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Calculating the Minimum Thickness of Rigid Foam Sheathing

In this case, the code is your friend — just follow the IRC’s foam thickness table

Posted on Oct 15 2010 by Martin Holladay

UPDATED on February 26, 2016 with a new table (Image #3)

If you plan to install exterior rigid foam on the walls of your house, how thick should the foam be? Although the 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.

Keeping walls dry

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.

Install thick foam and no interior poly

To sum up, there are two important points to remember about foam-sheathed walls:

  • The rigid foam must be thick enough to prevent moisture accumulation (“condensation”) in your sheathing or framing; and
  • This type of wall must be able to dry inward, so it's important to avoid low-permeance layers like polyethylene, vinyl wallpaper, or closed-cell spray foam on the interior.

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 (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.) 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 .)

The relevant table serves two purposes:

  • It gives permission to builders of foam-sheathed walls to use a minimal interior vapor retarder — one with the highest permeance values, known as a Class III vapor retarder. (Ordinary latex paint is all you need.)
  • It spells out the minimum R-values for exterior foam to be sure that moisture won’t accumulate in a wall.

All you need to know

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.

These days, the R-value shown on polyisocyanurate labels is usually equivalent to R-5.7 to R-6.0 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 or R-5 per inch. For more information on this issue, see In Cold Climates, R-5 Foam Beats R-6 and Cold-Weather Performance of Polyisocyanurate.

What’s my climate zone?

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.

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.

Here is a link to a web page with climate zone information for Canada.

What if I live in one of the warmer climate zones?

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.

What about walls with above-code levels of air-permeable insulation?

If you plan to install a thicker-than-usual layer of fluffy insulation, you'll also need to install a thicker-than-usual layer of rigid foam (to make sure that the proper ratio of rigid foam to fluffy insulation is maintained). The table reproduced as Image #3, below, includes the relevant percentages that need to be observed.

For more information on this issue, see Combining Exterior Rigid Foam With Fluffy Insulation.

What about flash-and-batt jobs?

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 . 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.

Why doesn’t every cold-climate wall have rotten sheathing?

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?

Is there a similar chart for unvented cathedral ceilings?

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:

  • Climate Zones 1-3 — R-5
  • Climate Zone 4C — R-10
  • Climate Zones 4A and 4B — R-15
  • Climate Zone 5 — R-20
  • Climate Zone 6 — R-25
  • Climate Zone 7 — R-30
  • Climate Zone 8 — R-35

For more information on this topic, see How to Install Rigid Foam On Top of Roof Sheathing.

For more information

More information on Table N1102.5.1 can be found in a useful article posted on the Building Science Corporation Web site, .

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 instructions on installing rigid foam on the exterior side of wall sheathing, see How to Install Rigid Foam Sheathing.

Finally, for an assessment of the risks associated with too-thin foam, see Rethinking the Rules on Minimum Foam Thickness.

Last week’s blog: “Solar Versus Superinsulation: A 30-Year-Old Debate.”

Tags: , , , , , ,

Image Credits:

  1. Image #1: Ty Keltner, Cold Climate Housing Research Center
  2. Image #2: U.S. Department of Energy
  3. Image #3: Martin Holladay

Oct 15, 2010 1:00 PM ET

up here in canada
by Jennifer Corson

Does anyone know if there is a similar section to IRC Table N1102.5.1 in the Canadian Residential Building Code.

Thank you for the great series of articles on this topic.

Oct 15, 2010 9:28 PM ET

additional insulation in the stud cavities
by J Chesnut

Does it matter with these numbers whether the stud cavities are insulated or not?
A project I was on in climate zone 6 went through a WUFI analysis and the results depended on whether there was insulation in the wall cavity.
I think in that case the concern with the insulation in the stud cavity was drying potential to the interior. But I recall from other contexts that the more you insulate the wall cavity the colder the sheathing. Isn't there the rule of thumb 2/3 R-value on the outside 1/3 on the inside?

Oct 16, 2010 4:41 AM ET

Response to J Chesnut
by Martin Holladay

The IRC table assumes that the stud bays are insulated.

