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Old 01-24-2012, 02:04 PM   #1
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Is this fiberglass insulation safe (touching heating duct)?

So the upper level of our house extends about 18 inches further than the lower level (there's a small "overhang" with soffits outside). As such, this heating air duct extends into that 18" soffit area where there's a 90 boot that goes directly into the upstairs floor vent.

With no insulation in that 18"x15" area between the joists, there's freezing cold air coming in -- so it clearly needs to be insulated. But I was a little concerned around this duct and wasn't sure if I should be. This is just R19 fiberglass insulation, cut in half to fit around the duct (18" deep to fill the cavity), with the kraft facing along the top (bottom of the floor) and facing the downstairs room.

Are there any issues with heat, with the insulation tucked right against the duct? Fire hazard with the kraft paper? Fiberglass particles/fibers getting into the duct and blowing upstairs? (The duct is well sealed with aluminum tape, but still...) Or is this all good for this setup? Any advice would be great. Thanks!
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Old 01-24-2012, 03:32 PM   #2
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No fire hazard there. Looks well sealed, so no fiber blowing there.

Just make sure you insulated the back side of the elbow so you don't permit condensation within the duct. Since you've sealed off the warm air entering that void you now have an un-insulated duct traveling through a cold space. The warm, moist air within the duct will come in contact with the cold duct wall and potentially condense. Ensuring the entire bay is filled with insulation will help to avoid this condensation.
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Old 01-25-2012, 01:53 PM   #3
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Great, thanks.

It should be pretty well insulated all the way around the duct. I was a little worried about the heat and the kraft paper before I put the ceiling panels back up, but if we're all good here then it looks like I can finally finish up this room.
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Old 01-26-2012, 02:27 PM   #4
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You are not heating that duct to anywhere near the ignition temperature for paper.
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Old 01-26-2012, 04:14 PM   #5
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I agree with the others.. Insulate/seal that duct from the cold as best you can, then button it up.
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Old 01-26-2012, 07:46 PM   #6
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If the duct is getting that hot you have bigger problems.
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Old 01-27-2012, 07:18 PM   #7
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The cantilevered joists over the exterior wall are under-insulated if that one piece of fiberglass batt (in each cavity) is blocking air movement over the wall and there is no more insulation behind them. The outside joists should be insulated with cavity insulation, and foamboard (then plywood/siding) to stop the thermal radiation to the ground outside;
First add foamboard at the cantilever rim joist as solid wood is only R-1.25 per inch......

The room ceiling joists should have been covered with drywall, then your existing drop ceiling, due to the open "V"'s between the vertical ship-lap siding wall application (behind the ceiling grid end rail on the wall). These open raceways will spread a room fire to anywhere the joist travels above, very effectively. 1/4" wood paneling is fine, though because of the moisture coming through the cantilever area, the panels appear bowing;
That wood burns very fast due to the poly finish on it.

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Old 12-13-2018, 10:33 PM   #8
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Hello. We recently took down a piece of drywall in our laundry room, and discovered that the precious owner had put in fiberglass with Kraft paper right up next to our heat vents. The paper side is what is next to the vents, and it was warm to the touch (the heat was on at this time.) can the vents get hot enough to cause the paper to catch fire? Our basement is finished and I'm worried they stuff the insulation all over in the ceiling of the basement. It is covered in drywall so there is no way to check without it being a financial burden. Thank you.
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