HVAC Duct Insulation

[ManCave]

I am finishing my basement and will have to put the main trunk(s) of my HVAC system in a soffit. I would like to insulate the duct work both for energy efficiency and to try to keep some of the noise from the basement from traveling up the ductwork to the upstairs. Do I need to use special insulation? Can I use regular unfaced fiberglass insulation or does it have to be foil faced? If foil faced, does the foil facing need to be flame-resistant FSK?

I’ve seen the usual R-6 foil faced fiberglass stuff at the big box stores but it seems overpriced and I’d prefer more sound control than that little blanket will likely give me. I really can’t find any good resources on the subject. Any help/advice would be greatly appreciated! Thanks!

[Dana]

If the ducts are used for cooling as well as heating, if you went with air-permeable insulation you'd run into condensation issues and possible corrosion of metal ductwork unless you keep the basement at a VERY low humidity. Foil facers (even those with perforations) limit air-exchange between the insulation and humid room air, limiting the amount of condensation forming on ducts.

Before you insulate it, be sure to seal every seam & joint with duct-mastic. Half the noise from metal ducts is the hissing & whistling of leak points.

There's no ignition-barrier requirement for fiberglass insulation or it's facers on HVAC ductwork, as long as it meets minimum clearances from flues.

A flash-foam of 1" closed cell goods (FrothPak, TigerFoam etc) would air-seal and insulate to R6, but WOULD need an ignition barrier between the foam and conditioned space. Closing in the soffit with half-inch gypsum would meet code. It's a pretty expensive way to go if you have a lot of duct to insulate, but it's air-impermeable and a good sealant on it's own.

[ManCave]

Thanks again for your help Dana!

Ducts are used for both heating and cooling. I have sealed all the joints as best I can.

So you're saying I'm ok using regular foil faced insulation (whether it is FSK or not)? None of the insulation should get close to any flues and the duct work will be closed up inside a drywalled soffit.

Thanks again.

[Dana]

FSK= "Foil Scrim Kraft", which is not a fire-rating, but a description of the facer material.

The key to avoiding moisture issues affecting ductwork is to make facer absolutely air-tight, which is very difficult to achieve with low & mid-density fiberglass batts designed for studwalls. Duct insulation is typically 2-3x the density , not nearly as compressible, making it easier to seal the edges & seams with FSK-tape.

If it's all going to be boxed into the soffit and thus behind an ignition barrier you might price out doing it with rigid foam, either 1" foil faced polyiso or 1.5" foil-faced EPS. Anything that you can't seal with FSK tape can & should be sealed with 1-part expanding foam (eg Great Stuff.) The key is to leave as little volume of air as possible between the insulation and the duct, and seal it well enough that it can't convect room air into the micro-gap.

[ManCave]

Thanks, Dana.

Should the polyisocyanurate have foil on both sides or is just one ok?

[Dana]

Foil on one side is fine, since a foil in direct contact the duct would have zero thermal benefit, and you want to keep it as tight to the duct as possible to limit the air-volumes from which moisture can condense.

I haven't seen single-foil iso at box stores in my area, but I've seen single-foil EPS. EPS will often be cheaper per unit-R than iso, but it takes ~50% more thickness- not a big issue for something you're going to box into a soffit.