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Helpful? 2

The 2012 Code Encourages Risky Wall Strategies

Building codes promote “R20+R5” walls in cold climates — a recipe for mold and possible rot

Posted on Jul 18 2014 by Martin Holladay, GBA Advisor

Builders who follow the prescriptive requirements of the 2012 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.) in Climate Zone 6, 7, or 8 are required to install a minimum of “20+5 or 13+10” wall insulation. What does this mean? According to an explanatory footnote in the code, the “First value is cavity insulation, [and the] second is continuous insulation or insulated siding, so ‘13+5’ means R-13 cavity insulation plus R-5 continuous insulation or insulated siding.”

Here in Vermont (a Climate Zone 6 state), builders have been framing walls with 2x6s for at least 35 or 40 years. Nobody installs R-13 insulation in walls in Vermont, so the most likely way that Vermont builders will comply with this code provision is to install R-20 insulation between the studs and R-5 rigid foam insulation on the exterior side of the wall 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. .

While this approach meets minimum code requirements, it violates a tenet of good wall design: namely, that any rigid foam installed on the exterior side of wall sheathing needs to be thick enough to keep the sheathing above the dew point during the winter. (For more information on this issue, see Calculating the Minimum Thickness of Rigid Foam Sheathing.)

If you want to eliminate an interior vapor barrier, the code requires thick foam

Cold wall sheathing is more likely to be damp than warm wall sheathing, so the “20+5” requirement is problematic. There’s more, however: the 2012 IRC still maintains antiquated vapor barrier requirements. In section R702.7, the code notes that “Class I or II vapor retarders are required on the interior side of frame walls in Climate Zones 5, 6, 7, 8 and Marine 4.” That requirement has always been unfortunate, but it has proven to be hard to change.

Of course, if you’re building a wall with exterior rigid foam, the wall can no longer dry to the exterior; it needs to be able to dry to the interior. An interior Class I vapor barrier prevents inward drying, and a Class II vapor retarder reduces inward drying to a considerable extent. Fortunately, code writers have come up with an effective but clumsy way to address walls with rigid foam; since 2007, the IRC and IECC International Energy Conservation Code. have allowed builders in some cases to omit an interior Class I or Class II vapor retarder (substituting a Class III vapor retarder — in other words, ordinary latex paint).

In Climate Zone 6, for example, the code allows builders to omit the Class I or Class II vapor retarder if the walls include “insulated [exterior] sheathing with R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. ≥ 7.5 over [a] 2×4 wall” or if the walls include “insulated [exterior] sheathing with R-value ≥ 11.25 over [a] 2×6 wall.” (The provisions are found in Table R702.7.1 of the 2012 IRC, which is reproduced as Image #2, below.)

These code-mandated minimum R-values for exterior rigid foam correspond to the recommendations I made in my article, Calculating the Minimum Thickness of Rigid Foam Sheathing. Unfortunately, though, the code doesn’t allow builders to eliminate the Class I or Class II vapor retarder if they’re building a code-minimum 20+5 wall.

Worst-case scenario

Why does the code’s prescriptive table (Table 402.1.1 in the 2012 IECC and Table N1102.1.1 in the 2012 IRC) require an insulation method — 20+5 — that most building scientists consider risky? (The table is reproduced below as Image #3.)

The short answer is that the code was not written by building scientists; its provisions are historical accidents resulting from political battles and compromises.

I first pointed out the serious problems with the current code's wall insulation requirements for Climate Zones 6, 7, and 8 in a 2011 article, An Overview of the 2012 Energy Code. Unfortunately, in the three years since that article appeared, there hasn't been any organized effort to fix the “20+5” problem.

To summarize the problems:

  • The code establishes a minimum prescriptive requirement for walls that mandates a layer of rigid foam that is too thin to keep the wall sheathing above the dew point during the winter. If builders choose to follow this minimum code requirement, wall sheathing can accumulate moisture and eventually rot.
  • The code requires the installation of an interior Class I or Class II vapor retarder — often interpreted to mean a polyethylene vapor barrier — on walls meeting the minimum code requirement of 20+5. In many ways, this is the worst-case scenario: you end up with a wall that can’t dry outward and can’t dry inward, with exterior foam that is too thin to keep the sheathing above the dew point.
  • The code does not include any provisions for the use of smart vapor retarders.

Imagining a better code

Vermont regulators recently held public hearings to discuss possible changes to Vermont’s building code. Although I was unable to attend the hearings, I heard that one of the topics under discussion was: How do we reconcile the requirements of the 2012 IRC and 2015 IRC with building scientists’ warnings about the dangers of thin exterior foam?

Fortunately, the energy consultants who have been attending these hearings understand the dilemma they face. One of these consultants, Richard Faesy of Energy Futures Group, told me, “We want to get the exterior insulation detailing and building science right.”

Several approaches are possible. Here are some suggestions.

Remove code requirements for interior vapor retarders. Any attempt to come up with rules to regulate the vapor permeance of building materials is fraught with problems. On houses with certain (forgiving) types of wall assemblies, an interior vapor barrier may not cause any problems. However, houses with less forgiving wall assemblies, especially air-conditioned houses, may develop problems if they include an interior vapor barrier.

Since interior vapor retarders (especially Class I vapor retarders) on walls and ceiling cause as many problems as they solve, there is no logical reason for building codes to require them. All such requirements should be eliminated — especially in light of the fact that an increasing number of Climate Zone 6 homes include air conditioning.

Would this code change allow some builders to design bad wall assemblies? Of course. But they’re already doing that. To minimize potential problems, building codes should do a better job of regulating the airtightness of building envelopes.

Require builders who install exterior rigid foam to specify foam that is thick enough to keep the wall sheathing above the dew point in winter. This principle is the basis for existing requirements found in Table R601.3.1 in the 2009 IRC and Table R702.7.1 in the 2012 IRC (Image #2, below).

If code writers want to retain the format of the prescriptive table (Table N1102.1.1 or R401.1.1 in the 2012 codes) listing minimum R-values for walls (Image #3, below), then a logical way to describe minimum R-values for walls with rigid foam in Climate Zone 6 would be: “20+11.25 or 13+7.5.” (The corresponding requirement for Climate Zones 7 and 8 would be: "20+15 or 13+10.")

Even better would be a paragraph that explains what this means this in plain English.

