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Musings of an Energy Nerd

How to Design a Wall

Common-sense advice for people who can’t decide how to build their walls

Double-stud walls interrupt thermal bridging through the studs by providing room for insulation between two parallel stud walls.
Image Credit: Lucas Durand

Builders love to talk about walls. Almost all of us are willing to argue about the best way to build a high-R wall, and we love to debate whether certain wall details are environmentally friendly enough to be considered “green.”

Although these conversations can be fun, our obsession with wall details is often misplaced. Details that inflame our passions are often irrelevant. In most cases, we should just choose a relatively airtight easy-to-build wall with good flashing details — one with an R-value in the range of R-20 to R-40 — and be done with it.

I probably shouldn’t admit this fact, but it’s true: when a GBA reader posts a question proposing a new type of wall assembly, I sometimes sigh. I wonder whether it’s time to dial back our wall discussions and to spend more time talking about air barriers or windows.

Two popular approaches

Now that I’ve gotten my rant out of the way, I’ll provide some advice on walls. For readers who don’t have time to get bogged down in details, here’s the short version of my wall advice:

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  1. C. B. | | #1

    This is one of the best articles written on GBA in a while. Important. Detailed (without being overly so). Thorough.

    In my humble opinion, this type of article needs to be seen more frequently on GBA and fewer articles on green energy philosophy.

    Nice job, Martin!

  2. GBA Editor
    Martin Holladay | | #2

    Response to C.B.
    Thanks for your feedback. A fairly complete list of all of my "how to" articles can be found on this page: How To Do Everything.

  3. Expert Member
    Malcolm Taylor | | #3

    I agree
    Great article. However if Martin thinks this will in anyway stem the flood of wall innovations from posters I think he is going to be disappointed.
    The photo of Lucas Durand's build made me realize we never got to see the end result. I hope he drops in here sometime to update us on how it turned out.

  4. Ethan ; Climate Zone 5A ; ~6000HDD | | #4

    Builder, architect, CPHC?
    Thank you for the information... Perhaps we could modify the assertion that "[t]he best type of wall is the one your builder prefers" to read "[t]he best type of wall is the one your builder, architect, or CPHC prefers. I've had too many builders value engineer all the extra insulation and fancy sheathing out of my walls to be comfortable with that assertion as currently written.

  5. GBA Editor
    Martin Holladay | | #5

    Response to Ethan Timm
    If a residential construction project has a team that includes an architect, a builder, and a homeowner -- and in some cases a certified Passive House consultant -- then of course everyone has to get on the same page, ideally during pre-construction conferences. Everyone has to be on board: the builder, the designer, the homeowner, and any consultants who are part of the team.

    If, in your experience, you have had jobs where the "builders value-engineer all the extra insulation and fancy sheathing out of my walls," you are talking about a project (and a team) with serious communication problems. On jobs like that, the team has to come together to resolve the communication problem, or you will end up with many more issues than a problem with missing "fancy sheathing."

  6. KEVIN ZORSKI | | #6

    Martin - Nice review article.

  7. Eric Habegger | | #7

    A further simplification
    Martin, since we are on the subject of simplification perhaps a further simplification can be made. I'm sure some people will disagree with my assessment but here goes: If you live in zone 4 or warmer then ditch the double wall approach. It's a needless complication and expense. It's so obvious maybe you felt you didn't need to say it.

    I would go one step further and use a sheathing in those zones that integrates the sheathing with the insulation, waterproofing, and an integrated flashing technique. Just choose the sheathing with the proper level of insulation for your zone and the level of insulation between the studs. One more thing, since it seems one can never depend on people using common sense, make sure that your team is up to date on the very slight changes in flashing required for a system like that, such as the zip system. Otherwise one negates all of the very real advantages of using that system. I can easily imagine that happening if one is too over confident.

  8. GBA Editor
    Martin Holladay | | #8

    Response to Eric Habegger
    Excellent comments. I will edit my article to reflect some of your recommendations.

