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Community and Q&A

Rigid insulation on the interior face of the wall

architect_sean | Posted in Energy Efficiency and Durability on

I am considering putting 2″ of rigid insulation on the INSIDE of my perimeter wall in a home I am preparing to construct. This will serve as a vapor retarder and add r10 to the wall assembly that will include 5.5″ of dense cellulose (r20 + r10 is not a bad wall). Here is the breakdown of the wall I am proposing: hardi Plank on 1×4 furring strips, Tyvek or similar air barrier, plywood sheathing, 2×6 framing at 16″oc, 2″ rigid (xps) with taped joints, GWB.

By using plywood sheathing instead of OSB and including a 3/4″ drainage gap I expect the wall will be able to dry to the outside if water gets inside the wall and should avoid rot. Thoughts?

The real question in my mind is the impact of the interior rigid. I have never seen this done and am trying to determine why. The pros of this are clear: limits thermal bridging, issues of fastener sagging are eliminated (see explanation below), interior GWB meets code requirement for fire protection, insulation work can be done from the interior standing on the floor, stockpiling insulation inside the house is easy fast and cheep. Cons are casing extensions for Windows, electric outlets needs to be well sealed or located elsewhere. What else? The material cost won’t change BUT the labor is cut considerably (especially since I could do it myself before the GWB sub arrives. Installing it myself on the exterior is too much for a average skilled diy guy like myself.). So what am I missing here? What is the downfall of the assembly I am proposing?

I have gotten great advice from this forum in the past. Please share your thoughts again if you don’t mind.

Here is some background on the project that might help:
I am an architect preparing to build a house for my family south of Boston. I’ve posted questions about vapor retarders and insulation on perimeter walls previously but the mystery of this topic continues for me. The dialogue and advice has been terrific. I released the drawings for bid and have spent the last couple weeks analyzing the numbers and talking it through with the contractors. The guys I am talking with are folks I’ve worked with multiple times and the repore is good so I am continuing to learn more and more about these current thoughts on building science and sustainability (that’s why they call it practice). The conclusion that we’ve reached is that to add rigid insulation on the exterior is an expensive proposition both material wise (no surprise) and labor wise (bigger surprise than I expected). Working from staging, lifting material two stories, temporary fastening, concerns about screws sagging / furring shifting under the weight of cement board planking (if the rigid is more than the 1.5″ min thickness (I was proposing 3″) the moment on the screw can be significant and if spaced far apart it is likely they will cut into the rigid and sag a bit in a few years), and eastern MA labor costs are making this assembly unachievable on my budget.

I’ve designed many houses over the years and I consider myself to be a thoughtful advisor to my clients. Designing a house for myself over an extended period of time has given me the opportunity to consider every decision much more carefully than is typical in practice and the cost is much more of a personal issue. I’ve been an environmentalist since I was a cub scout in the early 70’s (yes, I am in my mid fifties and have practiced for 30 years) and want to do the right thing here but I have to admit I am realizing the financial cost of some of my sustainability decisions is very high and a challenge for my financial abilities. $13000 (material cost) worth of rigid insulation buys a lot of electricity and the electricity doesn’t need to be financed through a mortgage. Banks only lend 30% of your gross income so additional big ticket materials like this insulation cost are challenging to fit into a budget if excess cash is not available. These are more difficult choices than they seem and help explain why the average home is vastly less sustainable than I would like it to be.

I want to insulate this house really well (my original design called for a 12″ double stud wall which was totally out of the question price wise) but I need to design a way to limit the cost of that insulation as much as possible to make it financially achievable. Is the assembly I propose above that solution or is their an inherent flaw in it that I am missing?

Thanks for your thoughts and advice.

Replies

  1. Expert Member
    MALCOLM TAYLOR | | #1

    Sean,
    Here is a link to an article Martin wrote on the subject that may be useful:
    https://www.greenbuildingadvisor.com/articles/dept/musings/walls-interior-rigid-foam

    Have you considered some other wall assemblies where the work is performed from the interior and also eliminate thermal bridging - like Mooney Walls?

  2. architect_sean | | #2

    Malcolm, first thanks for reading the longest post ever put up on GBA! Second, I must have missed that article so Thanks for the link. I will check it out promptly. I am not familiar with a Mooney wall. i will do some research on the topic. Thanks again.

  3. Expert Member
    MALCOLM TAYLOR | | #3

    Sean,
    I'm not sure the Mooney wall system is what you are looking for, but it's worth looking at a few alternatives.
    http://www.builditsolar.com/Projects/Conservation/MooneyWall/MooneyWall.htm
    Mike Smith, who helped develop it, posts on this forum:
    http://forums.delphiforums.com/Breaktime_3/messages/?start=Start+Reading+%3E%3E
    He is a very good guy and I'm sure would answer any questions you have.

  4. architect_sean | | #4

    I've added those references to my reading list. Thanks again.

