musingsheader image
9 Helpful?

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 Table R702.7.1.)

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.

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 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 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.

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

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

Mar 1, 2012 9:51 AM ET

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

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.

Mar 1, 2012 2:01 PM ET

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?

Mar 1, 2012 2:35 PM ET

Edited Mar 1, 2012 2:44 PM ET.

Zip sheathing
by Martin Holladay

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?

Mar 1, 2012 10:21 PM ET

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.

Mar 2, 2012 6:09 AM ET

Sealing plywood seams
by Martin Holladay

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.

Jun 29, 2012 11:48 AM ET

What did you choose?
by C Butler


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.

Nov 8, 2012 10:39 AM ET

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.

Nov 8, 2012 10:47 AM ET

Edited Nov 8, 2012 10:48 AM ET.

Response to Jon Lobo
by Martin Holladay

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).

Nov 8, 2012 11:58 AM ET

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?

Dec 22, 2012 4:31 PM ET

Edited Dec 23, 2012 11:39 PM ET.

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?

Dec 24, 2012 7:44 AM ET

Response to Jon Lobo
by Martin Holladay

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.

Dec 24, 2012 7:50 AM ET

Response to Mark Fredericks
by Martin Holladay

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.

Jan 17, 2013 8:15 PM ET

Edited Jan 17, 2013 8:20 PM ET.

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?

Jan 17, 2013 8:32 PM ET

Response to Keith Miller
by Martin Holladay

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.

Feb 23, 2013 11:19 PM ET

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!

Mar 4, 2013 5:23 PM ET

Response to Scott Rock
by Martin Holladay

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.

Mar 6, 2013 3:38 PM ET

Unvented roof
by Michael Gartley

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.

May 20, 2013 7:02 PM ET

Edited May 20, 2013 7:08 PM ET.

LowR XPS sheathing error on garage
by Mike Gagne


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 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.



May 21, 2013 7:10 AM ET

Response to Mike Gagne
by Martin Holladay

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.

May 21, 2013 6:42 PM ET

by Mike Gagne


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.

May 22, 2013 5:35 AM ET

Response to Mike Gagne
by Martin Holladay

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.

May 27, 2013 10:42 AM ET

Great White North
by Shawn L


Thanks for the great article! You may have saved me a significant amount of headache in the future.

I am replacing the siding on a house built in 1980 in Calgary Canada (I assume climate zone 6 or 7? It is very dry here). The house has poly on the inside of the wall behind the drywall.

I would like to install a cement fiber board over some rigid foam however the manufacture of the fiber board dictates a maximum foam thickness of 1”. Am I going to run into problems with only 1” (R6) of foam in my area?

Will installing a product like drain wrap between the OSB and the foam help mitigate any condensation problems?

Another option is to install 1.5” of foam with furring on the outside of the foam. The additional thickness may present some issues for installation however. Will 1.5” of foam be acceptable for my area?

Thanks again for all of your great advice.

May 27, 2013 10:55 AM ET

Response to Shawn L
by Martin Holladay

You should use the table on this page, and you should figure that you are in Climate Zone 7. I don't know whether your house has 2x4 walls or 2x6 walls.

In either case, you're clearly going to need more than 1 inch of foam. In order to install fiber cement siding, you will need to install furring strips.

More information here: How to Install Rigid Foam Sheathing.

The article includes many additional links that will help answer almost any question related to exterior foam sheathing.

Jun 18, 2013 7:32 PM ET

Foam insulation to retrofit a cathedral roof
by Cliff Stainsby

I live on Vancouver Island in British Columbia. My house was built in the 1980's and has a cathedral roof which suffers the problems you identify in your article "How to Build and Insulated Cathedral Ceiling'; it allows condensation to drip into the living room. It has 2x10 rafters. The vapour barrier, just above the drywall, is shot. The roof, also, has insufficient insulation which is fibreglass bats.

Will my roof work if I remove the fibreglass insulation, put blocking between the rafters over the walls (so no air movement from the soffits), put in 3 or 4 layers of rigid foam, put 2x4 strapping across the rafters, then cover the rigid foam with 2 inches of closed cell spray foam which will cover the rafters except where they intersect the 2x4 strapping, and then fasten steel roofing to the strapping?