Needless to say, the code needs to distinguish rigid foam products that have a low vapor permeance (foil-faced polyisoPolyisocyanurate foam is usually sold with aluminum foil facings. With an R-value of 6 to 6.5 per inch, it is the best insulator and most expensive of the three types of rigid foam. Foil-faced polyisocyanurate is almost impermeable to water vapor; a 1-in.-thick foil-faced board has a permeance of 0.05 perm. While polyisocyanurate was formerly manufactured using HCFCs as blowing agents, U.S. manufacturers have now switched to pentane. Pentane does not damage the earth’s ozone layer, although it may contribute to smog. , 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., and 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.) from mineral wool panels that have a high vapor permeance. There is no need for exterior mineral wool insulation to adhere to minimum R-value requirements.

One possible objection to my proposal: by requiring thicker exterior foam, this proposed code change will increase minimum R-value requirements for walls. There are two possible responses to this objection:

  • So what? Is that a bad thing?
  • The use of rigid foam shouldn't be mandatory. If code writers want to allow R-25 walls, that’s fine. There are ways to build R-25 walls that don’t cause rot. But code writers will have to come up with consistent method for calculating the R-value of walls — one that is easy for code enforcement officers and builders to understand and use. Up until now, no one has wanted to do that. (This task is complicated — but it’s possible.)

Include a “fanfold” exception. Thin rigid foam is sometimes acceptable, especially if it is somewhat vapor-permeable. Some brands of 1/4-inch-thick or 3/8-inch-thick fanfold foam insulation that are typically used under 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). siding are made of uncoated EPS and are therefore fairly vapor-permeable; there is no reason to ban the use of such products. Unfortunately, enforcing this fanfold exception would be tricky, because it would require building inspectors to know how to research the vapor permeance ratings of building products.

Simplicity is a virtue

The code changes I have proposed are fairly simple, and that’s good. A prescriptive code should strive for simplicity. Any builder who wants to build an assembly that doesn’t meet prescriptive requirements should be free to comply with the code by following an alternative performance path. (In Vermont, regulators propose to establish maximum HERSIndex or scoring system for energy efficiency established by the Residential Energy Services Network (RESNET) that compares a given home to a Home Energy Rating System (HERS) Reference Home based on the 2006 International Energy Conservation Code. A home matching the reference home has a HERS Index of 100. The lower a home’s HERS Index, the more energy efficient it is. A typical existing home has a HERS Index of 130; a net zero energy home has a HERS Index of 0. Older versions of the HERS index were based on a scale that was largely just the opposite in structure--a HERS rating of 100 represented a net zero energy home, while the reference home had a score of 80. There are issues that complicate converting old to new or new to old scores, but the basic formula is: New HERS index = (100 - Old HERS score) * 5. Index goals for builders who prefer to pursue a performance path.)

It might be argued that thinner exterior rigid foam could work as long as a home is equipped with a smart vapor retarder. But any code that tries to reflect that fact will need to define a smart vapor retarder, and that’s more complicated than it appears.

It can also be argued that thinner exterior rigid foam will work as long as homeowners successfully maintain a low level of interior relative humidity. That’s true — but good luck with enforcing that method of compliance.

Readers are invited to propose their own code changes

Code writing is tricky. I’m not an expert at code writing, but I know bad writing (and bad advice) when I see it.

Here are the hallmarks of a good building code:

  • It is written in plain English, using the same vocabulary usually heard at construction sites. (And I don’t mean, “Remember to install the friggin’ vapor barrier, you idiot!”)
  • It is flexible enough to allow designers and builders to come up with new ways of achieving desired goals.
  • It achieves regulators’ goals without any undesired side effects.
  • It is simple enough for builders and code enforcement officials to understand.
  • It is easily enforceable, without the need to look up permeance values, R-values, or U-factors on manufacturers’ web sites.

My proposals are probably flawed and are unlikely to be implemented. I look forward to reading better suggestions from readers.

In the meantime, one fact is undeniable: any building code which describes the prescriptive requirements for a wall in Climate Zone 6 as “20+5” while simultaneously requiring this wall to have an interior Class I or Class II vapor retarder is nuts.

It’s time to fix the code.

Martin Holladay’s previous blog: “Every House Needs Roof Overhangs.”

Click here to follow Martin Holladay on Twitter.



Image Credits:

  1. Owens Corning
  2. International Code Council

1.
Fri, 07/18/2014 - 09:10

Edited Fri, 07/18/2014 - 09:11.

cavity insulation
by Lecole Vielle

Helpful? 0

Mr. Holladay, for the purposes of your discussion, are you assuming that the cavity insulation is fiber glass batting, cellulose, or some other comparable insulation? That is, would you have the same concerns, if the cavity insulation were closed cell foam insulation?

(I'm thinking in particular of Zip R-Sheathing over closed cell foam cavity insulation).

Many thanks for your helpful explanations!


2.
Fri, 07/18/2014 - 09:19

Response to Lecole Vielle
by Martin Holladay, GBA Advisor

Helpful? 0

Lecole,
Closed-cell spray foam insulation solves one problem -- vapor diffusion from the interior to the cold sheathing during the winter -- but introduces another: it prevents damp sheathing from drying inward.

In general, most experts recommend against this kind of "foam sandwich" that prevents the OSB or plywood sheathing from drying in at least one direction.


3.
Fri, 07/18/2014 - 09:55

Edited Fri, 07/18/2014 - 10:42.

Prevention of damp sheathing from drying
by Lecole Vielle

Helpful? 0

If I have understood you correctly, Mr. Holladay, prevention of damp sheathing from drying is a problem anytime one uses closed cell spray foam insulation behind sheathing (in turn behind some kind of vapor barrier). That is, the problem is that there are effectively two vapor barriers: the R-Sheathing's ployiso insulation and the closed cell spray foam.

At http://www.greenbuildingadvisor.com/community/forum/energy-efficiency-an..., you said "Zip-R sheathing has a permeance that is below 1 perm; for all intents and purposes, it is a vapor barrier.

You are correct that the polyiso layer of Zip-R sheathing faces inward. Since any building with Zip-R sheathing can't be expected to dry outward, adding closed-cell or open-cell spray foam on the interior side of the polyiso layer doesn't change anything -- at least not with respect to the drying direction.

So you can go ahead and use spray foam between the studs if you want to."

Perhaps, the better alternative is to opt for regular Zip sheathing and closed cell foam, as opposed to Zip R-Sheathing. That might allow any moisture to dry out. The HuberWood website says, "The ZIP System® Sheathing overlay protects against water intrusion while providing an optimal permeance level (12-16 perms) to allow panels to properly dry out."

