I finished off my tub of mastic, sealing a lot of secondary "shop" seams in the ductwork, then sealed any remaining longitudinal seams with aluminum tape.
Then I took the plunge: purchased a roll of ductwork fiberglass insulation, 1" fiberglass with FSK (Foil-Scrim-Kraft) backing, and rolls of FSK tape.
I decided to clean the crawl space first, before getting into the insulation. There was a heavy plastic vapour barrier on grade, in pretty good shape, a few tears, but there was a lot of sawdust, dirt, and 30 plus years of grime and cobwebs.
Then, I took a couple of utility tables down there, and got to work on the insulation. There's some info online, guidelines on how long to cut pieces, depending on the duct diameter. Also guidance on cutting 2" wide flaps.
The insulation rolls are 4' by 100' and comes with one such 2" flap, running the whole length on one side. Then when you cut a piece you're supposed to create a second such flap at the cut edge, so that the FSK backing is always overlapping seams. There's guidelines for how long to cut a pice for wrapping. In a nutshell:
Round ducts: circumference plus 7"
Square ducts: perimeter plus 6"
Rectangular ducts: perimiter plus 5" (near square? Split the difference??
(All of the above include 2" for flap. For the round ducts for example, you need 5" overlength plus 2" flap allowance. These values stretch/compress the insulation slightly, theoretically to around 3/4" thickness.)
I used 3" wide FSK tape. It comes with a backing paper that you peal off as you apply it. It's good practice to go over after with a spatula edge to press it firmly in contact.
I wore old clothes, gloves and used a good quality dust mask, without exception. The gloves have to come off though, when applying tape, and finessing the insulation into tight quarters.
I got into a routine: get the insulation roughly positioned, then "tack" it with thin strips of tape at intervals along the seams. Finally apply tape for full length of all seams. Transition pieces were tricky, required making some templates.
Anyway, here's a few pics, before insulation with mastic and some tape applied, and after.
Here's the main plenum, sealed:
A round duct, coming directly off the plenum, sealed:
A couple of locations where electrical wiring ran in/out of the return air circuit, now sealed:
(This one I filled the big void with styrofoam, then caulked around. The caulk is clear, not all that visible)
A typical rectangular trunk line seam, sealed:
One of two registers for warming the crawl space, as needed, just sealed. The lever opens/closes the register via an internal metal damper disc.
A round duct, sealed:
(Note, in cases like this, with an elbow, I disassemble the duct, so that I could the mastic right into the joint. Where not possible, I used fiberglass mesh to reinforce the seal.)
The roll of insulation, about half used up:
(I ended up buying a second roll as well, and got about half way through that one)
The plenum, after insulating:
Note the tuck tape patches on the ground vapor barrier. I took the time to clean the whole crawl space, and patched the tears in the vapor barrier, both to tighten it up and to prevent dirt sifting through again.
Insulated rectangular trunk line, with a vertical deflection, and a round duct coming off:
A couple of round ducts coming off a trunk line, insulated:
Close up of round duct meeting boot at underside of register:
A bit of sprayed-foam is noticeable in this pic. I sealed all the sub-floor boot junctions with foam-in insulation (also caulked the boots against the sub-floor, from above). First I used Dow Great Stuff, then switched to a similar DAP product, one that was latex basedj. The latter was much friendlier to use, easier to clean up. The can can be used multiple times without clogging. Also, I was abple to put a flexible extension tube on it, to improve reach.
Prior to starting the insulation, if I set a temperature probe in a particular outlet, with the thermostat raised several degrees centigrade, so that the furnace would do a sustained, fully warmed up run, the temperature got up to around 38~39 Centigrade. After the insulation, the temperature levels out at 47~48.
This page has a number of relevant publications:
(in particular, the second one down: "A Guide to Insulated HVAC Systems (AH121)"