Aluminum-hulled sailboats are light, easy to maneuver and affordable, but they are cold if they aren't insulated. If you're remodeling a cruising sailboat that has an aluminum hull, insulate it with open-cell spray foam insulation that's sealed in plastic bags. While closed-cell spray foam insulation can be installed on sailboats, using open-cell foam in plastic bags will provide adequate insulation and make future repairs easy.
Closed-cell vs. Open-cell Spray Foam Insulation
There are two different kinds of spray foam insulation that you could insulate your sailboat with: closed-cell and open-cell. Closed cells have their own cell walls, not sharing any with adjacent cells. Open cells share their cell walls with other cells they are next to. Closed- and open-cell spray foam insulations are both used in residential and commercial buildings, and they both can be installed on an aluminum-hulled sailboat.
Both Kinds Insulate Against Heat and Cold
Both types of insulation will provide protection from the heat and cold. Closed-cell spray foam insulation's R-value, which measures a material's ability to insulate against temperatures, tends to be 2.3 higher than open-cell insulation's.
Unless you're boating along Canada's northern shores in the winter or fly down to the Caribbean in the summer to sail, though, open-cell spray foam insulation's R-value of 3.7 per inch will provide sufficient protection from moderately hot and cold temperatures. As long as you're a recreational sailor who only uses their boat during the main boating season, you won't see extreme temperatures that call for closed-cell's higher R-value.
Closed-Cell Spray Foam Insulation Deflects Water
Although closed-cell spray foam insulation has another advantage over open-cell, it also has a disadvantage
On the one hand, because closed cells don't share cell walls, closed-cell foam won't let water seep through the insulation. Open-cell spray foam insulation, in contrast, won't prevent water from seeping through the foam. Once water collects inside of foam insulation, it lowers the foam's insulating ability and promotes mildew and mold growth.
On an aluminum-hulled sailboat, closed-cell spray foam insulation's ability to prevent water from seeping in is especially important. In most residential and commercial construction, insulation is protected from water by exterior walls. On aluminum-hulled boats, however, dew regularly collects on the inside of the boat's hull. As your boat sits overnight, the water it's in will often drop the temperature of its hull below the dew point. Once the hull is below the dew point, condensation will collect on it—right next to your boat's insulation.
Closed-Cell Spray Foam Insulation Can't Easily Be Removed
On the other hand, it's difficult to remove closed-cell spray foam insulation. If you ever have to make a repair to the hull, closed-cell foam can't easily be removed. It adheres to the materials next to it. To make a repair in the future, you'll have to rip out the insulation, repair your boat, and re-install new spray foam insulation.
Plastic Bags Form Impermeable Barriers
Alternatively, you can use open-cell spray foam insulation that's contained in plastic bags. Although open-cell foam won't stop water from seeping through it, putting the insulation in a plastic bag will protect it with an impermeable barrier. This solution has the added benefit of making future repairs easy. If you need to repair something behind your sailboat's insulation, simply remove a bag of insulation, make the repair and put the bag back in place.
If you're remodeling an aluminum-hulled sailboat, give thought to how you're going to protect your boat from cold, heat and condensation. Closed-cell spray foam insulation will provide the best protection against cold and heat, but you likely never see temperatures that require its insulating ability. Open-cell spray foam insulation will adequately insulate against the temperatures you see during the boating season. Placing it in plastic bags will stop condensation from seeping through it and make future repairs easy.