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Musings of an Energy Nerd

Insulating Walls in an Old House With No Sheathing

You’ll need to create an air space behind the siding before you insulate between the studs

This older building has horizontal shiplap siding, without any sheathing. The building paper is so old that it is crumbling. As part of the rehab work, the studs were beefed up with new vertical lumber strips to accommodate more insulation.
Image Credit: Image #1: GBA / Ell Aire

When the owners of an old wood-framed building say that they’re doing a “gut rehab job,” that usually means that they’re demolishing the lath-and-plaster walls to expose the studs — the first step of renovation work that usually includes new wiring, new plumbing, and new insulation.

Uninsulated stud bays are common in older homes, so workers who pull down the lath and plaster expect to see the back side of sheathing boards on the exterior side of the studs. But in some cases, there isn’t any sheathing: the workers find themselves staring at the back side of the siding (see Image #1 on the right).

So how do you insulate stud bays from the inside if the building has no wall sheathing or water-resistive barrier (WRB)?

One “solution” to this situation is to install spray polyurethane foam insulation directly against the back side of the siding. Why is this is a bad idea?

There are several possible ways to insulate this type of wall. No matter what type of insulation you choose, though, you need to create an air space between the back of the siding and the insulation. (For more information on the benefits of having an air space behind siding, see All About Rainscreens.)

If you’re planning on filling the stud bays with air-permeable insulation (fiberglass batts, blown-in fiberglass, mineral wool, or cellulose), the material you use to create the air space (in most cases, a layer of rigid foam) needs to be an air barrier.

The easiest way to create the necessary air space is to install vertical ¾” × ¾” or 1″ × 1″ “sticks” in the corners of each stud bay. These vertical sticks need to be tacked in place toward the exterior, up against the siding.

Once these sticks are in place, rectangles of rigid foam can be…

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  1. User avater
    Ethan ; Climate Zone 5A ; ~6000HDD | | #1

    How about coming in from the exterior?
    With all this work: demo lathe and plaster, cobble together an air gap and insulation layer, then refinish interior... I'm tempted to go the other way and remove exterior siding in this instance and reinsulate, resheathe, and refinish exterior.

  2. User avater GBA Editor
    Martin Holladay | | #2

    Response to E.T.
    The article discusses strategies for insulating open stud bays from the interior. Of course, if you're willing to remove the siding and install new sheathing, this article is irrelevant.

  3. Peter Rogers | | #3

    I'm left wondering how many
    I'm left wondering how many houses like this have been blown in from the outside with cellulose or fibreglass by ignorant insulation contractors... scary

  4. Malcolm Taylor | | #4

    My sister's for a start...

  5. Torsten Hansen | | #5

    We do encounter these walls
    We do encounter these walls from time to time but they are quite rare in our area, mostly confined to vacation cabins that are being converted for year-round use. In my experience, the various approaches Martin describes all work well. As a foam contractor my preferred method is 15# felt between the siding and the foam; it is much faster than using foam board so it makes sense when labor cost is a factor.

  6. Kohta Ueno | | #6

    Cellulose Retrofit with no Sheathing/WRB
    Not to beat a dead horse, but here is Joe's case study on investigating a house that was retrofitted with cellulose in the stud bays, but had no sheathing or water resistive barrier/water control layer (i.e., clapboards on studs). Yep, the clapboards got wet from interior-sourced moisture (also possible exterior-sourced precipitation), resulting in paint blow-off and siding moisture damage.

    BSI-028: Energy Flow Across Enclosures

  7. Malcolm Taylor | | #7

    Not meaning to hijack this discussion, but have you come to any conclusions as to what approach might be useful to combat the damp roof sheathing problems you've been looking at in Vancouver roofs.

  8. Bennett Sandler | | #8

    air gap for DP cellulose
    For health and climate reasons I try to avoid foam. If done right cellulose could work well here I think. A product like Mortairvent, though designed to be used in masonry walls, can be used to create an air gap against siding (or a roof deck in a cathedral ceiling.) It combines a spongy spun polypropylene matrix with an insulweb-like backing and comes in various thicknesses from .25-.80. Seems like it would work just as well with spray foam if it was important to have that larger air gap.

    And if there are not structural considerations, cross strapping the added interior 2Xs would nearly eliminate thermal bridging.

  9. User avater GBA Editor
    Martin Holladay | | #9

    Response to Bennett Sandler
    Thanks for sharing your helpful suggestions.

  10. Jean Terwilliger | | #10

    Board sheathing
    So if you pull the plaster and lath off and find board sheathing behind the clapboard, is it safe to blow cellulose- up to 12" with a new stud wall built to the interior? Or would you still suggest an air gap and/or air barrier against the sheathing? I like the ventilation of the Mortairvent idea, but its expensive and isn't an air barrier.