You're right that in order to keep the inside face of the sheathing warm enough to prevent "condensation," the ratio between the insulation outside of the sheathing and the insulation inside of the sheathing matters. The more insulation inside of the sheathing, the colder the sheathing.

If the stud bays are empty, you really don't have to worry about minimum foam thickness. There really is no minimum in that case -- other than the existing code requirement for wall insulation. That's the beauty of the PERSIST system.

The old rule about 2/3 and 1/3 was devised in the 1980s to address thick walls with a poly vapor barrier in the middle of the wall. People don't build walls like that anymore, and in any case it was just a rule of thumb. But the idea was the same -- builders wanted to keep the poly warm enough to avoid condensation problems.

Oct 16, 2010 8:22 AM ET

What happened to section N1102.5 in 2009 IRC?
by Daniel Haedrich

I have been putting foam exterior of the sheathing, the thickness more set by builder preference than dew point consideration. That said, most of my work is now done under a vigorously enforced IRC 2009, so I figured I better make sure I was familiar with the code. Section N1102.5 (concerning moisture control) seems to be completely removed from the 2009 IRC. Did the 2007 supplement not get rolled into the 2009 IRC or did the whole section move? I checked the IECC and did not see it there either.

Oct 16, 2010 1:59 PM ET

eps interior and exterior
by Mike

Would there be any caveats to installing 1"polystyrene on the interior and also the exterior? The stud cavities are dense packed cellulose and are built above a block masonry wall (raised ranch lower level). By installing 2" eps on the block it offered the opportunity to continue this thermal and air barrier for the remainder of the interior wall. Future plan is to install new siding with foam insulation. Do you think this assembly could trap mositure due to the low permenace of the foam board?

Oct 16, 2010 4:46 PM ET

Response to Mike
by Martin Holladay

Most experts recommend against a "foam sandwich." It's best to choose which side of the wall you want to put your foam, and then add as much foam as you need or want -- all on one side of the wall. Then the wall can dry to the opposite direction.

Oct 17, 2010 10:44 AM ET

Moisture Accumulation vs. Condensation
by Gregory Behm


Would you please elaborate on your early comment about moisture accumulation, which you say 'is usually but incorrectly called "condensation").'? Are you talking about moisture accumulation being more than condensation alone, or that the use of "condensation" is incorrect altogether?

Oct 17, 2010 1:27 PM ET

Edited Nov 8, 2012 11:00 AM ET.

Response to Gregory Behm
by Martin Holladay

Condensation occurs when water vapor condenses on a cold, hard surface like glass or an aluminum beer can. Condensation generally does not occur on a hygroscopic material like wood framing, plywood sheathing, or cellulose. (However, frost can occur on these materials.)

If the material in question is below the dew point, it starts absorbing moisture: that is, its moisture content rises. This is a more accurate description of what happens -- more accurate than saying, "moisture condensed on the cold plywood."

For more information on this topic, see "Are Dew-Point Calculations Really Necessary?" (including the posted comments by Bill Rose).

Oct 17, 2010 6:20 PM ET

Drying of walls
by Doug McEvers

Martin, How do you get homeowners not to use low perm paints and wallcoverings? Does a disclaimer go with the house? This type of wall system is hardly robust in my opinion.

Oct 17, 2010 9:03 PM ET

Response to Doug
by Martin Holladay

That's an excellent question. I wouldn't worry about paint, since most painted walls will dry to the interior adequately. But vinyl wallpaper is usually a bad idea.

Anyone have any suggestions for addressing the scourge of vinyl wallpaper?

Oct 18, 2010 3:59 AM ET

Vinyl Wallpaper?
by Kevin Dickson, MSME

Does anyone with any taste still use the stuff? I know hotels and motels like it, but they have paid staff to evaluate things like that.

Martin, thanks for more helpful info about "condensation" on a hygroscopic material.

Oct 18, 2010 6:19 AM ET

More on vinyl wallpaper
by Martin Holladay

I agree that vinyl wallpaper is rare in residential applications -- fortunately.

Doug's question is similar to another question I hope to address in a future blog: If an existing cold-climate home built in the 1980s has interior polyethylene under the drywall, is it risky to install exterior foam sheathing during a re-siding job?

