Condensation Under Plywood Deck?

[piston broke]

We are presently building a 12m plywood catamaran. Do we need to insulate under a plywood deck to prevent condensation forming? Planning on cruising Tasmania and the Tropics.

Thank you.
Daren.

[UNCIVILIZED]

Simple answer, yes. Anytime there's much of a temperature differential between outside & inside you'll get condensation underneath of the decks. Particularly if there are humans inside generating moisture by doing things like breathing. And this will happen to varying degrees whether you're in a tent, or a boat with 6" of freezer insulation built into the decks.

On a boat, in order to save weight, especially a multi, you're better to just plan on building cored decks from the start, & not relying on one layer of ply to be your primary structural strength. Rather, do something like ply/core/ply, glass/core/glass, or ply/core/glass.

You can build an incredibly light, tough, relatively inexpensive deck pretty easily this way. And it'll also be a lot easier to put a nice finish on than ply, as well as more rot resistant.
A lot of multi's, & light weight racing mono's have decks built like this. And it's worth doing a bit of studying on.

[Delancey]

What kind of boat are you building?

Cored sandwich construction is great for the above stated reasons, but you are building one out of plywood which I assume has been designed that way.

Making a change from plywood-on-frame to cored construction will require altering the hull to deck joint and is easier said than done despite what some internet experts say. Before making any significant changes to the structure of the boat you would do well to consult the designer.

That said, if you want to simply add some insulation to the underside of the deck rather than making a radical change to the construction of the boat, sure. Go for it.

Just keep in mind that the deck only accounts for a small portion of the interior surface and that in cold climates condensation will occur on any surfaces where there is a temperature difference, so to be effective you would want to insulate the hulls as well.

Again, what kind of boat are you building? Maybe can give you more detailed feedback that way.

[GordMay]

Greetings and welcome aboard the CF, Daren.

[UNCIVILIZED]

Generally, switching to a cored deck from a plywood one needn’t alter a boat’s construction methodology by much. And if one’s gotten that far along in building a boat, making such a change can be done with little enough thought, based on one’s DIY boat building self-education to that point. As rounding over the edge of the core, & wrapping the layer of triax atop of it, to structurally connect the deck to the hull is generally more than sufficient.
It’s not as if we’re talking about building with aluminum honeycomb cored carbon fiber panels here. However, yes, if there’s any doubt, consult the designer.

With most plywood on frame boats, there’s a chine log where the deck (edge) meets the hull. And to this, a plywood deck gets screwed or nailed, & epoxied. Typically with a layer of glass cloth, biax or triax wrapped over the exterior edge of the joint to secure things. And to also serve as a permanent moisture barrier. Which, if adding core to the deck, instead of using just plywood for it, one would need something stronger than say 8oz cloth as might be the minimum used with some plywood decks for this. But a couple of layers of mid weight cloth, or a layer or two of biax/triax is more than sufficient. Considering that it would be stronger than either the deck, or the plywood used to build the hulls of most multi’s. Especially when combined with the attachment of the deck’s underside to the chine log at this juncture.

[DeepFrz]

For a much more comfortable boat you need insulation and ventilation. Be sure to use an insulation that wont get soggy. Don't use household fiberglass insulation for instance. A search for marine thermal insulation should get you some hits.

[Fore and Aft]

We launched our plywood yacht last year and used polystyrene under the deck then covered it with Ulrich exterior aluminium sign writing board. This is a great product, light weight, easy to cut and shape and a white glossy finish.
So far no condensation doing it this way.

[piston broke]

Thank you for the replies. We are building a Waller 1200. The saloon/cabin top is of glass.foam,glass construction and below the waterline is strip plank. We used these materials for the compound curves. The remainder of the construction is plywood for its cost effectiveness and minimal exposure to epoxy.

We are interested to know if anyone out there has a similar boat and has NOT put the insulation under the plywood deck and if they have experienced a condensation problem.

Thanks
Daren

[UNCIVILIZED]

One would think that the use of an antimicrobial agent would be something appropriate to use on insulation in which there are any voids, crevices, air gaps, or joints etc. Including mixing some into the paint to be applied to the overhead, before putting up the insulation. As any gaps or voids would probably look like attractive places to take up residence, for mold & mildew.

Which, it could go unnoticed for years, were it to live & grow in the space in between the overhead & loosely attached sheet foam insulation. Or were it to even begin to grow on the insulation itself. Given that there would probably be micro-condensation within such spaces almost regardles of how good a job is done adding insulation after the deck's been built.

And this idea is a big part of where/why my comment about building cored decks stemmed from. Since mildew will have a pretty tough time growing on the inside of a sealed, sandwich constructed, composite deck. Plus the weight of a cored deck tends to be less than that of an uncored deck, paired with seperate insulation & covering panels. And weight's obviously the enemy on multihulls.

Besides, who likes to build upside down twice, when you don't have to. Cored decks = once, uncored decks + insulation, & also insulation ceilings = twice, or 3x

[Djarraluda]

To get slightly technical, condensation is a factor of a surface that is at a temperature below the Dew Point temp of the air.
Dew point is a factor of the amount of moisture in the air and the air temperature.
Simplistically, the air at any given temp can only "hold" a certain amount of moisture. Warm air holds more, so for a given amount of moisture, the Relative Humidity is less than the same amount of moisture in cold air.
Surface condensation is a pain, especially the it drips on you in the night.

Interstitial condensation is the serious problem. This is where the moisture in the air penetrates a layer and then finds a surface too cold, it condenses-out in the middle of the material. We get this in buildings, especially cold stores, or in cold climates. Swimming pools suffer badly and the rot caused by this is what has caused roof collapses.

So, whilst insulation is a good thing, it keeps the surface warmer, it is also important to keep the moist air out of the sandwich construction. Closed cell insulations are good because they do this automatically, fibreglass batts from your roof are not, they allow open pathways for air to migrate. The is why air-conditioning insulation has the silver foil kraft paper finish as a vapour barrier.

In other words, yes, insulate the surfaces but also finish the insulation properly on the inside. When you cut holes, seal the edges just as you would externally.

Now you can visit Macquarie Island as well!
Roger.