  11. User avater GBA Editor
    Martin Holladay | | #11

    Response to Jean Terwilliger
    That's a tricky question. It depends on the siding -- vinyl siding is safer than stucco; the climate -- warm climates are safer than cold climates; and the roof overhangs -- wide overhangs are safer than stingy overhangs.

  12. Wayne Percifield | | #12

    Applying rigid foam on the interior
    I will be renovating a house with clapboard siding and no sheathing as described in this article.
    I plan to create the airspace as recommended and install fiberglass batts.
    I may also want to install a layer of rigid foam insulation on the interior before applying the drywall.
    In another article I believe Martin recommended using polyisocyanurate rigid insulation, as the most environmentally friendly, for this type of application.
    Is the foil membrane that encases the polyisocyanurate insulation a problem for an interior application, or
    can I purchase polyisocyanurate insulation without the foil membrane?
    My main concern is the "breathability" of the wall.
    Will the foil membrane create a moisture problem?

  13. User avater GBA Editor
    Martin Holladay | | #13

    Response to Wayne Percifield
    Here are links to two relevant articles:

    Walls With Interior Rigid Foam

    Choosing Rigid Foam

    Q. "Is the foil membrane that encases the polyisocyanurate insulation a problem for an interior application?"

    A. No. I addressed this question in my article, "Walls With Interior Rigid Foam." I wrote, "Will there be any problems if your walls can’t dry to the interior? The answer is no — as long as your walls can dry to the exterior."

    Q. "Can I purchase polyisocyanurate insulation without the foil membrane?"

    A. Yes. But you don't have to.

    Q. "My main concern is the breathability of the wall."

    A. Your concern is misplaced. As I wrote in How to Design a Wall, "Many green builders think that a wall has to “breathe.” While “breathing” is a poorly defined concept, it usually refers to a wall that is vapor-permeable. ... There is no truth to the belief that vapor-permeable walls perform better than walls that include a vapor barrier. In fact, limiting the flow of vapor through floors, walls, and ceilings is often essential."

    Q. "Will the foil membrane create a moisture problem?"

    A. No. Again, a fuller explanation can be found in my article, Walls With Interior Rigid Foam.

  14. Wayne Percifield | | #14

    RE: Applying Rigid Foam on the interior
    Thank you, Martin.
    You have a wealth of information.
    I am much more comfortable about attacking my project now.

  15. Matt W | | #15

    Southern California
    I am in a similar situation with a home in Southern California. Outside is stucco. Would geography change the answer?


  16. User avater GBA Editor
    Martin Holladay | | #16

    Response to Matt Wade
    You wrote that you are "in a similar situation." That would be similar to who or to what?

    I'm guessing that you have a stucco-clad home without any wall sheathing. If so, the advice in this article applies to your house -- even in southern California.

  17. Kevin Hood | | #17

    Keeping the plaster!
    Martin, I have a 100+ year old house in Detroit, Michigan, plaster lathe walls, balloon framed, shiplap 1"x10" sheathing, some type of partially degraded water barrier (builders felt?), cedar bevel siding covered in lead paint. I was planning to insulate with rigid foam polyiso on top of my roof, and rigid roxul comfortboard on my walls on the outside right over the existing siding. Here is my current plan, from the outside to inside, cementitious siding, windscreen furring strips, tyvek, rigid mineral wool board @ 8"-10", existing siding/felt/sheathing, empty stud bays for fishing/running mechanicals, existing plaster.

    In my understanding this creates a wall that is vapor permeable in both directions.

    I am concerned about water getting trapped where in a similar application with Polyiso

    Do you think my plan is a good idea or do you think some type of Polyiso application could work better?

  18. User avater GBA Editor
    Martin Holladay | | #18

    Response to User-6933276
    First, can you tell us your name?

    Your idea will work, as long as you figure out a way to flash your windows. It's easier to get the window flashing details right if you replace your existing windows with new windows. If you can afford that step, it will allow you to install flashing (including sill pans) in the window rough openings.

    Polyiso would also work. A wall with exterior polyiso doesn't need to be able to dry through the polyiso. All that is necessary is for the materials on the exterior side of the polyiso to be able to dry outward, and the layers on the interior side of the polyiso to be able to dry inward. So you can use either mineral wool or polyiso for your wall. Most contractors find polyiso to be easier to deal with, because it doesn't compress as much as mineral wool when the furring strips are fastened down with screws.

  19. Kevin Hood | | #19

    Hi Martin,

    My name is Kevin Hood. Thanks for the recommendation.

    P.s. I tried updating my name in my profile earlier but it doesn't seem to be working yet.

  20. dvschris | | #20

    What about doors?
    Hi there,

    This has been a tremendously helpful article, so thanks for that. I’m currently converting an attached carport to additional living space in a 1953 wood frame house in central Texas, and will be employing your foam-and-fiber method on one of the walls with existing shiplap siding without any sheathing or underlayment, as I wanted to avoid tearing it out.