Going with the alternative would mean suffering from thermal bridging, of course. Boy, it's never easy.


4.
Fri, 07/18/2014 - 11:22

easy solution & better solution
by Gregory La Vardera

Helpful? 0

The easy solution is for the code to prohibit foam exterior insulation where the R value of the exterior layers is less than 1/3 of the total R value of the exterior + cavity. This would be easy to implement, one line in the code, perhaps a foot note on the two charts shown.

A better solution would be to require a design certification by a Building Scientist for any wall system using exterior foam insulation layers. Because one thing you've made clear is that its complicated, and that you really do need an expert to design an assembly correctly with exterior foam. And it seems right to have those building science experts take responsibility and liability for those assemblies. Then problems will be greatly reduced and when foam is put outside, it will be done right.

And builders who don't wish this extra expense can use interior side vapor control as they always have, and use permeable exterior insulation where they need those extra layers. And no special certification would be needed for these well understood systems.


5.
Fri, 07/18/2014 - 13:35

Edited Fri, 07/18/2014 - 13:37.

Second response to Lecole Vielle
by Martin Holladay, GBA Advisor

Helpful? 0

Lecole,
You wrote, "Prevention of damp sheathing from drying is a problem anytime one uses closed-cell spray foam insulation behind sheathing."

That's not a hard-and-fast rule. Many types of wall sheathing and siding allow drying to the exterior, for one thing. For another, even when there is no drying to the exterior -- for example, in the case of a cathedral ceiling with vapor-impermeable roofing above the sheathing -- it sometimes makes sense to install closed-cell spray foam on the interior side of the sheathing, even though there is no way for the sheathing to dry. If the sheathing is very dry on the day that the spray foam is installed, the risks may be worth taking.

The situation with Huber Zip R-Sheathing, which you bring up, is a special case. As you pointed out, the rigid foam layer of this sandwich product is on the interior side of the Zip R-Sheathing, so installing closed-cell spray foam on the interior side of this product isn't particularly risky. The OSB layer faces the exterior, and can still dry (somewhat) to the exterior.


6.
Fri, 07/18/2014 - 13:41

Mineral Wool Semi-Rigid exterior insulation
by James Steel

Helpful? 0

In Canada, similar requirements are in place in cold climates. In B.C. in particular, the exterior R-5 is most often met with semi-rigid mineral wool board insulation. It's vapor permeable. A vapor-permeable WRB is applied to the sheathing inside of the mineral wool exterior insulation. This has worked well to both limit thermal bridging and allow wood frame structures to dry to the outside, as well as inside. Foam isn't the only way to achieve R-5 C.I. (continuous insulation) on the outside of the building.


7.
Fri, 07/18/2014 - 13:48

Edited Fri, 07/18/2014 - 13:50.

8.
Fri, 07/18/2014 - 13:56

it's not that easy (response to Gregory LaVardera)
by Dana Dorsett

Helpful? 0

A one size fits all solution would be at once too much and not enough.

A 1/3 of total-R requirement would be more than needed for US climate zones 5 & lower, and would be insufficient protection for zones 6 & up, and would even violate the current IRC prescriptives for zone 6 & 7 per Table R702.7.1.

http://www.greenbuildingadvisor.com/sites/default/files/Table%20R702.7.1...

Putting R19 or R20 cavity fill in a 2 x 6 wall would imply a requirement of only R10 on the exterior using the 1/3 rule, whereas R702.7.1 prescribes a minimum of R11.25, an R15 for zones 7 & 8. The R13 + 5 works fine for dew point control in US climate zones 4 & 5 (if a bit on the minimalist side for zone 5.)

Rigid rock wool under rainscreened siding rather than foam is a reasonable solution when cheating Table R702.7.1, and R6 Type II EPS is a better solution than R7.5 foil faced polyiso or R5 XPS for compliance in zone 5, due to the much higher vapor permeance of rock wool & EPS at those thicknesses. At R6/1.5" Type-II EPS is as vapor permeable as many grades of exterior house paint (paint that could be legally & safely applied without a rainscreen over 2x6 R20 in most of zone 5), and nearly as vapor permeable as interior latex paints. The fact that R6 EPS over an R20 cavity fill actually performs north of R7 whenever the outdoor temps are low enough to be concerned about sheathing dropping below the interior dew point temps helps too. R20 + 6 EPS(unfaced) makes it with or without a class-II or class-I in zone 5, despite violating Table R702.7.1 prescriptive, and R20 + R7.5 foil faced polyiso (which meets the prescriptive) is much riskier, due to the far-below-rated-R performance of polyiso at sub-freezing foam temps, and the extremely low vapor permeance of the facers.

But it's hard to include this much nuance in a simple & clear code prescription.


9.
Fri, 07/18/2014 - 14:05

Re: Response to James Steel
by James Steel

Helpful? 0

Martin,
Thanks for your response. I didn't miss your mention of mineral wool. And I've read most all the GBA articles on mineral wool. My point is that, if I understand the new code correctly, the requirement is for R-5 continuous insulation. Not R-5 foam. I agree that further clarification on what type of exterior insulation should be used would be great. But this is also self regulating. If you don't want your building to rot, probably shouldn't build a hybrid wall assembly with R-5 exterior foam. If the code were to read "R-5 vapor permeable exterior insulation", the foam industry would have a conniption. After all, if enough exterior foam is on the outside then there isn't a problem... No? So the problem isn't foam?


10.
Fri, 07/18/2014 - 14:13

Edited Fri, 07/18/2014 - 14:15.

Second response to James Steel
by Martin Holladay, GBA Advisor

Helpful? 0

James,
Thanks for your comments. You are right, of course.

In particular, you are correct to point out that ambiguous or deliberately obfuscatory code language is often the result of squawking by manufacturers and other so-called "stakeholders" with economic interests to protect.


11.
Fri, 07/18/2014 - 15:01

Re: Second response to James Steel
by James Steel

Helpful? 0

Martin,
You read into that exactly where I was going!


12.
Fri, 07/18/2014 - 17:57

Edited Fri, 07/18/2014 - 17:58.

Well the discussion is
by Gregory La Vardera

Helpful? 1

Well the discussion is convincing me that the way to go is for exterior foam assemblies be certified by a building scientist on a project by project basis. Too many variables for a simple code prescription.

The code should only prescribe assemblies with exterior permeable insulation, drying to outside, with interior vapor control, and that vapor control sheet of the variable permeability type if there is air conditioning.

Makes it more complicated for builders on the one hand, but keeps the code clean and simple.