    My only quibble: I'm reluctant to advise builders to use sheathing (Zip-R sheathing) from a single manufacturer. While all reports indicate that Zip sheathing and Zip-R sheathing are excellent products, it's hard to predict whether problems will arise with these products in the future, or whether Huber Engineered Woods will still be in business in 10 years.

  9. Charlie Sullivan | | #9

    Are they really so easy?
    I generally agree with the theme here, that we should stop re-inventing high-r-value wall assemblies for each job, and that people should pick one of these two, based on what the team (particularly the builder) is comfortable with and proceed. But the reality is that both are still tricky and can be unnecessarily expensive. I think there's a real need to keep refining the systems for each and sharing and discussing experiences.

    For example, I used to be a true believer in dense packed cellulose, based on it's ability to "fill all of the wall's nooks and crannies, without leaving any voids," which I'd heard contrasted to the need for meticulous installers to install batts without major deficiencies, a very real problem. But cellulose doesn't magically install itself -- when I tried to get my deep double stud wall packed by what was supposedly the best crew in the area, it took more visits than I can count, plus the help of a nationally recognized expert (Bill Hulstrunk) before we got a a halfway decent result.

    That experience increases my appreciation for the Lstiburek Ideal Double-Stud Wall Design, because the plywood in the middle divides the cavity into two smaller cavities. Each of those is easier to fill because it's more like what installers are used to and because you don't need as high density to prevent settling in a cavity that's not as deep. Furthermore, you can use an air-permeable netting (insulweb) to retain the cellulose on both sides, avoiding the challenge of blowing insulation into an air-tight cavity that has Intello membrane on the inside. And you have the additional real-world advantage that the mid-wall air barrier requires less coordination between trades in the process of getting to an air-tight wall.

    And once you pick a wall construction method, you also need to figure out how to install the windows, which results in another round of re-inventing the wheel, and, once you decide what to do, a round of building complex stuff on site that I think would have been better to build in a window factory. Window manufacturers, particularly those that specialize in high-performance triple-pane windows, should realize that a lot of their customers are re-engineering the installation systems. They should offer the option of windows that come ready to install in deep walls, with a straightforward flashing system with pre-cut pieces that anyone can install right. I think that part of the reason they don't do that is that there are so many different approaches to high R-value walls that it doesn't make sense for them to engineer a different system for every one. Maybe the result of Martin's article will be more standardization of wall construction, enabling windows to be engineered to match.

  10. David Gadbois | | #10

    Martin, what can we say? Many
    Martin, what can we say? Many of the folks around here are often creative types thinking outside of the box. I am considering adapting a medium-gauge steel frame system with Joe L's perfect wall. No wood, no drywall. There is no blueprint or even precedent when you get creative. So we need to ask the smart folks here about it.

  11. Expert Member
    Malcolm Taylor | | #11

    You wrote: "There is no blueprint or even precedent when you get creative."

    I don't think that is true on any field, and certainly not in construction, which is an applied science. Creativity allows the advancement of any field of human culture by building on past achievements - and that can only be done if you have a firm knowledge of the foundations of the field you are trying to advance. It is a true for building science as it is in physics, software development, poetry or painting. The important innovators put in the hard work to understand what is at stake and what is at play. What you are calling creativity is simply idle speculation.

  12. GBA Editor
    Martin Holladay | | #12

    Response to David Gadbois
    If you plan to put all of the wall insulation on the exterior side of the sheathing -- following the PERSIST method -- then there won't be any thermal penalty from using steel studs. You can do that.

    The main downside of the PERSIST approach is that some builders are tempted to skimp on R-value. The best PERSIST walls exceed code-minimum R-values. In cold climates, that requires a lot of rigid foam.

    I'm not sure what you mean by "No drywall." I guess that means either (a) that you will finish the interior side of your steel studs with a material other than drywall -- perhaps plaster? -- or (b) that you want to look at the empty steel stud bays. If (b), make sure that it is legal to leave your wiring exposed -- and get ready for lots of regular dusting.