    I’d like to add a new door to this existing, sheathing-less wall, but I realized today that all of the waterproofing methods I’ve seen rely on having sheathing on which to apply a membrane and appropriately layered felt paper.

    Would y’all have any recommendations on how to flash for an exterior door in this situation? I’ve cobbled a few ideas together but nothing that seems sufficient or code-correct...yet!

    -Chris D.

  21. Malcolm Taylor | | #21

    You may find this blog from a long time GBA contributor, who built a house without sheathing, helpful:

  22. dvschris | | #22

    Thanks for the link!

    Looking through his posts, it seems that the strategy he employed was to place the underlayment directly on the studs. Unfortunately, my siding is already installed on the studs, so that wouldn’t be an option without removing the siding (at which point I could just install sheathing!).

  23. User avater GBA Editor
    Martin Holladay | | #23

    Response to Chris D.
    You'll just have to do the best you can under the circumstances. You'll need a sill pan in the rough opening, of course -- that's a code requirement. Install housewrap or asphalt felt on the rough jambs, with the material lapping over the vertical legs of the sill pan, and then install your door. The exterior door casing will simply be installed with caulk -- not ideal, but the best you can do.

    The best detail to protect this door is an adequate roof overhang. If your building has a stingy roof overhang, make sure that you install a special roof directly over this door to protect it from the weather.

  24. dvschris | | #24

    Thanks, Martin
    That makes sense, thanks for the affirmation!

    Luckily, there's a decent overhang (~18" +/-) as well as a covered patio extending off this section of the roof line that provides good coverage for about 90% of the doorway, but I'll plan to re-evaluate the integrity of said roof (the previous owner appears to have been handy, but not the best steward of good building science) now that It'll be playing a role is keeping the house dry.

    Thanks again!

  25. Megan Matthews | | #25

    Hot Humid Climate
    I recently bought a house built in 1949 in Austin, Texas with clapboard over felt paper on the exterior and no insulation in the cavity. The interior drywall is at the end of its lifespan so when I replace it, I'd like to take the opportunity to add insulation to the cavity. It has been a cold winter!

    I'm intrigued by the closed cell spray foam/felt paper option listed above because it seems to be the fastest to install while offering the best air sealing and I wouldn't need to add spacers to increase the cavity's depth. But, I'm worried that since closed cell is a vapor barrier (or retarder? I'm not sure) that it'll trap moisture in the gap between the siding and the new felt paper. Am I completely off base here, or will this assembly work in my hot-humid climate?


  26. User avater GBA Editor
    Martin Holladay | | #26

    Response to Megan Matthews
    You didn't tell us whether your walls have any sheathing. Because you chose to post your comment on this page, I'm guessing that your walls don't have any sheathing.

    If you install strips of asphalt felt between the studs, followed by closed-cell spray foam, you won't trap moisture. Everything on the exterior side of the spray foam will be able to dry outward, and everything on the interior side of the spray foam will be able to dry inward.

    The spray foam is an air barrier as well as a vapor barrier, so it will prevent moisture from finding any cold surfaces. It's a safe wall.

  27. Megan Matthews | | #27

    Re: Hot Humid Climate
    Hi Martin,

    You're correct - we have no sheathing. Our assembly from inside to outside is drywall, studs (no insulation.. yet), felt paper, and wood clapboard.

    Thank you!

  28. williepaul | | #28

    1930s home in Northern California
    Hi Martin,

    First off, thank you for your post. I know it has been a couple of years now, but the topic is still relevant, and there is surprisingly very little information about retrofitting very old homes without insulation--or at least it's difficult to find.

    There was a video posted recently about this very subject, found here:
    The method used in that video seems very similar to what you recommend, with some improvements and specific materials. Basically, to maintain the air gap, the author is using Dorken's Delta Dry rainscreen (dimple mat), and on top of that, their Delta S product, which is a waterproof/airproof material. They drill through the floor and ceiling studs and add tubing through it to provide airflow into the crawlspace/attic. Their recommended insulation is closed-cell spray foam to seal and meet code requirements, but also provide alternatives--either rigid foam+canned spray foam to seal around the gaps, or strong tape to seal the Delta S material to the studs and rockwool to fill between the studs.

    I have a 1930s home in Northern California (East Bay area), and am considering using this method to insulate during a kitchen remodel where the walls have been exposed. The wall layers from outside in consist of stucco -> metal lath -> building paper (crumbles when touched) -> wood cladding -> studs. I'm told a vapor barrier is not needed in this climate; and anyway, I couldn't find any in stock at local hardware stores. Would the method in the video be appropriate here, and would you use closed-cell spray foam or stick to rockwool or fiberglass? I'm hesitant to go with the rigid foam because of the amount of blocking, cross-bracing, etc. Are there additional steps you recommend taking?