13.
Fri, 07/18/2014 - 19:09

Edited Fri, 07/18/2014 - 19:12.

The code could have complete prescriptive plan details sets
by aj builder, Upstate NY Zone 6a

Helpful? -1

If the code had a book of allowed plan detail sheets builders, subs, archs homeowners and inspectors all would have life easier and have more time for tubing summer rapids and shooshing the slopes or surfing the waves off Maui.

How to start this? Make one set of complimentary home plan details for each climate. Add more each year. Any manufacturer could also write up complete home detail sheets, as they would then be liable for there success.

The problem has always been that homes are not built from one companies detail sheets. Some one of us starts pulling together this detail and that manufacturer and a little GBA and a little Joe L. and a little ajbuilder...

We're lucky any homes are built properly. Here's an example of whatever... In my area every production home is built with poured concrete foundations, no foam under the slabs or on the walls, what they do is build a stud wall off the concrete a few inches and then they fiberglass insulate this wall, then they put roll metalized paper over this wall two rows horizontally applied, no taped seams, and at the top of the wall the air space is open to the underfloor. The rim joist... 1/4" of closed cell spray foam. The electric panel is mounted to the concrete wall no insulation, wall insulation interrupted air gapped all around. It all looks great to the untrained average homeowner but myself or Joe L or Martin would not think any of it was right at all. AND THIS IS STANDARD HERE AND IS EXACTLY WHAT OUR CODE OFFICERS WANT TO SEE. WE HAVE TO HAVE A VAPOR BARRIER OVER OUR PINK INSULATION IN OUR BASEMENTS. Try to find Martin saying that is the way to do it.

GBA should publish complete home plan details, a package of details that work together. With a list of to do's and not to do's. Sell the set of details for any of us to design a home with or give to our home designer. Joe L. should do it. But really THE PRESCRIPTIVE ASPECT OF THE CODE SHOULD BE A COMPLETE DETAIL LIST FOR A COMPLETE HOME. We all still can build other than prescriptive via the performance part of the code.

Prescriptive codes need to be complete sets of unified details.

All the options we have as builders is where any of us get in trouble. I know many of us love having options and freedom to do our own ideas. I do. That is always possible now and would be with my idea of how to fix the codes.


14.
Fri, 07/18/2014 - 19:37

Edited Fri, 07/18/2014 - 19:38.

Response to AJ Builder
by Martin Holladay, GBA Advisor

Helpful? 0

AJ,
Green Building Advisor is not in the business of developing or selling home plans; plenty of other companies already do that.

But GBA has a very large library of details, fully accessible for downloading by any GBA Pro member (that is, a subscriber).

Here is the link to our detail library: GBA Detail Library.


15.
Fri, 07/18/2014 - 23:39

Response to AJ
by Malcolm Taylor

Helpful? 0

Our code does just that with wall and ceiling assemblies that are required to meet a certain standard for fire and acoustic separations. It provides pages of different assemblies using various materials. I don't see why they couldn't do the same for exterior wall and ceiling assemblies required to meet new energy
codes.


16.
Sat, 07/19/2014 - 10:32

Edited Sat, 07/19/2014 - 10:34.

Martin disconnected detail sheets add to the problem
by aj builder, Upstate NY Zone 6a

Helpful? -1

Anyone who publishes home construction details without adding a tool that groups compatible details is making things worse not better.

Like Malcolm says, the code in NY has us submit a passed Reschek. All that would have to be done would be to add detail sheets to reschek that then it either allowed or disallowed the group of detail sheets selected.

We not talking Mars mission planning... It could be done the same way building code is developed and published. I could ever do it, PHD not.

Must go now and help eat a pig and monitor proper completion of all beer on site.
Aj


17.
Sat, 07/19/2014 - 21:07

Sounds crazy.
by Gregory La Vardera

Helpful? 1

No, the idea of creating a code book of wall assemblies is crazy. Every state adopts the code at a different rate, each would have to adopt each wall assembly. There are so many products, so many possible assemblies, and so many local climate variations that defy the climate zones. For exterior insulation interior drying walls the climate zones are not granular enough to broadly adopt standard assemblies, as you can for interior side vapor control in cold climates.

Think about it - what you are describing is like the UL Listing book of fire rated assemblies. Except now they are going to be indexed over 6 climate zones plus marine and arid overlays. It would be a monumental undertaking. Its not rocket science, but it is a whole lot of science, by a whole lot of industry players, who will no doubt dig in heels to oppose such regulation. They really don't care if you get mold or rotted sheathing, only that you buy their product.

Exterior side insulation/vapor control/interior drying is an unforgiving and non-resilient assembly. Its easy to get it all wrong, and even when its a little wrong it can work very badly. Certification by qualified Building Scientists is the only reliable way to have some assurance that the assembly will be properly designed for the application. Even that measure would be greatly resisted. Face it, we are likely to just have the confusing mess outlined in this article.


18.
Sat, 07/19/2014 - 22:22

Edited Sat, 07/19/2014 - 22:28.

Nuts Greg, nuts.
by aj builder, Upstate NY Zone 6a

Helpful? -1

The detail sheets exist today. Assembling a set for all climates would take Joe L. ten minutes.

And you still would have the right to build a performance based home that you had a professional put together the plan. which is your building scientist idea.

I love the idea.

Joe L has really already done this if you buy his regional books you can see he is just about doing this with those publications which is my building scientist/building code idea.

I would have no problem with handing the whole inspection system to Joe. Done.

If you have used Rescheck to certify a home, you would see that it with just a bit of added programming could be used to self collate a group of compatible thermal moisture details.

I should get this aj-Rescheck-building detail grouping software built and sell it.


19.
Sun, 07/20/2014 - 00:09

mmmm ..
by Jin Kazama

Helpful? 0

""""""""""""
Unfortunately, enforcing this fanfold exception would be tricky, because it would require building inspectors to know how to research the vapor permeance ratings of building products.
""""""""""""""

Is this an intended joke ??

buildings inspectors = n00bs ??!!


20.
Sun, 07/20/2014 - 05:15

Edited Sun, 07/20/2014 - 06:13.

Response to Jin Kazana
by Martin Holladay, GBA Advisor

Helpful? 0

Jin,
Some fanfold insulations have facings. It can therefore be hard to determine the vapor permeance of a sample of fanfold insulation by looking at it.

Of course, the federal government could pass a new federal law, the Permeance Rule (along the lines of the R-Value Rule which requires insulation products to be labeled with R-values). If such a law were passed, manufacturers of certain building products, including fanfold foam insulation, could be required to label their products with a perm rating.