  5. architect_sean | | #5

    Malcolm, I just did a bit of reading on the Mooney wall. It has a lot of potential. If I substitute eps for xps in the wall I proposed the r value would not exceed that of the dense cellulose. That said, the Mooney wall provides the same R value as I was considering but with better fastening potential. Additionally, by inserting a square of sill seal or 1/4 xps at the overlap point of the stud and strapping of the Mooney wall I couple eliminate the thermal bridge pretty much all together.

    I am going to continue to think this through. Thanks again. Sean

  6. STEPHEN SHEEHY | | #6

    Sean: lots of sources for reclaimed insulation in MA. Using it can save you a pile of money.

  7. JC72 | | #7

    It appears with the Mooney Wall you still have thermal bridging occuring through the floors. Also it seems like a lot of work vs using dense mineral wool board (i.e Roxul Comfort Board IS) on the outside of the taped sheathing) with a rain screen. At least the MW will cover the outside of the mud sill/rim joist. MW is vapor permeable so you'll get drying to the outside.

  8. Dana1 | | #8

    Interior side foam makes all of the structural wood colder (= higher moisture content), is harder to air seal than exterior foam due the greater amount of cutting & fitting required for everything from electrical boxes & plumbing penetrations to floor & ceiling joists.

    A 2x4 + 2x2 Mooney Wall is a half-inch thinner than a 2x6 wall and slightly higher performance but higher labor. A 2x4 + 2x3 Mooney wall is a half-inch thicker than a 2x6 wall, but would allow you to insert a smart vapor retarder between the studs & girts while leaving space for the electric runs to be inside the vapor retarder. Installing 2" of reclaimed roofing polyiso on the exterior would be more than sufficient dew point control for the sheathing on either assembly, at about 1/4-1/3 the cost of virgin stock.

    But it's less work to install a 2x6 wall + 3" of reclaimed roofing polyiso foam, which would hit about ~R30 whole-wall which would have HUGE dew point margin at the sheathing. Some prefer to put 2-3" of reclaimed foam on the sheathing, then use foil-faced half-inch or 1 inch foil faced virgin stock on the exterior for ease of air sealing, despite the fact that an exteiror 1" costs as much a 3" or more of reclaimed roofing foam.

    Using cap screws or cap nails for the initial hanging of the foam is quick & reliable.

    For the same thickness wall it's higher performance and higher dew point margin to put 4" of foam on the exterior of a 2x4 wall than putting 2" of foam on a 2x6 wall, but either has sufficient dew point margin to skip the interior vapor retarders.

    Local resources for relcaimed & factory-blem foam:

    http://www.greeninsulationgroup.com/ (Worcester)

    http://www.nationwidefoam.com/ (Framingham)

    Pouring the foundation with insulated concrete forms ( ICF) and building the walls with the exterior foam co-planar (or slightly proud of) the ICF foam provides a complete thermal break at the foundion sill, and keep the foundation sill fully protected from interior moisture drives. An EPDM sill gasket is always a good idea for limiting ground moisture migration into the foundain sill. If opting for innie windows, use EPDM flashing tape as the Z-flashing to direct moisture out where the wall foam meets the ICF foam.

  9. GBA Editor
    Martin Holladay | | #9

    Sean,
    Dana gave you good advice.

    For me, the four main disadvantages of your suggested approach (as Dana correctly mentioned) are:

    1. There are far more penetrations on the interior than the exterior, complicating air sealing work.

    2. Your approach doesn't address rim joists.

    3. Your approach doesn't address partition intersections.

    4. With your approach, the sheathing stays cold and damp during the winter instead of warm and dry.

  10. architect_sean | | #10

    Regarding insulated rims: I planned to set the face of the stud wall so it overhangs 1" beyond rim and then sheath the rim joists with 1" of rigid to prevent bridging. Additional spray foam on the inner face of the rim should keep the cold out of the floor.

    If the spray foam at the rim is 8" thick you could effectively maintain / continue the plane of insulation of the Mooney wall.

    Electrical boxes are still a hiccup but I had already planned to eliminate all penetrations (other than doors and windows) in the outside wall by putting them in the wood framed floor and saling them well.

    lOts of good food for thought here. Thank you.

  11. architect_sean | | #11

    I have never looked into reclaimed insulation as I imagined it would be damaged and a nuisance to work with requiring patching and mending. If my fears are unsubstantiated I will definitely look into it. Savings there might offset the labor just enough and I will be able to keep the sheathing warm.

    Thanks again.

  12. Reid Baldwin | | #12

    In #5, Sean said "by inserting a square of sill seal or 1/4 xps at the overlap point of the stud and strapping of the Mooney wall I couple eliminate the thermal bridge pretty much all together."

    It sounds like you are thinking about thermal insulation like electrical insulation. With electrical components, the conductivity difference between conductors and insulators is many orders of magnitude, so a thin layer of insulation can effectively isolate conductors at very different voltages. With thermal insulation, that is not true. The conductivity of your conductor (wood) and your insulator (foam) differs by a factor of only 4-5. Therefore, a fraction of an inch of insulation doesn't make a substantial dent in the thermal bridging.