I am not a builder or carpenter and have been reading and asking question until my head is spinning, so an answer will likely help both my roof and my head.

It has also been suggested that I put a radiant barrier (from Radiant Technologies) in with an air space on either side. I cannot find any support for this idea and so, unless you suggest otherwise, I will ignore this suggestion.

Jun 19, 2013 6:37 AM ET

Edited Jun 19, 2013 6:38 AM ET.

Response to Cliff Stainsby
by Martin Holladay

It's unclear from your description whether you intend to install spray foam above the rigid foam or below. I'm going to guess that you will be opening up the roof from above, and that you intend to install the spray foam above the rigid foam.

Your plan will work, as long as the total R-value of the rigid foam plus the spray foam is adequate for you climate zone. I don't know the requirements of your local Canadian building code, but I would imagine that you want at least R-38.

Most roofers would feel more comfortable if you installed a plywood deck on top of your 2x4 strapping, and then a layer of asphalt felt on the plywood. Roofing underlayment (asphalt felt) is required by most building codes.

Your roof assembly does not need a radiant barrier.

Jun 19, 2013 4:08 PM ET

by Cliff Stainsby

Hi again
Just a brief note to say thank-you. You guessed correctly about the approach I intend to use and you have made me feel much more comfortable about it.

Jul 1, 2013 5:48 PM ET

Foam insulation to retrofit a cathedral roof
by Cliff Stainsby

Hi again
I don’t wish to overstay my welcome. However, the first idea I presented to you, which you thought would work fine, turns out to be considerably over my budget. I am retired on a partial pension.
I am trying to repair a cathedral ceiling/roof and I wonder whether you think the following approach would work.
The rafters are 2*10, so I have approximately a 9.5 inch cavity to work with; I am working toward an unvented roof.
I propose to:
1. place 2*10 blocking between the rafters over the outside walls
2. apply a 2 inch layer of spray foam between the rafters and up the inside of the blocking (to seal the lower ends), which apparently provides a complete vapor and air barrier as well as more than R12
3. put in 7.5 inches of high density fiberglass batts
4. cover the rafters with 1.5 inches of ISO board
5. cover that with ½ inch plywood, titanium 30 underlay (or something similar), and a steel roof.
This should give us an R50 (49.9) roof.
Do you think this will work? If the spray foam and blocking are applied properly is moisture likely to be an issue? Should I seal the ridge or leave a ridge vent? If a ridge vent, should it just be for the plywood and steel, or should it continue through the ISO board to allow moisture (if any) to escape from the fiberglass?
If these questions appear silly, please remember I am an amateur, not a roofer, not even a carpenter.
Thank you again.

Jul 2, 2013 5:56 AM ET

Response to Cliff Stainsby
by Martin Holladay

The most serious problem is your suggestion to install rigid foam above the rafters. That won't work. The rigid foam needs to bear on a continuous surface like plywood or OSB. If you put it on top of your rafters, it will compress.

However, your suggested solution will work as long as you reverse the order of the rigid foam and the plywood. So, once you have insulated your rafter bays, you install plywood on top of your rafters, followed by roofing underlayment (asphalt felt or synthetic underlayment). Then you install a layer of rigid foam. Then you install your purlins or furring strips (1x4s or 2x4s, parallel to the ridge -- usually 24 inches on center), and then your steel roofing.

Jul 2, 2013 12:10 PM ET

by Cliff Stainsby

Thanks again Martin. Very helpful. I expect you saved me some grief.
Any comment on the question about a vent at the peak -

Should I seal the ridge or leave a ridge vent? If a ridge vent,r should it continue through the ISO board and plywood to allow moisture (if any) to escape from the fiberglass?
ps. I sure am glad I found your site as in it I find an endless series of interesting articles. I have spent much of my life fighting climate change and promoting energy efficiency, so your site is like manna from heaven for me.

Jul 2, 2013 12:17 PM ET

Edited Jul 2, 2013 1:11 PM ET.

Response to Cliff Stainsby
by Martin Holladay

The roof assembly you have described is an unvented roof assembly, so you don't need a ridge vent. For more information on this type of roof assembly, see How to Build an Insulated Cathedral Ceiling.

