Lakeside Building Systems has used rigid foam board as have all builders for years. We used to use XPS, extruded polystyrene, the green, pink, blue, yellow stuff everyone used to use because that is what we could get. We found out several years ago at a conference that XPS is not what it should be.
XPS is a 2 part foam, which means over time the cells break down releasing the gas in the cells. Air then replaces the gas which means the R-value drops. The R-value warranty is 90%, which means that claimed R-5 is now 4.5. What R-value it is in 15- 20 years no one is saying. That gas coming out of the cells is also very bad for global warming. The Global Warming Potential, GWP, of XPS is over 1000 times worse than CO2. The manufacturers haven’t shown much interest so far in changing this. Every contractor we have talked to has dug up XPS and found it very heavy with the water in it. Now with foam put on the outside of walls, perm ratings also have to be addressed. 1.5” of XPS is treated as almost impermeable by code, 2” is impermeable.
EPS and GPS are the types of foam board that do away with the major issues of XPS. EPS is expanded polystyrene, the white stuff, and GPS is EPS with graphite in it. This makes the GPS gray with a higher R- value. EPS is R-4.1/ inch and GPS is R-5. They have 100% R-value warranty. Both of theses foams can be cut and molded into endless shapes. We have had shapes cut out for thickened edge slabs so you just butt the pieces together on the ground and there is your form. They can cut out shapes to match steel roofing profiles. Both foams do not hold water like XPS so thermal performance is much better. The perm rating is higher so you can put it on a wall with higher thicknesses. Coatings stick better to EPS and GPS because they have a different texture than XPS, and the GWP of EPS and GPS is around 8 compared to over 1000. Passive House designers and builders take into consideration Life Cycle Analysis which is a study of materials from cradle to grave. EPS and GPS can be recycled and are the materials of choice for these professionals. For certain applications different coatings, like a plastic film or foil film, can be applied to EPS and GPS. When XPS burns it is highly toxic, and when EPS and GPS burn it is like burning wood. These 2 foams can also be made in densities up to 200+ psi and will hold screws if needed. The lower the density the less the cost so you get what you need for the application and save money. EPS blocks of foam are used as fill under runways and highways because they hold weight better in soft ground than other types of fill.
Which brings up cost. In general, XPS costs more than GPS and EPS costs less than GPS. For in ground applications where a little additional thickness doesn’t matter, EPS is the way to go. Where higher R-value is desired in a smaller thickness, GPS is the answer. There are more options to consider when ordering these foam boards and it just adds flexibility to your project along with cost savings.
Call us with questions and we can get give you answers. You can use better products and save money at the same time.