We live in a very old house with an unfinished basement. The heat for the entire main floor is from a furnace in that basement that blows through the square style ductwork to vents placed in each room on the main floor. These ducts have no insulation on them at all, and get quite warm to the touch near the furnace itself, but decay with distance (obviously). Right now our water heater is wrapped with probably pretty old insulation - I don't see any labels on it, but it's the yellow style with a white paper backing. It's held to the water heater with duct tape. We have a water heater insulation tube recently purchased, and are wondering if we could repurpose the fiberglass insulation on the ductwork.

The two main concerns we have come up with are:

  1. Overheating the insulation --> starting a fire.
  2. The insulation somehow makes its way into the duct through some tiny crack (currently we have the fancy duct tape for heating vents around all the cracks, but this is likely imperfect) and gets into our air.

We think these are both unlikely but wanted to ask. Having trouble finding this exact question on the internet. We know this is not an optimal solution, but it's available now and free, so our main concern is safety. Can we get away with this, or is it critical to purchase the foil-backed insulation for use with duct work for heat?

If it matters, these ducts only connect to the furnance; there isn't any air conditioning set up in this house, which was built in the early 1800s (for sure before 1830).


1 Answer 1


All fiberglass insulation is white and dyed a colour (so there's no functional distinction among colours). The melting point of fiberglass is about 1,300 F. But the paper backing is, obviously, not so flame retardant. Yes, foil-backed insulation is better in every respect but batt insulation (minus the paper backing) will function. BTW, foil-backed bubble wrap should never be used (and for the cost and claims, is a pseudo-fake product) ...if it's still on the market.

Fiberglass batt insulation works best if not compacted (ie, by being taped tightly with duct tape...which begins to melt at 200 F w/ toxic smoke). There's also a safer product specifically for taping ductwork and duct insulation that is made of metal foil.

  • Hey, thanks for this very nice reply. :) Could I just put the insulation on with the paper side facing out then?
    – mmallek
    Commented Jan 2, 2017 at 18:33
  • I tend to be overly cautious about what can go wrong with things so where the paper could touch anything hot I would say, "no". But as long as it's about 12" (0.3m) or so from the hot water tank and furnace, I'd say that's okay (use without paper backing any closer than that). Commented Jan 3, 2017 at 6:04
  • Cool, thanks! We have the metal tape that we used to seal cracks in the duct work already, so we can use that to attach some insulation. Very much appreciate you responding.
    – mmallek
    Commented Jan 3, 2017 at 13:27

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