  13. Dana1 | | #13

    Reclaimed foam varies a bit in quality- I've seen stuff that looked perfect after 25+ years of service, and I've seen frost-damaged insect bored misery (and priced accordingly). Assume there will be a few dinged corner and dents (which can often be worked around by selecting the orientation while cutting around windows & doors etc), but on par the scrap rate due to sheets beyond use or in need of patching is under 10%. A few dents & dings in facers aren't a significant performance hit. When going for thick layers (6" +) and more than two layers there can sometimes be flatness issues due to variance in thickness due to long term shrinkage, but that's not usually a problem at 1-2 layers.

    An inch of polyiso cuts the heat transfer through the framing fraction of a 2 x 6 wall roughly in half, but that's still a substantial heat loss. Three inches cuts it by 3/4.

  14. STEPHEN SHEEHY | | #14

    I got some 4" and 2" reclaimed XPS from Green Insulation Group in Massachusetts.

    http://www.greeninsulationgroup.com

    Even after delivery to Maine, I saved a lot of money. The stuff was dusty, but otherwise pretty much perfect. The company was a pleasure to deal with.

  15. architect_sean | | #15

    Martin,

    In point #4 of your comments above you note the sheathing will stay wet & cold in the winter.

    I think Condensation is the wetness you are referring to. The sheathing will be plywood not OSB. Do you really see this as a problem if a substantial drainage gap is included to aid in drying? Is this condition different than the one in the article Malcom referenced?

    As for junctions of interior partitions and exterior walls I would propose to install the insulation before the interior partition is erected. The insulation would be continuous in that case.

    Any additional thoughts are appreciated.

  16. GBA Editor
    Martin Holladay | | #16

    Sean,
    Cold sheathing tends to be damp. The moisture comes from several sources. Some of the moisture comes from the exterior air. Some comes from wind-blown rain. Some comes from the interior of the house, via the mechanism of vapor diffusion. And some comes from the interior of the house, transported by exfiltrating air.

    Condensation doesn't really occur, however. The moisture that accumulates on the interior side of the sheathing in winter sometimes occurs as frost. If the weather is too warm for frost to accumulate, what you get is sorption, not condensation.

    If you install a continuous layer of insulation (usually rigid foam or mineral wool) on the exterior side of your wall sheathing, you'll keep your sheathing warm and dry all winter long.

    If you don't want to install exterior insulation, you should definitely include a ventilated rainscreen gap between your siding and your sheathing. That rainscreen gap will go a long ways toward keeping your sheathing safe.

    If you are planning to build a wall that will have cold sheathing in winter -- and it sounds like you are -- then your decision to specify plywood sheathing rather than OSB sheathing is a good one.

    Here are links to articles that discuss these principles in greater depth:

    How Risky Is Cold OSB Wall Sheathing?

    Monitoring Moisture Levels in Double-Stud Walls

    The Return of the Vapor Diffusion Bogeyman

    All About Rainscreens

  17. user-5474617 | | #17

    Sean, there is a system were you can have your contractor install the exterior foam and the OSB & an exterior poly film facer all with one panel. It has no structural defects because it attaches wood sheathing to wood stud. It utilizes 1 5/8" of clear poly-faced 2nd generation graphite enhanced (Neopor resin by BASF) EPS foam that is fully laminated to high quality aspen-based OSB. It is attached with gun that drives the nails to the face of the OSB. The r-value of the system is R8.75 and if you want to add more foam to the exterior to get to an R10 rating you could ask the manufacturer to skip the poly facer on the laminated panel & install 1/2" of poly-faced LCi15 (15 psi) or LCi25 (25 psi) Chrome for another R2.5 so this exterior applied wall assembly would end up adding an R11.25 to your overall wall R-value and more importantly is that you put it to the exteririor were it does the most good for your structure adding to the durability of the wood sheathing and dimensional framing by keeping it warmer and drier. The exterior poly facer makes taping and sealing the exterior wall assembly pretty easy, especially, if you are building your walls on the deck & lifting them up into place. While the walls are still down on the 1st & 2nd floor deck you can tape most of the seams fairly easily. If your contractor prefers building the frame & installing the insulating structural sheathing in a separate step he most likely will use a cherry picker (lift) or assemble scaffolding and tape can be applied when the panels are installed.

    The wall panels I'm suggesting are affordable and come in 4x8, 4x9 & 4x10 panel sizes. If you need more information on this insulating structural panel or the poly faced rigid foam go online & search Atlas EPS ThermalStar Products. Best of luck with your project, however, I don't think you will need much luck with all the excellent feedback you have already received from some very knowledgeable/experienced individuals here on GBA. Plus your own thought process were you are examining several different options & the fact you are open to different ideas I think helps you get to were you want & need to be.

  18. GBA Editor
    Martin Holladay | | #18

    The product that Robert Murphy is talking about is called ThermalStar LCi-SS™ (Laminated Continuous Insulation- Structural Sheathing). It's a type of nailbase panel that is designed to be installed with the OSB facing the studs and the rigid foam facing the exterior.