In the absence of such a law, I don't see it happening.

And I don't think it's a good idea to have a code requirement that is hard to verify when an inspector visits a job site. So this issue is more complicated than it first appears.


21.
Sun, 07/20/2014 - 08:00

Edited Sun, 07/20/2014 - 08:01.

Mineral wool
by Mark Attard

Helpful? 0

This exactly why we recommend using mineral wool in a rain screen application. First the mineral wool is vapor diffuse. A properly applied air and moisture barrier is the first step. Beyond that the mineral will dry to the outside of the wall assembly if it does get wet. Second mineral wool's thermal performance holds its performance over a wide variety of temperature extremes. This helps eliminate a moving dew point target. Lastly, a good 3/4" to 1" air gap created with vertically applied furring strips allows for bulk water run off and air movement behind sheathing, siding, etc. This allows for proper drying and the prevention of heat build up.


22.
Sun, 07/20/2014 - 14:04

Why codify it?
by Andrew Michler

Helpful? 0

I think the key phrase in Martin's article for me is that the code is simply adopting building science-less rules of thumb based on light frame construction which is proven to be cheap when your only using 2x4s and 2x6s. Now that we are getting beyond what those cavities can take for r-values the prescriptive approach is not very viable. And code struggles with non prescriptive assemblies by nature. Greg's suggestion that we treat these higher r-value assemblies with a more specialized review and design strategies seems to be the natural evolution. The alternative is many rotting boxes and lawsuits, which is what drives code to a large extent historically.

Why not treat wall assemblies like we would structural loads and have them properly engineered on each and every job for the location? Sure it drives the cost up but much less so as engineers learn the physics of vapor diffusion and retention. And of course a lot cheaper than failed buildings.

I also think this should force us to reevaluate the foam wall system to get to where we want to go- robust, durable and low cost assemblies. Double walls, larson trusses, or other built out cavities which can be blown with low cost products like cellulose combined with smart vapor barriers starts to make a lot more sense than mindlessly adding expensive foam to the old fashioned 2x wall.

Come to think of it this sounds like what they have been doing in Europe for some time... humm.


23.
Mon, 07/21/2014 - 06:28

simple solution?
by Dustin Harris

Helpful? 0

Sure, the situation is complex and different assemblies will work well in one situation and fail miserably in another. Why not prescriptively allow work-anywhere solutions like PERSIST, but require others to get analyzed and stamped?


24.
Mon, 07/21/2014 - 07:03

Edited Mon, 07/21/2014 - 14:40.

Response to Dustin Harris
by Martin Holladay, GBA Advisor

Helpful? 0

Dustin,
Thanks. Your suggestion has merit. But any code change that appears to favor one wall system as the default solution is likely to face resistance from those who prefer a different wall system -- especially if the new default is one like PERSIST, which doesn't resemble the type of residential walls that are usually built in the U.S.

It's interesting to contrast your suggestion with Gregory La Vardera's suggestion. The two suggestions are mirror images of each other, just as an ICF wall is a mirror image of a Thermomass wall.


25.
Mon, 07/21/2014 - 09:54

SIPS
by Joel Cooper

Helpful? 0

How do SIPS (structural insulated panel system) rate in this "risky wall strategies" discussion. I am considering SIPS for a building site (zone 7) that is not vehicle accessible (and therefore, reduce the number of trips taken to bring building materials to the work site).


26.
Mon, 07/21/2014 - 10:29

Response to Joel Cooper
by Martin Holladay, GBA Advisor

Helpful? 0

Joel,
Some SIP buildings have suffered OSB rot. When this occurs, the affected area is usually the exterior OSB facing, near a SIP seam. The cause of this rot is a poorly sealed seam that allows air leakage. Exfiltrating air carries moisture in it, and the moisture condenses when it reaches a cold surface (for example, asphalt felt or cold OSB) during the winter.

In general, the risk of this type of rot can be greatly reduced if the SIP installers pay attention to air sealing. In addition to using caulk and canned spray foam as directed by the SIP manufacturer, it's wise to also seal all SIP seams with high quality tape on the interior. For more information on this issue, see How to Protect Structural Insulated Panels from Decay.

Finally, the airtightness of SIP seams should always be verified as part of the construction process. This verification is done with a blower door and a smoke pencil (or, in come cases, with a blower door and an infrared camera).


27.
Mon, 07/21/2014 - 17:00

Why Not Just "R-25"?
by Richard Faesy

Helpful? 0

What if the code for walls said "R-25" instead of "R-20+5 or R-13+10"? This would remove the faulty building science (for 20+5), state a total minimum R-value that could be achieved through multiple approaches, and could be the basis for sound building science support through other market approaches. Could this solve the conundrum?


28.
Mon, 07/21/2014 - 17:50

Edited Mon, 07/21/2014 - 18:01.

Response to Richard Faesy
by Martin Holladay, GBA Advisor

Helpful? 0

Richard,
Your suggestion would work, of course, and it is one of the options I mentioned in my article.

I wrote, "If code writers want to allow R-25 walls, that’s fine. There are ways to build R-25 walls that don’t cause rot. But code writers will have to come up with consistent method for calculating the R-value of walls — one that is easy for code enforcement officers and builders to understand and use. Up until now, no one has wanted to do that. (This task is complicated — but it’s possible.)"

For example, do enforcement officials have to take thermal bridging through studs into account when calculating R-values? If they ignore thermal bridging, what do they do about buildings with steel studs? Does the code need to include a table listing the R-values per inch of a variety of insulation materials? How do you calculate the R-value (or measure the thickness) of bumpy and uneven applications of spray polyurethane foam? Is the R-value of insulation between studs treated identically to continuous insulation, or must its R-value be downgraded by a framing factor? Do code enforcement officials have to calculate a building's actual framing factor, or will there be a published default value for an average framing factor?

There is another issue to wrestle with if code writers accept your proposal: will 20+5 still be acceptable, or will the code decide to outlaw 20+5 in Climate Zone 6?


29.
Mon, 07/21/2014 - 18:12

Edited Mon, 07/21/2014 - 18:13.

simple
by aj builder, Upstate NY Zone 6a

Helpful? -1

After reading all the goofy ideas my idea of approved assemblies is by far the simplest and simple to implement.

