©2015 Green Building Advisor. From The Taunton Press, Inc., publisher of Fine Homebuilding Magazine.
UPDATED on August 26, 2014 with new information on flash-and-batt requirements in the 2012 IRCInternational Residential Code. The one- and two-family dwelling model building code copyrighted by the International Code Council. The IRC is meant to be a stand-alone code compatible with the three national building codes—the Building Officials and Code Administrators (BOCA) National code, the Southern Building Code Congress International (SBCCI) code and the International Conference of Building Officials (ICBO) code.
If you plan to install exterior rigid foam on the walls of your house, how thick should the foam be? Although the GBAGreenBuildingAdvisor.com Web site has addressed this question several times in our Q&A column and various blogs, the question continues to perplex readers. New questions along these lines come our way regularly.
The last time I answered the question was at the end of a long, very technical blog . In this blog, I'll cut to the chase.
When it comes to rigid foam sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. , thick foam is better than thin foam. Thin foam is dangerous, because it reduces the ability of the wall to dry to the exterior without warming the sheathing enough to prevent moisture accumulation (a phenomenon that is usually but incorrectly called “condensation”).
Fortunately, building scientists have calculated the minimum foam thickness required for different wall thicknesses and different climates. By following their recommendations, your wall sheathing (or the interior face of the rigid foam) will stay warm enough to prevent moisture accumulation during the winter.
Because foam sheathing reduces the ability of a wall to dry to the exterior, all foam-sheathed walls must be able to dry to the interior. That means you don’t want any materials with a very low permeance on the interior of a foam-sheathed wall or between the studs. If you are building this type of wall, you should not include interior polyethylene or vinylCommon term for polyvinyl chloride (PVC). In chemistry, vinyl refers to a carbon-and-hydrogen group (H2C=CH–) that attaches to another functional group, such as chlorine (vinyl chloride) or acetate (vinyl acetate). wallpaper, nor should you install any closed-cell spray foam between the studs. It's perfectly acceptable to fill the stud bays with open-cell spray foam, however, since open-cell foam is vapor-permeable.
To sum up, there are two important points to remember about foam-sheathed walls:
Of course, foam-sheathed walls must comply with existing building codes. Until recently, that was difficult, because some building inspectors insisted on the need for interior polyethylene — even on foam-sheathed walls, where poly definitely does not belong.
Fortunately, the 2007 Supplement to the International Residential Code (IRC) came to the rescue. Since that Supplement was adopted, the IRC has allowed certain cold-climate walls to dry to the interior. The code now includes a table, Table N1102.5.1, listing which types of wall assemblies have minimal requirements for an interior vapor retarder. (In the 2009 IRC, these provisions can be found in section R601.3; the new designation for the table is Table R601.3.1. In the 2012 IRC, the relevant provisions can be found in section R702.7; the new designation for the table is Table R702.7.1 .)
The relevant table serves two purposes:
Here is the essential information from Table N1102.5.1 that applies to foam-sheathed walls:
|Climate Zone||Minimum R-Value of Foam Sheathing|
|Marine Zone 4||R-2.5 for 2x4 walls; R-3.75 for 2x6 walls|
|Zone 5||R-5 for 2x4 walls; R-7.5 for 2x6 walls|
|Zone 6||R-7.5 for 2x4 walls; R-11.25 for 2x6 walls|
|Zones 7 and 8||R-10 for 2x4 walls; R-15 for 2x6 walls|
Once you know the minimum required R-value for your foam sheathing, you can determine your foam thickness. To do that, you need to know the R-value per inch of your foam. The most common type of expanded polystyrene (EPSExpanded polystyrene. Type of rigid foam insulation that, unlike extruded polystyrene (XPS), does not contain ozone-depleting HCFCs. EPS frequently has a high recycled content. Its vapor permeability is higher and its R-value lower than XPS insulation. EPS insulation is classified by type: Type I is lowest in density and strength and Type X is highest.) has an R-value of about R-3.6 per inch, while extruded polystyrene (XPSExtruded polystyrene. Highly insulating, water-resistant rigid foam insulation that is widely used above and below grade, such as on exterior walls and underneath concrete floor slabs. In North America, XPS is made with ozone-depleting HCFC-142b. XPS has higher density and R-value and lower vapor permeability than EPS rigid insulation.) has an R-value of R-5 per inch.
