Image Credit: All images: Home Innovation Research Labs
More Musings of an Energy Nerd
Production builders in the U.S. love 2×4 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 2×6 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 NAHB 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 2×6 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 2×4 walls on 2×6 plates. The 2×6 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 sheathing 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…
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8 Comments
Vapor barrier
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.
Response to Randy Williams
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."
Ripping 2x10s
If you want to use 2x6 studs, wouldn't it be easier (and better) to use thicker foam than to rip the 2x10 plates?
Response to Reid Baldwin
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.
Cost?
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?
Response to Stephen Sheehy
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.
Would this assembly meet 2012/2015 IECC?
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?
Response to Bill Burke
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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