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, GBA Advisor

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Thick foam is better than thin foam. If you're installing rigid foam on the outside of your walls, make sure the foam is thick enough to prevent moisture accumulation, and make sure that your wall is able to dry inward.

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.

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 — especially polyethylene — on the interior of a foam-sheathed wall.

Install thick foam and no interior poly

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

Make sure the foam is thick enough to prevent moisture acculmulation (“condensation”) in your sheathing or framing; and
Avoid low-permeance layers like 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 on the interior so the wall can dry inward.

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.)

Table N1102.5.1 (also known as Table R601.3.1) 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; 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; and polyisocyanurate has an R-value of about R-6.5 per inch.

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.

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.

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 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.

Although the fiberglass batt in a flash-and-batt stud bay will be thinner than the fiberglass batt in a wall with exterior foam sheathing, this thinner batt moves 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).

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.

The 2009 IRC (Section R806.4) allows unvented roof assemblies insulated with a combination of rigid foam insulation above the roof sheathing and air-permeable insulation in the rafter bays. (The 2009 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

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.”

Tags: condensation, foam insulation, foam sheathing, foam thickness, rigid foam, Walls, Insulation

Image Credits:

Ty Keltner, Cold Climate Housing Research Center
DOE

51.
Thu, 03/01/2012 - 09:51

A high-R wall for upstate New York
by Martin Holladay, GBA Advisor

Elizabeth,
It's hard to beat a 12-inch-thick double stud wall insulated with dense-packed cellulose for simplicity, relatively low cost, buildability, and energy performance. If you decide to build such a wall, I recommend that you use plywood sheathing rather than OSB sheathing.

52.
Thu, 03/01/2012 - 14:01

High R wall for Upstate NY
by Elizabeth Kormos

That is pretty much the wall we are planning. I generally don't like OSB (I specified plywood on my current home). My husband does mold assessments and says that wet OSB is very susceptible to mold because the wood fibers are broken.

We are considering using the zip system since you can seal the seams easier than with plywood and it is sealed. Do you see any downside to using Zip?

53.
Thu, 03/01/2012 - 14:35

Edited Thu, 03/01/2012 - 14:44.

Zip sheathing
by Martin Holladay, GBA Advisor

Elizabeth,
Zip sheathing is a type of OSB. It's up to you to determine what type of sheathing you prefer.

Here's an article on the "cold OSB" debate: How Risky Is Cold OSB Wall Sheathing?

54.
Thu, 03/01/2012 - 22:21

Worried
by Elizabeth Kormos

Now you have me worried. Do I run a risk of moisture collecting on the interior (not coated) side of the zip sheathing from interior moisture with the 12" walls?

If we use plywood, as you recommend, for the sheathing, how do we effectively seal the seams? I want a tight house.

55.
Fri, 03/02/2012 - 06:09

Sealing plywood seams
by Martin Holladay, GBA Advisor

Plywood seams can be effectively sealed. For more information on tapes, see Air-Sealing Tapes and Gaskets.
One product recommended for sealing plywood seams is 3M All Weather Flashing Tape (8067) . The two importers of European tapes, Small Planet Workshop and Four Seven Five, sell high quality tapes suitable for plywood seams.

56.
Fri, 06/29/2012 - 11:48

What did you choose?
by Christian Butler

Elizabeth-

Which exterior wall system did you wind up choosing? I'm working on a project in Denver, CO, and am considering a wall similar to the one you described. I'm curious if you decided to use ZIP sheathing or not.

57.
Thu, 11/08/2012 - 10:39

How strict are the climate zone requirements?
by Jon Lobo

Hi Martin,
I'm in Bennington Co VT in the SW corner of the state. I'm technically in zone 6, but I'm a few miles away from zone 5. My builder designed 1.5" R9.8 foam board over 2x6 walls with fiberglass insulation. I see from the chart that zone 6 should have R11.25 and zone 5 should have R7.25 over 2x6 walls. It looks like I'm just on the edge. Do you foresee moisture problems with this setup?

Thanks for your help.

58.
Thu, 11/08/2012 - 10:47

Edited Thu, 11/08/2012 - 10:48.

Response to Jon Lobo
by Martin Holladay, GBA Advisor

Jon,
The recommendations in the building code (and in this article) result in a safe wall assembly that is relatively robust. These recommendations take into account variations in wall construction quality and variations in indoor humidity levels.

If your wall leaks air, the risk level rises. If your interior humidity level is high, your risk level rises.