Simply aj


30.
Tue, 07/22/2014 - 08:44

R25 center-cavity is not R25
by D Dorsett

Helpful? 0

The reason R20+ R5 continuous insuiation (not thermally bridged) is due to the fact that R25 center-cavity as a low-densty batt in 2x8 framing is also R25 nominal, but dramatcally underperforms R20 in a 2x6 framed wall with R5 continuous insulation. The "whole wall" R of R20 cavity fill is about R13 after factoring in the thermal bridging of the framing. Adding the sheathing, gypsum & siding adds anotyher R1-R1.5 or so, bringing it up to R14-R15, then adding R5 of continous insulation brings the whole assembly up to about R20 performance. R25 cavity fill in a 2x8 16" o.c. assembly without continuous insulation barely makes R17-ish performance after the thermal bridging. Energy codes are about the total whole-wall U-factor/R-value, and not center cavity.


31.
Tue, 07/22/2014 - 09:51

Edited Tue, 07/22/2014 - 15:27.

Response to Dana Dorsett
by Martin Holladay, GBA Advisor

Helpful? 0

Dana,
You wrote, "Energy codes are about the total whole-wall U-factor/R-value, and not center cavity."

Really? What makes you think that? I don't know of any evidence to support your contention.

In regions where the prescriptive code calls for a minimum of R-13 wall insulation, code enforcement officials have always deemed builders to be in compliance with the code if the builders install batts labeled "R-13" between the studs.


32.
Tue, 07/22/2014 - 11:49

The evidence is found in Table R402.1.1, with a bit of analysis
by Dana Dorsett

Helpful? 0

The U-factors/whole-wall-R of the alternate assemblies are very similar.

Take the wall prescriptives for zones 3- 5: At typical framing fractions an R20 wall comes in at about R14-R15 after factoring thermal bridging and adding other layers, R13 + 5 comes in at about R14.5-R15- a nearly identical U-factor, and which very similar to the R13/R17c.i. mass wall prescription for zone 5. The prescribed center cavity R is either R20, or (R13 + R5=)R 18. That's a 10% difference in R value measured at the center cavity, but nearly identical whole wall performance.

For zones 6 & higher they prescribe R20+5, which comes in around R19-R20 whole-wall, after framing + other layers, as well as R13 +10, which also comes in at R19-R20 whole-wall, which is identical to the prescriptive R20 (if more than 50% of the insulation is on the interior) of a mass wall for zone 6, or R19/R21 for zones 7+. R20+5 has a center cavity R of R25, whereas R13 +10 comes in at R23, but the whole-wall performance is within R0.5 of one another.

In zones 1-2 the prescriptive is R13, which is merely the most commonly available batt insulation that fits in 2x4 cavites. They could have prescribed R11s which are slightly cheaper but I suspect the rationale for the denser R13 batts had to do with infiltration. Wall-R has very little effect on cooling loads, but air infiltration does.

BTW: The R25 2 x 8 framed wall example may only hit R17 at a 25% framing fraction (16" o.c. spacing), but would come close to R20 at a 15% framing fraction (as can be had using some advanced framing techinques at 24" o.c. stud spacing.).


33.
Tue, 07/22/2014 - 12:40

Response to Dana Dorsett
by Martin Holladay, GBA Advisor

Helpful? 0

Dana,
I understand your analysis, but I think you are being extremely charitable when you attribute a concern with whole-wall R-values to code writers.

The most charitable version I can come up is that there is no consistency in how the code approaches wall R-values. For decades, the code has referred to walls with R-13 batts as R-13 walls.

Now that the code has introduced 20+5 and 13+10, your analysis makes sense. But to this day, the code does not include any guidelines for code enforcement officials to tell them how to calculate the R-value of wall insulation.

In some cases, the logic behind the numbers can be inferred (as you demonstrated). But for most of the country, for decades, code enforcement officers (and builders) have said, "The code says that you need an R-13 wall" or "The code says that you need an R-19 wall."


34.
Wed, 07/23/2014 - 15:08

Yes, there is a wealth of ignorance out there...
by Dana Dorsett

Helpful? 0

... amongst builders, inspectors, and (apparently) code writers too.

But I would presume that drafters of IECC codes prescriptions would have a handle on how to calculate the U-factors at typical framing factors, or use other energy use simulation tools (as is clearly going on in the mass-wall prescriptives) to come up with codes that make at least some sense, given that financially rational energy conservation is their mission.

It's arguable that architects & building designers SHOULD be able to figure this out, but it's clear those that can are the exception. I have no hope of turning builders or inspectors into engineers, which is why the codes are so simplified.

IIRC the energy aspects of building codes in Sweden are performance based rather than a simplified list of prescriptive-Rs, with penalties assessed on the architects &/or builders for underperforming buildings (depending on whether it's found to be a design vs. implementation issue). You can bet THOSE architects & builders can all run U-factor calculations in their sleep, (and don't get much sleep if a design under construction is too close to the line.)

The simplification of using cavity-R for ~25% framing fraction walls as the shorthand vernacular for wall performance is part of what lets ICF & SIP manufacturers get away with using outlandish performance numbers. An R13 wall is really an R9-R10 wall, and R19 wall is really an R12-R13 wall when comparing performance to continuous insulation assemblies. It kinda matters that (almost) NOBODY would choose to build a 2 x 12 studwall with a 25% framing fraction for an "R40" wall (with R40 cavity fill), so it's not a credible assembly for comparing with ICF or SIP construction.


35.
Wed, 07/23/2014 - 17:19

R20+5 insulation code
by Anders Lewendal

Helpful? 0

Lecole asked about using closed cell foam in the wall cavities with closed cell foam panels outside of the sheathing. As Mr. Holladay pointed out, that is a bad idea. You could, however, put 1" or 2" of closed cell foam panels on the outside of the studs and then spray closed cell foam against the foam panels giving you a solid 5" to 7" of insulation. You might ask where the shear wall goes. If your home is fairly simple, you can save 50% to 75% of the shear wall materials if you ask a structural engineer to spec out shear walls nailed on the exterior wall studs from the inside. I have done this a few times with very good results. Plus, no issues with dew points. I am also in zone 6. Some would call it outsulation. Good luck.


36.
Wed, 07/23/2014 - 19:27

Disaster waiting to happen
by Katy Hollbacher

Helpful? 0

Another timely post—I was shocked when I learned about the new code and have been bracing myself for the fallout that's going to happen. Some of our projects are in CZ 6 & 7, and the fact that code not only allows but requires exterior insulation along with interior VBs as before is true insanity! I already know of some projects that have installed 1-2" exterior XPS, with R-21 batts, in a climate with average winter temps in the teens. SCARY.

