I just installed a roof vent for an island hood, when it rains the sound transmits right down the duct. Is there any thing I can use on top of the vent to deaden the sound? Thanks -Ed
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If there's a substantial length of the duct in the attic, replace part of it (2-3 feet) with a suitable flex duct. (Not sure what "suitable" would mean here -- it probably has to be metal, still, for fire resistance.)
Flex will both reduce sound conduction through the metal duct wall and help to diffuse sound coming through the duct proper.
Offsetting the vent (with a sort of S-shape duct) will also help, but you probably don't want to move the hood or cut a new hole in the roof. Adding a sort of "U" (like an expansion U in a steam pipe) to the duct would do the same thing, though.
Another trick I've used on problem metal surfaces (such as the top of the roof vent) is to glue on a piece of cheap self-stick floor tile (or any scrap of vinyl flooring you have around). (Even if you use self-stick, be sure to use some additional adhesive, to help it resist the weather.)
The tile/vinyl serves as a fairly good damper for vibrations. (I've got chunk of it on the top and side of my computer here, right next to my left knee, to keep the case from vibrating.)
happy?
Guys, thanks for the suggestions. I can't add flex, the solid duct is just 4 feet long (ranch house). I was thinking of maybe a piece of rubber roof material glued to the top. -Ed
The (more or less) ideal damping material, as it happens, is alternating layers of heavy aluminum foil and a rubber-like material. In aircraft they use rubber layers that are are maybe 2-4 times as thick as the foil -- about as thin as practical.Pure EPDM (or similar) should work fairly well, but if you can layer it with something (thin almuminum flashing material, eg) it'll work a lot better.Of course at some point you're essentially polishing a turd, so whatever works.
If ignorance is bliss why aren't more people
happy?
I'll second the vote for a heavy piece of tile stuck to the cover. Should be able to find something that looks OK if it will be in plain sight. Also make sure the back flow damper is working properly, that should baffle some of the leftover noise.
Any kind of flex ducting, listed or not would scare me on a grease producing appliance. The same with adding any more ells than needed to make the run.
The tile would act much like the material they put under stainless sinks to reduce the vibration noises.
The vinyl tile sounds like a good idea. I've also used a piece of roofing shingle secured with roofing tar. It should last at least as long as the roof.
-Don