Jul 28, 2013 1:28 AM ET

Edited Jul 28, 2013 5:24 PM ET.

Drywall, poly, 2x4, fiberglass batt, 3/8 sheathing, +2" iso ???
by Iain Matheson

I've done some reading and it appears I'm in a bit of a spot. the poly is already in the wall.

1973 house, with the a fore mentioned wall construction, plus building paper over the 3/8 exterior sheathing, I want to put 2" polyiso board, strapped with 3/4 strapping and then Hardie siding. the Iso board I can get locally is for roof construction and has tiny wholes in the paper on both sides, No foil, and is supposed to breath (somewhat, not sure how much, but am looking into it) what are your thoughts on this for a north west, marine coast, Olympic peninsula, environment,
thank you

Aug 1, 2013 2:20 PM ET

Response to Iain Matheson
by Martin Holladay

Many energy experts have worried whether it's a good idea to install exterior foam on a house with interior polyethylene. Although it would be better if the poly wasn't there, the fact is that tens of thousands of Canadian homes with interior poly have been retrofitted with exterior rigid foam, and there haven't been any reports of widespread problems. According to building scientist John Straube, all indications show that these retrofits are "not so risky as most people think. These homes will probably be fine."

That said, the installation of exterior foam is not advised on any home that has suffered wet-wall problems like leaking windows, condensation in stud cavities, or mold. If you plan to install exterior foam during a siding replacement job, keep an eye out for any signs of moisture problems when stripping the old siding from the walls. Investigate any water stains on housewrap or sheathing to determine whether the existing flashing was adequate.

If there is any sheathing rot, determine the cause -- the most common cause is a flashing problem, but condensation of interior moisture is not impossible -- and correct the problem if possible. If you are unsure of the source of the moisture, hire a home performance contractor to help you solve the mystery.

If your sheathing is dry and sound, I don't think you need to worry about adding exterior foam. Adding a rainscreen gap will certainly go a long way toward avoiding future moisture problems. Of course, it's important to be meticulous with your details when you are installing your new WRB and window flashing. It's also important to keep your interior relative humidity within reasonable levels during the winter. Never use a humidifier.

To summarize, here are four caveats:

1. Be sure that your foam is thick enough to keep the wall sheathing above the dew point in winter. Read more on this topic here: Calculating the Minimum Thickness of Rigid Foam Sheathing.

2. When the siding is being removed, inspect the existing sheathing carefully for any signs of water intrusion, and correct any flashing or housewrap problems.

3. Install rainscreen strapping so that there is a ventilated gap between the new exterior foam and the siding.

4. Keep your interior humidity under control during the winter; if the interior humidity gets too high, operate your ventilation fan more frequently.

Sep 11, 2013 4:42 PM ET

exterior foam
by aaron whiteman

I am in zone 5 2x6 walls batt insulation osb sheathing and housewrap. My budget is shot and I can only afford 1" extruded foam board instead of 1.5". Would it be better to use 1" or nothing at all. Vinyl siding on exterior. Certainteed voids the warranty if I use a rainscreen. Siding must have 100 percent contact with wall.


Sep 11, 2013 5:01 PM ET

Response to Aaron Whiteman
by Martin Holladay

If you choose foil-faced polyiso instead of XPS, you'll get R-6 without an air space, or R-7.5 if you use furring strips on the exterior side of the polyiso. Either approach is pretty close to what you need, although it's best if you can go with the furring strips.

CertainTeed is nuts to void the warranty when furring strips are used. You might just go ahead anyway -- the warranty isn't worth much. More about vinyl siding and furring strips here: Can Vinyl Siding be Applied Over Furring Strips?

Sep 12, 2013 6:06 AM ET

Edited Sep 12, 2013 8:57 AM ET.

Exterior Foam
by aaron whiteman

If I am unable to find 1" polyiso in my budget would using the 1" xps be better or would it be safer to not use any foam. I am right in between Erie and Pittsburgh. I am not sure about siding on furring strips without filling in between. The Certainteed rep told me that the siding will become wavy when it starts to expand and contract as well as I will see "zebra lines", a term which they used to describe when there is dew on the outside of the house and is noticeably different on the siding where it lays on the furring strip as opposed to where there is no contact. Right now I am just worried about the foam.