John Straube's answer is basically, the jury is still out, but all signs are, "Not so risky as most people think." Thousands of houses with interior poly in Ontario and Quebec (and northern New England, for that matter) have had retrofit exterior foam installed, and we are not seeing large numbers of failures associated with this retrofit work. Struabe says, "These homes will probably be fine."

Oct 18, 2010 10:17 AM ET

Should tile and shower type surfaces be avoided?
by Nelson Labbe

Should tiled or other types of shower walls also be avoided on the interior of foam sheathed walls? Should we be especially concerned with waterproof membranes such as kerdi underneath tile?

Oct 18, 2010 11:46 AM ET

Response to Daniel Haedrich
by Martin Holladay

Daniel Haedrich,
I have ordered a copy of the 2009 IRC but have not yet received it. I just got off the phone with Joe Lstiburek, who assures me that the table in question has been integrated into the 2009 IRC, but in a different location and with a different table number. As soon as possible, I will track down the reference and share that information here.

Code officials have decided to remove almost all requirements having to do with vapor retarders from the IECC, so you won't find this table there. All references to vapor retarders have been moved from the IECC and put into the IRC.

Oct 18, 2010 11:51 AM ET

Response to Nelson Labbe
by Martin Holladay

Nelson Labbe,
Any time that you have an impermeable membrane on the interior of your wall and foam sheathing on the exterior of the wall, you raise the stakes for the builder. In most cases these walls will work well, even in a cold climate, but you need to be meticulous about your details to stay out of trouble.

A rainscreen siding gap will go a long ways toward improving the chances that wind-driven rain won't enter the wall cavities.

Meticulous air sealing on the interior will go a long ways toward improving the chances that moisture-laden air won't enter the wall cavities from the interior.

Finally, be sure that your sheathing and studs are dry when you seal up the wall. If everything got soaked in a rainstorm, and then builders rush to install exterior foam and an interior waterproof membrane behind a tiled shower, you are trapping moisture in your stud bays right from the start.

So use your head, do a good job, and you should be fine.

Oct 19, 2010 3:03 PM ET

Table Location
by T.C.

IRC '09 adresses vapor retarders and the allowance for class III vapor retarders over varying R-values of exterior rigid insulation (based upon wall cavity depth) in R601.3 and table R601.3.1. That is where the above information was integrated. Interestingly, IRC also recognizes the benefit of vented cladding in this section/table.

Oct 19, 2010 3:49 PM ET

Response to T.C.
by Martin Holladay

Thanks for the references!

Oct 20, 2010 4:33 PM ET

table location
by Raff

thanks for this info. I just finished enclosing a timberframe with 2x4 REMOTE walls. With 5.5" of foil faced polyiso and no cavity fill, even our PEng. had a hard time stamping the idea. This was over a year ago, glad to see IRC is finally catching up

Oct 20, 2010 5:21 PM ET

2 x4 and 2 x 6 foam sheathing thickness
by Kevin Hanlon


Thanks for the update on N1102.5.1. That table in CZ6 requires foam sheathing R7.5 for 2 x4 walls and foam sheathing R11.25 for 2 x 6 walls. Is the extra sheathing thickness and Rvalue required on the 2 x 6 wall due to the fact that higher R value is assumed to be installed in the cavities, requiring more Rvalue for the 1st condensing / moisture accumulation surface?

Oct 21, 2010 4:05 AM ET

Response to Kevin Hanlon
by Martin Holladay

Yes, you've got it. The extra insulation in a 2x6 wall (compared to a 2x4 wall) further separates the wall sheathing from the warm interior -- thereby keeping the sheathing colder. So 2x6 walls need thicker exterior foam than 2x4 walls.

Oct 21, 2010 10:59 AM ET

Martin, Thanks for the
by Hallie

Thanks for the article! It's good to know that if I keep up with GBA, I'll get the information I need to know!

Oct 22, 2010 8:32 AM ET

Other climate zones?
by Daniel Ernst


So, based on this table, exterior insulation thickness is a moot point for climate zones 4A & 4B?

I realize there are other methods of calculating foam thickness (based on HDD, average wintertime temperatures, etc.), and designers should be informed before making a decision on this subject. BUT, from the IRC's perspective, people building in a zones 3, 4A, & 4B don't have to worry about winter time moisture accumulation?