    I'm in the process of looking for but am also hesitant to use "professionals" in the area. My next door neighbor had spray foam blown in without any consideration of the age of the house and how the exterior walls were constructed. I worry about the mess he might have in the future. This topic doesn't seem to be one of common knowledge amongst folks here, and since the home prices are so high, there is a tendency for some to ignore potential long term issues with the excuse that "we could always just sell" (good luck).


  29. User avater GBA Editor
    Martin Holladay | | #29

    Response to Willie Paul (Comment #28)
    Q. "Would the method in the video be appropriate here?"

    A. I disagree with some of the elements of Matt Risinger's approach in the video. The air flow created by drilling holes in the plates is unnecessary and possibly counterproductive. For example, pulling air from a crawlspace is risky -- that might be humid air. More importantly, the wood siding shown in the video dries readily to the exterior, so it's unlikely to stay wet and need ventilation drying behind it. Finally, air flow between the studs might result in enough wind-washing of the insulation to reduce the thermal performance of any fluffy insulation installed between the studs (if fluffy insulation is used).

    In short, stick with the advice provided in this article.

  30. Marc Montoya | | #30

    Hey Martin! Thanks for this post!

    I know this isn't exactly what your writing about, but many parts seem similiar.
    My 1920's English Bungalow, in Dallas TX, (zone 8a) from exterior to interior, is Brick Vaneer, small air gap, shiplap, studs, empty cavity, shiplap, sheetrock. I'm currently performing a remodel on an exterior corner of the house and have removed the sheet rock and the first layer of shiplap, leaving exposed wall cavities. The gaps in the outer layers of shiplap are pretty large in some places. There seems to be signs of some water entry, mainly under the window, and possibly from the kitchen that used to be there. There will be a shower in the corner, with plumbing on the exterior wall. I'm also (foolishly perhaps) installing a wall mounted toilet in the wall cavity under the window. As this is a difficult place to keep dry, i'm a bit baffled by what steps I should take to both keep it dry, and keep it from freezing in the winter time. There is about a 1/4" gap between the wall carrier (frame and toilet tank)and the shiplap. The toilet tank is partially covered with styrofoam from the factory. Obviously not doing the plumbing fixtures here would be ideal, but that's not whats happening. I might be able to add 1/2"-1" additional cavity space, but there's problems with that too. (tiny house). The tyvec as been up in there for a couple years with the door locked while I mustered the courage to tackle this project, I understand that's not the best idea? Thanks again for all you do!

  31. User avater GBA Editor
    Martin Holladay | | #31

    You are at risk of suffering water damage to your house.

    1. You need to address the "signs of water entry, mainly under the window" by removing the window(s) and installing flashing in the window rough opening, followed by the installation of a new window (or re-installation of the existing window). You can perform a search on "flashing a window rough opening" if you don't know how to do this work.

    2. It's insane to install a toilet tank in an exterior wall cavity. That wall cavity needs to be filled with insulation, not plumbing. Move the toilet. The record low temperature for Dallas, Texas, is -8°F. If you get a cold spell like that again, your toilet tank will be frozen solid.

  32. Marc Montoya | | #32

    Thanks for your quick reply!

    1. Yes. It's not going un-adressed. I bring it up because I understand that airflow in these walls is what has kept this thing from more serious water damage. Once the window has been addressed, I've got to insulate these walls in a manner that doesn't cause more problems. What would you do differntly in this situation? Do I have different choices because there are bricks? Is felt still the best option, or could I adhere something to the ship lap? If the Brick Vaneer keeps the moisture off the shiplap, will I still have condensation to contend with?

    2. Got it. There's been plumbing there for over 80 years, a sink to be exact. Yes a toilet tank in the wall is not ideal, perhaps insane, but when we have cold spells like you mention, we open the cabinets and leave the water running, etc., the same thing could be done with this toilet tank, remove the cover, maybe even adjust it so it runs a bit for the duration of the cold spell, or empty it and turn it off. The alternative to having a toilet here, for me, is no toilet. I'd prefer to turn it off and let it sit empty for a couple weeks then not have it here. This is why I didn't ask you, "do you think I should put a toilet here". Specifically, I'm looking to maximize my chances at having success in this difficult situation.

  33. Leanne Bridge | | #33

    Hi Martin,
    Would vinyl siding over the old wood siding have any bearing on this method of insulating the cavities? I hope to one day remove the vinyl and restore the wood, but that might be a ways off.
    Thanks, Leanne.

  34. User avater GBA Editor
    Martin Holladay | | #34

    If your walls lack sheathing, and if you "hope to one day remove the vinyl and restore the wood," then you should follow the advice in this article.

