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Posted by still just me on December 24, 2007, 11:42 am
Heat engineering question:
Background: I have some insulted flex hot air ducts in my attic
supplying heat to the second story from the ceiling. They currently
lose a fair amount of heat, as evidenced by an ice dam problem above
each duct location. I heat mostly by wood so the hot air runs only few
times a day, usually twice in the morning to bring the temps up and a
couple runs overnight when the wood stove is burning low.
I know of a variety of alternative ways to deal with the dams from the
carpentry, roof, and venting disciplines, but I had another idea that
seemed intuitively like it might be a better choice (but maybe not,
intuition isn't engineering :-).
If I was to insulate the ducts heavily ( they currently have the thin
built-in flex duct insulation only and sit on 6" of fiberglass )
would this reduce the loss of heat to the attic appreciably? Or would
it just slow the loss so that it happens over a longer time period?
I'm guessing that having more insulation between the ducts and the
zero to 10 degree air above (when the dams form) would reduce loss
while running, but most of the time the heat in there is just
residual... or perhaps back-feeding from the room. Would insulation
cut the loss to the attic itself?
Thanks,
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Posted by on December 24, 2007, 12:01 pm
On Mon, 24 Dec 2007 16:42:25 GMT, still just me
>Heat engineering question:
>Background: I have some insulted flex hot air ducts in my attic
>supplying heat to the second story from the ceiling. They currently
>lose a fair amount of heat, as evidenced by an ice dam problem above
>each duct location. I heat mostly by wood so the hot air runs only few
>times a day, usually twice in the morning to bring the temps up and a
>couple runs overnight when the wood stove is burning low.
>I know of a variety of alternative ways to deal with the dams from the
>carpentry, roof, and venting disciplines, but I had another idea that
>seemed intuitively like it might be a better choice (but maybe not,
>intuition isn't engineering :-).
>If I was to insulate the ducts heavily ( they currently have the thin
>built-in flex duct insulation only and sit on 6" of fiberglass )
>would this reduce the loss of heat to the attic appreciably? Or would
>it just slow the loss so that it happens over a longer time period?
Same exact thing, just stated 2 different ways.
>I'm guessing that having more insulation between the ducts and the
>zero to 10 degree air above (when the dams form) would reduce loss
>while running, but most of the time the heat in there is just
>residual... or perhaps back-feeding from the room. Would insulation
>cut the loss to the attic itself?
Seal all air leaks. Religiously. THEN add insulation.
>Thanks,
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Posted by lp13-30 on December 25, 2007, 10:01 am
Do not mess with trying to add insulation to old flex. If you are going
to do anything with the ducts, replace them with R8 flex. You will spend
way more time and money patching at the old ones, and still have a bunch
of crap. Larry
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Posted by Zyp on December 25, 2007, 5:03 pm
lp13-30 wrote:
> Do not mess with trying to add insulation to old flex. If you are
> going to do anything with the ducts, replace them with R8 flex. You
> will spend way more time and money patching at the old ones, and
> still have a bunch of crap. Larry
Still Just me;
Or you can just call up a "blow in" insulator who can bury the ducts in
insulation. [Under Title 24 Energy Design - this is called "deeply buried
ducts."]
--
Zyp
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Posted by still just me on December 26, 2007, 2:20 pm
On Tue, 25 Dec 2007 14:03:24 -0800, a variety of posters wrote:
Thanks for all the info.
One last question: I noticed that the metal adaptor/box (6" round to
4x10)at the end of the flex isn't insulated at all. Do these normally
get wrapped with something ?
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>Background: I have some insulted flex hot air ducts in my attic
>supplying heat to the second story from the ceiling. They currently
>lose a fair amount of heat, as evidenced by an ice dam problem above
>each duct location. I heat mostly by wood so the hot air runs only few
>times a day, usually twice in the morning to bring the temps up and a
>couple runs overnight when the wood stove is burning low.
>I know of a variety of alternative ways to deal with the dams from the
>carpentry, roof, and venting disciplines, but I had another idea that
>seemed intuitively like it might be a better choice (but maybe not,
>intuition isn't engineering :-).
>If I was to insulate the ducts heavily ( they currently have the thin
>built-in flex duct insulation only and sit on 6" of fiberglass )
>would this reduce the loss of heat to the attic appreciably? Or would
>it just slow the loss so that it happens over a longer time period?