easier since the stuff is square. You could wrap it in EPS, XPS, polyiso, or do the cellulose box route that I suggested above. If you are cheap you could even bust out fiberglass rolls and wrap flex or ductboard in that but no promises on its performance since you can't install fiberglass in an ideal way to get the most R-value by wrapping it around something square or round.
|