  35. Gary Hello | | #35

    Hi Martin, Thanks for addressing this issue. I currently have our 1906 farmhouse gutted with open stud cavities, backside of original pine sheathing exposed. Unfortunately the situation gets more complicated at the exterior. Working out from pine sheathing: Tar paper, asbestos shingles, a very thin (1/4" or less) yellow foam board insulation, then vinyl siding. We intend to replace vinyl in next few years (It is well past useful life), but will have to leave asbestos. Does the asbestos change recommendations in article? Rock wool is my "fluffy" insulation of choice. Any recommendations for new exterior siding over asbestos after vinyl replacement? Thanks

    1. User avater GBA Editor
      Martin Holladay | | #36

      The advice in this article doesn't apply to you. This article is about houses without wall sheathing. Your house has wall sheathing.

      In your case, you're just talking about installing new siding on an older house. All you need to do is install a water-resistive barrier -- housewrap would be the usual choice -- and verify that there is good flashing at windows and doors. Then proceed with the siding of your choice.

      Possible upgrades include the addition of a continuous layer of exterior rigid foam and a rainscreen gap.

      1. Gary Hello | | #37

        Thanks for the quick response.
        I was confused about the sheathing, as my 1x6 pine looked a lot like the pictures. It's an old house with many repairs/additions over the years. One section of the house actually had wood siding and no sheathing, but we were able to remove it as no asbestos was present. At least 3 different scenarios on exterior walls, so far. Trying to strip back as much as we can, and do it right this time. Good to hear the suggestions for the asbestos. Your suggested upgrades are on my list.

  36. Ryan Loney | | #38

    My name is Ryan from Vassar, MI.

    Thanks for this post all of the other great posts.
    My wife and I purchased a 1950s brick ranch home a few years ago and have been doing reno projects from day one. I was hoping someone could help us come up with a wall assembly that will work for our house. We've rebuilt a couple of our exterior walls where we had access to the interior and exterior sides of the wall. These 2x4 walls were constructed of some type of fiberboard sheathing, see attached photo of our bathroom walls, and interior foil faced paper batt insulation. The old insulation barely filled half of the stud cavity depth. We did new unfaced batt insulation, sheathing, wrb, and exterior rigid xps foam insulation with a lot of attention to air sealing. The new walls came in with an r value just under 30. We are now doing a reno in our full bathroom and are trying to figure out an insulation solution for the exterior wall. The old fiberboard sheathing is very air leaky and we'd like to seal up the cavity. Its not very feasible to remove the original brick and old sheathing to install new sheathing and siding. I've reviewed this article but I'm not sure if it applies to my situation. I was wondering if I can use 2 inch XPS foam to air seal the cavity then fill the rest of the cavity with rockwool batt insulation? If so I'd plan to seal the edges with canned spray foam and high performance tape. I'd like to avoid any solutions that I'm not able to complete on my own such as spray foam or packed cellulose. I'm concerned about how this approach will perform given we have fiberboard sheathing then about a 3/4 inch to 1 inch air gap before the brick. Do you have any ideas on how to best air seal the sheathing side of the cavity? We live in zone 5 but border 2 counties that are in zone 6. I've read about the Bonfiglioli wall assembly and am hoping to incorporate it into our house to achieve an thermal stud break and increase cavity insulation values.

    We appreciate any help.


  37. Anukeen Warda | | #39

    This is Anukene from Oakland, CA. We have gutted walls with nothing but the back of the beautiful redwood siding. The studs are 2"x3.5", 16" apart, diagonal bracing running between.

    I am trying to decrease costs and complication. Being in Zone 3, can I skip the cobbled WRB recommenced in this article? My hope is to put mineral wool in the cavity ,which will should allow the siding to dry out, and do an air seal with the interior drywall. No freezing in this part of the world.

    Thoughts? Better ideas? Thanks for the time.

    1. User avater GBA Editor
      Martin Holladay | | #40

      The need for a water-resistive barrier (WRB) has little to do with the climate zone. Factors that affect whether you can get away with omitting the WRB include (a) if your climate is very arid (low levels of rainfall), or (b) if your roof overhangs are very wide, or (c) if your house has one story (a factor that allows the roof overhangs to work better than if the house had two or three stories).

      Water entry into walls can cause rot, which is why houses without a WRB can be risky. Assessing all the risks at your house is a matter of judgment, but you should start by assessing the factors I list. It would also be a good idea to see if there are any signs of mold or incipient rot at the lower corners of the window rough openings -- something that is easier to assess when the walls are open, as yours are.

      1. Anukeen Warda | | #41

        Thanks Martin.

        Regarding my climate zone, I was just saying that (I think) I don't need to worry about condensation.

        Re: WRB, my understanding is that water that does enter the siding will either be wind-driven or moisture moving through the wood. If given a rainscreen, it will drain along the back or dry out, as it has in this house for a hundred years. My understanding is that the moisture will not travel to the drywall/plaster, or effectively wet any future fibrous insulation that might be pushed against it. If this is the case, I'm not sure what the WRB is protecting. Please set me straight.

