Rigid foam insulation, like any particular method of insulation you could choose for your home, has its advantages and disadvantages. Rigid foam is good at insulation under ideal circumstances. It is very powerful when judged on its ability to block heat and keep it where it belongs. It is also waterproof, and it can even help make your walls stronger by supporting them. However, other than these great advantages, there are some questions about whether rigid foam insulation can cause moisture problems. While the answer is slightly more complicated than a simple yes or no, the following should give you all of the information you need if you are concerned about moisture problems that relate to your rigid foam.
The short answer to the basic question is yes. Rigid foam insulation can cause problems with moisture. However, the truth in practice will vary a great deal, depending on the actual situation that the rigid foam is installed in, how it is installed, and the surrounding climate and location.
Rigid foam is installed in large sections. These sections are solid and rectangular. While they are good at filling your walls, they do not necessarily fill up any gaps or cracks that may exist in your walls. Because of this, it is possible that there will be a passageway in your home from the inside to the outside. No matter how small this passageway is, if air can get in and out, it may cause problems. Air traveling in and out of your home can bring moisture with it. This can lead to any of the multitudes of potential problems caused by moisture.
Rigid foam is a perfectly adequate insulation material. However, its minor problems with air leaks can be magnified under certain circumstances. If your rigid foam is measured incorrectly and does not fit in your walls perfectly, or is installed badly and does not seal off correctly, it will allow even more air to enter and exit your home. Larger air leaks can lead to more moisture problems, so it is very important that you make sure your rigid foam insulation is installed as well as possible.
While rigid foam can fail to protect your home from moisture-related problems, the severity of the problems will vary wildly depending on the conditions your home is exposed to. In dry areas with low humidity, the problems will be minimal—though you will still have to worry about air leaks, which can make your home's heating and cooling systems less efficient by allowing heated or cooled air to escape to the outside.
In contrast, more humid areas may have more problems. If the area you live in has medium or high humidity, it is important for you to install rigid foam carefully, or possibly avoid it altogether.
In particular, basements are vulnerable to moisture. This is because the walls of a basement are exposed to the surrounding soil, which will always contain water in all but the driest conditions. Rigid foam insulation may not be a good choice for basements if you wish to avoid mildew and other moisture-related problems.