Rethinking the Rules on Minimum Foam Thickness

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Rethinking the Rules on Minimum Foam Thickness

Exterior rigid foam doesn’t always have to be as thick as the rules recommend

Posted on Nov 3 2017 by Martin Holladay

When builders ask for advice about installing rigid foam on the exterior side of a wall, I usually refer them to one of my articles, “Calculating the Minimum Thickness of Rigid Foam Sheathing.” The article explains that the R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. of the rigid foam layer needs to be high enough to keep the OSB or plywood sheathing above the dew point during the winter. For example, a house with 2x6 walls in Climate Zone 6 would need rigid foam with a minimum R-value of R-11.25.

Here’s the reasoning behind the advice: thinner foam would be risky, because the thinner foam doesn’t have a high enough R-value to keep the sheathing dry during the winter — but is still thick enough to limit the rate of outward drying during the spring and summer.

So far, so good: Just follow the rules and you’ll stay out of trouble.

Recently, however, building scientist Joseph Lstiburek — the guy who explained these rules to me in the first place, a decade ago — has been stirring the pot. In a 2016 article called Lstiburek wrote, “I grew up with this wall in Ontario [Climate Zone 6] — 2x6 wall with R-5 foam sheathing coupled with unfaced fiberglass batt cavity insulation covered with an interior 6 mil poly air/vapor barrier. This wall is currently being built all over Minnesota and Wisconsin. It is still being built in Ontario. And it works. We know that it works because we have been building it for so long without problems.”

In this article, I’ll explore the following question: If a 2x6 wall in Climate Zone 6 with R-5 exterior rigid foam works well, why have I been advising Zone 6 builders to install foam with a minimum R-value of R-11.25?

Three relevant issues

There are at least three factors to explain the disparity between my advice and Lstiburek’s “we know that it works” statement:

  • Over the years, Lstiburek has changed his advice on this issue somewhat — in emphasis if not in all details.
  • While most experts, including Lstiburek, have advised builders who install exterior rigid foam to omit interior polyethylene, it turns out that including interior polyethylene in this type of wall actually reduces risk — at least in some circumstances and climates.
  • Simple rules favoring robust wall assemblies are easier to explain and defend than complicated rules that include exceptions and qualifications — which means that even if Lstiburek is right (and he is), my rules might still make sense.

Lstiburek’s change in emphasis

As an example of Lstiburek’s change in emphasis, note these two passages:

: “Check out the walls [in the illustrations — namely, walls with exterior foam] … They all have ‘vapor barriers’ [foam insulation] on the exterior. They also can work everywhere in Canada (and the U.S.) because of the thermal resistance outboard of the condensing surfaces. Adding interior vapor barriers to these … walls is completely unnecessary and quite frankly risky. You want these walls to be able to dry in at least one direction. They can’t dry outboard due to the material properties of the insulation boards … So they need to be allowed to dry inward. No interior poly please.”

: “So houses with foam sheathing in cold climates with interior vapor barriers with good interior air sealing that are not humidified and pressurized to hospital and art gallery performance metrics work.”

Lstiburek’s 2013 advice was absolute: “No interior poly” on walls with exterior rigid foam.

Lstiburek’s 2016 advice was more nuanced: “Houses with foam sheathing in cold climates with interior vapor barriers [polyethylene] … work” as long as the builder follows three rules: (1) provide good interior air sealing; (2) don’t humidify the interior; and (3) don’t pressurize the interior with respect to the outdoors.

What about interior polyethylene?

All wall designs should strive to avoid problems from damp OSB or plywood wall sheathing. Most experts agree that there are two proven methods for avoiding problems with damp sheathing:

  • The old-fashioned approach: make sure that the sheathing can dry outward in the spring and summer. This approach depends on having no low-permeance barriers on the exterior side of the wall sheathing. Since most brands of housewrap and most types of siding are vapor-permeable, this approach is fairly easy to implement.
  • The newer approach described in my article (“Calculating the Minimum Thickness of Rigid Foam Sheathing”): keep the sheathing warm and dry by protecting the exterior side of the wall sheathing with a thick enough layer of rigid foam to keep the sheathing warm enough during the winter to avoid moisture accumulation.

