Insulating Ductboard Ducts in Attic

[J]

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 good thing.

Many thanks, Jim

[CBHVAC]

Out.

Junk.

Neither is ductboard ducts.

Alumaguard 60 if you must.

[Bob]

I've seen a lot of people just simple cut pieces of fiberglass insulation and lay them perpendicular to the ducts. If the attic isn't used much, it stays put.

[J]

Bob,

Thanks.

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!

Thanks again... Jim

Bob wrote:

[m Ransley]

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

[Bob]

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.

[nicksanspam]

Would you have any evidence for this article of faith?

How would you possibly prove it? :-)

Nick

[J]

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! Jim

Bob wrote:

[Stretch]

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.

Stretch

[m Ransley]

Nick READ reflextic site READ the package sold, READ, do your own research , you learn that way.

[m Ransley]

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.

[nicksanspam]

Again, you were the one who made the claim.

How would you prove it? :-)

Nick

[Nick Pine]

How do you know?

Nick

[WM]

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.