    Here are links to the relevant documents:

    Brochure

    Installation guide

    Information sheet

  19. GBA Editor
    Martin Holladay | | #19

    ThermalStar also manufactures two other interesting products:

    1. ThermalStar X-Grade, a type of EPS for below grade installation that includes termiticide.

    2. ThermalStar Inter-Grade, a type of EPS for insulating the interior side of basement walls and crawl space walls; the manufacturer claims that this product "Meets fire code compliance if left exposed - Fully tested."

  20. GBA Editor
    Martin Holladay | | #20

    ThermalStar products are manufactured by:

    Atlas EPS
    8240 Byron Center Ave SW
    Byron Center, MI 49315
    616-878-1568
    http://atlaseps.com

  21. architect_sean | | #21

    Martin & Robert, Thanks for the informative product info. It looks like it has the possibility of reducing that labor cost a bit. Today I was looking at the Insofar product line. https://insofast.com/products/ex-panels/ also very interesting because of the nailing built-in "stud" that allows the installation of furring without having to go all the way through to the structural studs (see concerns for sagging described in my epic entry above).

    Once again, thanks for the GBA Community for some real thought provoking discussion.

  22. architect_sean | | #22

    How do SIPs address the cold sheathing issue? A SIP is certainly a well insulated panel but the outside is OSB. Does the sheathing get wet & damaged due to the lack of exterior rigid insulation?

  23. Dana1 | | #23

    SIPs can and do become damaged from interior drive moisture accumulation at the seams if not properly sealed. This is more common on SIP roof ridges than walls though, since the stack effect makes leaks at the lower half of the wall infiltration points bringing cold dry winter air in, and the ridge leaks the exfiltration point, where more humid conditioned air is leaking out.

    Back-ventilated siding (rainscreen or vinyl siding) and roof overhangs to limit direct wetting is sufficient to protect SIP walls, as long as the window flashing & WRB are properly installed.

  24. Irishjake | | #24

    Sean,

    I am in the middle of building a zero energy home, with LOTS of insulation. I was able to source my EPS from Nationwide Insulation. I saved at least $30,000 dollars doing it that way (I bought 5 semi-truckloads of insulation). I did this after researching virtually every other insulation, cost, performance, etc. This approach will, hands down, be your best value, and the quality is great. This savings should allow you, as it has for me, to build the right way and get what you are looking for.

    I can also assure that SIPS will leak at their seams, even when taped and spray foamed. If you follow Dana's advice you won't go wrong.

  25. architect_sean | | #25

    I will pay a visit to Nationwide & Greeninsulation Group soon and look into this avenue. As you point out: it will let me build it the right way and get what I want.

    I presented this project to a group of college students recently. The idealism of a student is refreshing but there is so little time to address construction costs when they have the opportunity to see the cost implication of design decisions they are blown away. I walked through how to finance a project and how much the average Joe can finance. Then we went through the costs of the most efficient systems and insulation and they were charged up to find a way to solve the problem. I will definitely pass the info regarding recycled insulation on to them in my next follow-up. Thanks All.

  26. rmcarch | | #26

    White Paper 20+6fci wall 14Nov2
    PDF
    1 of 1
    Page 3 of 110
    White Paper 20+6fci wall 14Nov2.pdf
    14Nov23

    Full text w/ attachments, illustrations here: https://www.dropbox.com/scl/fi/hf23xgkwmdjrzb89qynh0/White-Paper-20-6fci-wall-14Nov2.pdf?rlkey=q6zr9y6r13nuprkehdjtepji6

    Energy Performance Compliant Alternates to Code Required R20+5ci Wall Assemblies
    by Gregory La Vardera, Hans Breaux, Micheal Maines
    Contents