Either approach works. Questions arise, however, when we try to analyze walls like the one discussed by Lstiburek: namely, a 2x6 wall in Ontario with foam that is “too thin.” This type of wall is at risk if the sheathing stays damp for a long time. Clearly, it’s important for builders who choose this wall to lower the rate at which interior moisture migrates toward the cold sheathing during the winter. To accomplish this, the builder must:

  • Ensure the interior side of the wall is relatively airtight, to reduce the rate at which interior air leaks out of cracks in the wall. (Exfiltrating interior air carries a lot of moisture with it.)
  • Install an interior layer that reduces outward vapor diffusionMovement of water vapor through a material; water vapor can diffuse through even solid materials if the permeability is high enough. through the wall; in decades past, many builders installed interior polyethylene to fill this role.

Paradoxically, the interior polyethylene that Lstiburek warned against in 2013 helps save the Ontario wall with too-thin foam. But what about the fact that (in 2013) Lstiburek called interior polyethylene “quite frankly risky”? Shouldn’t that warning worry builders?

As it turns out, maybe not. Everything is relative and climate-dependent.

So where is interior polyethylene risky?

The anti-polyethylene bias that Joe Lstiburek began sharing in the 1990s had its origin in a cluster of memorable wall failures that he investigated in Florida. Lstiburek had been called in to inspect hotels in Florida with 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. In some hotel rooms, the vinyl wallpaper was beginning to peel. Behind the vinyl wallpaper was a gruesome sight: damp drywall covered with mold.

To a building scientist like Lstiburek, the failure mechanism was obvious. In Florida, the outdoor air is hot and humid for most of the year. The interior side of these walls is kept cool by air conditioning. The direction of the vapor drive through these walls is inward — from the hot damp exterior to the cool dry interior. When the moisture hits the back side of the vinyl wallpaper, it condenses, soaking the drywall.

The moral to this story is clear: You don’t want interior polyethylene or vinyl wallpaper in Florida. In fact, Lstiburek warned, polyethylene can be risky in any building with significant hours of air conditioning. The only question concerns the extent to which builders can generalize this principle — “no interior polyethylene in buildings with significant air conditioning” — and the degree to which builders can apply the principle to areas of the continent that are colder than Florida.

In , Lstiburek included illustrations of three different wall assemblies with interior polyethylene. The caption read: “Figure 1a: The ‘Old Plastic Wall’ — The wall that kinda worked with fiberboard but not with OSB. Don’t use this wall with air-conditioning. Figure 2b: The ‘New Plastic Wall’ — The wall that works with OSB because of the ventilated claddingMaterials used on the roof and walls to enclose a house, providing protection against weather. . Don’t use this wall with air-conditioning. Figure 2c: High Performance Plastic Wall — This wall works well. Just don’t use it with air-conditioning.”

The message is clear: Interior polyethylene should only be used in a climate without any air conditioning. In a later passage in the same article, Lstiburek elaborated: “The good news is that the ‘polyethylene cult’ did not spread too far south of the border — only Minnesota and parts of Wisconsin seemed to get affected. The interior polyethylene sheet has a big liability in buildings that are air-conditioned — it results in a vapor barrier on the wrong side of the wall.”

Two things have changed since Lstiburek wrote those words: (a) Lstiburek has softened his hard line against interior polyethylene, and (b) Air conditioning has moved north, and can now be found in many parts of Ontario. Figuring out where these lines cross — the advisory line of Lstiburek’s softening injunctions and the geographical line of risk associated with increasing hours of air conditioner use — can be quite tricky.

Simple rules — or WUFI?

Here’s the bottom line:

