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My new kitchen included a new exterior flute for a new range hood.

Few months later, I opened the cover for some maintanace reason and saw that the flexible flute is basically going through the ceiling via a hole cut into the ceiling.

The circular hole has about 1-2" space around the pipe... see picture.

How do I air-tight this? seems like all my house heating can just fly up into the attic this way...

alt text

I thought to use expanding foam, but not sure if that is the "right" way, also, if I do, can I use the cheap stuff or do I need the "fire" rated one? any ideas? pictures are most welcomed if you have something to show me...

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3 Answers

up vote 2 down vote accepted

Hernan asked me via e-mail for my recommendation and I'm replying here for everyone's benefit. The proper way to seal the range vent flue penetration through the drywall ceiling is is Type B Gas Vent Fire Stop/Support/Wall Plate. See this link: http://www.ventingpipe.com/heat-fab-sc03fs-3-saf-t-vent-sc-8-x-8-firestop-support-wall-plate/p999815

The fire stop plate provides clearance for the vent from the drywall and ceiling joists, while sealing the hole to block drafts, smoke and fire. Choose a fire stop with the appropriate dimensions and install according to the mfg's instructions (see the above link). The fire stop plate can be sealed to the drywall with silicone caulk and to the flue duct with aluminum foil HVAC tape for an air tight bond. In this range vent hood exhaust application, the air temperature will be very mild compared to a gas water heater or furnace exhaust where a fire stop is normally installed.

Do verify the exhaust vent has a damper installed somewhere to prevent backdrafts on windy days.

Take care, Bob Jackson

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Great Stuff makes a fireblock sealant that is minimal expanding. It's actually specifically designed for penetrations between floors and through fire walls (required in commercial situations, but works equally well here).

Great Stuff Fireblock

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no need for some rubber thing or some other solution to hold the tube from moving? –  csmba Oct 27 '10 at 19:10
    
As soon as the foam starts to set (in ~15 minutes) it will hold it in place. Just make sure it's spread evenly around, and keep an eye on it while it's setting so it stays somewhat centered (not that it's a huge deal if it isn't centered, so long as it's sealed). If you've never used this foam before, be careful: it hardens basically permanently. Use gloves (it takes days to get it off your hands completely), and don't get it on anything else (like your stove, below) –  gregmac Oct 27 '10 at 21:05
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My first thought was expanding foam as well. DAP Kwik Foam claims to contain a fire retardant, but it doesn't list any specific ratings.

If you want to be extra safe, you could use something like Abesco FP200, which is fire-rated for ASTM E-814. (Resistant to 2 hours of exposure to flame.)

Otherwise, if you want it to look pretty, you could go find some flashing for a vent base that fits your duct. If the duct isn't easy to disconnect, you could cut something like this in half to fit around the duct where it is and caulk the seam afterwards.

alt text

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I would probably not bother with the flashing - cutting that tube is likely to be way more of a pain than it's worth, and it's covered anyways. –  gregmac Oct 27 '10 at 3:20
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