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JasonCN35 10-29-2009 10:32 AM

NEED advice...steel boat insulation
I recently purchased a Canadian Northern 35 steel yawl and am beginning the process of removing the old custom interior. When I bought it the owner told me she was insulated, but I have discovered that the entire thing was done with Great Stuff Expanding Foam (from a can). Is this OK? I am worried about it soaking up the condensation from the hull... I have never heard of anyone using off the shelf spray foam to insulate a hull...only for spraying in gaps around windows and doors.

I am worried that I am going to have to scrape all of it off....:(


pedcab 10-29-2009 11:42 AM


I know two guys who have built their own boats here at the club, one out of aluminium the other made of steel, both Van de Stadt 30+ footers and both of them insulated their hulls using a blue type of closed cell foam (extruded polystyrene) that they cut to fit from commercially available blanks normally sold for residential wall/ceiling/roof insulation purposes. Expanded polystyrene is normally used as surfboard blanks.

It was an entedious task (because this foam comes in blanks from which they had to cut as needed) but they made it with very good results in terms of sound/thermal insulation properties.

The bad thing about the polyurethane (PU) foam you mention, which is the type of expandable foam normally available at DIY stores in aerosol cans, is that it is of the open cell type and although it shows excelent insulating properties it also does soak water and should, in my opinion, be avoided for marine use applications.

Here is the stuff they used



merc2dogs 10-29-2009 12:26 PM

If the foam is adhered to the hull, there won't be any condensation at the junction, simply becasue there is no air space. As long as the 'skin' is undisturbed after application it is waterproof.

That said, I don't think I'd want it sprayed into a steel hull simply bacause it can't be removed for inspection etc.

The sheet stuff works great because it can be removed easily and is waterproof for the most part. Bad part is that since it is a sheet there WILL be airspaces between foam and hull so you can easily get condensation.

A method I've used with spray foam to make it fit tightly yet be easily removed, is to make sure the cavity it's going into is smooth, then spray the foam into plastic 'bags' you make from sheet plastic and tape. The plastic keeps the foam dry and prevents it from adhering to the hull, so it's easy to pull out for inspection or maintenance.


Artz 11-03-2009 02:25 PM

It's expensive but it also works very well. It's good old cork. since it is a wood product it breaths letting the condensation escape while maintaining a nice sound barrier. I worked on a 45 foot motorsailer that was built in Holland right after WWII. They used cork and it was still in pretty good condition after nearly 50 years of service.

pedcab 11-03-2009 04:05 PM

That is indeed the best piece of advice I've read here in a while, especially because Portugal is probably the largest producer of cork and derivates in the world!

We use it a lot from pin up boards, to shoe cushioning, to thermal insulation on walls and ceilings and as soud insulation in studios, as flotation aid in boats, as GRP core material and, of course, to seal wine bottles :)

Here in Portugal it's a lot cheaper than any kind of synthetic foam and, most important, it is an eco friendly product. It is exctracted from the tree (sobreiro) without damaging it...

Go for for cork! At least you'll be improoving our export rate :)



luckytroll 07-27-2013 09:19 PM

Re: NEED advice...steel boat insulation
The boat was insulated with a professionally sourced closed cell poly - it came in large cylinders with replaceable hoses and heads, etc. The great stuff was only used on edges for the refrigeration. The forward part is done with replaceable Styrofoam panels, which may be glued in place.

If I had to do that insulation again, would do it with removable panels and leave the spray foam out of it, except for the edges of the fridge compartment.

The most important thing isnt so much preventing condensation, but managing flow to the bilges with limber holes, etc.. My current boat has stringers that lack those, and they are going to need replacement before they rust out from accumulated water.

You are lucky in that your boat has minimal longitudinals to catch water this way.

(Former Owner)

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