  • My simple rules concerning the minimum recommended R-value for exterior rigid foam still work.
  • Walls with exterior rigid foam that is thinner than the recommendations in my article won’t necessarily fail. Walls with too-thin foam are made safer by including a good interior air barrierBuilding assembly components that work as a system to restrict air flow through the building envelope. Air barriers may or may not act as a vapor barrier. The air barrier can be on the exterior, the interior of the assembly, or both., some type of interior vapor retarder, and a method of keeping the indoor relative humidity in a reasonable (low) range during the winter.
  • While interior polyethylene helps these walls dry during the winter (by limiting outward vapor diffusion), interior polyethylene may create problems during the summer, especially if the house is air conditioned. The extent to which polyethylene is risky depends on many factors; one of these factors is climate, and the other factor is the thickness of the exterior rigid foam. In almost all climates, a so-called “smart” vapor retarder (a vapor retarder with variable vapor permeance, like MemBrain) is safer than interior polyethylene.
  • A wall with too-thin foam and some type of interior vapor retarder or vapor barrier may work or it may not. One way to analyze the performance of this type of wall is with a hygrothermalA term used to characterize the temperature (thermal) and moisture (hygro) conditions particularly with respect to climate, both indoors and out. software tool like WUFI. In the right hands, a WUFI analysis answers the question of risk. In the hands of an unskilled user of WUFI, however, this analysis results in an answer that is wrong and therefore worthless. (For more on this issue, see WUFI Is Driving Me Crazy.) That’s why I rarely advise builders to perform a WUFI analysis.
  • Most buildings are designed to last a long time, and builders have little control over how occupants will operate the home’s ventilation system or whether the occupants decide to install a humidifier. In light of these facts, builders should favor wall assemblies that fall on the robust side of the risky-to-robust spectrum.

For the time being, I’m sticking by my rules. Builders who follow my rules won’t get into trouble.

Martin Holladay’s previous blog: “Living Without Electricity.”

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Image Credits:

  1. Image #1: Fine Homebuilding

Nov 3, 2017 9:10 AM ET

flashing leaks
by Charlie Sullivan

In addition to the considerations mentioned, people considering an interior vapor barrier plus thin exterior foam should consider how confident they are in their window flashing. Water that gets in through a flashing leak will lead to persistent dampness if drying is impeded in both directions. Poor planning and poor execution of flashing are both rampant problems.

Nov 3, 2017 9:14 AM ET

ASHRAE Fundamentals or WUFI
by Armando Cobo

Learning to calculate wall and roof assemblies condensation through ASHRAE Fundamentals or WUFI, teaches you that it’s not just about the ratio of exterior and cavity insulation with in a climate zone, but also a correlation between R-values with indoor and outdoor temperature and relative humidity. I believe that the Code (2013 IRC R702.7.1) and any rules of thumb can be misleading if not completely understood.
If the Temperature and/or RH on the inside of your house is maintained much higher than your neighbor’s house (assuming both houses are built the same), the amount of exterior insulation will need to be higher, regardless of your climate zone. I don’t know many homeowners that are taught by their Architects/Builders/Realtors/HVAC Contractors the importance of maintaining proper temperature and RH in their houses, in relation to how the house was built.

Nov 3, 2017 11:44 AM ET

Building Science
by Malcolm Taylor

Joe's work, and that of John Straub have been game-changing in the way in which we look at and build houses. I do wish they were a bit less strident about their preferred methods and knowledge at any given time though. The whole "you are an idiot if you don't build our way" stuff isn't always helpful.

Nov 3, 2017 12:41 PM ET

> keep the sheathing above
by Jon R

> keep the sheathing above the dew point all winter long

Picking R19 insulation in a stud wall and R11.25 of exterior insulation, we get 37%.

Pick a simple temperature of 0F (zone 6 gets much colder than this).

.37 * 70F = 26F

70F air at 30% RH (lowest recommended level) has a dew point of 37F, much higher than 26F.

In this generous example, R11.25 of rigid foam fails to keep the sheathing above the dew point all winter long.

Nov 3, 2017 1:35 PM ET

Edited Nov 4, 2017 1:36 PM ET.

Interesting and seldom
by Jon R

Interesting and seldom discussed is Lstiburek's comment "If the outside is “leakier” than the inside we reduce the risk". I interpret this as:

a) the common practice of taped exterior sheathing or foam with no interior side air barrier increases risk.

b) As Martin writes, a smart vapor retarder and interior side air sealing would offset the need for some unknown amount of exterior foam (ie, maintain the same risk level).

I'll add that somewhat vapor permeable external insulation (eg, EPS) is lower risk than impermeable (eg, foil faced). Perhaps another offset here.

It would be great if there were a simple, proven rule such as "with a smart vapor retarder and interior side air sealing, one can reduce the recommend levels of foam by 50%". But until then, perhaps a simple approach for anyone wanting to use less than the recommended thickness of foam is to put it on the interior side. Tape it. Keep the exterior side perms high.