  13. GBA Editor
    Martin Holladay | | #13

    Response to Malcolm Taylor
    In your comments, you're implying that David Gadbois may not have a firm understanding of building science principles. I'm inclined to give him the benefit of the doubt on that point, and to address his creativity without impugning his knowledge.

  14. GBA Editor
    Martin Holladay | | #14

    Response to Charlie Sullivan (Comment #9)
    You've raised some important issues. I'll address them one at a time.

    1. The types of walls described in this article are "unnecessarily expensive." This is a real issue, especially for Passivhaus builders. These days, energy is cheap. Moreover, PV systems continue to drop in price. Old rules of thumb -- for example, "cold climate builders need to aim for R-40 walls" -- no longer apply. Trying to figure out whether to invest in an expensive wall when energy is cheap is quite tricky. The answer depends on many factors, including how long you expect to live in the house and your predictions for future energy costs. Suffice it to say that you're right -- we need to avoid unnecessary expense. In some cases, it may make sense to build a wall that barely meets code minimum R-value requirements -- especially if the builder is willing to pay close attention to airtightness.

    2. In some areas of the country, it's hard to find contractors who are experienced at installing dense-packed cellulose. This is, indeed, a real problem. If you see signs that your insulation contractor is in over his (or her) head, it may be time to specify a different type of insulation.

    3. Lstiburek's double-stud wall recommendations make sense. I agree with you, which is why I mentioned this approach and included a link to an article describing it.

    4. Figuring out how to install windows is complex. I agree, and I said as much in my article on the topic (Installing Windows In a Foam-Sheathed Wall). I can't think of an obvious way to solve this problem, though, short of the manufactured housing solution. Lots of things in life are complex, including a few necessary skills (like, say, performing an appendectomy) that experts go to school to learn. Sometimes we just have to acknowledge that mastering a few complex skills is part of our job as builders.

  15. Expert Member
    Malcolm Taylor | | #15

    Some of the window innovations you'd like to see are already occurring. Several of our local manufacturers offer "rain screen windows" where the flange is set further back so the frame covers the rain screen gap. I would imagine that once the innovation slows and several wall systems are widely adopted by large national builders, then manufacturers will begin to market products that will cover deeper layers of exterior insulation.

  16. Expert Member
    Malcolm Taylor | | #16

    My comments weren't meant as a criticism of David, but rather a common approach to building design I often see here on GBA, which I think works on flawed logic. The thinking seems to work something like this: Since existing building assemblies aren't energy efficient enough, that leaves me free to make up my own. This extends from wall assemblies to other elements of the house, and often to the architectural design of the house itself.
    At the risk of hurting a few feelings, I wonder if it wouldn't be worth looking at all the important innovations that have occurred in building science over the last few years and seeing if any of them are the result of this "creativity"? Do the houses that come out of this end up better from a building science and design standpoint? Does each house need a different set of assemblies thought up by the owner, or would it be more fruitful to look to the people doing the hard slogging and see what they have come up with?

  17. GBA Editor
    Martin Holladay | | #17

    Response to Malcolm Taylor
    You seem to distinguish between "people doing the hard slogging" and people who are interested in "creativity." I imagine, however, that a Venn diagram of these two groups would show a lot of overlap.

    Here are GBA, we certainly do our best to explain the findings of building scientists. We're all learning: scientists as well as the rest of us (their disciples). If our logic here at GBA is occasionally flawed -- and I don't doubt it sometimes is -- someone almost always shows up to point out the flaws and urge that the flaws be corrected.

  18. Expert Member
    Malcolm Taylor | | #18

    Perhaps it's generational thing, but "creativity" and "talent" used to be attributes that were bestowed by others, not claimed by individuals or groups.

  19. David Gadbois | | #19

    To clarify
    To clarify for Malcolm, my comment was responding to "when a GBA reader posts a question proposing a new type of wall assembly, I sometimes sigh."

