Most Closed cell foams are resistant to water but over time can outgas there cells allowing moist air in. My preference for boat refrigerator insulation is still Polyurethane controlled density boards encapsulated in 6 to 10 mill builders plastic.
There are many insulation comparisons on the web but none I know of are reliable reporting for boat refrigeration. This is a good sample of R values:
Foam Board Insulation R Values
Written by Todd Fratzel.
Foam Board Insulation Types
I’ve written several posts about how to insulation basement walls in which I promote the use of foam board insulation as the first line of defense against moisture and mold. Because of this I often get questions about which type of foam board insulation to use and what R values these products provide.
There are three basic foam insulation board products on the market produced under several different manufacturer names. The basic types of foam board insulation include: polystyrene, polyurethane or polyisocyanurate.
They include expanded polystyrene, extruded polystyrene, and polyisocyanurate unfaced or foil faced. DOW products has lots of information on their site about different foam board products here. I also recommend you read a recent article about Open Cell Vs Closed Cell Foam products to understand the differences between the two product types.
Expanded Polystyrene Foam
Expanded polystyrene foam (EPS) is the cheapest and least used foam board product on the market. This product typically has an R value of 3.6 to 4.0 per inch of thickness. Expanded polystyrene insulation is similar to the foam used for packing “peanuts” and it’s typically used for insulated concrete forms also knows as ICF’s. It is also sometimes used on commercial buildings for roof and wall panel insulation which is typically sandwiched between light gauge metal.
Cost = Cheapest of the foam insulation boards.
Extruded Polystyrene Foam
Extruded polystyrene foam (XPS) also known as blue board or pink board comes in many different thicknesses and edge profiles. This insulation board is probably one of the most widely used foam board insulation products in the residential construction industry. XPS has an R value of 4.5 to 5.0 per inch of thickness.
This is the product that I typically use to insulate basement walls. It’s reasonably priced, light weight and easy to use. This product is also used to insulate the outside of foundation walls and even under slabs.
Cost = This product is the middle of the road for these types of foam board insulation products.
Polyisocyanurate and Polyurethane
Polyisocyanurate also known as polyiso is seen in all kinds of commercial building applications and more recently with residential building projects. Polyiso is typically used with a foil facing and it has an R value of 7.0 to 8.0 per inch of thickness. The reflective foil facing makes it an excellent insulation board when radiant heat is involved. The foil facing also makes it very easy to seal with good quality foil faced tapes.
Cost = Polyiso is the most expensive of the foam board insulation products however it’s the highest R value.
Polyurethane and polyisocyanurate are both closed-cell foams. They contain low-conductivity gases in the cells (usually one of the HCFC or CFC gases.) The higher R-Values (R 7.0 to 8.0) are the result of thermal resistance of the gases in the cells. This can lead to a couple of disadvantages including: off gassing of HCFC or CFC gases, and reduced R Value over time as the gas escapes.
Closed Cell Foam
Closed cell foam, a.k.a. medium density foam, expand to about 25 time it’s liquid size. Closed cell has such an advantage because it forms a complete envelope sealing out moisture, preventing mold and providing an R-value of 7.5 per inch!