        All of this being said, I am likely going to go with the recommendation in this article. But I just want to make sure I understand what the WRB is doing in this case.

        The assembly I'm thinking is 3/4" spacers on corners, 1/2" polyiso, canned spray foam, batt insulation, gypsum.

  38. user-7495975 | | #42

    Hello and thanks for all the information on your articles. I’m looking for some direction. I’m in the midst of adding insulation to a 1920 home. Zone 4. Never any insulation in the walls. Reading this article makes me wonder if this approach to the rehab is something I should heed?
    The walls are 2x4 studs, balloon framed, that had plaster & lathe on walls. Somewhere in the 1990’s someone placed 3/8 drywall over the walls. Which I took all down to put mineral wool insulation in.
    Like the interior walls. Someone has resided the home without removing the original siding. When looking at pic. You can clearly see large gaps between the boards on the exterior. There appears to be roofing/ felt paper on the other side of these boards. Then the aluminum siding on top of the original siding. What I can’t tell is if the boards are the original siding or just old time sheathing the siding is attached too.
    I have to extend the studs out to meet the hardwood floor which is one of my girlfriends favorite parts of the house.
    Do I need to follow this Articles path or can I air seal the cavities & then place the batts in there? I’d rather do this once. Not sure if in the future we will do exterior of the house yet.
    2nd I was contemplating running furring strips & comfortboard at right angles across studs to help with thermal bridging and than capping with MemBrain & drywall. To bring the wall out to meet the hardwood floor. Any suggestions will certainly help me keep my girlfriend from the deep end of the pool. Thanks again!


    1. User avater GBA Editor
      Martin Holladay | | #44

      Those horizontal boards you see are sheathing boards, not siding. The felt paper on the exterior side of the sheathing boards are your water-resistive barrier (WRB). So your house has sheathing, and you can ignore the advice in this article -- since the advice in this article does not apply to houses like yours.

      1. Johanna Carr | | #70

        Martin, can you refer people with a situation like this (old, gapped board sheathing) to any articles on how to insulate from the interior? Thank you.

        1. User avater GBA Editor
          Martin Holladay | | #71

          In general, you would insulate such a wall the same way you would for new construction: choose your favorite insulation, and pay attention to airtightness during installation.

          If you're worried about thermal bridging through the studs, you might want to consider installing a continuous layer of interior rigid foam: "Walls With Interior Rigid Foam."

          Note that if this type of wall has a water-entry problem due to the lack of a WRB or bad flashing, the problems can't be solved from the interior. In some cases, there is no substitute for siding replacement.

  39. user-7538235 | | #43

    Hello Martin,

    Thanks for the article and replies to posts. I'm going to read it all again, but I'm not clear on a couple of things.

    My single story creole cottage (located NW FL) needs full infrastructure reno (still has live knob and tube wiring!) so I decided to demo the lathe and plaster in most of the exterior walls to do that and add insulation. There are lots of crazy angles and added 2x4s to furr out walls for sheetrock and to support termite damaged framing so whatever I do, it will be labor intensive. 

    The lap siding has lots of large gaps and the floor plates inside walls were practically nonexistent (3-5" holes looking down to the ground). House is on an open crawlspace. I *really* don't want the walls to provide access to critters so GC recommended we block them (and maybe seal with foam). In hindsight, maybe leaving it more open and screening would be better for drainage and airflow? 

    I haven't seen definitive info on why stone or rockwool can't just be used (with nothing else or possibly vapor barrier on the sheetrock side?) or does that not allow enough ventilation and evaporation inside the wall? 

    If that's the case, how about furring strips, foam or asphalt barrier, and then the stone wool? 

    I'm getting so much conflicting advice, it's boggling my mind. Thanks so much for any clarification!


  40. User avater GBA Editor
    Martin Holladay | | #45

    In no case do you want to encourage air flow into or out of your stud cavities. Your walls need to be air sealed.

    Nor do you want a path for insects or rodents to enter your stud cavities.

    So your contractor is correct: You want blocking, not insect screening, at the base of your stud cavities. Each piece of blocking should be installed in an airtight manner.

    In an old house like yours -- a house without sheathing -- you should follow the advice in this article. The main reason that you don't want to just stuff the stud cavities with rock wool and call it good is that your house lacks a water-resistive barrier (WRB). That means that wind-driven rain can easily enter the stud cavities. You need a plan to address that problem.

  41. user-7538235 | | #46


    Thanks very much for the clarifications! This has been very helpful.


  42. user-7538235 | | #47

    Hi Martin,

    Me again... I provided my GC with the details of the approach outlined in your article. Fyi, I am leaning away from spray foam for a number of reasons so we were focusing on the rigid barrier etc. option.

