duct silencers

I've seen plans around here for duct silencers built from MDF boxes lined with duct liner. Here's my question, would it be wise to paint the mdf interior of the silencer with exterior paint, or line it with tar paper to waterproof and seal it somehow. MDF dissintegrates in contact with water, room exhaust can be damp. is this not such a big deal? any ideas? thanks

Also, I don't think MDF will pass code for fire barriers. In commercial construction, can you use this stuff in HVAC? Rod? -ashley

John, as far as the humidity goes - it's all relative (pun intended). NO big deal - the MDF should remain stable just like it would anywhere else in the building - it's only when it gets wet from an outside source that it becomes a problem - but not at RH inside a space. The fire issue is a seperate thing. In a lot of commercial applications fire seperation is not an issue - and it would be fine i those cases. It would not be fine where fire seperation was an issue. But - In THOSE cases - it is no big deal to construct a box with the same inside dimensions with an outside frame that is a rated assembly to work with - OR - if the seperation requirements are only one hour then you could make it a metal duct plenum - same details as the MDF chamber - line it - and then surround it with any other assembly (MDF OK in this case) just to stop the possible transfer of ambient noise in the space from making it into the duct chamber. Rod

I built plenum boxes of mdf lined with duct liner board. The duct liner is foil-covered, both sides. I sealed all the edges with foil tape, so I would say they are, by definition, vapor-sealed. Foil is considered a good vapor seal. And I used acoustic sealant to filet all the edges, too, which applied mass to the board and gave further stability to the seal of the corners and edges. On top of that, the edges are bound together with drywall corner bead, to keep if from further drifting apart, stitched together with my little 1-1/4 in nail gun. As long as the air is moving and there is no condensation, the moisture in the plenum should remain constant. If you made it of 3/4 in plywood, it would be meet the requirements of a one-hour fire stop. In my case, the wall cavity is enclosed with 5/8 in drywall and stuffed with fiberglass which meets the prescriptive requirements for fire-spread, in case they should ask. This box is made to stand up inside the wall cavity, the only place there is room. Original topic: Ventilation plenum boxes inside the wall cavity? http://www.johnlsayers.com/phpBB2/viewtopic.php?t=10240 That's my best guess. Listen to Rod. Cheers, Kevin

that silencer looks so cool! it's huge too. i'm a little worried that you installed the duct liner backwards. with the foil on the inside, the insulation does no good to reduce the noise indside the duct. others that i have seen building plenums like yours have put the foil side on the outside next to the mdf and the insulated side on the inside, with the microbial black facing keeping the fiberglass fibers from dislodging

You can also ask a local duct manufacture to create a silencer for you. I got a metal built silencer, with over 650 mm length for 250 € where I live.