Insulate only to the waterline?

[GilStump]

Howdy folks,

I am in dire need of insulation. Most articles I read say to only insulate to the waterline, but they don't say why. My tiny brain can't figure it out. Anyone? If I have insulated below the waterline do I deserve a Darwin award?

[GilStump]

I just stumbled onto this. Anyone have any experience with it on a Fiberglass boat?

Mascoat Products — Worldwide Leaders in Insulation Coatings

[scottyt]

well stump, if you are in warm waters then you are trying to keep warm air temps out. if you are in colr water then you are keeping cold air temps out. normally the water is a pretty good temp buffer, always either warmer or cooler than the air out side.

also it has to do with condensation which happens under the water line really easily, even with insulation. it is much easier to fight it above the water line. so basicly one reason is to avoid getting the insulation wet with condensation

[dman]

It would be nice to see where a leak was coming through...instead of ripping up insulation all over the boat.

[GilStump]

Both of you make sense. Which is why I never thought of it. So, maybe the insulating paint is a good idea? Then I can see where leaks are coming from and keep the areas above the water line dryish. Which would be niceish.

[arf145]

Stump, there was a good article in Good Old Boat in 2008, maybe last December, about insulating a boat on the cheap. What they did was use Celotex insulation sheets--the 1/2" stuff with the foil face that is put on the exterior of houses. They then covered the foil face with fabric for a nice look. He did a series of kerf cuts--cutting the celotex right up to the foil face--to help it curve to the hull. You get an R value of something like 3.5 with celotex.

[AlanInSeattle]

I'm an architect and spoke with Mascoat reps about using their product on a building project in Michigan. The project had steel spanning in and out of the building and we were concerned about condesation forming on the interior steel in the winter. Although we didn't use the product b/c we dicided it wouldn't be effective enough - I learned a bit about it. You might have to ask yourself what the goal is of insulating your hull.... to keep you warmer? To keep from condesation building up? My understanding of the paint is that it lowers radiant heat gain/loss by lowering conductivity. So - this is enough to provide some comfort and usually lower chance of condensation. Also, the paint is on the expensive side.

[mitiempo]

Stump
I just read the Mascoat data including the application instructions. I got a clue when they talked about noise reduction and the application instructions confirmed it. It's for steel and aluminum. Fibreglass isn't mentioned.
What I use is Ethafoam in 1/2" thickness. It's not expensive, is closed cell so doesn't absorb water, easy to cut with a razor knife or scissors and I glue it on with 3M Super77 spray adhesive. My friend on the dock has it in his boat as well and it works well. His boat was done 20 years ago. It's durable and white in color. In lockers I wouldn't necessarily cover it. Where you'll see it you could glue fabric over it or slats for a more traditional look. My friend only has problems where it is exposed in the quarter berth and his cats scratched it. His below waterline lockers are done as well - under the seats etc. But not the bilge. Hope this helps.

[GilStump]

You guys rock!

[GaryHLucas]

Ethafoam is a good choice. It is VERY important to not leave an air space between the hull and the insulation! If you do it will sweat behind the insulation and you'll have a great place to grow mold.

Gary H. Lucas