The R-value shown on polyisocyanurate labels is usually equivalent to R-6 or R-6.5 per inch. However, the actual performance of polyiso decreases at cold temperatures. Concerns about the cold-temperature performance of polyiso are real, so GBA recommends that cold-climate builders use caution when choosing a rigid foam designed to keep wall sheathing above the dew point during the winter. Either EPS or XPS is probably a safer choice for this purpose than polyiso, unless you derate the performance of the outermost layer of polyiso to about R-4 per inch. For more information on this issue, see In Cold Climates, R-5 Foam Beats R-6 .
If you’re not sure what climate zone you live in, you can look it up on the Department of Energy’s climate zone map. The map is posted here on the GBA website; click here  to see it.
Once you’ve visited the site, you can bookmark the page for future reference. I have also included the climate zone map on this page (Image 2 at the bottom of the blog); just click the image to enlarge it.
If you are building a house in one of the warmer climate zones — zone 1, 2, 3, or 4 (except for 4 Marine) — you don't have to worry about the thickness of your foam. Any foam thickness will work, because your sheathing will never get cold enough for “condensation” (moisture accumulation) to be a problem.
Builders following the flash-and-batt method — that is, a hybrid insulation system using a thin layer of closed-cell spray polyurethane foam against the interior side of the wall sheathing, with the balance of the stud bay filled with fiberglass batts or cellulose — can follow the recommendations in the table above for the minimum thickness of the spray foam. Closed-cell spray polyurethane foam has an R-value ranging from R-6.5 to about R-6.8 per inch.
The 2012 IRC specifically endorses this approach to flash-and-batt calculations in Footnote a to Table R702.7.1 . The relevant footnote reads, "Spray foam with a minimum density of 2 lb/ft3 applied to the interior cavity side of wood structural panels, fiberboard, insulating sheathing or gypsum is deemed to meet the insulating sheathing requirement where the spray foam R-value meets or exceeds the specified insulating sheathing R-value."
The table can also be used as a minimum foam thickness guide when following the cut-and-cobble method (insulating between studs by combining a layer of rigid foam installed against the interior side of the wall sheathing with fiberglass batts in the rest of the stud cavity).
Although the fiberglass batts in a flash-and-batt stud bay will be thinner than the fiberglass batts in a wall with exterior foam sheathing, thinner batts move the wall in the direction of more safety rather than more risk, since thinner fiberglass keeps the interior surface of the cured foam warmer (and therefore less likely to collect condensation).
If you want to sharpen your pencil, you can get away with thinner foam for a flash-and-batt job than an exterior-foam job. As long as you retain the ratio of foam R-value to fluffy-insulation R-value shown in the table, you should be OK. For example, the table recommends R-5 foam for a 2x4 wall filled with R-13 fiberglass insulation in Climate Zone 5 (38% foam and 62% fiberglass). For a flash and batt job, you could get away with R-3.6 foam and R-9.5 fiberglass insulation. However, in most cases you don't really have to sharpen your pencil quite this much.
Since most homes don't have foam sheathing, what keeps the cold sheathing on a typical home from developing moisture problems?
Good question; the answer can be found in another blog, How Risky Is Cold OSB Wall Sheathing? 
The same logic used to calculate the minimum thickness of foam wall sheathing can also be applied to unvented cathedral ceilings.
Recent versions of the IRC allow unvented roof assemblies insulated with a combination of rigid foam insulation above the roof sheathing and air-permeable insulation in the rafter bays. The relevant code provisions can be found in section R806.4 of the 2009 IRC and in section R806.5 of the 2012 IRC. (The IRC defines air-impermeable insulation as “an insulation having an air permeance equal to or less than 0.02 L/s-m² at 75 Pa pressure differential tested according to ASTMAmerican Society for Testing and Materials. Not-for-profit international standards organization that provides a forum for the development and publication of voluntary technical standards for materials, products, systems, and services. Originally the American Society for Testing and Materials. E 2178 or E 283.” Although spray foam insulation and rigid foam insulation can meet this standard, dense-packed cellulose cannot.)
The code requires that “rigid board or sheet insulation shall be installed directly above the structural roof sheathing as specified in Table R806.4 for condensation control.” These values are:
More information on Table N1102.5.1 can be found in a useful article posted on the Building Science Corporation Web site, Insulating Sheathing Vapor Retarder Requirements .
If you are a masochist, and want to delve deeper into the intricacies of dew-point calculations, you can check out my earlier blog on this topic, Are Dew-Point Calculations Really Necessary? 
For installation details, see How to Install Rigid Foam Sheathing .
Last week’s blog: “Solar Versus Superinsulation: A 30-Year-Old Debate.”