I'd appreciate hearing your thoughts on this topic!

Oct 22, 2010 8:56 AM ET

Response to Daniel Ernst
by Martin Holladay

You're correct.

If you live in climate zones 1, 2, 3, or 4 (except 4 Marine), there is no danger of wintertime "condensation" (moisture accumulation) because your sheathing will never get cold enough to cause a problem, regardless of whether or not there is exterior foam, thick or thin.

Oct 28, 2010 11:08 AM ET

Southern Paradigm
by Anonymous

Every time I see this darn map and other DOE statistics I shake my head. It still seems to be calculated from the aspect of conserving "home heating oil". A typical residence in Central Texas runs the air conditioning (electricity) for ten months of the year. Additional insulation does allow for more comfort and less dependence on the cooling cycle. So I agree that we do not need the insulation for protection against cold, but we do need it for protection against heat.

Oct 28, 2010 11:18 AM ET

Response to Anonymous
by Martin Holladay

So why do you shake your head? It's just a climate map. Surely you agree that the climate in Texas is different from the climate in Minnesota?

No one is saying that you don't need insulation in Texas. Of course you do. But the climate map tells you many things -- including the fact that in Texas, you don't need to worry much about wintertime condensation or moisture accumulation in your wall sheathing.

Oct 28, 2010 11:48 AM ET

Interior Foam insulation
by Wayne

I have a building with a "stepped" ICF wall. Above the ICF I have a 2x6 studded wall on 24" centers with the studded wall flush with the outside of the ICF. I am considering using 4" of rigid foam on the inside to insulate and make the inside of the studded wall flush with the inside of the ICF. I have 1/2" plywood on the outside of the studs, with 30# building felt and board and batten siding. I plan to have the wall dry to the outside. I live in the Pacific Northwest. This building is a large shed and I will not likely be heating it over 60 degrees. What issues might I have with moisture content in the walls?

Oct 28, 2010 11:56 AM ET

Response to Wayne
by Martin Holladay

While many builders have had success with walls similar to the one you describe, building scientists are beginning to note that such walls are risky. You've got cold exterior sheathing; that sheathing is at risk of moisture accumulation.

So, you need to do one or two things to lower the risk. The most important thing you can do is to create an effective interior air barrier (for example, using the Airtight Drywall Approach). Don't forget to use airtight electrical boxes and to take measures to avoid air leaks at these boxes. These measures keep warm humid interior air out of your wall.

The second measure is optional, but it lowers your risk further: include a ventilated rainscreen gap between your sheathing and your siding. Such a gap will help your plywood to dry quickly if it ever gets damp.

Nov 6, 2010 11:17 AM ET

IRC 2009 table N1102.1 and Online Codes for all
by Bob Ellenberg

Martin--Two things.
First, I understand this pretty well and assume they factor in conservative estimates for moisture generated inside a tight house during the winter. But how do you reconcile table N1102.1 in the 2009 IRC that states zone 6 can have R13 batts in the wall and R5 foam sheathing on the exterior and footnote h. even goes on to say that you can have as little as R2 foam on the exterior? This seems to be in conflict with R603.3.1
Secondly, you mentioned you had ordered a new copy of the Code and were waiting for it. There are two websites with a lot of codes on line including many states and local municipalities--an invaluable resource. They are:

Nov 6, 2010 1:47 PM ET

Response to Bob
by Martin Holladay

The code lists minimum performance requirements; however, it's perfectly possible to build a code-compliant house that doesn't perform well or fails quickly. There's nothing in the code to keep you from screwing up. The code doesn't necessarily keep you out of trouble. (For example, it still allows the use of interior poly in many situations when its use is ill-advised.)

So, "Table N1102.1 in the 2009 IRC that states zone 6 can have R13 batts in the wall and R5 foam sheathing on the exterior." That's the code minimum. I don't suggest you do that just because it is legal. You should do better.

Nov 6, 2010 2:29 PM ET

Code conflict
by Bob Ellenberg

Maybe I wasn't clear. I totally understand the concept, agree with you and don't intend to put 1" foam on a 2x4 wall in zone 6. I was wanting to point out that it appeared to be a conflict within the code and wanted to see if you read it the same way. It appears to me that N1102.1 states you can do something that R603.3.1 says you can't. Is that how you read it?