    I do believe that there are plenty of wonderful energy-efficient wall designs out there. What makes people want to go off the beaten path? Well, many things can. I'm in a mountainous area of California, where we have a wildfire hazard. So I favor non-combustible materials for most things. Second, the frame system I selected (Blue Sky steel frame, only in California so far) already puts me well off the beaten path. They have a baseline wall design they are used I can adapt that and tweak it so that I have a Pretty Good House. Third, well, there are just certain materials I'd like to avoid, I don't need to rehearse the weaknesses of wood and gypsum board. To sum, there are many factors besides energy efficiency in play.

    New products and new techniques will always drive creativity.

    I am an aerospace engineer, BTW, who voraciously reads GBA so...I hope I am at least conversant in building science?

    Martin, to your points I'm actually trying to find a fiber cement panel that is appropriate for interior walls. And, yes, the Roxul board will all be outside of the studs.


  20. Mitchell Costa | | #20

    Maximizing useable under roof space
    Martin, Excellent article and thanks for simplifying the choices. I'm building a home on a limited lot space and am looking for a good wall that uses minimum space on the main floor (walk out basement will be ICF walls because space isn't as critical down below). I'm in an official zone 5 area in CA, but really closer to zone 4 at the edge of the zone where we only very rarely dip below 25F, or go above 90F. Everything I've read indicates that 2x4s on 16" centers with OSB siding for shear strength provides a very strong wall, and I plan to top that with 2" EPS on the exterior. That gives R-8 from the EPS plus R-13 from fiberglass bats inside for an R-21 wall with great airtightness and little thermal bridging. Upgrading to polyiso takes it up to R-24 and adds better fire and water resistance, but also significant cost. The assembly comes out to mass production standard 2x6 wall thickness to simplify finishing somewhat. The extra useable space provided by 2x4s vs 2x6s in this assembly seems like it would be good for many situations. Is simplification the only reason you didn't include the 2x4 option, or is there another drawback to 2x4s that I'm missing?

  21. GBA Editor
    Martin Holladay | | #21

    Response to Mitchell Costa
    Thanks for your valuable comments, and thanks for the reminder that 2x4 wall framing is often perfectly adequate in warmer climate zones. I have edited my article to reflect that fact.

    One quibble: When R-13 batts are inserted between 2x4 studs, you don't end up with an R-13 wall. Depending on the framing factor and the quality of the installation work, the whole-wall R-value of a fiberglass-insulated 2x4 wall may be as low as R-9.7.

    If you add R-8 of rigid foam to the exterior side of this type of wall, you end up with a wall rated at about R-18, not R-21.

  22. Expert Member
    Malcolm Taylor | | #22

    You are right and I wasn't seeing it in that light. Of course we should modify our houses to fit the demands each region or climate makes on them. What I find unproductive is the starting from zero each time approach of so many posters. As an engineer you know your whole profession is built up on a carefully tested knowledge base. You don't just dream up components, you come from a place of experience and presidents. At least I hope you do, or I'm going to be a lot more nervous when I fly :)

  23. Steve Knapp CZ 3A Georgia | | #23

    Suggestion for David Gadbois
    Have you considered using MGO board ( as an alternative to drywall?

  24. Eric Habegger | | #24

    It's interesting when
    It's interesting when discussing building construction in fire country. I happen to live in Lake County Ca , where we had a conflagration this past summer which destroyed over a thousand homes and caused (I think) four fatalities. About 1/10 of the homes in our small county were destroyed. I don't speak of it much because though I wasn't affected directly if you live in this county then you are affected emotionally. You can't help but be.

    On the subject of new construction in this county. Well, to me the best advice is not to live in the most rural parts of the county that is farthest from local fire fighting resources. The fire that engulfed the homes here was so hot that its not entirely clear that one could survive even in a home that was invulnerable to fire. That is simply because of the heat generated by the combustion of the forest, and other homes, that were next to you.

