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Condensation on HVAC system in attic

Posted by mosquitogang201 (My Page) on
Fri, Jun 6, 14 at 22:51

The air supply plenum in my attic has a lot of condensation building up and dripping through my ceiling. It is wrapped in foil backed insulation. I noticed a ~3 inch gash in the insulation. I suspect that I may have accidentally cut it while I was in the attic running new coax cable over the winter. Now that the summer heat and humidity are back, I think that moisture is getting in the gash, condensing on the plenum, settling in the bottom of the insulation and dripping through. The insulation feels room temp on the sides and top but ice cold on the bottom. But I'm not an HVAC expert - does this sound like what could be causing the condensation? Is there anything else I should check before tearing off the insulation?

Is installing the insulation as simple as it looks... just lay it up and tape all the seams? Or are there any tricks to it I need to know?

Thanks in advance for your help.


Follow-Up Postings:

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RE: Condensation on HVAC system in attic

I think that the condensation is more from leaky duct
takeoffs & plenum to equipment connections that are poorly
sealed rather than a gash in ductwrap.
although you don't say how large or where the gash is
located on the supply plenum ductwrap.

and...no it isn't as easy as it looks...if you do it properly.

first remove all duct insulation.
seal each duct take off with mastic or Hardcast brand mastic #1402 tape. (surfaces must be clean & dry) paint on mastic
will take time to dry, mastic tape must be firmly pressed in
place to make seal. paint on mastic should be thickly applied..
like thick as a nickel.


you'll have to cut the ductwrap to fit around the duct takeoffs.
I usually take ducts off, so that I can put the ductwrap with precut
holes for takeoffs.
take time to seal plenum to equipment. I find that the mastic tape works best for this.
you don't say if plenum is upright or trunk line...so most common would be upright.

wrap the ductwrap around the plenum, leaving top for seperate piece.
tape covers duct staples.
so you'll need a duct stapler, duct staples & fsk tape (foil scrim kraft tape...looks like ductwrap without the insulation)
double row of staples, fsk covering staples..and I then staple down fsk.

ductwrap is sold in 4' wide rolls. buy R-8 only.
one side of the ductwrap is blunt cut...meaning ductwrap
and insulation are same...the other side the insulation is cut
back less than the insulation...this is the side that is stapled to
the ductwrap. staples can't penetrate ductwrap & insulation to
attach to the ductwrap under the lap..so any areas to be stapled
need to be cut back in the same manner.

once you get ductwrap on sides of plenum installed,
then cut piece for top cap of plenum. make sure insulation is in
full contact with the top cap & make the lips (ductwrap without
insulation) about 3" wide. use these lips to staple to wrap
on sides of plenum. fsk tape, staple.

then re-attach the ducts.

insulation of ductwrap has to be in full contact with metal
of plenum...or it will continue to condensate.
make sure you pull it tight enough to be in full contact
but don't compress the insulation.
duct & plenum to equipment connections has to be
completely sealed or it will continue to condensate.

ducts attached to takeoffs on plenum...insulation in
full contact with both start collar of takeoff & plenum.
inner liner (flex) mastic taped to start collar.
outer liner of flex covering insulation at plenum & mastic
sealed to plenum.

some people use lock ties (pundit straps) be aware
that compressing insulation with lock ties will cause
condensation, and that lock ties alone on start collars
will work loose...even with racheting tool...over time..

I like to mastic seal the outer liner (the vapor barrier of the
flex) to the vapor barrier of the ductwrap. for ease of install
& to save time...mastic tape.

it is a hot, uncomfortable job that requires patience &
much attention to detail...or it will condensate.

look to spend $100+ for ductwrap
$50 for duct stapler plus staple costs
$30 for hardcast mastic 1402 tape
$20 for a bucket of cci 181 mastic & brushes
can't remember price of lockties or tool to tighten
as I hardly ever buy lockties or use the tool I've
had for years.

buy two rolls of Hardcast 1402. this tape seals to many materials
as long as they are clean & dry...
use the other rolls to seal supply boxes, if you take off
a supply grill inside the house, you'll see the oversized cut
where the supply box penetrates. keep tape within area
covered by supply grill.
also take down bath fan covers & use mastic tape to
seal those oversized cuts.

other tapes don't seal like this particular brand of tape,
over the years I've tried them all. hardcast 1042 is what
works & lasts.

if you hire this out...make sure that the work is done
as described above..if you diy & have questions..
post or email me.

and if existing ductwrap is cold on the bottom..it is probably
wet inside.

best of luck.


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