Two sealing questions, vent install

[josrulz]

Hi all,
Sealants are often discussed here. I have two questions, and I don't think these have been discussed (or at least I couldn't find answers when searching). First the background...

I'm installing a Nicro Day/Night vent on the cabin top, which has some camber (within tolerances according to Nicro). The vent is for a composting head, and is stainless on top, but the base where it seals to the deck is plastic. It's going through a cored deck (yes I'm sealing the core with epoxy) with a stiff fiberglass headliner (see illustration below).

Question 1:
My first question is about what sealant to use with the plastic-based vent to the deck. The instructions from Nicro state that silicone should be used on a flat surface, but on a cambered surface (like ours) one should use 3M5200 or Sikaflex (they don't not specify which one), along with the rubber gasket they include. No way am I using 5200! I'd like to be able to remove the vent some day if needed.

Although I'm a big fan of butyl tape, this vent is only screwed into the deck (3 screws), not through-bolted. I'm wondering if this is one of those applications where we might want more adhesive properties than butyl tape can give, but still a good amount of flexibility due to the camber. Maybe Sikaflex 295UV (seems flexible and strong but not too strong)? Other?

Question 2:
My second question is about how to seal off the space between the deck and the liner. It's about 1" vertical space, and with the vent all assembled that space is not sealed off from air going to the vent. Since this is for a composting head, we need to make sure that air going to the vent only goes out of the boat. Any suggestions on what to seal it off with? I thought of window/door foam (it doesn't expand much and is soft, not hard), but I can't figure how to use it in a space with no "back". In other words, I could end up filling a LOT of headliner space with foam shooting out of can deep into the space. I only need to seal off right around the vent, not the entire headliner. Suggestions on what to use?

As always, thanks for the input!
-J

[Siamese]

I'd keep thinking about the butyl tape. Even though there are only 3 screws, it's just a vent, not a cleat.

Not sure about the other sealing issue. Even though there's no "back" to it, I'd still be thinking about the expanding foam.

[saillife]

In response to your 2nd question since this isn't structural cut some wood, ply to make a ring, cut it into pieces so that you can slide it into the hole and glue that in as a backer with plain old caulk to seal the joints?

[Maine Sail]

I used butyl tape on mine. Bone dry now for the last four years...

[josrulz]

Originally Posted by Maine Sail

I used butyl tape on mine. Bone dry now for the last four years...

Thanks Maine Sail. Did you use the gasket too, or just butyl tape? I'm on a cambered deck, if that makes a difference in your recommendation.

Originally Posted by saillife

In response to your 2nd question since this isn't structural cut some wood, ply to make a ring, cut it into pieces so that you can slide it into the hole and glue that in as a backer with plain old caulk to seal the joints?

Not a bad idea.... Thanks!

[meteuz]

At a good hardware store you can find rolls of closed cell foam with a circular cross section. It is meant to be pushed into a crevice before caulking so that the caulk has something to go against and has the right shape. If you can get one large enough, it might be a simpler solution than a plywood ring.

[josrulz]

Originally Posted by meteuz

At a good hardware store you can find rolls of closed cell foam with a circular cross section. It is meant to be pushed into a crevice before caulking so that the caulk has something to go against and has the right shape. If you can get one large enough, it might be a simpler solution than a plywood ring.

I like that idea. I did some googling...is this the type of stuff you mean?

Sealants, Adhesives, Flashing, and Foam Products | Premier Industrial Supply

It's called "backer rod". If I can find some locally, it might do the trick. Thanks!
-J

[josrulz]

I looked at the vent base again, and it looks like butyl tape or no, you need the gasket, because the bottom of the base isn't flat. At each screw hole, the base sticks "down" a little. On the gasket there's a cut-out hole at each screw hole location to match up. So without the gasket, the bottom of the vent doesn't really stay in contact with the deck.

Maine Sail, not sure if your vent was the same, but did you use the gasket along with the butyl? It appears to be neoprene or similar...

[JimPendoley]

The very best way to deal with a Nicro Vent install is to buy a different product. I installed one in my head for the same reason, failed within months. This was the Day/Night unit. They are flimsy-the motor is not up to the task. There have been numerous folks who have chimed in on this site to report similar frustrations. Worst $149 investment I ever made on my boat. They move a very, very small volume of air. I'd strongly recommend you look around for a different solution.

[josrulz]

Well, it's going in, so I guess we'll see. Everything's cut for it, and I need the vent now so I can use the head. I'll have a muffin fan already in-line with a switch, so I can use that if the vent fails.