The Offshore Voyaging Reference Site

Q&A: What Do You Think About Spray Foam Insulation For A New Aluminum Boat?

Question: We are planning to insulate the interior of our new aluminum boat with spray foam, any comments? Is your hull foamed? Was it primed with paint before or sprayed directly onto the metal?

Answer: Our hull was primed first and then sprayed, but I’m not sure this is really required. Talk to the manufacturer of the foam system. Also, you may want to look at self-adhesive foam sheet systems rather than spray on.

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Neil McCubbin

We sprayed Milvina with 50 mm polyurethane foam and are happy after 6 years cruising. I feel that spray is the best bet, since I have seen significant condensation buildup under stick on and loose slab foam when the boat is in cold, or cool, water.

We did NOT paint before spraying foam. No signs of problems to date.

Before spraying mask VERY WELL with plastic sheet, not just paper. The foam is VERY sticky, particularly to wood and plexiglass.

Trimming the excess thickness is a major PITA, but has to be done.

One good-news item, is the we have discovered that we can weld to the outside of foam sprayed surfaces.

I suggest spraying all living areas, but not the bilge, or any sail lockers or other storage areas remote from the moisture given off by people and cooking.

Andrew Troup

Has anyone tried adhering end-grain balsa to the plating in the bilge region, and then sheathing with fibreglass, sanding smooth and painting? It seems to me this would be a good way of dealing with the otherwise problematic area beneath the waterline, which can be a major source of “cold sole syndrome” in high latitudes vessels.
I seem to remember Evans Starzinger mentioned he had also thought about doing this, but at that time I don’t think he had actually tried it.

Dave Benjamin

Although it wasn’t an aluminum boat, when I was installing a hydronic heating system on a Freedom 39, I put a few loops of copper under the sole in the master stateroom where your feet go after getting out of bed. This proved popular as did a perforated towel rack with copper loop under in the head. Apparently heated towels score brownie points.


I have a similar hot liquid heating system onboard. Very comfortable heating indeed. However, since my vessel is made from aluminium, the piping system is made from plastic pipes with quick-fit connection. Easy to install and leak free – if such a thing exists.

Iris btw is spray insulated like many Dutch alu vessels. Works well, but covers everything permanently, so inspection is difficult. I once looked at a JFA vessel from a French yard. They use heavy foam mats, which appeared to be a both good and flexible solution facilitating easy removal.

Dave Benjamin


Before I installed my first heating system I did heaps of research. I am very sold on hydronic heating. Liquid is a much better conductor of heat than air. Instead of running large ducts, you’re running heater hose or copper. You can use the system to heat domestic hot water quite efficiently. A heat exchanger for water heating is about the size of a shoebox. A “summer loop” can be plumbed so you can run the system to make hot water without circulating coolant mixture throughout the boat. You can do zone heating very easily.

Dave Benjamin

For the sort of high latitude sailing you’ve been doing, I can understand the emphasis on reliability. There are more components involved with a hydronic system but the heart of it is a simple diesel boiler. The one thing I’d be concerned with would be some sort of failure of the electronic control. For us if the heat stops working it’s an inconvenience as we only sail in moderate climates. For someone in the high latitudes it could have a more dramatic effect.

Neil McCubbin

My original post failed to emphasise how much condensation I have seen behind loose or imperfectly glued on foam on other boats.
I do not see any way to do a good job of gluing the balsa. In any even it would be a HUGE amount of work, and never as complete as a sprayed foam.

Neil McCubbin

The discussion has moved from insulation to heating
We chose hot air because
1) Simpler, as long as you are happy with a well heated saloon and foul weather locker. If I wanted American quality home heating I would stay home
2) It introduces lots of warm low humidity air to the boat, which picks up damp and leaves. Drying is at least as important as temperature for comfort. If I had hot water heating, I would want at least one heat exchanger to take in cold outside air, heat it and blow into boat.

Like most design decisions, ther are pros and cons to balance, as well as personal preferences.

Dave Benjamin

I experienced just the opposite. Everything felt quite dry. I lined the lockers with cedar and had the hydronic hoses passing through underneath. I provided a vent bottom and top of each locker. The boat really felt bone dry and clothes stayed as nice as if they were in the closet of a nice home.
A dehumidifier is a nice addition to any boat while dockside.
I spent a lot of time in the Pacific Northwest where hydronic heating is quite popular and the environment notable damp.


About 20 years ago I had the pleasure of meeting Rolf Bjelke and Debora Shapiro from the steel Joshua class “Northern Light. On their first visit to Antarctica in 1984 they had no insulation on the boat at all, and only an intermittently functioning kerosene stove to cook on. When they left Antarctica they made a non-stop passage to Sweden, partially because they had less than $50 dollars left to their name!

Now that is adventure cruising!


Hi Johnm
People don’t realize how mild the temperatures are on the Antarctic Peninsula, especially in the summer. Even in winter they rarely see temperatures as low as we do in Jackson Hole.

Since they were planning to winter over on their next Antarctic voyage, I imagine at least one of the partners insisted on insulation that time around! (LOL)

Rene Blei

Hi John,
Interesting topic. My Dutch built alu boat too has spray foam. When I come onboard in winter (Seattle), it takes a couple of days for the system to heat up, with all build-in
electric heat, which is safe and efficient and use a 500watt radiant heater under the saloon table where I do my computer work which makes it very comfortable.
The 35 year old teak deck showed signs of wear and tear, yet wasn’t leaking, but too good to rip it all out and instead coated it with 4 coats of light grey KIWI Grip. The lighter deck color made it much more comfortable downstairs in summer and no A/C needed.

When we built our house near Calgary, we had the usual forced air heat and had the hydronic pipes placed in the basement floor, but it wasn’t operational until about 4 years later. When hooked up to the boiler, we noticed the house was much more comfortable, no more that basement feeling and total nat-gas consumption dropped by about 13% and we keep the downstairs thermostat at a constant 20¨C. Am very happy with the combination of both systems. Most windows are facing south providing passive solar heating and even with a minus 15¨F the forced-air furnace doesn’t come on. Did make the house pretty well air-tight with quite a few cans of spray foam, when being built.