Martin, I do have a question re: "...the code needs to distinguish rigid foam products that have a low vapor permeance (foil-faced polyiso, XPS, and EPS) from mineral wool panels that have a high vapor permeance..." Absolutely, and this was my first thought when I heard the code details—for the code not to take it a tiny step further, and clarify the TYPE of exterior insulation allowed, is just unbelievable.

But my question is, do you really consider EPS to have "low vapor permeance" at modest, most common thicknesses of 1 or 1.5" (and their associated perm ratings of 5 or 3)? That's a far cry from what I consider to be low permeance (<1 perm) XPS or vapor barrier polyiso, and I generally wouldn't expect to see issues with using only 1 or 1.5" EPS even in very cold climates.
**
Also, something I'd like to hear more about is how the inclusion of what I like to call "quasi-rainscreen" functionality between the drainage plane and back of rigid insulation impacts condensation risks. Grooved EPS, Tyvek StuccoWrap, HomeSlicker kind of thing (ie, an approach that provides some kind of air movement & related drying potential, but not so much that it ruins the thermal performance of the insulation). I have yet to come across a thorough study that has looked at this in-depth. Anyone?


37.
Wed, 07/23/2014 - 20:56

Edited Wed, 07/23/2014 - 21:14.

Excellent discussion, but...
by Ryan McCoon

Helpful? 0

I'm a bit surprised no one has mentioned one solution would be to remove the organic material layer that can/will potentially mold (OSB or plywood). Why couldn't the code state that if exterior continuous impervious insulation is used, wood structural sheathing would not be allowed. Hence, older traditional forms of wall bracing could be used - i.e. let in bracing. I know this still opens up the opportunity for potential condensation at the wall stud, but won't it be very minimal or possibly non existent?

My fear is for a builder like myself that uses a 1" continuos exterior layer of polyiso (R Max - formerly Dow SIS) and CC flash & BIB cavity, that now my method will become 'illegal' with the new code. I essentially have 3-1/2" of CC foam on the exterior of the wall with this system, but could potentially be non-conforming?!

Something tells me a few PHIUS folks are reading through this discussion and laughing histerically.


38.
Thu, 07/24/2014 - 00:29

Mold and Wood
by George Hawirko

Helpful? 0

Insistence on the use of wood limits the solutions, if there were any in this situation. There is no help for those who won't listen to the solution.


39.
Thu, 07/24/2014 - 04:00

Response to Anders Lewendal (Comment #35)
by Martin Holladay, GBA Advisor

Helpful? 0

Anders,
Your suggestion -- that walls without exterior OSB, braced by alternate means, could safely include R-5 exterior rigid foam and R-20 spray foam between the studs -- is correct. It points to the fact that there are many ways to build a wall, complicating the task of writers of any prescriptive code.


40.
Thu, 07/24/2014 - 04:11

Edited Fri, 08/08/2014 - 13:18.

Response to Katy Hollbacher (Comment #36)
by Martin Holladay, GBA Advisor

Helpful? 0

Katy,
Q. “Do you really consider EPS to have ‘low vapor permeance’ at modest, most common thicknesses of 1 or 1.5 inch (and their associated perm ratings of 5 or 3)?”

A. Depending on the type of EPS we are talking about, the permeance of 1 inch of EPS ranges from about 2.0 to 5.8 perms. A sample that measures 1.5 or 2 inches thick would have a lower permeance, of course.

I'll rephrase the question: "Assuming that EPS has an R-value of R-4 per inch, and that a builder chooses to comply with the 20+5 code provision by installing 1.5 inch of EPS on the exterior side of a wall, would the EPS be permeable enough to keep the wall out of trouble?"

That's a difficult question to answer without either (a) using and trusting the results of WUFI, or (b) building 100 houses in a cold climate and seeing what happens. As a builder, I wouldn't trust outward diffusion through the foam layer to provide enough drying every March and April to make up for the dependable wetting occurring every February.


41.
Thu, 07/24/2014 - 04:19

Edited Thu, 07/24/2014 - 04:28.

Response to Ryan McCoon (Comment #37)
by Martin Holladay, GBA Advisor

Helpful? 0

Ryan,
Like Anders Lewendal, you have suggested that damp OSB can be avoided by building a wall without exterior sheathing, instead using alternate bracing methods. That method will work, as long as the builder doesn't attempt to comply with the 20+5 provision of the code by installing R-20 fluffy insulation between the studs and R-5 of rigid foam on the exterior side of the studs.

The result of the scenario I describe is that condensation will occur on the interior face of the rigid foam sheathing. The moisture will either saturate the outer layer of fluffy insulation, or will run down the face of the foam and form puddles on the bottom plate.

You are avoiding that scenario by installing 2.5 inches of closed-cell spray foam (rather than fluffy insulation) on the interior side of the 1-inch thick polyisocyanurate -- a similar approach to the one suggested by Anders. That approach will work. I agree with both you and Anders that whatever solution is arrived at by code writers, their solution should allow the type of wall assembly you describe.

I'm not sure exactly what you meant when you wrote, "Something tells me a few PHIUS folks are reading through this discussion and laughing hysterically." Perhaps you mean that PHIUS has always advocated for walls with a higher R-value than the values included in the prescriptive code, or that PHIUS (unlike you) favors walls without much rigid foam or spray foam. If that's what you meant, you're right.

Like PHIUS, GBA has always recommended that builders choose walls with a high R-value, and has also emphasized that any cold-climate wall assembly must avoid the kind of moisture accumulation that is possible in a 20+5 wall.

But editors at GBA aren't laughing hysterically at the 20+5 problem, even though GBA readers are unlikely to be tripped up by minimum code provisions. I think that energy codes should make sense, so that even builders who aim to barely meet the code should end up with safe walls.


42.
Thu, 07/24/2014 - 04:32

Response to George Hawirko (Comment #38)
by Martin Holladay, GBA Advisor

Helpful? 0

George,
You wrote, "Insistence on the use of wood limits the solutions."

You're right, of course. The 20+5 provision in the building code assumes that the wall under discussion is framed with 2x6 lumber. If a builder chooses (for example) to build an ICF wall, then the problems discussed in my article won't occur.

One solution -- perhaps one that you would support -- is to outlaw walls that are framed with lumber. Since most homes in the U.S. have 2x4 or 2x6 walls, however, that suggestion is unlikely to gain much traction.


