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Just so we know I got a 2009 Skyline 14 x 80. I'm working on changing my heating/cooling system this summer. I currently have R-8 Insulated Flex Ducts ran to each room for return air pulls. Biggest pain in the butt crawling in that tiny ceiling! Anyways, as for the ductwork I will be looking to change it out. I wasnt sure if I could do Fibgerglass Ductboard underneath the home outside of the belly wrap because I dont want to tear that open. I was going to use a large flex duct and make 2 distribution boxes with fiberglass ductboard. Just want to know if I can use it under there without concern?

Never mind I finally got a call back from Owens Home Corning which the brand of fiberglass board I am using. They said it is okay to use this under mobile homes without concerns besides critters tearing it apart. I don't really have critter problems over here so hopefully nothing tears it to shreds! Just to be clear it has to be the fiberglass board with the foil face on it. And I would recommend using at least a thickness of 2" for a R-8 value and maybe even wrap it for better insulation.

Hi, JP
Sounds like you got your duct figured out. The only other thing i would add is that you use a good foil back tape. Venture tape 1580 is one that has been tested for use on Ductboard Insulation. You want to use this anywhere you have a joint or connection.
Bill

I would recommend foil tape only to build the duct board distribution boxes. When connecting each flex duct to its start collar or vent boot, panduit straps (large zip-ties) are the time-proven and professional method. Fiber reinforced mastic should then be used to properly seal every connection.

Yeah I got some good Extreme Weather Foil Tape thats UL approved and what not. Got plenty of the bands to hold the flex on the vents. I'm not a fan of the mastic stuff. I had duct systems that beat California code by a lot for air loss as we have to now pressure test our duct systems here in PA. As long as you use quality tape and connect everything correctly shouldn't have problems.

Foil tape is excellent when building the duct boxes, but it doesn't hold up well (the adhesive) to the Florida heat when using it to seal the flex connections. PA obviously is a different climate.

I agree with you on that though. Makes sense, although mastic probably isn't a bad idea either way. It doesnt hurt to makesure its really really sealed good.