insulation and airflow

[yacht_planb]

I am currently refitting a 1976 Hudson Force 50 and I am putting 2 inch blueboard in the ceiling for insulation. This provides approx an R10 value. We are in south florida where humidity is 90% or higher. The boat has a lot of 4 inch deck supports that run from port to starboard that are made of mahagony. The ceiling panels will cover the insulation both of which are sandwiched between these horizontal beams. My question is this, should I make sure the insulation is tight against the wooden beams to prevent the moist air from staying stagnent or is it better to have a 1 inch gap to allow air next to the wood?

Any advice would be much appreciated.

[jdoe71]

Originally Posted by yacht_planb

I am currently refitting a 1976 Hudson Force 50 and I am putting 2 inch blueboard in the ceiling for insulation. This provides approx an R10 value. We are in south florida where humidity is 90% or higher. The boat has a lot of 4 inch deck supports that run from port to starboard that are made of mahagony. The ceiling panels will cover the insulation both of which are sandwiched between these horizontal beams. My question is this, should I make sure the insulation is tight against the wooden beams to prevent the moist air from staying stagnent or is it better to have a 1 inch gap to allow air next to the wood?

Any advice would be much appreciated.

Seems like so far no one has any thoughts on this. I am not positive on this either, but in building houses in our cold climate you try to keep the humidity out of the insulated walls with a vapour barrier. The vapour barrier goes on the warm side of the wall and nowadays quality builders seal it tight to the framing with acoustic sealant to try and avoid air leaks at all cost. It is sealed around electrical receptacles and so forth. The cold side has an air barrier that is moisture permeable so that any moisture that does manage to get in the wall from the inside can pass through and out of the wall to avoid rotting the framing. I believe but not 100% certain that rigid insulation is vapour tight, but since most houses use fibreglass batts in the walls the vapour barrier is always there. We have 2" of rigid insulation outside a 6" wall full of resin infused blown in fibreglass, and we still have a tightly sealed vapour barrier inside all that and a vapour permeable air barrier outside all that. It seems the thrust is to keep the moisture out from whatever side is the more humid side.

That's the only similar situation I can think of and I'm not sure how or even if it applies in your situation. If you had air con inside the boat then wouldn't the warm moist side be at the outside and you would therefore want a vapour barrier at the top above the rigid blue panels? But that's going to be very hard to seal with those beams already there. But if you didn't have air con then both sides are going to be warm and moist so then what?

Just guessing, but if I was doing this I *think* I would seal the rigid panels as tight to the roof and beams as possible and try to keep humid air out, if that's even possible. Hopefully someone who actually knows how this might be done properly in your climate will jump in here.

[David M]

I have an aluminum boat with sprayed on foam insulation. It is perfect in the sense that water (condensation) will never get trapped between the hull material and the foam since the foam is in direct contact with the hull in 100% of the surface area. I also never get condensation on the surface of the foam. The foam also acts as a heat insulator and dampens noise. I would look into spray on foam first.

The foam board you are considering is going to be a place where water will get trapped causing mildew and accelerated corrosion to any metal. The foam board is also going to make it more difficult to run wire.

[GordMay]

As jdoe71 indicates, the general rule of thumb is to install vapour barriers on the 'warm' side of the insulation; which places it on the inside of the insulation (between wall finish & insulation) in Cold climates.

In Hot Humid climates, such as S. Fla, place the vapour barrier on the outside of the insulation.
Your impermeable (I hope) decks will form an excellent external vapour barrier.
I would fit the rigid closed cell foam insulation tight to the mahogany stringer/beams.

[yacht_planb]

Thanks for the feedback. I was doing more research on the Blueboard and I found the following from the DOW site...

"Styrofoam Square Edge (extruded) - This product is often called "blueboard" due to its light blue color. In spite of its relatively modest "R" value (4.92 per inch), Styrofoam SM is our overwhelming recommendation for foam ice box insulation in marine applications. Unlike its expanded cousin (above)and virtually all other foam insulation, Styrofoam "Square Edge" is completely impervious to moisture. This is a huge plus. While many other foams start out as better insulators, they inevitably suffer significant decline as they absorb moisture from the surrounding air. This is not the case with Styrofoam SP. Another benefit of its hydrophobic nature is that no "air gap" is required when installing it. This means that more insulation can be packed into a smaller space. If is available in many thicknesses and two sheet sizes."

[hellosailor]

Blueboard does not require a vapor barrier, since it is not porous like fiberglass batting. In fact, blueboard itself will be a vapor barrier.

But in Florida, moisture and fungus will take hold in any "crevices" in the boat, so I would make sure the blueboard is 100% sealed at all edges so nothing can take hold in between the panels and the hull. For the mahogany ribs, I don't think it would matter if you sealed the blueboard to them (a filled glue joint, and not using any caulk that fungus can eat) or left open space for air circulation--as long as the circulation was adequate. If there's going to be a cover over the spaces, I'd rather run the blueboard up to the ribs and seal it to them. The self-expanding urethane foam might be a good adhesive for this, AFAIK mold doesn't eat it and it should be easier to work "upside down" with it.

[PUDDLE JUMPER II]

Just to update this thread--- A 2" roll of AP Armaflex has an R Rating of 8.0, and is vapor, moisture, and impervious to mold... If I were in Florida-- I would be using this product because of the mold + moisture issues down there... I have read several posts from folks finding this product pricey--- however, it lives up to its name. If you find an online dealer willing to sell you a 18' Roll for less than $300--- LET ME KNOW... One roll weighs 60lbs-- so finding a local dealer or driving to source might be advantageous in keeping the costs down. I plan on buying a roll at a time-- and when I have enough to do a large section-- i'll break it open and lay it down. At the moment-- I have not found that MAGICAL dealer willing to sell it for less than $300 a roll...

[hellosailor]

The Armaflex would appear to be way overkill for interior insulation on a boat, simply the wrong material at the wrong price.

In an engine bay, to resist heat and fumes, or out on deck to resist UV exposure, sure. But in the sheltered environment of a boat interior, all you need is a water-resistant closed cell foam. Ensolite, or the blue stuff (can't remember the name) that is used for exercise pads, or other nitrile or neoprene, lots of options on closed cell "foam" insulation. Although I'd bet a gas-blown neoprene would actually cost as much or more for that size and thickness. Cheap foams from anonymous sources tends to shrink, burn, crumble, or turn to goo. All making it anything but cheap.