    Even if your home survived that fire the very reason many people lived in these remote areas, the beauty of the scenery, no longer exists in many cases. To me it begs the question if one really wants to put all your resources into building a fire safe home. It seems to me that it might make more sense to mitigate the condition that cause these types of intense fires. And where that can't be done then stay out of those areas if you can.

  25. Eric Habegger | | #25

    One more thing
    I don't think even concrete construction would save a home if the fire is intense enough. The homes that burned here burned all the way to the foundation with virtually nothing left. There weren't many partially burned structures. But the concrete foundations below those stick built homes were decimated. It's not obvious visually but on further inspection the foundations that are left mostly have no structural integrity. They now will just crumble from the damage done by the heat. A home does not have to combust to be damaged severely in a fire as long as the heat is intense enough.

    The message: the common sense solutions to a fire safe structure do not work in all situations.

  26. Robert Swinburne | | #26

    Non-integrated team approach
    As an architect, I am often designing for owner-builders with not much experience. I also do stock plans for the national market and I do projects where I am not much part of the building process and where the builder may have little or no experience outside of 2x6 walls with fiberglass and plastic. (You do what you have to do to feed the family.) I find that sending links to informative and relevant GBA articles and super simple double stud or exterior foam detailing go a long way toward making these projects successful. I find myself preaching air sealing and ventilation much more than any particular wall system.

  27. GBA Editor
    Martin Holladay | | #27

    Response to Robert Swinburne
    "I find that sending links to informative and relevant GBA articles and super-simple double stud or exterior foam detailing go a long way toward making these projects successful."

    Thumbs up! That's good to hear.

  28. David Gadbois | | #28

    MAG board
    Good suggestion, Steve. I'm open to mag board but I don't see it much around these parts...not that that is a deal-breaker. I admit I am not precisely familiar with the pros and cons of mag board vs. fiber cement.

  29. Burke Stoller | | #29

    Wall Data in Marine Zone 4C
    Many of the articles that are referenced to on GBA regarding building science studies on wall assemblies come out of the Northeastern US and Eastern Canada. I am a builder on Vancouver Island, and am therefore in a climate alike to Seattle. Can anyone offer any links to comprehensive building studies using test walls showing RH, temperature, moisture content, etc. of test wall assemblies built in the Marine 4C climate? I would love to see data about double-stud wall assemblies, since this is what we are most interested in compared to 2x6 walls with exterior mineral wool insulation (which we know are "safe" from a moisture standpoint, but which are more costly to detail and build than a simple double-stud wall). Thanks for any links to data/studies!

  30. GBA Editor
    Martin Holladay | | #30

    Response to Burke Stoller
    I would start by reviewing the data gathered at the Coquitlam Test Hut. Here are some links:

    The Vancouver Test Hut Facility

    Mark Gauvin: Coquitlam Test Hut

    Vancouver Test Hut

  31. Burke Stoller | | #31

    Vancouver Links
    Thanks Martin. Exactly what I was hoping for!

  32. Douglas Epperly | | #33

    Sheathing Options

    We really appreciate this informative article - thanks. So, for double stud wall sheathing with a rain screen is OSB a reasonable option, or should it still be avoided? Do Zip Panel's adhered WRB with a rain screen reduce/eliminate the potential for moisture to effect the OSB in Zip Panels?

    Thanks in advance for your advice and thoughts.


  33. GBA Editor
    Martin Holladay | | #34

    Response to Douglas Epperly
    I stand by the advice I gave in this article. If I were building a double-stud wall, I wouldn't use OSB sheathing -- even Zip System sheathing.

    The sheathing on a double-stud wall has to be robust enough to withstand regular moisture cycling. The sheathing is going to get damp every February, and then it will begin to dry out in April. You want a sheathing that can hold up to this type of moisture cycling for 50 or 100 years.

    Of course, my advice is conservative. Plenty of people ignore my advice and install OSB on their double-stud walls. That way, they save some money. (If you decide to ignore my advice and install OSB, I agree that Zip System OSB is more robust than conventional OSB.)