Nov 7, 2010 6:34 AM ET

Second response to Bob Ellenberg
by Martin Holladay

Bob Ellenberg,
1. I agree with you that the code could be written better. You make a good point -- Table N1102.1.1 implies that it's OK to install R-5 foam over a 2x4 wall in zone 6. The table should be re-written or should include a footnote warning against the practice.

2. However, remember that minimum code requirements for insulation are NOT recommendations for best practice. These minimum requirements are simply a way of saying, "If you do anything less than this, you've broken the law."

Nov 9, 2010 2:24 PM ET

Best Practice query
by Joe W

I understand that it is not necessary for exterior rigid foam in areas like mine (Zone 3) for walls. What about on unvented roofs? Or are roof questions ruled by a different table of commandments? And I'm interested in both walls and roofs as "best practices," not in terms of minimums for the minute.

My question comes from Dr. Joe's book on Mixed Humid climates where he designs an unvented roof for Dallas TX, also in a mixed humid climate w/HDD and climate similar to Atlanta's. He places R-5 rigid insulation on the exterior of the roof to avoid the potential for "condensation on the underside of the roof sheathing."

Is this a contradiction of the points made here, or am I (more likely) not understanding the difference between roofs and walls for rigid exterior foam?

Joe W

Nov 9, 2010 2:52 PM ET

Response to Joe W
by Martin Holladay

Joe W.,
Good question. Although I'm not a building scientist -- and I'll do my best to contact one to get more information on your question -- here's my understanding: unvented insulated roofs are particularly problematic if they aren't detailed carefully, since there are often few opportunities for damp roof sheathing over an unvented assembly to dry out.

Roof sheathing over an unconditioned attic dries readily to the interior. If the rafter bays are stuffed with insulation, however, the roof sheathing won't dry as quickly toward the interior. Since the roof sheathing is covered on the outside with underlayment and roofing, it doesn't dry much to the exterior, either.

Roof sheathing over an unvented insulated roof doesn't dry as well as wall sheathing. After all, wall sheathing is better able to dry to the exterior than roof sheathing.

I assume that Joe Lstiburek recommends the use of R-5 foam on top of an unvented insulated roof in Zone 3 to keep the roof sheathing warm (and therefore dry) during the winter. This foam isn't necessary for walls in Zone 3, because the wall sheathing is able to dry to the exterior during the summer.

Nov 9, 2010 6:00 PM ET

Zone 3 roofs vs walls
by Joe W

Thanks Martin. That makes real sense. As always I appreciate the help. And yes, keeping the sheathing warm is his objective.

Joe W

Nov 11, 2010 2:18 PM ET

I'm learning much from these
by Jeff W.

I'm learning much from these forums! Would it always be considered a best practice to have two layers of foam as opposed to one so the joints could be staggered then taped?

Nov 11, 2010 2:21 PM ET

Response to Jeff W.
by Martin Holladay

Jeff W.,
Most experts agree that two layers of rigid foam with staggered seams is better than one layer of the same total thickness.

Nov 28, 2010 8:30 PM ET

Confusion by Clarification
by Riversong

Martin is finally differentiating between condensation and moisture accumulation in this blog and his "How To Avoid Condensation in Your Walls" blog, but I'm afraid the discussion is creating more fog than sunshine. I'm composing a Q&A thread, which will be posted very soon, to attempt to bring more light to this complex and highly misunderstood topic.

But my primary concern about this blog is that it seems to suggest that the best if not the only way to prevent condensation and moisture accumulation in sheathing (at least from the inside) is to keep it warm with exterior foam (which is Martin's oft-stated bias).

Though it's true that this blog is only about how thick a layer of exterior foam is necessary to keep the sheathing relatively safe, it's revealing that Martin abbreviated IRC Table N1102.5.1 (which he copied in its entirety on his other blog).

What the full table ( reveals is that sheathing moisture problems can be avoided in one of two ways in almost all of the lower 48 states: either by keeping it mostly above the dew point temperature with exterior foam or by using permeable exterior sheathings and cladding assemblies.

