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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.

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  1. Expert Member

    Here is a link to an article Martin wrote on the subject that may be useful:

    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

    I'm not sure the Mooney wall system is what you are looking for, but it's worth looking at a few alternatives.
    Mike Smith, who helped develop it, posts on this forum:
    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: (Worcester) (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

    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.

    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


    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

    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:


    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

  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. 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


    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.

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