‘Extended Plate and Beam’ Walls

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‘Extended Plate and Beam’ Walls

A new wall system designed for production builders

Posted on Oct 13 2017 by Martin Holladay
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Production builders in the U.S. love 2x4 walls. They also love keeping the cost to build their homes as low as possible.

When energy codes ratcheted up in the 1980s and 1990s, cold-climate home builders eventually switched to 2x6 studs. But most production builders are still reluctant to install exterior rigid foam or furring strips.

In Climate Zones 6, 7, and 8, new codes are forcing builders to consider the implications of the “R-20 + R-5” requirements for walls. But many builders are unhappy with current options for building high-R walls.

Responding to builders' concerns, engineers at a research facility associated with the National Association of Home Builders (the Home Innovation Research Labs, formerly known as the NAHBNational Association of Home Builders, which awards a Model Green Home Certification. Research Center) have developed a new wall system called the “extended plate and beam” system. The main developers of the system were Vladimir Kochkin and Joe Wiehagen. (Wiehagen recently left his job at the Home Innovation Research Labs). Kochkin and Wieghagen wanted to come up with a wall that performs better than a typical 2x6 wall, but that isn't expensive or scary enough to disturb production builders.

Cantilevered plates

At its most basic, here’s the idea: builders should frame 2x4 walls on 2x6 plates. The 2x6 plates should be flush with the 2x4s on the interior, but should be proud of the studs on the exterior. The protruding plates leave room for 2 inches of rigid foam to be installed on the exterior side of the studs (see the close-up image below).

The OSB or plywood 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. is installed on the exterior side of the rigid foam. In this respect, an “extended plate and beam” wall resembles a wall with Zip R sheathing. (For more information on Zip R sheathing, see “Nailbase Panels for Walls.”)

In a recent phone interview, Kochkin told me, “The idea was to figure out a wall system that integrates foam sheathing but works more like a traditional wall system with the OSB on the outside.”

Kochkin and Wiehagen reasoned that builders would appreciate the fact that this wall system doesn’t require furring strips. It allows siding to be attached to a continuous layer of OSB or plywood, an approach that most builders are familiar with.

This wall system has a weak link, though: the rigid foam does not cover the top plates or the bottom plates; nor does the rigid foam cover the rim joists (see the close-up image at right). That means that this approach results in more thermal bridgingHeat flow that occurs across more conductive components in an otherwise well-insulated material, resulting in disproportionately significant heat loss. For example, steel studs in an insulated wall dramatically reduce the overall energy performance of the wall, because of thermal bridging through the steel. (and in most cases, more air leakage) than a system with continuous exterior insulation.

Laura Dwyer, the manager of the DuPont Building Knowledge Center, is a fan of the “extended plate and beam” system. In a video interview, Dwyer explained that the system “reduces 80% of thermal bridging” through a wall.

Advantages of this approach

Kochkin notes several advantages to this method of building walls. According to calculations made by the Home Innovation Research Labs, the system costs less to build than either a 2x6 wall with continuous exterior rigid foam or a double-stud wallConstruction system in which two layers of studs are used to provide a thicker-than-normal wall system so that a lot of insulation can be installed; the two walls are often separated by several inches to reduce thermal bridging through the studs and to provide additional space for insulation. (see Image #2, below).

Kochkin also notes that wall system is more adaptable to use by panelization plants than other types of high-R walls.

What about bracing?

Since this wall system includes a layer of rigid foam between the studs and the sheathing, conventional assumptions about wall bracing need to be reconsidered. After extensive testing, researchers concluded that this type of wall will be adequately braced as long as the sheathing is fastened with 3½-inch nails. Nails should be installed every 3 inches on panel edges and every 6 inches along studs.

While this approach works in most of the country, the developers of the system note that this type of wall is “not approved for high-seismic or high-wind areas.”

You may need an engineer to help you get code approval

This system is not yet recognized by the International Residential Code. Kochkin told me, “Code approval? We don’t have it yet.” Kochkin is hoping to get approval for this system at upcoming code hearings.

Until codes change, local code officials may give this approach the thumbs-down unless a builder submits engineering documents to justify the worthiness of the system.

What about 2x6 studs?

This system uses 2x4 studs and 2x6 plates. What if builders want to use 2x6 studs?

They can, but there’s a problem: 2x8 plates won’t be wide enough to work. The system requires 7½-inch plates. These can’t be purchased, so builders will have to rip their own plates by running 2x10s through a table saw. That’s labor-intensive. (If you try this, make sure that your table saw has a sharp blade and a short extension cord with high-gauge wire.)

What about rim joists?

Home Innovation Research Labs has produced a guide to this system called A Builder’s Guide: Extended Plate & Beam Wall System.

The guide provides several options for rim joists — noting, for instance, that builders can omit window headers and door headers if they double up their rim joists (as long as they remember that any joist installed above a window or door needs joist hangers).

