HVAC duct insulation question, in unconditioned attic

[Venkman]

I have 40 feet of square metal ducting in an attic. It is subject to extremes of temp and humidity. Currently it is insulated with a pitiful 1/2" of fiberglass wrap and vinyl.

The ducts sit 6" above the ceiling joists. Total distance from ceiling drywall to bottom of ducts is approximately 12". The duct is held off the ceiling joists by 2x6 spacers. and metal ties.

There is plastic vapor barrier on the attic side of the ceiling drywall.

FG batts combined with blown in FG insulation fills the space from the ceiling up to the underside of the ducts.

I intend to strip the existing insulation and seal the ducts (which was not done during installation).

I then want to make a 3 sided structure of 2" foil faced polyiso insulation board over the ducting, anchoring it to the ceiling, which would be the 4th side. I will seal the joints with gap and crack foam. This would make a box that brings the ducting into the conditioned space of the house and isolates it from the unconditioned attic.
I intend to leave the present attic insulation in place under the ducting when I enclose it.

The 2" polyiso will be R-13+. The recommended R value for attic ducts in my climate zone is R-13.

My primary question is, can I use 1" polyiso board to fill in the space between the ceiling joists, beneath the ducting? The 1" is only R-6 but it will be backed up by the insulation that is under the ducts now, that I will enclose. I'm not sure what the effective R value of that arrangement would be.
My main concern is to avoid condensation on the surface of the Polyiso board when running the ac on a 72 degree dew point day.

[TinyTool]

From what I can see, your current insulation setup looks pretty solid. The blown-in fiberglass is covering the ducts well, so you’ve got a good starting point.

Using 1-inch polyiso board between the joists under the ducts sounds fine. That R-6 layer, along with the batts and blown-in insulation already there, should easily get you to around R-13 or more. It’s hard to give an exact number because of how materials layer, but you’re in a good range.

The main thing is sealing it all well to keep humid attic air out. Make sure you tape all the seams and foam the edges and any gaps. Your plan to box in the duct with 2-inch foil-faced polyiso is smart. It’ll help make that ductwork act more like it's in a semi-conditioned space, which improves performance and helps it last longer.

Looking at the photos, it seems like a few of the duct wrap seams could be sealed up a bit tighter, especially around the joints. Also, just double-check that you’re using foil tape rated for ducts and attic conditions.

Overall, your plan looks solid. But yeah, it wouldn’t hurt to get a second opinion from an HVAC or insulation pro just to be sure everything’s on track.

[Venkman]

I appreciate your answer TinyTool.
I plan to strip all that old insulation off the ducts. It's only like R2!. That way I can seal all the ducting before covering it with the new polyiso.

[TinyTool]

Glad to hear you're going that route. Stripping the old wrap and sealing the ducts first is definitely the right move. Once you get the polyiso in place and everything sealed up, you’ll have a much tighter and more efficient setup. Keep us posted on how it turns out!