    He consulted with a number of insulation and waterproofing experts (one of whom serves as an expert witness on construction lawsuits related to this general area apparently). The group deviated from the air gap plan with the following alternative recommendation, which I want to run by you and others for input. I believe their consensus was formed around the fact that our location is prone to a lot of storms and hurricanes that can produce a lot of windblown rain.

    Their proposal is to use fiberglass tape over any large daylight gaps in the siding and sealing them with BASF AWB900, and then going over the entire inside of the siding with a Synergy wood waterproofing product. Followed by rigid foam also sealed in with AWB900, and lastly filling the remaning air cavity with net/blown in insulation.

    If that doesn't sound like a viable alternative to the air gap approach to you, can you give me some points to further discuss with them or other sources I could contact?

    I was comfortable with the approach outlined in your article and now this throws a wrench into it that I'm not sure about.

    Thanks again,


    1. User avater GBA Editor
      Martin Holladay | | #50

      I don't think that it's possible to make lap siding watertight from the interior by the use of fiberglass tape and elastomeric sealant. The siding moves too much. (Moreover, spreading elastomeric sealant on the back side of lap siding will probably interfere with the ability of the lap siding to dry out.)

      That said, I've never tried it.

      I would be reluctant to attempt the approach suggested by your GC. I would use one of the methods described in my article. But the decision is yours.

  43. Nicholas I | | #48

    I am concerned about the flammability of rigid foam. Would it be practical to replace the rigid foam with comfortboard 80, or would that fail to provide an adequate air barrier? If a separate air barrier is needed, would a plastic film like Membrain suffice?


    1. User avater GBA Editor
      Martin Holladay | | #49

      If you don't want to use rigid foam, you could use asphalt felt, as explained in the article. The disadvantage of the asphalt felt is that it isn't an air barrier. If you choose to install asphalt felt on the exterior side of your insulation, make sure that the interior side of your insulation has a good air barrier. (Usually, that means carefully installed drywall -- installed according to the Airtight Drywall Approach -- along with airtight electrical boxes.)

  44. user-7538235 | | #51


    Thanks. I had concerns as well so we went back to the drawing board. We will replace siding to reduce daylight gaps, use the 1" air gap and rigid foam, plus add some venting since we are otherwise sealing the attic and floor plates. Given the high humidity here and "rising damp" issue, I'm considering encapsulating and dehumidifying the crawlspace (which is an open, pier foundation) but that is TBD due to all the added expense (of everything). Apparently, quite a few historic homes in the area have done that to achieve better control over that weak point.

    I appreciate your input.


  45. user-7538235 | | #52

    Hi all,

    So, per my last post, the encapsulation of the crawlspace is possibly a good idea but is adding $6500 to an already over budget project. Any ideas?

    I'm replacing some rotted subfloors (but not all), the house is on an open pier foundation (but less than 30" clearance required for spray foam). Just doing a barrier (no dehumidifiers) is still like $5k.

    Is this necessary, worth doing, and/or is there an alternative?

    The premise is by doing all the rest, the HVAC will pull moisture up through the old floors which will cause issues, odors, and reduce overall efficiency and proper conditioning.




  46. User avater GBA Editor
    Martin Holladay | | #53

    Q. "Just doing a barrier is still, like, $5 K."

    A. I'm not sure what you mean by "barrier." If you mean "air barrier," then my advice is, "All sides of your house need an air barrier -- including the floor assembly." The air barrier isn't optional -- you don't want air leaks.

    You have rotted subfloors. Why did they rot? Where did the moisture come from? Is it exterior moisture (water vapor) that collected on the subfloors that were made cold by air conditioning? You have to understand the mechanism for the failure before you can repair the problem.

    If you house is built on piers, with less than 30 inches of clearance to grade, there is no perfect solution (although jacking up the house to provide better clearance is often a good idea -- it's just expensive). You may get some ideas from this article: "Crawl Spaces vs. Skirts."

  47. Deleted | | #54


  48. user-7538235 | | #55

    Hi all, we're getting closer to a plan for the historic cottage. But, the devil is in the details still...

    Using the advice here with 1" air gap and rigid foam approach, my question is about venting. The GC plans to block lower wall floor openings with pressure treated lumber (per the photo) and then cut weep holes in the bottom of each stud bay. Since the attic will be spray foamed and we will be sealing the top wall plate off from the attic side, we're also planning some venting to the outside at the top of the walls too.

    1) what size and spacing is recommended for the weep holes?
    2) product recommendations for bug screening over the holes? I know I've seen some in blogs, but I can't find them. I want something with small grid that isn't easily chewed or rusted
    3) per articles on rigid foam and ants, should we tape the raw edges of the foam? (we don't have lots of risk factors for carpenter ants, but they could be a possibility)
    4) recommendations for the upper vents (size, products, etc.)?

    As always, any feedback is much appreciated!