While most “extended plate and beam” walls include standard rim joists — that is, rim joists without any exterior foam — builders can modify that approach if they want (see Image #3, below). The guide notes, “Insetting a double or single rim by 1 in. also meets 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. performance targets in lab tests, and improves thermal performance by making room for a continuous insulation layer of exterior rigid foam. A final option allows a 2 in. inset if the WSP [“wood structural panel” — that is, the OSB sheathing] spans the entire wall/rim assembly, and the scheduled fasteners connect to the sill plate.”

Siding attachment

Most types of siding can be fastened to the OSB or plywood sheathing without the need for long fasteners that extend back to the studs.

If you plan to install heavy siding, however, you may need long fasteners. The guide notes, “The IRC has included a table specifying attachment of siding weighing 3 psf or less (most fiber cement siding qualifies) to wood structural sheathing. … Alternately, siding or other types of exterior finishes may use nails or screws of sufficient length to attach through both the wood structural sheathing and 2-in. foam layer to the studs with penetration to the depth required by IRC Section R703.3.3, at the prescribed frequency.”

Why not just use Zip R?

Attaching OSB on the exterior side of rigid foam is similar to using Zip R sheathing — which raises the question, “Why not just buy Zip R?” That way, the rigid foam can extend over the plates and rim joists, not just the studs.

I posed the question to Patricia Gunderson, a research engineer at Home Innovation Research Labs. Gunderson responded, “Zip R is quite expensive. Potentially this [the ‘extended plate and beam’ wall] could be a more acceptable solution. Builders have different needs: Do they want OSB or plywood? Would they like 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. or 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. ? With this system, they have choices.”

This wall is not for GBA readers

In our wide-ranging discussion, Gunderson directed some of her comments to the readers of Green Building Advisor. “At your web site, you’ve attracted a small cohort of builders who have been pushing the envelope,” Gunderson told me. “This is not the wall for them. The wall does not provide enough of improvement for that cohort. They have already solved their wall problems. The idea with this wall is to pick up a large sector of the market, the builders who are now kicking and screaming. This will give them something that is doable with a much better result than what they are building now. This is not going to be a PassivhausA residential building construction standard requiring very low levels of air leakage, very high levels of insulation, and windows with a very low U-factor. Developed in the early 1990s by Bo Adamson and Wolfgang Feist, the standard is now promoted by the Passivhaus Institut in Darmstadt, Germany. To meet the standard, a home must have an infiltration rate no greater than 0.60 AC/H @ 50 pascals, a maximum annual heating energy use of 15 kWh per square meter (4,755 Btu per square foot), a maximum annual cooling energy use of 15 kWh per square meter (1.39 kWh per square foot), and maximum source energy use for all purposes of 120 kWh per square meter (11.1 kWh per square foot). The standard recommends, but does not require, a maximum design heating load of 10 W per square meter and windows with a maximum U-factor of 0.14. The Passivhaus standard was developed for buildings in central and northern Europe; efforts are underway to clarify the best techniques to achieve the standard for buildings in hot climates. wall; it’s not going to be a wall for a net-zero-energy house. Remember, there is a huge chunk of builders who will never adopt the strategies that you are promoting.”

More information

Designers or builders seeking more information on the “extended plate and beam” wall system should check out these documents:

Martin Holladay’s previous blog: “Building a Low-Cost Zero-Energy Home.”

Click here to follow Martin Holladay on Twitter.


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Image Credits:

  1. All images: Home Innovation Research Labs

1.
Oct 13, 2017 8:22 AM ET

Vapor barrier
by Randy Williams

I'm in zone 7, Northern Minnesota, where building inspectors are requiring a vapor barrier. I'm assuming the best option would be a "smart" vapor barrier. Would the 2 x 4 wall then get R-13 fiberglass bat giving the wall a total of R-23? I'm an energy auditor trying to convince builders to change the way they are approaching insulating and air sealing. This wall assembly may be a good start for some of them.


2.
Oct 13, 2017 9:01 AM ET

Edited Oct 13, 2017 9:28 AM ET.

Response to Randy Williams
by Martin Holladay

Randy,
Answers to both of your questions can be found in one of the documents I linked to (“A Builder’s Guide: Extended Plate & Beam Wall System”).

R-value calculations for the wall system can be found on page 3 of the Builder's Guide. The combination of R-10 rigid foam plus R-13 batts yields a calculated whole-wall R-value, according to the guide, of R-21.7. This value is less than R-23 because of thermal bridging through studs and plates.