  34. Douglas Epperly | | #35

    Thanks for Sheathing Clarification

    Thank you for the clarification. I was just uncertain about whether the rain screen made enough difference. We'll start evaluating the other sheathing options mentioned in the article. Of those, do you have a recommendation?


  35. GBA Editor
    Martin Holladay | | #36

    Response to Douglas Epperly
    Plywood works fine and is easy to tape. If your sheathing doesn't have to be an air barrier, and if your local building code official has no objection, you can use diagonal board sheathing from a local sawmill.

    For more information, see Wall Sheathing Options.

  36. Douglas Epperly | | #37

    Thanks Martin!
    Appreciate your time and for letting me know about the Wall Sheathing article. Best wishes.

  37. Tony Tibbar | | #38

    Mineral wool
    I like mineral wool. It's quality material and bugs hate it or at least don't like it as much as foam insulation.

    I can't find any (nearby) store that sells Rockwool.
    Home Depot sells Roxul Comfort batts. When bought at 15 pack quantities good stuff for a fair price. (meaning I haven't found a cheaper place)
    So I'll use Roxul Comfort batts as cavity insulation.
    I would also like to use Roxul (or Rockwool) on the exterior side of my wall.
    Unfortunately I can't find a place where they sell the boards. Roxul support doesn't even bother to answer emails. And if they do, they want to send a sales droid; not their exact wording :-)

    Are Roxul Comfort batts also suited for the exterior side of the wall? Under siding ofcourse.

    Mineral wool insulation can be substituted for rigid foam insulation on the exterior side of wall sheathing. One advantage of mineral wool over rigid foam: because mineral wool is vapor-permeable, it doesn’t inhibit wall sheathing from drying to the exterior. That means that builders can install mineral wool of any thickness on the exterior side of their walls. You don’t have to worry whether exterior mineral wool meets any minimum R-value requirement. (Of course, thicker insulation always does a better job of resisting heat flow than thinner insulation.)

    Does the above mean no vapor retarder/barrier is needed?

  38. GBA Editor
    Martin Holladay | | #39

    Response to Tony Tibbar
    The trickiest part of installing mineral wool on the exterior side of wall sheathing is the squishiness of the mineral wool, which makes installing furring strips a little tricky. The denser the mineral wool, the easier it is to install. There are lots of articles on the topic on GBA -- start with this one, and then read all the articles that show up in the "Related Articles" sidebar on the same page: Installing Mineral Wool Insulation Over Exterior Wall Sheathing.

    As you know from reading my article on vapor retarders (where you posted another question), interior vapor retarders are required by code in colder regions of the country. In general, a vapor retarder like vapor-retarder paint or MemBrain causes fewer moisture problems than polyethylene.

  39. LeakyRoofOntario | | #40

    My builder built our home in 2017 in Ontario.

    Exterior cladding is EIFS with a 1.5 inch EPS foam with NO DRAINAGE. Just flat EPS foam 1.5 inch thick mechanically fastened to the sheathing/Studs

    Sheathing is OSB and there is a WRB between the EPS and OSB.

    Interior insulation is "Flash and Batt" I think 2 inch of closed cell spray foam and the remainder is batts and then drywall (no plastic sheets). Its a 2x6 framed walls.

    Am I going to have OSB rot? Should I spend the money to rip out all the EIFS and redo with a more breathable eifs with drainage gaps.

    Pls advise I am torn on what to do...

    1. GBA Editor
      Martin Holladay | | #41

      These days, EIFS manufacturers and insurance companies in the U.S. don't allow the installation of EIFS by anyone other than a certified installer. Was the installer of your EIFS certified?

      Barrier EIFS is no longer installed in the U.S., as far as I know, having been replaced by water-managed EIFS. Perhaps the situation is different in Canada.

      Q. "Am I going to have OSB rot?"

      A. It's hard to say -- but maybe. There are lots of factors to consider -- for example, the width of your roof overhangs, and whether or not your window rough openings have sill pans.