If you build the wall you describe, you can lower your risk by (a) paying attention to airtightness when installing your drywall (in other words, following the Airtight Drywall Approach), and (b) maintaining a reasonable indoor humidity level in the winter (in other words, don't install a humidifier, and remember to use your exhaust fan when you take a shower).

59.
Thu, 11/08/2012 - 11:58

interior walls
by Jon Lobo

Thanks for the quick response Martin. So breathable interior wall is good because it facilitates drying, but big air gaps in interior wall are bad because they allow hot air to reach cold sheathing? I would think that a bit of convection in wall helps drying, but overall it's a net minus?

60.
Sat, 12/22/2012 - 16:31

Edited Sun, 12/23/2012 - 23:39.

exposed floor - Climate Zone 7
by Mark Fredericks

I'm looking to insulate the underside of a porch and I wonder how these wall figures apply to an exposed floor.
The porch is conditioned space that's attached to the main house and sits on two concrete piers, so the underside of the floor is exposed. On the inside of the house, this floor area is very cold in the winter. I have enough foam from a previous project to apply 4" of continuous EPS (R 14) to the bottom of the porch joists and I'm unsure if I should add batt insulation between the joists, or if I should buy more foam? Is an exposed floor any different than an exterior wall?

61.
Mon, 12/24/2012 - 07:44

Response to Jon Lobo
by Martin Holladay, GBA Advisor

Jon,
Q. "So breathable interior wall is good because it facilitates drying, but big air gaps in interior wall are bad because they allow hot air to reach cold sheathing? I would think that a bit of convection in wall helps drying, but overall it's a net minus?"

A. What you call a "breathable" interior wall is more accurately called a wall that is vapor-permeable on the interior. This is very different from a wall with air leaks. It's possible (in fact, it's desirable) for a wall finish to be vapor permeable but also airtight.

You are correct that air leaks in your wall assembly are a "net minus." The energy waste and the potential for moisture buildup during the winter are much worse than any theoretical benefits.

The one part of your assembly where you can use air movement to your advantage is in the ventilated air gap between your siding and your sheathing. This type of air movement is not associated with energy losses or any problems from moisture buildup in your wall assembly.

62.
Mon, 12/24/2012 - 07:50

Response to Mark Fredericks
by Martin Holladay, GBA Advisor

Mark,
Q. "I'm looking to insulate the underside of a porch."

A. I suggest that you read the following article: How to Insulate a Cold Floor.

Q. "I wonder how these wall figures apply to an exposed floor."

A. The same principles apply, in theory. However, in practice, you are unlikely to have moisture problems in a floor, because the stack effect prevents air leaks from carrying interior moisture into your floor assembly. That's why the use of thinner foam than recommended in this article shouldn't lead to any floor moisture problems.

Q. "I have enough foam from a previous project to apply 4" of continuous EPS (R 14) to the bottom of the porch joists and I'm unsure if I should add batt insulation between the joists, or if I should buy more foam?"

A. Go ahead and install the batt insulation if you want; it will do no harm. If you want to buy more foam, that is also a good idea. In either case, the most important thing you can do is pay attention to air sealing.

63.
Thu, 01/17/2013 - 20:15

Edited Thu, 01/17/2013 - 20:20.

Zip R Sheathing
by keith miller

does it create plywood drying issues?
should you put a tyvek type product over it?
would it work then with fiberglass batts or would that allow air to pass to the interior side of the foam and not dry out in the wall bays?

64.
Thu, 01/17/2013 - 20:32

Response to Keith Miller
by Martin Holladay, GBA Advisor

Keith,
Yes, many people have used it.

It isn't plywood. It's a type of engineered wood resembling OSB, sandwiched to a layer of foam.

The R-value isn't much. There are two versions -- one is R-3.6, and the other is R-6.6. Unless you choose a sheathing product that meets the minimum R-values recommended in this article, your wall could have moisture issues. In other words, Zip-R sheathing is a warm-climate product, not a cold-climate product.

65.
Sat, 02/23/2013 - 23:19

Best Insulation for Climate Zone 4 - KS?
by Scott Rock

I am having a house built and trying to determine the best approach for exterior wall insulation. I'm in Kansas, climate zone 4, but about 60 miles from zone 5. We are using 2x6 walls. The three options I'm looking at are (1) flash & batt with 1/2" closed sprayed foam and R19 batt, (2) 1/2" exterior foam -- Dow SIS or Rmax Thermasheath SI -- and R19 batt inside wall, or (3) same as #2 but with 1" foam. I'm curious to know which method you think would be the best? According to your article, foam thickness doesn't matter in my climate, so would either of the 1/2" options be just fine and have no condensation concerns? Unfortunately very few builders and suppliers have used exterior foam so I can't find much good information locally. Thanks!

