Which Rigid Insulation Should I Choose?
Get the details right, and a small increase in wall thickness can make a big difference in energy performance.
Rigid-foam insulation packs a lot of R-value into a thin package, but not all rigid foam performs the same. Choose insulation wisely, and consider the effect its characteristics will have on the performance of the walls.
Foam isn’t the only way to build a super-insulated home
If you’re still in the planning stage of your construction project, just be aware that exterior foam insulation isn’t the only way to build an energy efficient home. take a look a the article Six Proven Ways to Build Energy-Smart Walls if you want to explore your options.
3 rigid-foam choices
Expanded polystyrene (EPS)
EPS is the insulation used most widely in insulated concrete forms and structural insulated panels. EPS has the lowest average R-value of the three types of rigid-foam insulation, around R-4 per inch. At about 19¢ per sq. ft. for a 1-in.-thick 4×8 sheet, it also costs the least. Although EPS is acceptable for ground contact and can be treated to resist insects, it does absorb water. When applied as sheathing, EPS should be used over housewrap. Most EPS is unfaced, which means it is fragile to work with and is considered semipermeable, so it does not create a vapor barrier.
Extruded polystyrene (XPS)
Easily recognized by its blue, green, or pink color, XPS falls in the middle of the three types of rigid-foam insulation in both cost and R-value. At about R-5 per inch, XPS costs around 42¢ per sq. ft. for a 1-in.-thick 4×8 panel. XPS comes unfaced or with a number of different plastic facings. Unfaced 1-in.-thick XPS has a perm rating around 1, making it semipermeable. Thicker and faced XPS is stronger and can have a lower perm rating, but either way, it is considered a vapor retarder, not a vapor barrier.
ISO panels are expensive, costing as much as 70¢ per sq. ft. for a 1-in.-thick panel, and they pay off with (aged) R-values as high as R-6.5 per inch. (R-values start around R-8 and degrade slightly over time.) Because ISO starts as liquid foam and has to be sprayed against a substrate to form a rigid panel, all ISO panels are faced. A few different facings used on ISO affect the performance of the panel in both durability and perm rating. Foil-faced ISO panels are considered impermeable. Because applying these products as sheathing creates an exterior vapor barrier, they never should be used with an interior vapor barrier. More permeable ISO panels are faced with fiberglass and can be used without creating a vapor barrier.
Read the complete article: Save Energy with Rigid-Foam Insulation
More about rigid-foam insulation choices and construction details:
Get the Right Rigid Foam – Foam-board insulation can boost R-value, slow thermal bridging, and control condensation—but you better choose the right type.
Is Your Exterior Rigid Foam Too Thin? – Whether it’s to meet code or to adhere to a “green” target, more and more builders are adding a layer of rigid foam insulation on the outside of homes. The idea makes sense, if it’s done correctly. Learn if you’re applying exterior rigid foam correctly.
Detailing Walls With Rigid Foam – Builder Steve DeMetrick shares construction and design details for efficient and trouble-free installation of exterior foam sheathing. His method employs Zip System sheathing for structure and air-sealing, 2-in. foil-faced foam for exterior insulation, and a felt-paper weather-resistive barrier behind a rain screen.
Choosing the Right Thickness of Exterior Foam – Rigid foam must be thick enough to prevent condensation on the roof sheathing.
Insulate Your Basement, Part 3 – In this episode, Justin Fink explains the why and how of using foam insulation on masonry walls, and demonstrates how to properly insulate the rim joist area with rigid foam and batt insulation in your basement.