      Q. "Should I spend the money to rip out all the EIFS and redo with a more breathable EIFS with drainage gaps?"

      A. Probably not, unless you have evidence of a problem.

      1. LeakyRoofOntario | | #42

        Thanks so much for your response. Doubt it was a certified EIFS installer.

        The thing is, I dont want to wait to find out my OSB is rotting. By time i find evidence of a problem, then it would be too late and I'd have to not only rip out the EIFS and re do it, but also possibly have to replace a bunch of rotten OSB and framing members.

        How do I monitor this situation, should i cut little inspection holes say under windows and inspect to see of there is any rot or moisture?

        My thinking was Id rather spend $30k now replace the EIFS with breathable drainge EIFS vs potentially replace the EIFS + OSB/framing memebers etc down the road.

        And we have no sill pans on our windows. I just need to now whats a good way to check for rot/problems..

  40. Mike Ferraro | | #43

    Regarding not using closed cell foam in stud cavity when applying rigid foam to the exterior.... if you do not use exterior sheathing against the 2x6/2x4 studs, then is it ok to use the closed cell in the cavity. Specifically, if you apply the rigid foam directly to the stud, then tyvek, a rain screen, then the LP siding material, it seems there is no issue of trapping moisture. Your thoughts please.

  41. Frank Fuller | | #44

    How about this 2x6 wall? Cement board, rain screen, Zip R-6, 2” close cell foam and 3 1/2” Rockwool.

  42. Dan Showalter | | #45

    Question regarding the section on Mineral Wool below. Does this also apply to roof assemblies as well? Specifically thinking about a ventilated cathedral assembly in Zone 5.

    One advantage of mineral wool over rigid foam: because mineral wool is vapor-permeable, it doesn’t inhibit wall sheathing from drying to the exterior. That means that builders can install mineral wool of any thickness on the exterior side of their walls. You don’t have to worry whether exterior mineral wool meets any minimum R-value requirement. (Of course, thicker insulation always does a better job of resisting heat flow than thinner insulation.)"

    1. GBA Editor
      Martin Holladay | | #46

      If you are planning a ventilated roof assembly, you won't have any continuous insulation on the exterior side of the roof sheathing -- neither rigid foam nor mineral wool. Remember, most types of roofing prevent roof sheathing from drying to the exterior. The only way roof sheathing can dry is by either (a) ventilation drying in the case of ventilated roof assemblies, or (b) toward the interior if there is no interior rigid foam or interior polyethylene.

      If you're talking about a ventilated roof assembly with fluffy insulation between the rafters (either mineral wool or fiberglass), then the assembly will work fine with mineral wool.

  43. Dan Showalter | | #47

    Martin, thank you for the response. We are running fluffy insulation (mineral wool) between the rafters. So from your last comment are you saying that we could run exterior mineral wool on the roof or that it would compromise our ventilated assembly?

  44. GBA Editor
    Martin Holladay | | #48

    A continuous layer of exterior insulation above the roof sheathing -- whether rigid foam or mineral wool -- is a waste of money in a ventilated roof assembly. The ventilation channel invites cold outdoor air to flow through a channel under the roof sheathing. All of the insulation needs to be on the interior side of this ventilation channel.

    The ventilation channel not only moves air (and the moisture in the air), carrying it away; it also moves heat.

    If you install insulation on the exterior side, it's a waste. It doesn't do anything. You might as well hang the insulation on a clothesline in your back yard.

    1. Dan Showalter | | #49

      Appreciate the response and explanation. I feel a little foolish for having essentially the same question posted on two different articles. I can be a little slow sometimes, so maybe it was good I read the same advice twice!

      1. Expert Member
        Malcolm Taylor | | #50


        If you are in the slow camp, I am too. Some of the most basic building science concepts took me ages to understand - and the consequences were larger as I was designing houses for other people. All that's important is you get it before actually building your house.

        1. Dan Showalter | | #51

          I couldn't agree more!

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