I'm in the process of re-taping the joins in the old duct-work in the very hot attic. They are quite cool to the touch so I'm considering insulating the ducts with Reflectix or even fiberglass batts.
Waste of time & money?
Ideas experiences opinions?
My own experience has been positive.
I had my oversized furnace replaced with a Rheem modulating unit (wonderful). I have close to 50' of trunk with takeoffs.
One section, about 30', has a rather noticeable temperature drop from one end to the other. Once the house temperature stabilizes from setback, the furnace usually runs at a very low speed (can't hear it), with the result that the conductive heat losses were high. I was heating the basement.
The last run at the end of the trunk is quite long and feeds the 2nd story master BR. I estimate the run to be 40'+. Needless to say, the heat loss (trunk and run) was excessive.
So far, I have insulated that section of trunk and the accessible portion of the duct, with the result that the delivered temperature is 4 - 5 degrees higher.
Before insulating, holding my hand above the trunk, got quite hot. Afterwards, there was a slight feeling of heat in places.
Reflectix provides an R value of 3. With a proper airspace, it is about R6. It was not terribly difficult to work with, and got easier and better as I went along. I was even able to 'artfully' cover curved transitions.
Before I started the project, I purchased a package of the tubular fiberglass insulation to see how it would go, as an experiment. I had to cut through it lengthwise to use (didn't want to dismantle the duct. It was difficult to work with, and not as neat.
With regard to the airspaces, I used double strips of Reflectix (about 1.5" and wrapped them around the trunk/duct at 3' intervals. I used a heavy aluminum tape (about 3" wide) available from Home Depot.
When measuring the cut, allow plenty of extra for overlap and for (air space) slack. You should be able to press on it and have good movement, before you contact the duct.
I think that the foil double foil surfaces will work well in your attic.