If I were to insulate my R-4 ductboard ducts for my HVAC system in my
attic, which way should the vapor barrier face? I had thought to glue
"Comfort Therm" type insulation to the main ductwork with contact
cement. Thus, the vapor barrier should be in direct contact with the
duct face and allow for no moisture to accumulate.
I've also been looking into the "foil bubble wrap" products and DIY
spray on foam.
Anyone do duct insulation and have good results? My ducts are "not a
problem", but I imagine that R4 insulation in the northeast is not a
Yes, my first thought was to do this. But then there is all the
paranoia (reality?) about mold and moisture. The duct is for
heating/cooling, so I am worried about condensation. It is well sealed
and constructed (new construction and accessible). Because it is
sealed, fiberglass w/o a vapor barrier may be the best choice; that is,
cheap, easy and no moisture buildup (hopefully, effective :+).
Someone mentioned Alumaguard, which I had not discovered by "googling".
This group is the best!
Just a few things ive seen is insulated flex ducts are fiberglass
insulation with plastic inside and out. With enough insulation I
dought condensation could occur. Bubble wrap stuff reflextic is
overpriced , non substantionated-verified R values. its a Radiant
barrier. Foam is expensive. I might consider wide 6 fiberglass batts.
The probem is getting good seam- tape seal with batts, probably the
aluminum heat rated tape is best
It woulds be better to replace R-4 ductboard with R-6 ductboard or R-8
ductboard. Properly installed it is very effective at sealing out air
leaks. I would rather have ductboatd than sheet metal as it is
thermally more efficient in most installations.
With wrapped metal duct, you must seal every seam in the metal duct,
then wrap it with insulation and seal every seam in the insulation.
This includes both transverse and longitudinal seams in both the metal
and the wrap. ductboard only needs sealed once, using UL-181 A-P tape
; or duct mastic & glass fabric tape. DO NOT use duct tape.
NOTE, this is NOT a DIY job.
If rou put more insulation over ductboard, condensation can occur
inside the insulation, depending on conditions and where the dew point
occurs. Insulayion is not an air barrier. I do not recommend it. BTW,
due to its density, ductboard IS an air barrier.
As for reflectix, new data seems to support its claims. I did not
believe it at first, but some very knowledgable people I know say it is
good stuff, which agrees with my experience.
Relextic has no independant verifiation with standardised industry
accepted R value testing. They show no test done on R value by
standardised industry testing. Their claim is it performs " like".
Interesting how their thin product varies in R value on their literature
from aplication to aplication. My IR Thermometer tests prove to me
Reflextic and insulating paint claims missleading at best.
Just " like" insulated paint performs "like" R values advertised by
insulating paint salesman.
Deceptive advertising is all it is. "Like" my 96.7% 100000 btu carrier
infinity. Or pure clean air out of o3-Ozone producing air filters like
the Ionic Breeze.
If you're worried about moisture under the insulation, then you need to
ventilate your attic. It's almost impossible to get a 100% seal when
insulating existing duct work. Humidity can condense on the duct work and
cause a lot of problems.
Being an attic insulation in the NE US, I'm guessing it's only for air
conditioning. Lay R-19 batts on top of the ducts and put in a
thermostatically controlled fan.
The ductwork is for both heating and A/C. The blower is in the attic
(second zone) heated hydronically by an oil boiler in the basement. I
would guess condensation is an issue only for cooling.
I think I will go with fiberglass batts. Cheap and relatively easy. I
also plan to spread some mastic to ensure no air leaks (I do not think
there are any).
All, thanks for the help!
I was losing several degrees between my unit and the back bedroom
outlets due to poorly insulated ductwork (R4). I built a trough of
fiberboard for the long run (30 feet) and filled it with blown in
cellulose 10 inches deep. Lowes had everything I needed including the
bucket of mastic.
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