Vapor retarder recommendations are found on page 13 of the Builder's Guide. The guide notes, "According to IRC Section R702.7 and Table R702.7.1, Climate Zones 1-4 do not require vapor retarders. Climate Zones 5-8 and Marine 4 require a Class I or II vapor retarder on above-grade walls unless certain conditions regarding vented cladding or continuous insulation are met. Because an EP&B wall includes a 2-in. layer of continuous insulation, in most cases a Class III vapor retarder may be used. ... If the configuration you’ve chosen does not qualify for Class III interior vapor retarder, then Home Innovation recommends a Class II vapor retarder, such as Kraft facing on batt insulation. Proprietary “Smart” vapor retarder products have perm ratings that rise with increasing relative humidity from 1 perm or less at normal conditions (Class II) up to 35+ perms (vapor permeable) in high humidity, and represent a “belt and suspenders” approach. Home Innovation discourages the use of polyethylene sheeting as an interior vapor retarder as it may create a double-vapor barrier condition, trapping incidental moisture and limiting drying."


3.
Oct 13, 2017 10:25 AM ET

Ripping 2x10s
by Reid Baldwin

If you want to use 2x6 studs, wouldn't it be easier (and better) to use thicker foam than to rip the 2x10 plates?


4.
Oct 13, 2017 10:49 AM ET

Edited Oct 13, 2017 10:50 AM ET.

Response to Reid Baldwin
by Martin Holladay

Reid,
A wall with 2x6 studs on 2x8 plates would require 1 3/4 inch foam (thinner foam than the 2 inches suggested). That's possible -- if done in two layers (a 1-inch layer followed by a 3/4 inch layer).

A wall with 2x6 studs on 2x10 plates would require 3 3/4 inch foam (thicker foam than the 2 inches suggested). That's possible, perhaps, if done in two layers (a 3-inch layer followed by a 3/4 inch layer). It should be noted that not many lumberyards carry 3-inch foam, although roofing supply houses probably do.

There are several problems with your suggestion:

1. The whole idea is simplicity -- one layer of rigid foam and you're done with the foam -- and low cost.

2. The wall bracing depends on the idea that the rigid foam is no thicker than 2 inches. Thicker foam would require new bracing tests and fastener schedules.

A builder who wants thicker foam is unlikely to adopt the extended plate and beam approach, with all of its disadvantages. Such a builder will simply install thicker foam over the entire wall, including over the plates and rim joists, and then install furring strips.


5.
Oct 13, 2017 4:04 PM ET

Cost?
by stephen sheehy

Can anyone estimate the cost of using ZipR compared with this system or using sheets of 2" foam over studs? I understand that ZipR costs more in material, but is it enough more that the labor savings don't make up for the additional cost?
And why no rain screen?


6.
Oct 13, 2017 4:38 PM ET

Edited Oct 13, 2017 4:39 PM ET.

Response to Stephen Sheehy
by Martin Holladay

Stephen,
The answer to one of your questions can be found by consulting the bar graph reproduced as Image #2.

According the that graph, based on calculations made by the developers of the extended plate and beam wall system, an ordinary 2x4 wall with 2 inches of exterior rigid foam costs $21.67 per square foot, whereas an extended plate and beam wall with 2x4 studs and 2 inches of rigid foam costs $21.12 per square foot.

The reports do not consider walls with Zip R sheathing. When I interviewed Kochkin and Gunderson, I thought that their responses to my questions about the Zip R alternative were rather weak.

The answer to "why no rainscreen?" is simple. They wanted to develop a low-cost wall for production builders.


7.
Oct 18, 2017 8:08 PM ET

Would this assembly meet 2012/2015 IECC?
by Bill Burke

Would this wall meet the requirements of the 2012/2015 Residential IECC? About half the population of the country, at least on paper, live in states that require that level of performance or greater. See https://www.energycodes.gov/status-state-energy-code-adoption.

I'm all for facing reality and moving the market, but can get depressed when I read, "The idea with this wall is to pick up a large sector of the market, the builders who are now kicking and screaming... Remember, there is a huge chunk of builders who will never adopt the strategies that you are promoting.”

Is the message that energy codes aren't enforced? They are enforced to a significant degree here in California. But Florida and Texas have adopted 2012/2015 IECC. Do production builders there meet code with 2x4 walls with insulation between the studs? I realize there are many states that haven't adopted recent code updates. So maybe I should feel good about this?


8.
Oct 19, 2017 4:17 AM ET

Response to Bill Burke
by Martin Holladay

Bill,
Q. "Would this wall meet the requirements of the 2012/2015 Residential IECC?"

A. There are several ways to comply with the IECC. Most builders choose the prescriptive path, but there are performance alternatives for builders who don't want to follow the prescriptive path.

As I noted in Comment #2, and extended plate and beam wall combining R-10 rigid foam plus R-13 batts yields a calculated whole-wall R-value of R-21.7. That satisfies the prescriptive requirements of the 2012 and 2015 IECC in Climate Zones 1 through 5, but not in Climate Zones 6 through 8.


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