Thanks again,

Sep 12, 2013 6:30 AM ET

Response to Aaron Whiteman
by Martin Holladay

Only you can make this decision.

The right answer is that the minimum R-value you need for your exterior rigid foam is R-7.5. That doesn't mean that your walls will rot if you decide to install R-5. It means that you have chosen a risky wall assembly.

If your indoor relative humidity stays low, your walls will probably be OK. But if you choose to build a wall assembly that is risky, you have to be willing to accept the risk that accompanies your decision.

Oct 14, 2013 2:48 PM ET

values for 12" cellulose walls
by michael pi

This table is specific to 2x4 and 2x6 walls, but I'm wondering if it is possible to extrapolate an amount of exterior foam insulation necessary for 12" dense packed cellulose walls from the information here?

The recommended R values increase from 4" to 6" walls, so does it also increase for 12" walls, or does this not necessarily follow?

Oct 14, 2013 3:06 PM ET

Edited Oct 14, 2013 3:10 PM ET.

Response to Michael Pi
by Martin Holladay

It's usually a bad idea to try to put foam on the exterior of a double-stud wall filled with 12 inches of cellulose. As you guessed, the higher the R-value of the insulation between the studs, the thicker your foam has to be. By the time you get to 12 inches of cellulose, the foam gets ridiculously thick -- so thick that it is hard to attach.

If you want to do the necessary calculations to determine how thick your foam needs to be, this article explains how to do the calculations: Are Dew-Point Calculations Really Necessary?.

Soon, it becomes clear that if you are going to have very thick exterior foam, it makes sense to keep your stud bays empty -- that is, to put all of of your insulation on the exterior side of the wall sheathing. This is an excellent approach; it is called the PERSIST method. For more information on PERSIST, see Getting Insulation Out of Your Walls and Ceilings.

It's a little unclear to me why you want to put any insulation on the exterior of a 12-inch-thick wall insulated with cellulose. Do you just want more R-value? If that's the reason, why not just build a 14-inch-thick wall or a 16-inch-thick wall and fill it full of cellulose?

The idea with a thick cellulose-insulated wall (a classic double-stud wall) is to keep the exterior sheathing relatively vapor-permeable, to encourage drying to the exterior. So -- usually -- no exterior foam for this type of wall.

Oct 15, 2013 9:24 AM ET

Edited Oct 15, 2013 9:30 AM ET.

Response to Martin Holladay
by michael pi

Martin - Thanks, that's what I thought.

I don't want to put foam on my walls per se, but it was one strategy outlined in numerous of the great studies discussed here about how to deal with potential moisture migration issues. I thought this model wasn't recommended for thicker walls, and was validating that.

It's currently my understand that using plywood or fiberboard sheathing with a vented rainscreen gap along with airtight drywall looks like the best approach in this scenario (upstate NY zone 5 right on border of zone 6). I had read the Straube 2009 research which pointed to this.

More recently, I discovered and read the NREL Moisture Research - Optimizing Wall Assemblies paper.

There is a section for high R walls, including a double stud dense packed cellulose wall (evaluated in zones 5a and 7). They modeled with and without vented cladding. Without vented cladding, the wall I'm looking at increased in moisture over a 3 year period, but dried out with it. Then there is additional discussion of condensation potential which i don't entirely follow. And then section 4.2.4 including "all the high-R walls fail this criterion by a significant margin when the ASHRAE 160-2009 interior conditions with cooling are assumed. All the cases fail the 30 day running average." 4.2.5 - "For all high-R walls, the ASHRAE 160-2009 indoor conditions result in isopleths that indicate the potential for mold on both the interior surface of the drywall an the interior face of the first condensing surface."

They conclude by discussing vented cladding being essential, but i'm not clear on whether and in what conditions the concerns/failures above their concluding discussion continue to be relevant, versus in what cases they are solved by using vented cladding. This is undoubtedly just due to gaps in my subject matter expertise here. But some of this was concerning to me, and I intended to follow up with these study authors to gain some clarity.