    Summary

    Background

    Issues Inherent in R20+5ci assemblies

    Energy Performance Compliant Alternate - R20+6fci

    Recommendations

    Appendix
    Summary
    Recent Editions of the IBC and IRC include an expanding requirement for wood frame wall
    assemblies with a layer of R5 continuous insulation, often designated by the nomenclature of
    +5ci (R5 continuous insulation). Where these recent Code Editions are adopted it brings about
    a situation where +5ci insulation was once an option required only for 2x4 based walls, and
    now is a requirement for 2x6 based walls. The majority of builders and contractors working in
    these regions now face a non-trivial challenge of building new wall assemblies and adopting
    new methods to integrate continuous insulation into their work.
    While it is possible for these builders to avoid some of these new challenges by using
    alternate wall assemblies with performance equal to or exceeding the +5ci assemblies called
    for in the code, there is no recognition or validation of these alternate assemblies discussed or
    described in the Code text or tables.
    Alternate assemblies with equal performance can provide a more conventional building
    sequence, allowing builders and contractors to continue with their established methods at the
    exterior weather side of the wall assemblies. Instead of adding continuous insulation at the
    exterior, a layer is added at the interior allowing them to meet the energy requirements of the
    code with less construction complexity and reliable time and cost anticipation.
    Our purpose is to demonstrate the equal or better energy performance of an interior furred
    continuous insulation assembly utilizing an R6 furred semi-continuous layer at the interior side
    of a wall assembly – in code nomenclature this would be 20+6fci (R6 furred continuous
    insulation). For the sake of this demonstration we will focus on 2x6 based wall assemblies
    relying on R20 insulation in the main stud cavity. The principles can be applied to other model
    walls.
    We wish to request State and Local Code O
    ffi
    cials to recognize the compliance of this
    R20+6fci assembly, and work to include these configurations in future versions of the Code
    requirements, as well as advocate for their incorporation into the compliance documentation
    tools o
    ff
    ered to builders and designers (DOE’s Rescheck & Commcheck).
    Background
    While some regions have been working under the code requirement for R20+5ci and
    R13+10ci for some time, other areas in warmer climate zones have been for many years
    working under the requirement for only R20 or R13+5ci. In these areas builders have almost
    universally favored the R20 requirement because it avoids the complexity involved in adding
    the +5ci layer to a 2x4 R13 wall. Even though 2x6 stud walls cost more than 2x4 walls, the
    complexity of properly flashing and making a continuous exterior insulation layer, properly
    flashed and water shedding, made the R13+5ci assembly more costly in dollars and time. The
    R20 wall was by far the more popular choice.
    As the performance requirements of the code have increased, large regions have faced a
    transition where the R20 wall assembly was no longer compliant, and required the addition of a
    +5ci layer in order to meet code. One such transition has been the adoption of the 2021 code
    version in locations formerly under the 2018 version in climate zones 4 & 5.
    Ongoing in some regions has been the continuing requirement for R20+5ci and R13+10ci
    where exterior insulation work has been needed on the majority of stud framed structures in
    climate zones 6 & 7.
    These regions are all experiencing higher construction costs to meet these standards, and
    when in transition to these requirements are more vulnerable to building errors and failures due
    to the more challenging methods needed for weather tightness and water shedding in a
    continuous exterior insulation assembly. These walls are harder to detail and execute
    successfully.
    While alternates to wall assemblies with continuous exterior insulation exist, they are not
    described or offered as compliant options in the 2021 Table N1102.1.3 (R402.1.3)
    Minimum Insulation Values. Nor do the compliance documentation tools ResCheck and CommCheck offer assemblies in alternate configurations. This absence adds hardship to adopting alternate assemblies, as evidencing compliance to local code officials for each building project becomes a repetitive and costly task. Further there is no guarantee that local officials will believe or honor documentation of performance for alternate assemblies which can be simpler to build, and less costly in dollars and time. Issues Inherent in R20+5ci Assemblies
    While the Code does not stipulate that a +5ci insulation layer must be located at the
    exterior side of a wall assembly, this is the most obvious location for a continuous insulation
    layer. See Figure 1 (Code described R20+5ci wall assembly)

