silencer box: flexduct inside? or without?

So I'm just about finished with my first silencer box. It's 3 layers of 5/8" drywall with the inner layer being 1/2" plywood. I was thinking of priming/painting that plywood, attaching my duct liner and calling it good. However, now I'm wondering if it might be a better idea to run the "rat maze" with insulated flex duct instead. That way, if there were ever any moisture issues, I could crack open the box and simply remove the flex duct as opposed to the potential moisture issue affecting the wood of the box (that's why I was planning on priming/painting as well). For the record, this silencer will have an outgoing fan connected so I'm not 100% sure as to what moisture issues could be a problem. Just trying to cover all my bases. Any advice is much appreciated. Thanks.

you can run the flex duct inside and fill the gaps around it with some pink insulation.

Thanks, gullfo. Just out of curiosity, what is the purpose of striving for larger airflows within the box? Say if you have a 6" fan but you want the silencer space to have a 12" space to cut down on the noise of the air movement? If one uses the flex duct, doesn't that negate the need for larger air space within the box? EDIT: Also, I've got a really beefy fan. Moves 384 cfm. I wonder how it's going to behave with flex duct.

I think you should be useing oversized flexduct, otherwise, as you point out, you'll have no reduction in velocity of air flow. Snaking the flexduct is just a variation or alternative to using baffles. The principle remains the same. I still think you will get better performance i.e. improved silencing using baffles as the flexduct acts as a guide and will impart less resistance and offer less obstruction, therefore less sound will be absorbed. As the air flows through the silencer and bounces off the baffles, a portion of the sound energy is imparted to the baffle (actually the sides are involved too) and converted to heat. It is this aborption of the sound energy being carried in the air which makes the silencer silence. A flexduct will not offer as much obstruction as it's nature dictates curves instead of abrupt angles, therefore will be less efficient.

I was thinking the same thing, BriHar. My only concern with this 'direct exposure' method is the likelihood of moisture-related problems (mold, mildew, etc.) Thoughts? Thanks.

Most products (duct liners) I've looked at, are treated or have moisture control to prevent mold growth. Most products are also cleanable so if you make the units such that you can pop the top or side, you can vacuum and wipe it out as needs be. This is the construction method I'm planning to use.