Aside from clarifying some of the points in the study above, I suppose my overall concern is whether this wall is "high risk" - in the sense that it wouldn't react well to some sort of failure like a big leak, tape or caulk (or other "mechanical") failures, if the rainscreen gap somehow got plugged up or compromised, etc. At the end of the day though - not sure this matters because i don't know what a lower risk wall would be. All high insulation strategies seem to have potential for failure or risk if bad stuff happens. Not sure one has less than another or is in some way more adaptable to murphys law.

Oct 15, 2013 10:38 AM ET

Response to Michael Pi
by Martin Holladay

As long as you choose plywood sheathing, board sheathing, or fiberboard sheathing, and as long as you include a ventilated gap between the sheathing and the siding, I don't think that cellulose-insulated double-stud walls are risky.

For further discussion of the issues you raise, see How Risky Is Cold OSB Wall Sheathing?

Nov 1, 2013 4:20 PM ET

Further response to Michael Pi
by Martin Holladay

The issues you raise are discussed further in this article: Monitoring Moisture Levels in Double-Stud Walls.

Dec 4, 2013 11:47 AM ET

IECC 2012 C402.2
by Nathan Kipnis, FAIA

My current project is in ASHRAE Zone 5. As I pulled up the table in reference from the code, I noticed the code also states that for 'wood framed or other' walls, R20 + R3.8ci is allowed. What is your experience with this portion of the code and moisture issues.

Dec 5, 2013 6:55 AM ET

Response to Nathan Kipnis
by Martin Holladay

Building code regulations provide the minimum specifications that you can build to without being arrested or sued. Building codes do not define best practices.

If you install exterior rigid foam on your walls, you should follow the guidelines provided in the table in this article. The only way you could get away with R-3.8 continuous insulation is if you made sure that the insulation is vapor-permeable (for example, by using R-3.8 of mineral wool insulation on the exterior).

Dec 10, 2013 1:18 AM ET

Mineral Wool sheathing
by Pete Marthaler - Zone 7

Thank you for keeping this discussion thread alive.
I am considering using Mineral Wool insulation (Roxul Comfortboard IS) in a retrofit scenario rather than foam, as my walls were built using interior poly (Climate zone 7) and I want to maintain the ability to dry to the exterior.
I see that the header for Table N1102.5.1 in the article states that it pertains to foam sheathed walls. Is it the same if I were to use mineral wool insulation? As I understand it, the stated R value in the table is to try to minimize the moisture accumulation in the exterior structural sheathing and thus could be applied similarly to vapor open insulation that is applied exterior to the structural sheathing. I am aware that I would need to employ a rain screen for the siding installation. Sincere thanks in advance.

Dec 10, 2013 8:37 AM ET

Response to Pete Marthaler
by Martin Holladay

The recommendations in my article and the requirements in Table N1102.5.1 refer to exterior rigid foam. They do not apply to mineral wool insulation.

If you want to install exterior mineral wool insulation, thick insulation is still better than thin insulation, since thick mineral wool will keep the sheathing above the dew point (and therefore dry). If you choose to install thin mineral wool, however, the mineral wool has one advantage that rigid foam does not: it's vapor permeable, so that when the sheathing gets wet during the winter, it will be able to dry out to the exterior in the spring and summer.

On the other hand, mineral wool doesn't stop inward solar vapor drive. So if your wall assembly is susceptible to problems associated with inward solar vapor drive, rigid foam is a better choice.

Feb 5, 2014 10:19 PM ET

Aged Poly-Iso outboard + 'cold roof'
by Theodore Chabane

We are designing a three story house for Toronto. All insulation in the cathedral ceiling will be between the 12" , 24" o.c. rafters.. The roof will be built with a 1" vented airspace between the underside of the ply roof sheathing and 9" of poly-Iso foam topped with 1.5" of styrofoam. We were concerned about the ~1% (temp+time dependent?) shrinkage in new poly-Iso. However, we have access to perfect 12ft. x 4.5" poly-Iso slabs which are 11 years old. Can anyone direct us to information that quantifies shrinkage in this material vs. time? Has all of the time- dependent shrinkage already occured?

Lastly, where would you put the vapour barrier or air barrier?

Feb 6, 2014 5:59 AM ET

Response to Theodore Chabane
by Martin Holladay

It sounds like you are planning to insulate your cathedral ceiling with the cut-and-cobble method. I don't recommend this approach, because differential expansion and contraction between the rafters and the rigid foam can lead to air leaks.