Even if one uses the recommended exterior thickness (IMO, this isn't infallible), still consider the other factors that further reduce risk.

Nov 3, 2017 2:28 PM ET

It's the mean winter temp, not the peak low @ JonR, Martin
by Dana Dorsett

"...keep the sheathing above the dew point all winter long..." is clearly wrong. It's the mean temp at the sheathing over weeks/months that matter. Wood sheathing can take on quite a bit of moisture as adsorb before it becomes problematic, and cycling moisture into & out of the sheathing when it's cold as the temperature varies doesn't really matter much.

Five perm paint (standard latex on gypsum board) limits the magnitude of the hourly, daily, weekly moisture load swings in the sheathing, but if the mean temp of the sheathing over say 10-12 weeks is sufficiently below the dew point of the conditioned space air enough moisture can accumulate to become a problem once warmer mold-growing temperatures arrive.

While 0F events (or even 0F daily highs during a cold snap) are common in US climate zone 6, mean January temps in zone 6 are always above 10F, and the mean temperature of the 10 coldest weeks is usually north of 20F, even at the cool edge of zone 6, warm edge of zone 7, such as Duluth, MN:

Get a bit deeper into zone 7 and the winter temperature averages are lower, requiring more exterior R for dew point control:

Nov 3, 2017 2:36 PM ET

The argument against active humidification @ Armando Cobo
by Dana Dorsett

"If the Temperature and/or RH on the inside of your house is maintained much higher than your neighbor’s house (assuming both houses are built the same), the amount of exterior insulation will need to be higher, regardless of your climate zone."

That is SO true!

I can't count the number of times I've had to tell people to get rid of their whole house Aprilaire on their hot air heating systems (or at least turn it down) to avoid the springtime mold or wintertime window condensation problems (or how much push-back I get!) Some seem convinced that 50% relative humidity is the "right" number all year round, and anything less isn't healthy or comfortable.

Nov 3, 2017 4:04 PM ET

If air can flow from the
by Jon R

If air can flow from the interior, to the exterior sheathing and then back to the interior, then large amounts of moisture could be deposited during periods where the sheathing is below the dew point - independent of interior side perms. An interior air barrier plays a role in wall moisture.

As Lstiburek says, building pressure is also a factor. Tripling the average pressure is similar to moving RH from 30% to 60%. Keep Winter monthly average pressure negative and I expect that some highly non-recommended walls would work well.

Nov 4, 2017 12:32 PM ET

Edited Nov 4, 2017 12:35 PM ET.

by Malcolm Taylor

I agree, and that brings up an interesting question: How resilient do you make a house to the behaviour of the occupants?

There must be a sweet spot between designing a house that can function whether the occupants control the humidity and pressure or not, and one that works only if the occupants carefully maintain certain conditions.

A corollary question is to what extent should a house rely on mechanical systems for the robustness of its basic structure?

Nov 4, 2017 1:10 PM ET

Edited Nov 4, 2017 1:17 PM ET.

I think that control of
by Jon R

I think that control of important parameters should be automated, with alarms for when they malfunction. But this isn't new ground. Consider:

crawl space exhaust fans and dehumidifers
bathroom/shower fans
radon and HRV fans (important even if not a structure issue)
accidentally sealing off a furnace return (causing a perhaps 10x increase in room pressure and much more wall moisture)
FPS foundations that rely on building heat

Nov 4, 2017 3:20 PM ET

by Malcolm Taylor

Addressing the downside of relying on automated controls with alarms sounds like a really good idea.

Nov 8, 2017 9:41 PM ET

Is a foam with perm rating of
by Mai Tai

Is a foam with perm rating of 3.48 enough to make a difference and help moisture migrate to the outside? The assembly is a R-19 2x6 wall (R-19) with 2" R-10 foam in Zone 5, so it should be "safe" regardless, but it would be nice to have extra safety margin. According tho the manufacturer the 2" EPS foam is foil faced with micro-perforations, making it a decent water barrier while still being vapour permeable.

I investigated rock wool insulation, as I know this is safe (extra permeable), but it is prohibitively expensive in my area vs EPS.

Nov 9, 2017 12:05 AM ET

Edited Nov 14, 2017 3:44 PM ET.

breathable is better
by Jon R

In a cold climate, 1.74 perms (3.48/inch @ 2") to the exterior is better than say .03 perms to the exterior. In your "safe" case (although R10 is suspect), it might be reasonable to use WUFI to quantify the difference.