43.
Thu, 07/24/2014 - 10:33

It's not complicated for the
by aj builder, Upstate NY Zone 6a

Helpful? 0

It's not complicated for the code to have one prescriptive plan set as one example of a buildable home. This one set of allowed details would be in addition to the codes that already exist. Anyone could just attach the standard allowed detail sheets to their plan set. Done. Easy.


44.
Thu, 07/24/2014 - 10:53

Prescriptive vs. Performance
by John Semmelhack

Helpful? 1

Building off of Dana's comment regarding Sweden - the direction our energy codes need to move is toward performance-based standards rather than prescriptive. Presumably, the goal of the International ENERGY CONSERVATION code (IECC) should be to conserve energy. Prescriptive requirements will point a project generally in the direction of energy conservation, but they can't overcome certain design decisions such as 40% window to floor area ratio or very poor geometrical efficiency in a cold climate. These kinds of design decisions will still lead to excessive energy use, even with relatively stringent prescriptive requirements for the various assemblies.

Fortunately, the IECC is already headed in the direction of whole-house performance with the inclusion of an Energy Rating Index option (for instance, RESNET HERS Rating) in the 2015 code. You can read more about it here - http://www.resnet.us/professional/main/Hers_index_and_energy_codes While I have some issues with using the HERS Index for code compliance - for instance, it's a lot easier to get low HERS score on a house with a conditioned basement, compared to a similar house that is slab on grade - I think this is a huge step in the right direction.


45.
Thu, 07/24/2014 - 16:50

Impacts of rainscreen/rear-vented insulation
by Katy Hollbacher

Helpful? 0

Following up from my own question, which was essentially: Exactly how much does rainscreen functionality, when used in an assembly that includes exterior insulation, impact sheathing condensation risks? (and following from that, how should this information be incorporated into related building codes, types and levels of insulation allowed or req'd, etc.?)

The following papers/reports are compelling, but what I am really interested in seeing is the same types of studies applied to constructions that include various types and thicknesses of rigid exterior insulation. I don't see how this particular debate can be had without considering this topic.

The Effect of Air Cavity Convection on the Wetting and Drying Behavior of Wood-Frame Walls Using a Multi-Physics Approach
http://www.ibp.fraunhofer.de/content/dam/ibp/de/documents/Publikationen/...

Ventilated Wall Claddings: Review, Field Performance, and Hygrothermal Modeling
http://www.buildingscience.com/documents/reports/rr-0907-ventilated-wall...


46.
Thu, 07/24/2014 - 22:19

Change the Code - 2018 next opportunity
by Mike Guertin, GBA Advisor

Helpful? 0

I got a copy of the 2015 IRC a couple weeks back. Just checked the vapor retarder and insulation tables. Pretty much the same as the 2012 IRC.

Next opportunity for code changes will be the 2018 code hearings. Here's the ICC codes and standards calendar for the next few years: http://www.iccsafe.org/calendar/Pages/Calendar-CS.aspx

Given that many jurisdictions haven't even adopted the 2012 codes, it may be a long time before good building science based prescriptive insulation and vapor-control requirements make their way into common practice - unfortunately.


47.
Fri, 07/25/2014 - 06:56

Response to Mike Guertin
by Martin Holladay, GBA Advisor

Helpful? 0

Mike,
I agree that the three-year code cycle for updates and the cumbersome code change process means that we are going to stuck with these provisions in the International codes for quite some time.

It is nevertheless worth mentioning that the IRC and the IECC aren't legally binding on any jurisdiction in the country until they are adopted by a local authority. They are what used to be called "model" codes -- that is, they are models or suggestions for local authorities to consider when establishing local codes.

Any jurisdiction in the country has the option of adopting parts of the International codes while rejecting or modifying other parts of the code. As far as I understand, that's what's happening in Vermont right now. The Department of Public Service is considering the 2015 codes; if the authorities want to, they can adopt a modified version of the code.


48.
Fri, 07/25/2014 - 07:48

Response to Katy Hollbacher (Comment #45)
by Martin Holladay, GBA Advisor

Helpful? 0

Katy,
We certainly know that ventilated rainscreen gaps accelerate the outward drying of damp wall assemblies. Whether this accelerated drying is enough to tip a risky wall assembly with thin exterior rigid foam over the line -- making it an acceptable wall assembly -- is unknown.

One thing is for sure: if you are installing foil-faced polyisocyanurate, the rainscreen gap isn't going to accelerate outward drying of the wall assembly (although it will have other moisture management benefits). If you have EPS, however, it might.

While WUFI can provide an answer to your question, it's up the each builder or architect to decide whether WUFI results are enough to hang your professional reputation on. (For more information on this topic, see WUFI Is Driving Me Crazy.)

My own decision is as follows: I'm going to continue to follow Joe Lstiburek's recommendations for minimum exterior rigid foam thickness, rather than adopting thinner foam in hopes that the rainscreen gap keeps me out of trouble.

Like you, I would welcome more field studies that look into this question.


49.
Fri, 07/25/2014 - 09:46

Possible Solution
by Elyse Inglese

Helpful? 0

Martin,

I enjoyed reading this article, and think you’ve provided valuable information about the complexities of code and the issues that can arise from lack of understanding and misinterpretation. We are seeing an increase in questions related to assemblies with exterior rigid foam and our end users have concerns about the moisture content of this type of system.

What are your thoughts regarding a wall assembly using a drainable-type, vapor permeable housewrap overtop of sheathing, and underneath exterior rigid foam?

This would help to let trapped moisture escape while still maintaining the R-value of the foam. Would this type of assembly help to solve the moisture concerns associated with exterior rigid foam?


50.
Fri, 07/25/2014 - 10:06

Edited Fri, 07/25/2014 - 10:07.

Response to Elyse Inglese
by Martin Holladay, GBA Advisor

Helpful? 0

Elyse,
As Joe Lstiburek explained in one of his articles (Mind the Gap, Eh!), a crinkly housewrap can provide some hygric redistribution, although at a small loss in thermal performance. This hygric reidistribution is especially important when the insulation between the studs (for example, closed-cell spray polyurethane foam) doesn't allow any inward drying.

You are proposing another use of crinkly housewrap. It sounds as if you are proposing that builders might install crinkly housewrap in hopes that the hygric redistribution will tip a risky wall assembly (one with with too-thin foam) into safer territory. This proposal is similar in principle to the proposal made by Katy Hollbacher in Comment #45.

Count me a skeptic -- until someone conducts a field study that verifies the idea. I don't think it's worth violating the rules that have been established concerning minimum foam thickness -- even when we include a few tricks that might lower the risks.


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