    A continuous insulation layer at the interior side is problematic because interior finishes
    such as drywall, and cabinetry, electrical and plumbing fit-out of the interior side of walls rely
    on fastening to wood framing. A continuous insulation layer pushes that framing away from the
    surface making direct fastening of finishes impossible.
    Double stud wall assemblies can isolate a continuous layer in an internal gap between the
    stud layers. However such a wall with two stud layers of R13 will yield a R26+5ci wall assembly
    greatly exceeding code requirements.
    Hence exterior continuous layer of R5 is the most likely outcome of the R20+5ci
    requirement of the Code. As US building cladding and window components and materials are
    largely configured for direct application to building sheathing, the addition of a continuous
    exterior insulation layer between the sheathing and the cladding introduces greater complexity
    to the wall assembly.
    Exterior continuous insulation layers are typically made with rigid insulation materials,
    because the insulation layer is required to support the application of a cladding system. As
    such it must be strong enough to support the cladding and dimensionally consistent to
    produce unwavering lines in the overlaid cladding. This insulation is either of a dense fiber type
    - mineral or glass wool; or wood fiber; or a rigid foam insulation material - XPS or PolyIso. The
    former group is largely vapor open, while the latter foam insulations are a vapor retarder or
    barrier when foil faced. The choice of these will introduce other challenges.
    When working with a vapor open exterior insulation the practice is typically to make the air
    and water barrier and drainage plane at the face of the sheathing and lay the exterior insulation
    over this plane. A ventilation cavity is typically made between the face of the insulation and the
    back of the cladding. Any water getting behind the insulation must be flashed back out to the
    ventilation cavity to be drained out of the bottom of the assembly. Water behind the insulation
    can dry outward through the vapor permeable insulation.
    However window openings complicate this system. US windows are made to be mounted
    via a fastening flange to the outside of the wall sheathing. The windows typically project past
    the sheathing and create an edge where cladding can be terminated against the window frame.
    When a continuous insulation layer and ventilation space is added to the exterior side it
    disrupts the anticipated relationship between cladding and window frames. The cladding must
    either be trimmed with returns to close the gap between window and cladding, or the window
    opening must be projected through the exterior insulation layer with an additional face
    mounted frame. These modifications can create vulnerable points for water to enter the wall
    assembly.
    There is not yet one broadly adopted strategy for detailing of window openings through
    exterior insulation. Builders in climate zones 6 & 7 who have been under the R20+5ci
    requirements for many years are still using a variety of methods to deal with this. Builders in
    climate zones 4 & 5 are facing these issues for the first time under recent code updates, and
    the risk of building errors is higher than normal. The introduction of the exterior insulation layer
    upends the methods used for years by builders to ensure weather tight work at the exterior of
    their walls.
    When working with a vapor retarding exterior insulation such as the foams, a different set of
    challenges presents itself. Treating the vapor closed insulation in a similar manner to the vapor
    open insulation can have bad results. If water is allowed behind the foam insulation it will not
    dry outward as the insulation is a vapor retarder. This means the outward face of the insulation
    must be treated as the water & air barrier and drainage plane. As the surface of the foam can
    not be lapped like traditional house wraps or building paper this means builders must rely on various tapes to flash these conditions. Windows face similar dilemmas described for vapor
    open insulation. Window units can be potentially mounted at the exterior surface of the
    insulation if it can be relied on to provide a stable substrate to fasten the windows back to the
    framing. It is not ideal.
    Further complicating the use of a vapor retarding exterior insulation is mixed requirements
    for continuous insulation R-values in the 2018 IRC. For Climate Zone 6 the code called for a
    minimum of R11.5 for the continuous insulation on a 2x6 wall in Table R702.7.1. However that
    R11.5 requirement was not mentioned in the Insulation section of the code where the minimum
    wall assembly required by Table N1102.1.2(R402.1.2) was R20+5ci. The code fails to point out
    here that the higher R-value is called for when using a vapor retarding exterior continuous
    insulation to correctly manage water vapor. The apparent conflicting direction here adds
    confusion to working with vapor retarding continuous exterior insulation.
    While the regions who have long been under the requirement for R20+5ci have risen to the
    challenge, it no doubt has come with additional cost in dollars and time. Yet there are alternate
    wall assemblies available of equal or better performance than Code requirements. There is not
    any reason why these alternates can not be referenced in the Code to encourage more
    frequent use and an easier transition for regions recently adopting these requirements.
    Energy Performance Compliant Alternate - the R20+6fci wall
    Wall assemblies utilizing an interior side insulated service cavity offer similar performance
    to exterior continuous insulation layers. This wall element is referred to as a service cavity
    because it allows for wiring, electrical boxes, and limited plumbing to be run without making
    penetrations in the wall’s vapor control sheet which can be located at the interior side of the
    studs behind the service cavity. This greatly simplifies achieving better air tightness in the wall
    assembly as electrical boxes need not puncture the air-tightness membrane. On top of this the
    cavity provides an ideal location for a continuous insulation layer.
    We have made 3d thermal performance models of the Code described R20+5ci assembly
    with the continuous insulation layer at the exterior, and the proposed service cavity assembly
    with the furred insulation layer at the interior. These performance models confirm that the furred
    continuous insulation assemblies meet or exceed the performance of the Code described
    assemblies.
    The service cavity is formed by applying horizontal furring members across the interior face
    of the main wall studs. These furring members would be 2x2 furring strips (1 1/2” x 1 1/2”), and
    the furring is spaced at 24”oc vertically. This provides for conventional fastening for drywall
    panels and other finishes. See Figure 2 (R20+6fci wall assembly)

    The service cavity is insulated to its full 1 1/2” depth, typically R6 when insulated with
    medium to high density batt insulation such as mineral & glass batts. The furring strips do
    reduce the insulation level across their runs, and do create a minor thermal bridge where they
    cross and are fastened to the studs. However in comparison to continuous R5 insulation the
    increase to R6 will wholly overcome the performance reductions of the furring strips.
    Although the horizontal furring members do make these minor compromises to thermal
    performance, the insulation still provides the function of a near continuous insulation layer by
    breaking the thermal bridging of the studs from exterior to interior. As such we propose a new
    nomenclature for this wall configuration. Where “continuous insulation” is referred to in the
    code as “ci” we believe a new designation should be made for “furred continuous insulation”
    and indicated as “fci”.