It's heartening that the IRC, at least, is finally recognizing that exterior breatheability is as important as trying to keep sheathing warm, which can have the adverse effect of encouraging mold and decay growth (which cannot occur in cold or frozen sheathing).

Nov 30, 2010 8:45 PM ET

Foam Sheathing
by Catching Up

I have a small, cement block house in tbe wet Willamette Valley (Zone 4) of western Oregon. What thickness of exterior rigid foam sheathing should I use in that case?

Dec 1, 2010 6:12 AM ET

Response to Catching Up
by Martin Holladay

Catching Up,
According to the 2006 IRC, so-called "mass walls" in your climate zone should be insulated to at least R-13 (assuming continuous foam insulation rather than cavity insulation).

That means at least 2 inches of polyisocyanurate, or about 3 inches of XPS, or about 4 inches of EPS.

These are minimum code requirements. Many green builders prefer to exceed the minimum code requirements.

Sep 17, 2011 12:39 AM ET

foam sandwhich
by lyle zimmer

the minimum foam thickness charts don't seem right to me...experts recommend against foam on both sides of a wall, aka "foam sandwich" but does it really matter if you have poly and r22 bat insulation on one side of the wall sheathing and r15 foam insulation on the other side of the wall sheathing?? As this would be the case in a retrofit. Would it make sense on a 2x6 wall to use poly and r22 bat insulation on the inside of the wall sheathing and only 1" rigid insulation on the exterior as the 1" has enough perms to allow drying to the outside if necessary?? And there's no mention at all on this blog about tyvek or typar house wrap against the wall sheathing!

Sep 17, 2011 5:58 AM ET

Edited Sep 17, 2011 6:00 AM ET.

Response to Lyle Zimmer
by Martin Holladay

I'll try to address the points you raise.

Q. "Does it really matter if you have poly and R-22 batt insulation on one side of the wall sheathing and R-15 foam insulation on the other side of the wall sheathing?"

A. I'm not quite sure what you mean by, "Does it matter?" I think your question concerns the presence of polyethylene, but I'm not sure. If that's your question, here's the answer: the polyethylene is less than ideal, because it slows down the wall assembly's ability to dry to the interior. However, the wall may still work, especially if there are no water intrusion issues from wind-driven rain, and if the indoor relative humidity is kept low.

Q. "Would it make sense on a 2x6 wall to use poly and R-22 batt insulation on the inside of the wall sheathing and only 1 inch of rigid insulation on the exterior as the 1 inch has enough perms to allow drying to the outside if necessary?"

A. First of all, if you plan to install exterior rigid foam on your wall, it never makes sense to install interior poly. So, if it's new construction, be sure to leave out the polyethylene. If it is a retrofit job, and the poly is already there, you may have to live the with poly -- not ideal, but not necessarily a deal-breaker. Concerning the permeance of 1-inch thick foam: EPS permeance is between 2 and 5.8; XPS permeance is between 0.4 and 1.6; and foil-faced polyiso permeance is 0.05. These are low numbers, so you won't see much drying, if any, to the exterior. It makes far more sense to keep the sheathing warm enough to avoid condensation or moisture accumulation than to deliberately leave the sheathing a little bit cold and hope that the cold, wet sheathing can dry through the foam. Trust me -- it won't dry through the foam. It's better to ensure that it never gets wet than to hope it finds a way to dry.

Q. "And there's no mention at all on this blog about Tyvek or Typar house wrap against the wall sheathing!"

A. True. So what? If you have a question about Tyvek or Typar, I would be glad to answer it. Briefly, Tyvek and Typar are water-resistant barriers (WRBs). Neither is thick enough to keep sheathing warm, so they don't enter into this calculation. Both materials are vapor-permeable. If you are interested in learning more about WRBs, I suggest you read All About Water-Resistive Barriers.

Dec 2, 2011 1:19 PM ET

Do these minimum R-Values apply to SPF as well?
by Tyler Dotten

Being that I am in climate zone 4 here in Portland OR. Do I still need to achieve an R-15 minimum when encapsulating rafters with open cell foam?

Dec 2, 2011 1:24 PM ET

Reply to Tyler Dotten
by Martin Holladay

No, the guidelines presented here apply only to rigid foam sheathing installed on the exterior of walls.

