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Posted by Bob Pietrangelo on September 19, 2007, 7:09 pm
There are local and national codes that determine your minimum duct
insulation in an unconditioned space. There are lots of ways to achieve it.
--
Bob Pietrangelo
bobp3@comcast.net (home)
bob@comfort-solution.biz (work)
www.comfort-solution.biz
"Lloyd E. Sponenburgh" <lloydspinsidemindspring.com> wrote in message
>> Then I gather, Foil Bubble Wrap Insulation is not effective for A/C
>> ducts, which are located in the attic. Temp there can be well over 100
>> degrees. Is this a correct interpellation? What is the best
>> reasonable priced material to use for duct work trunk lines?. I do
>> realize you get what you pay for. Thanks
>
> You didn't describe the ducting. If it's made-up metalwork, you'd
> _better_ get some insulation on it. Most residential work seems to end
> up as flex or ductboard, primarily because it requires much less skill to
> install than metal. Most installers don't have the skills to make custom
> metal. Typically, they're using pre-fabbed components, and just hooking
> them up.
>
> Flex duct and ductboard are already insulated to one degree or another.
> Older stuff was about R-2.5; new material is usually R-6.
>
> You have two primary insulation issues to deal with, and an ancillary
> one. The first is the air temperature in the attic. That requires
> thermal insulation -- foam, fiberglass batt, or fiberglass pressed board.
> The second is radiant (IR) energy booming down from the roof surface.
> That requires a radiant barrier film. It's best placed on the underside
> of the attic rafters, but flex duct can have it as the outer barrier
> layer (Silvaflex, etc). Most ductboard typically has an aluminum radiant
> barrier outer layer.
>
> The third issue is to provide some sort of vapor barrier between the
> potentially humid attic space and the cool outer surface of the duct
> liner. Again, flex and ductboard accommodate this with the outer barrier
> layer, which, if it's taped and mastic'd properly will keep out moisture.
> Metalwork must be properly wrapped and sealed to prevent moisture
> traveling to the cool surface, and condensing there. If the thermal
> insulation layer gets wet, the R value essentially drops to nada in the
> wet areas.
>
> Commercial work in our area uses either a wrapped fiberglass batt topped
> off with a continuous layer of mastic, or a spray-on foam material.
>
> The foam is a superior insulator AND moisture barrier combined, but for
> commercial installs (here), it must be covered with intumescent paint in
> order to meet fire codes.
>
> You have a lot of options -- but first it boils down to the original
> question; What type of ducting do you have?
>
> LLoyd
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