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Rigid foam insulation boards, are they all the same?
- Luis Imery
It is often that I am asked if rigid foam boards are the same, it does not matter if it’s a builder client that I am working with to get a house certified under an energy efficiency program such as Energy Star, or a green building program such as EarthCraft House, or a weatherization job for which I have conducted an energy audit. I want to discuss this insulation alternative from both a performance and make up perspective.
In the world of rigid foam board you will find the expanded polystyrene, aka EPS, which will provide a lower R-value per inch and be more permeable than its counterparts. Then you have the extruded polystyrene, aka XPS that is more suitable for wet environment and with a higher R-value. Lastly, the Polyisocyanurate, or Polyiso, is the board that will give you the highest R-value per inch.
Now, which board is better? Well it depends on the application. My opinion is that if you are planning on insulating underneath your slab or the perimeter/edge you can consider using EPS. First, make sure that if you leave on a termite prone area that you get it treated with “Preventol” or a termite deterred agent. For below grade insulation and attic knee wall or hot walls, as they are called in some areas, I will consider using XPS. They are fairly priced, water resistance, and work excellent as an air barrier (as long as you seal the joints) and help you get a continuous insulation behind your stud framed walls. Lastly, Polyiso’s are an excellent choice for roof sheathing or when you like to have continuous insulation on your exterior walls and want to reduce the extra expense of trimming out around your wall penetration due to the extra thickness of your wall.
You can use this table as a reference for R-value, but always check with the manufacturer:
|
inch |
EPS |
XPS |
Polyiso |
|
½” |
2 |
2.5 |
3.2 |
|
¾” |
3 |
3.8 |
5 |
|
1” |
3.85 |
5 |
7 |
As for the green attributes of each type of board, they each have a different chemical makeup, and the expanding compound that is use for making the final product have been proven to be harmful to the environment into some degree, in particular the ozone layer. However, manufacturers have altered their formulas to substantially reduce this adverse effect. I encourage you that you go into your manufacture of choice website, whether it’s Owen Corning, Amoco, Atlas, etc and find out more on this particular subject.
In summary, when it comes to rigid foam boards, first determine the type of application, how much space you have to work with to achieve the sought out R-value, and then make sure you air seal the joints with some kind of expansion foam.
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