If you are installing open-cell spray foam between your rafters, all you have to do is meet your minimum code requirements for R-value. According to the 2009 IRC -- which may or may not apply in your jurisdiction -- the minimum ceiling R-value for those following the prescriptive path in your climate zone is R-38.

That means that you need to install at least 10.25 inches of open-cell spray foam.

Dec 17, 2011 2:48 PM ET

Above grade retro fit
by Bob Le Beau

Hello Martin and the Blog posters. I am interested in improving my energy insulation in a 2x4 studded walls with 40 y/o fiberglass R13 and Kraft faced insulation. I live in Zone 5 (Chicago).

After reading the great info on this site, I plan to re side and re pack the insulation with R15, then sheathing with OSB/Ply, rain/water barrier/proper flashing, then 2- 1 inch polyiso, seams taped and alternated, then furring strips, then fiber cememnt.

Does this sould appropriate? Do I need more than 2 inches of Polyiso (hard to come by for a DIYer).

Thanks in advance,


Dec 18, 2011 5:20 PM ET

Response to Bob LeBeau
by Martin Holladay

As the blog on this page explains, all you need is a minimum of R-5 for a 2x4 wall in Zone 5, so your 2 inches of polyiso will work fine.

Feb 29, 2012 7:00 PM ET

Is foam necessary?
by Elizabeth Kormos

Is foam on the exterior really needed? Wouldn't it be 'greener' to skip the foam on the exterior and do a double wall with cellulose using something like the zip system to seal up the outside. Thick foam seems to cause a lot of problems with exterior windows and detailing.

Am I missing something?

Feb 29, 2012 7:07 PM ET

Edited Mar 1, 2012 6:32 AM ET.

Response to Elizabeth Kormos
by Martin Holladay

I don't know where you got the impression that foam is necessary; this article certainly never said that foam is necessary.

GBA includes a great many articles on double-stud walls, Larsen trusses, and the installation of mineral wool panel insulation on exterior walls. See, for example:

Double-Stud Walls

Is Double Stud-Wall Construction the Path to Efficiency on a Budget?

All About Larsen Trusses

How to Install Cellulose Insulation

Installing Mineral Wool Insulation Over Exterior Wall Sheathing

Feb 29, 2012 7:40 PM ET

Sorry, just trying to figure it all out.
by Elizabeth Kormos

I've read all those articles and so far came to the conclusion that we didn't need foam on the outside. I am crazy or shouldn't there be a best practice (most energy efficient and affordable) way of constructing a wall per climate area?

Mar 1, 2012 6:39 AM ET

Edited Mar 1, 2012 6:41 AM ET.

Response to Elizabeth Kormos
by Martin Holladay

Q. "Shouldn't there be a best practice (most energy efficient and affordable) way of constructing a wall per climate area?"

A. GBA tries to provide information to readers to help them determine energy-efficient and affordable ways of constructing a wall. However, the type of wall that is affordable in your area may be different from the type of wall that is affordable in others areas. Variables include the cost of labor, the availability of skilled cellulose installers, and the proximity of the nearest SIP manufacturing facility. All of these factors, and many more, affect the cost of a wall.

The building code sets minimum prescriptive R-values for wall assemblies. If you build a wall with a lower R-value than the code, you are breaking the law. Of course, building a wall with a higher R-value than the legal minimum is perfectly OK, and it's usually a good idea.

Green builders usually try to build a house that is better than the legal minimum. That's why there are a variety of green building programs with more stringent requirements than the building code. You might also be interested in the discussion around the idea of the "Pretty Good House" -- an attempt to establish what might be called best practices.

Mar 1, 2012 8:57 AM ET

by Elizabeth Kormos

Hi Martin,

We are about to start design on our near passive house (or pretty good house) in zone 5. I've been an avid reader of all your blogs and articles. I think your recent discussions as to regionalization (which by the way LEEDS is now addressing too) is right on. Thank you for making all this information available to builders and homeowners.

If you were building my house (2,200 sf, 1,500 sf on first floor (universal design),triple windows, on a slab, vented roof, passive solar orientation) in upstate NY and had a limited budget what wall system would you use?

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