Flex duct through the wall

[GDA]

Hi everyone,

My basement studio is coming along nicely. Framing is up, and I’m building an inside-out ceiling next week along with drywall. I think I’ve addressed all the issues except for one.

Near the back of the studio is a flex duct that runs the HVAC up to the master bedroom’s closet. This duct comes from outside the studio and goes right through the outer leaf. It’s isolated from the inner room (although it lightly touches the framing), but I’m obviously most worried about the enormous hole in the outer leaf.

My question is, what’s the best way to seal around this? It may be tricky to take it out all together.

Any help is appreciated!

[John Steel]

Hello & howdo' GDA?

I’m obviously most worried about the enormous hole in the outer leaf

You are correct to be worried about this - certainly if you need to achieve any significant transmission loss. Forgive my unfamiliarity with your project - it sounds to me like you are building a room within a room, MSM design. Ordinarily, the way that T/L would be maintained across the transition between the inner & outer leaves would be to use baffle boxes or silencers. These are enclosures that incorporate 2 or more 180° turns and must be at least as massive as the leaves that they penetrate. They should also be lined with a class 'O' liner or foam. The dimensions of the silencer boxes need to be calculated accurately in order to provide your studio with sufficient airflow at the correct velocity to avoid your ventilation system being too noisy (or worse, not supplying sufficient fresh air). Typically, a high level of isolation will require a total of 4 silencers for a single room ( 1 x outer & 1 x inner leaf for the supply path + 1 x outer & 1 x inner for the exhaust path).
Fortunately there is an absolute wealth of information on this site that will help you to figure this out if you are not familiar with the principles involved here - try searching for "HVAC, baffle box, silencer design / build" and related terms. Apols if I'm repeating stuff you already know.
Good luck & best wishes, John.

[GDA]

Hi John,

Thanks for the reply! Sorry for the lack of background info.

it sounds to me like you are building a room within a room, MSM design.

You got it. Room within a room, with an inside-out ceiling.

Ordinarily, the way that T/L would be maintained across the transition between the inner & outer leaves would be to use baffle boxes or silencers.

I have thought about that, but this duct doesn't actually go into the isolated room. It just goes through the outer leaf, then up into the room above it. The duct itself doesn't need the isolation, but I don't quite know what to do about sealing around it.

I've thought about building a baffle box without the baffles. The turns inside aren't needed since I'm not worried about silencing the airflow. I'm just trying to seal the outer leaf.

I've also thought about drywalling as close to it as possible and using a combination of putty pads and spray foam.

Any other thoughts? Any more detail I can provide?

Thanks

[John Steel]

this duct doesn't actually go into the isolated room

I see - in the interest of clarity, would you be willing to draw a diagram? I'm finding it difficult to visualise your problem and why the duct can't be diverted or outer leaf altered. ATB John.