The 3 known types of rigid foam board insulation are: polystyrene, polyisocyanurate and polyurethane, with a different purpose for each of them. First, insulating an attic with polystyrene involves deep heat and pressure to fit in shape. Being the most expensive, it is also the most durable and strong.
The last type is the polyisocyanurate and polyiso foam boards. It makes it a best choice for attic insulation because it is very resistant to high temperatures. Also it has a moisture barrier if applied with aluminum foil.
So, before using rigid foam insulation, you have to consider some thing. First, the flame retarder; know that once this product lights up, even though it lights up hard, it will burn really hard. Because of safety regulations, you have to use flame resistant materials to cover the insulation.
Another thing to consider is the sunlight which affects the insulation as well. You can protect it with acrylic silicone, rubberized paint asphalt, roofing felt, or tar. Also be careful for bugs.
Moisture can soak in the insulation, so you should use a vapor diffusion retarder, to prevent wood damage and mold. Even though this type of insulation seems to be the cheapest you still need to make sure you install it correctly and safe. Don’t start until you read the safety regulations for your building.