For more information on the cut-and-cobble method, see Cut-and-Cobble Insulation.

For more information on foam shrinkage, see Using Rigid Foam As a Water-Resistive Barrier. (Look for the paragraph with the subhead "Do rigid foam panels shrink?" and read the sections that follow.)

The best way to use your rigid insulation is to install it as a continuous layer above the roof sheathing, rather than to cut it into strips and insert it between the rafters.

Feb 17, 2014 2:03 PM ET

Styro sandwiched between 2x4 walls.
by Kevin vesey

Hi all, I have posted this question/opinion in another thread. It also suits this thread and I would like all the feedback I can get.
I'm in zone 6a. I didn't include that in the original post.
I have also been considering your design 3 option for my build. I live in Ottawa which often sees -20c through the winter. My wall section would consist of:
house wrap (breathable drainage plane)
3/8 plywood
2x4 stud wall 16 oc with roxul batt. Framed to underside of floor decking.
1"-1-1/2" extruded styro with ship lap joints taped. Again to underside of floor decking. This will be my vapour barrier and thermal break.
2x4 stud wall 24" oc less joist height to support floor and run electrical. Will also insulate with fibreglass batt.
drywall and paint.
Note that I realize that the rule of thumb is 2/3 of the insulation should be to the cold side of the vapour barrier, and this design is close. I could easily achieve it by opting for 2x6 exterior, increased styro, or decreased batt to the inside. however I'm not to concerned as the design allows for drying to the warm side if or when condensation occurs.
My reasons for considering this design are:
-I will have quite a bit of exterior detail and I don't want the hassle of working over top and around insulation.
-I don't like the idea of a vapour barrier on the cold side of my wall for obvious moisture issues.
-I know that a continuous vapour barrier to the inside of a wall is virtually impossible after all the mechanical and fastening penetrations
-I like the fact that this design can dry to both the inside and outside.
With all the benefits that I see with this design I wonder why I can't find more about it. Is there something that I'm missing??? I realize that framing a second wall is more work but after factoring in the fact that you would have to strap over the entire outside of the house for something to fasten siding to (unless you plan to brick or stuco) I don't see that a second wall to the interior as such a big deal. In my opinion as a framer building walls is the fun part! If you have ever strapped and sided over styro you know that it's a pain in the a...
Maybe it's the r value that people are hung up on? But I would suggest that r means nothing if your walls leak... and something that I have encountered with these super tight wall systems is that the less leaks you have the greater concern they become. You can end up with a really high concentration of condensation where you have a small leak and most of these designs don't allow for the wall to dry in or out.
Does anyone have first hand knowledge of this design? or see an obvious flaw in my reasoning that I could be missing?

Feb 17, 2014 2:21 PM ET

Edited Feb 18, 2014 10:46 AM ET.

Response to Kevin Vesey (and others interested in the topic)
by Martin Holladay

Posting questions twice is confusing. For the sake of GBA readers, it's best if all of the answers (and the back-and-forth communication) are on the same page.

Anyone interested in following the thread raised by Kevin Vesey should post any comments or reactions on the page where Kevin originally posted. Here is the link: Wall design questions.

Feb 18, 2014 6:36 AM ET

Rigid Foam Sheathing
by Paul Fowler

I will start building a new house near St. John’s Newfoundland this coming spring. I cannot find anything about a climate zone so I will go with the US zone # 7 to be on the safe side. The wall itself will be constructed using 2x8 studs on 2x8 plates. ½ plywood on outside of stud and 3/8 on inside stud, creating a sealed air space. Cavity will have spider insulation. 2x3 stud wall for electrics and such, not sure if I will insulate this wall probably not. I will cover the outside ½ plywood with building wrap. Then fit Rigid Foam to R-15 followed by 1x3 strapping for rain screen and add spruce clapboard stained both sides for siding. My question, is the R-15 going to be able to keep this wall warm since it is a 2x8 and not a 2x6, if not what R value should I use? There is going to be no poly or any kind of retarder on the inner wall.

[Editor's note: to read the answer to this question, advance to Page 3 by clicking on the "3" below.]

Register for a free account and join the conversation

Get a free account and join the conversation!
Become a GBA PRO!