66.
Mon, 03/04/2013 - 17:23

Response to Scott Rock
by Martin Holladay, GBA Advisor

Scott,
Q. "Which method you think would be the best?"

A. Of the options listed, I would choose 1 inch of exterior rigid foam.

Be sure to pay attention to airtightness; I recommend the use of a blower door to track down air leaks.

67.
Wed, 03/06/2013 - 15:38

Unvented roof
by Michael Gartley

Martin,
At the end of this article you talked some about the code requirements for an unvented roof. I am a little confused about the way you defined the properties of insulation in the joist bays. You stated that code requires rigid foam above the sheathing, and "air-permeable" insulation in the joist bays. You went on to quote the IRC 2009 definition of "air-impermeable" insulation. Would you mind giving a little more clarification on that?
Which is the preferred method inside the joist bays, permeable or impermeable? Thanks for leading all of these helpful discussions.

68.
Mon, 05/20/2013 - 19:02

Edited Mon, 05/20/2013 - 19:08.

LowR XPS sheathing error on garage
by Mike Gagne

Martin,

I am in the process of converting a single detached garage into a workshop/storage venue, Winnipeg MB Canada, so Zone 7 I think (2 hrs north of Grand Forks ND). On one long wall adjacent to my neighbour's garage (2.5 foot separation between structures), I put building paper (i had 30 lb asphalt felt) over 5/8 ply, 1/2 in XPS (R3-5) to minimize thermal bridging, furring strips, then metal siding. The wall is 2x4 but with 2x horizontal bracing i had intended to put 1.5 in xps against the studs equal to the bracing but will just stick to bats now so it will be closer to 2x6 insulation (roxul or fibreglass pink -to be confirmed yet) covered by firestop drywall (fire code in wall next to neighbours garage as it is within 4 feet) then painted osb (to make shelf construction easier and so the drywall doesn't get too banged up). Thankfully I read your articles and blogs, and reread some of the building science.com stuff before doing the other 3 walls.

Before I deconstruct the exterior to either add rigid XPS to an R15 or remove the too-thin layer I have (most likely option) would it be an acceptable risk to just leave this wall as is even with the non -desireble thin foam sheathing given that the structure will be only be heated to sweater temp no more than 16 hrs a week max during nov-feb as a workshop (ie I'm not living in it).

For the other walls, is it acceptable to just skip the external xps insulation and go with a more traditional, batt insulation in the 2x4 cavity, 1.5 in XPS, then poly (not sure if this is required other than the code says it is here)' then osb?

I know I am over- thinking this garage/man cave thing a bit much, but the idea that the cold sheathing will frost up if (when) heat/vapour moves through the wall and contact it makes absolute sense and I really wish I had figured this out before now.

Thanks

Mike

69.
Tue, 05/21/2013 - 07:10

Response to Mike Gagne
by Martin Holladay, GBA Advisor

Mike,
Q. "Would it be an acceptable risk to just leave this wall as is even with the non-desirable thin foam sheathing given that the structure will be only be heated to sweater temperature no more than 16 hours a week max during Nov-Feb as a workshop?"

A. Yes, that is acceptable.

Q. "For the other walls, is it acceptable to just skip the external XPS insulation and go with a more traditional, batt insulation in the 2x4 cavity, 1.5 in XPS, then poly (not sure if this is required other than the code says it is here), then OSB?"

A. Yes. As long as this is a garage, and it is not converted to living space, I think that would be fine.

70.
Tue, 05/21/2013 - 18:42

Thanks
by Mike Gagne

Martin,

Thanks. Your website is extremely helpful, informative, and practical. It contains a wealth of knowledge and people like me can learn a little from your experience and expertise. Firstly, just to be clear about the wall in the first Q with the inadequate r3 XPS - should I skip the 1.5 inch XPS and poly on the interior beneath the gypsum to allow in to dry inwards? And second, I confirm that it is definitely not to be living space.

71.
Wed, 05/22/2013 - 05:35

Response to Mike Gagne
by Martin Holladay, GBA Advisor

Mike,
Q. "Should I skip the 1.5 inch XPS and poly on the interior beneath the gypsum to allow in to dry inwards?"

A. Yes. But don't worry in either case -- if the room isn't generating a lot of moisture, everything should be fine.

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Calculating the Minimum Thickness of Rigid Foam Sheathing