This isn't well researched, but I interpret page 9 to suggest that if you have less than 1 perm on the interior side (eg a smart retarder) and > 1 perm to the outside (eg <= ~2" of EPS and the right sheathing), then any amount of external foam is safe. Similar to any amount of Roxul being OK on the exterior.

Nov 10, 2017 10:58 AM ET

Laws of physics don't change
by Bruce Fergusson, CIH, PE

Thank you for what in IMHO is a balanced examination of the issues surrounding this construction detail and it's effects. FWIW, I think Joe L and John S perceived stridency is probably born of trying to stem the "tide" of incorrect "good-ole-boy" construction traditions and the resultant push-back. I laugh and tell clients that I know all the wrong ways to do things, being brought in after things "break." So I applaud the guys out there trying to build them right and participating in discussions like this. Thank you!

Nov 10, 2017 4:15 PM ET

The laws of physics are also self-enforcing... @ Bruce
by Dana Dorsett

... and there's no enforcement discretion that mere humans can apply.

Now if only some of the folks making policy had a clue about that...

'nuff sed. ;-)

Nov 18, 2017 6:15 AM ET

Response to Dana Dorsett (Comment #6)
by Martin Holladay

You're right that I shouldn't have used the phrase "keep the sheathing above the dew point all winter long," since the phrase is technically inaccurate. Thanks for keeping me on my toes. I have edited the article to reflect your point.

Nov 19, 2017 3:39 AM ET

Roxul Comfortboard?
by Stephen G

Am I wrong here, or would Roxul Comfortboard not work way better for this type of application? Or, at least, it's a lot more vapor permeable. If you're covering it with an air barrier like tyvek it would give the wall a lot of drying potential to the exterior without having to be worried about ensuring the wallboard has sufficient insulation to keep it above the dew point. You can get your thermal break cake, without having to eat the cost and complexity of 2 layers of 1.5" of EPS.

Nov 19, 2017 5:50 AM ET

Response to Stephen G
by Martin Holladay

Roxul Comfortboard (or other brands of semi-rigid mineral wool) is neither better nor worse than rigid foam. It's different.

Here's what I wrote in one of my articles, How to Design a Wall:

"Mineral wool insulation can be substituted for rigid foam insulation on the exterior side of wall sheathing. One advantage of mineral wool over rigid foam: because mineral wool is vapor-permeable, it doesn’t inhibit wall sheathing from drying to the exterior. That means that builders can install mineral wool of any thickness on the exterior side of their walls. You don’t have to worry whether exterior mineral wool meets any minimum R-value requirement. (Of course, thicker insulation always does a better job of resisting heat flow than thinner insulation.)"

It's true that you can use mineral wool of a lower R-value as continuous exterior insulation than is required for rigid foam. But if you go that route, remember that the sheathing will still be absorbing moisture in January and February. While the mineral wool will allow the damp sheathing to dry in April and May, it would probably be better to install a thick enough layer of continuous exterior insulation to prevent the moisture accumulation in the first place.

In other words, if you want to keep your sheathing dry, it's best to follow the minimum R-value rules, whether you are using rigid foam or mineral wool to provide the continuous exterior insulation. And rigid foam has a big advantage over mineral wool: it's less squishy, so it's much easier to install furring strips that are co-planar.

Nov 19, 2017 11:32 AM ET

Winter performance with thin mineral wool
by Charlie Sullivan


While I agree that following the minimum R-value rules with mineral wool will give more margin on keeping the sheathing dry than a thinner layer of mineral wool would, and in that sense it's best, I think your second to last paragraph above is a little overly pessimistic. Compared to s simple old-fashioned wall with sheathing on the outside, any thickness of exterior mineral wool will only reduce the mid-winter moisture accumulation in the sheathing. That's in contrast to a thin layer of foam, which can make things worse compared to no exterior insulation.

Nov 19, 2017 12:20 PM ET

Response to Charlie Sullivan
by Martin Holladay

I agree with your analysis. Thin mineral wool is better than no exterior insulation at all, and not quite as good as mineral wool (or rigid foam) that meets the minimum R-value levels designed to prevent significant moisture accumulation.

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