    See Appendix 1 for the performance modeling data for the Code described R20+5ci
    Assembly id1 - note U-Value of 0.041 Btu/hr ft

    See Appendix 2 for the performance modeling data for the proposed R20+6fci
    Assembly id3 - note U-Value of 0.039 Btu/hr ft

    This modeling data shows a slightly higher performance value for the service cavity
    equipped wall R20+6fci. This wall assembly will be compliant for any situation requiring the
    Code described R20+5ci configuration. The modeling was conducted with WUFI®Passive V.
    3.3.0.2 and is an industry standard for energy performance modeling of building assemblies.
    Small improvements to the R20+6fci assembly yield even better results:

    Utilizing R23 5 1/2” mineral wool batts. See Appendix 3 for the performance modeling data
    for the proposed R23+6fci, Assembly id2 - note U-Value of 0.037 Btu/hr ft

    Utilizing R23 5 1/2” mineral wool batts and studs at 24”oc. See Appendix 4 for the
    performance modeling data for the proposed R23+6fci, Assembly id4 - note U-Value of
    0.036 Btu/hr ft

    Comparing constructibility of the service cavity wall to the exterior continuous insulation
    wall reveals its simplicity and expediency. Because the exterior work is unchanged from
    traditional R20 only assemblies, the service cavity walls can be brought to dry-in in the same
    amount of time and work effort that was common prior to Code updates. And in fact there is no
    new work or construction operations required at the exterior.
    At the interior side of the service cavity wall, construction operations required to build out
    the service cavity consist of materials and operations previously used on simple R20 only
    walls. No new or unique work operations are required. Wood furring is nailed, batt insulation
    installed, drywall hung and finished, trimmed, and painted. The only additional work required is
    the installation of horizontal furring, and the install of 1 1/2” R6 batts. These are very simple
    operations, with no quality issues that could impact the weather tightness performance of the
    assembly. The increased energy performance comes with much less additional effort, and no
    new technical skills. The service cavity adds performance and value to the wall assembly much
    more efficiently than continuous exterior insulation.
    While the service cavity is very simple and does not demand new skills from trades, it does
    change the approach to electrical wiring on these exterior walls. At these locations it is
    necessary to mount devices in 4x4x1.5 square boxes rather than conventional deep wall
    boxes. This does not present any great difficulties as the 4x4 boxes typically have a greater

    interior volume than standard single gang boxes. When 4x4 boxes are used with a device plate
    reducer cover matching the depth of the drywall, these boxes can easily accommodate bulky
    devices such as dimmers or GFI receptacles. Wires are routed at the back of the service cavity,
    and covered by the R6 insulation, and where running vertically are passed behind the
    horizontal furring member providing the code required depth of cover along drywall screwing
    locations. This small change to electrical work at exterior walls takes no more time or cost than
    wiring in a conventional R20 only wall.
    Recommendations
    We recommend a formal acknowledgement from the State Code Department that the
    R20+6fci assemblies are Code compliant for all instances calling for an R20+5ci assembly. We
    believe that by acknowledging this the Department will facilitate the adoption of this approach
    and lift a hardship from both builders attempting to transition to higher performance
    assemblies, as well as offering a simpler option to builders who have been working under the
    R20+5ci requirement for some time. Further, an acknowledgement from the State Code
    Department will remove any doubts or resistance from local code officials to accepting these
    assemblies.
    We are aware that the new 2021 U-Factor requirement table N1102.1.2 (R402.1.2)
    Maximum Assembly U-Factors and Fenestration Requirements
    will already establish these
    assemblies as compliant. Both assemblies test below the code required 0.045 U-Factor,
    showing compliance. However the meaning of U-Factors and R-Values are not always
    understood by builders. The prior and continuing R-Value requirement table N1102.1.3
    (R402.1.3) is much more instructive to builders as well as owners, because it speaks directly to
    the layered nature of these higher performance assemblies. It describes the wall layers they
    should be building for compliance. It anticipates the types of walls that will be built and clearly
    states what will be compliant. We recommend that service cavity wall assemblies, and the
    furred continuous insulation nomenclature be adopted and presented side by side with the
    continuous insulation nomenclature, so that this approach and technique can be more
    accessible to builders and designers. This can be done by edits to the State adopted versions
    of the IBC and IRC, concurrently with advocating to the ICC that they adopt these additional
    format assemblies and nomenclature into the model codes. Until such a time that updates are
    issued the Department can also share this information through their newsletters and bulletins
    to ensure that Code Officials in the field are aware.
    We also recommend that the State Code Department advocate to the DOE that future
    versions of the ResCheck and CommCheck compliance packages add these furred continuous
    insulation wall assemblies to the software used by builders and designers to document
    compliance with the Energy Code. Again, by incorporating these assembly types into the tools
    we use it will help advance the use and knowledge of these effective wall types.

    Appendix
    (attached)
    Contents:

    Appendix 1 - Performance modeling data for the Code described R20+5ci assembly

    Appendix 2 - Performance modeling data for the Proposed R20+6fci assembly

    Appendix 3 - Performance modeling data for the Proposed R23+6fci assembly

    Appendix 4 - Performance modeling data for the Proposed R23+6fci @24”oc assembly
    7
    of
    7
    White Paper 20+6fci wall 14Nov2.pdf
    WUFI®Passive
    WUFI®Passive V.3.3.0.2: Project CO+OP/
    Page
    1
    Assemblies/window types
    Assembly (Id.1): Wall
    -
    Code (Nominal 20+5ci)
    Nr.
    Material/Layer
    (from outside to inside)
    U
    [lb/ft
    ³
    ]
    c
    [Btu/lb°F]
    O
    [Btu/hr ft °F]
    Thickness
    [in]
    Color
    1
    Roxul ComfortBatt
    2.25
    0.2
    0.0208
    1.25
    2
    8.12
    0.55
    1.3289
    0.039
    3
    28.72
    0.45
    0.0537
    5.5
    4
    Kraft Paper
    0.36
    0.2427
    0.039
    5
    Gypsum Board (USA)
    53.06
    0.21
    0.0942
    0.5
    Exchange materials
    6
    Low Density Glass Fibre Batt
    Insulation
    0.55
    0.2
    0.023
    ---
    Weather Resistive Barrier
    E
    a
    s
    t
    e
    r
    n
    W
    h
    i
    t
    e
    P
    i
    n
    e
    + Low Density Glass Fibre Batt Insulation
    7
    .
    4
    9
    White Paper 20+6fci wall 14Nov2.pdf
    WUFI®Passive
    WUFI®Passive V.3.3.0.2: Project CO+OP/
    Page
    3
    Assembly (Id.3): Wall
    -
    Proposed (20+6ci)
    Nr.
    Material/Layer
    (from outside to inside)
    U
    [lb/ft
    ³
    ]
    c
    [Btu/lb°F]
    O
    [Btu/hr ft °F]
    Thickness
    [in]
    Color
    1
    8.12
    0.55
    1.3289
    0.039
    2
    Oriented Strand Board
    40.58
    0.45
    0.0532
    0.5
    3
    28.72
    0.45
    0.0537
    5.5
    4
    0.039
    5
    28.72
    0.45
    0.0537
    1.5
    6
    Gypsum Board (USA)
    53.06
    0.21
    0.0942
    0.5
    Exchange materials
    7
    Low Density Glass Fibre Batt Insulation
    0.55
    0.2
    0.023
    ---
    8
    2.25
    0.2
    0.0208
    ---
    Kraft Paper
    E
    a
    s
    t
    e
    r
    n
    W
    h
    i
    t
    e
    P
    i
    n
    e
    + Low Density Glass Fibre Batt Insulation
    Weather Resistive Barrier
    E
    a
    s
    t
    e
    r
    n
    W
    h
    i
    t
    e
    P
    i
    n
    e
    + Mineral Fibre Insulation
    Mineral Fibre Insulation
    7.49
    0
    .
    3
    6
    0.2427
    White Paper 20+6fci wall 14Nov2.pdf
    WUFI®Passive
    WUFI®Passive V.3.3.0.2: Project CO+OP/
    Page
    2
    Assembly (Id.2): Wall
    -
    Proposed (23+6ci)
    Nr.
    Material/Layer
    (from outside to inside)
    U
    [lb/ft
    ³
    ]
    c
    [Btu/lb°F]
    O
    [Btu/hr ft °F]
    Thickness
    [in]
    Color
    1
    8.12
    0.55
    1.3289
    0.039
    2
    Oriented Strand Board
    40.58
    0.45
    0.0532
    0.5
    3
    28.72
    0.45
    0.0537
    5.5
    4
    0.039
    5
    Eastern White Pine
    28.72
    0.45
    0.0537
    1.5
    6
    Gypsum Board (USA)
    53.06
    0.21
    0.0942
    0.5
    Exchange materials
    7
    2.25
    0.2
    0.0208
    ---
    Kraft Paper
    Weather Barrier
    E
    a
    s
    t
    e
    r
    n
    W
    h
    i
    t
    e
    P
    i
    n
    e
    + Mineral Fibre Batt Insulation
    Mineral Fibre Insulation
    7.49
    0
    .
    3
    6
    0.2427
    White Paper 20+6fci wall 14Nov2.pdf
    WUFI®Passive
    WUFI®Passive V.3.3.0.2: Project CO+OP/
    Page
    4
    Assembly (Id.4): Wall
    -
    Proposed (23+6ci)
    -
    24OC
    Nr.
    Material/Layer
    (from outside to inside)
    U
    [lb/ft
    ³
    ]
    c
    [Btu/lb°F]
    O
    [Btu/hr ft °F]
    Thickness
    [in]
    Color
    1
    8.12
    0.55
    1.3289
    0.039
    2
    Oriented Strand Board
    40.58
    0.45
    0.0532
    0.5
    3
    Eastern White Pine
    28.72
    0.45
    0.0537
    5.5
    4
    0.039
    5
    28.72
    0.45
    0.0537
    1.5
    6
    Gypsum Board (USA)
    53.06
    0.21
    0.0942
    0.5
    Exchange materials
    7
    2.25
    0.2
    0.0208
    ---
    Weather Resistive Barrier
    E
    a
    s
    t
    e
    r
    n
    W
    h
    i
    t
    e
    P
    i
    n
    e
    + Mineral Fibre Insulation
    Mineral Fibre Insulation
    Kr
    a
    f
    t
    P
    a
    pe
    r
    7.49
    0
    .
    3
    6
    0.2427

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