    1. User avater GBA Editor
      Martin Holladay | | #57

      If I were you, I wouldn't include vent holes. The assembly can easily dry to the exterior, because the sheathing, WRB, and siding should all be vapor-permeable. There is no need for vent holes.

  49. user-7592432 | | #56

    Hello Martin –

    I recently uncovered a water infiltration problem in a mid 90’s built house in Minnesota. The wall construction from outside in is stucco on metal lath, 2 layers of felt, 5/8” fiberboard sheathing, 2x6 wall studs. I discovered that water had been infiltrating the wall cavity at the roof level and getting trapped between the WRB against the stucco and the vapor barrier placed behind the interior drywall leading to dry rot of the structural framing and mold on the framing and insulation. The wall is a 2 story space with no windows and no cracking in the stucco so I’m confident the water source was from the roof and has now been properly sealed off. The felt and fiberboard sheathing on the first floor level are brittle and cracking and need to be removed. I have replaced the structural framing and I’m trying to determine the best way to reinsulate the wall that no longer has viable exterior sheathing. I’ve read the suggestion of placing a WRB against the stucco and spray foaming the wall cavity with a closed cell product. I don’t know if that will work with my situation, the inside face of the stucco is littered with staples and nails that are embedded in the stucco, I’m fearful of removing them and causing cracking to the stucco.

    Is a WRB required if I were to apply a closed cell spray foam insulation directly to the inside face of the stucco?

    Is a product like Mortairvent applied to the stucco and then apply closed cell spray foam a better solution?

    Can I install a GreenGuard siding underlayment to the stucco and then spray foam?

    I’m trying to save the stucco from being removed and the entire wall rebuilt in the winter months.

    Any suggestions would be greatly appreciated.


    1. User avater GBA Editor
      Martin Holladay | | #58

      In Minnesota, you absolutely need an air space between the stucco and the sheathing -- no exceptions. In my opinion, you'll have to remove your stucco and correct the problem. For more information, see this article: "To Install Stucco Right, Include an Air Gap."

  50. user-7538235 | | #59

    Update and question re: final insulation

    We are a month in and the air gap and R5 rigid foam are in place. The GC decided to use BASF AWB 900 to seal the edges to the studs instead of spray foam.

    The plan was to then fill the rest of the cavity with BIBS. Now, the insulation contractor says there isn't enough space (I don't think he accounted for the 1" air gap) and he wants to use open cell spray foam.

    I really want to avoid that unless it is the *only* option. I have concerns VOCs if not done 100% correctly and fire also. Building out the stud bays is also not feasible.

    Any other options or am I stuck with spray foam?



  51. User avater GBA Editor
    Martin Holladay | | #60

    Open-cell spray foam will work. So would fiberglass batts. (A 3.5-inch-thick batt can be split into two batts, each about 1.75 inch thick. Remember that fiberglass is compressible, so the insulation can start out thicker than the space you are filling.)

    1. user-7538235 | | #61


      Thanks, I'll discuss that alternative with my GC.


    2. user-7538235 | | #62


      Still researching... are there any reasonable alternative spray-in options for this shallow application other than open cell SFP?

      I'm looking at Rockwool Premium Plus or cellulose... I am really uncomfortable with surrounding myself with the SPF. I have chemical sensitivities and if anything goes wrong, I am sunk. And, I have already used both the XPS and some closed spray so that's enough chemicals.

      With that said, I am willing to give up some thermal performance for a truly healthy green option (if the code insp will let us). I know fiberglass batt is an alternative, but I'm just hoping there is something better to fill all the irregular shapes and depths of the old stud cavities and a spray other than foam makes sense to me.

      I know the Rockwool PP also has good sound and fire ratings which are also important to me but not sure about a minimum depth for that product or how bad the adhesive is yet (undisclosed chemical makeup).

      Is there anything else to consider doing?

      Thank you,


      1. User avater GBA Editor
        Martin Holladay | | #63

        You can look into the use of either Spider insulation (spray-applied fiberglass insulation) or damp-sprayed cellulose.

        For more information on Spider, see "Getting to Know Spider Insulation."

        For more information on damp-sprayed cellulose, see "How to Install Cellulose Insulation."

        1. user-7538235 | | #65

          Terrific, I'll check them out.

      2. Rick Evans | | #64

        Hey Robin,

        Fiberglass is the Rodney Dangerfield of the green building community ("It gets no respect...") But, BLOWN-IN fiberglass is a great product. I used it for my own home and have no regrets. It's not itchy like fiberglass batts and is far less dusty than cellulose. The stuff literally feels like little cotton balls. Mineral wool has a huge carbon foot print compared to cellulose and fiberglass.

        Blown-in Fiberglass ranks high in terms of "healthy insulation" and is similar to cellulose and Rockwool for flame spread. Rockwool ranks poorly for health due to its formaldehyde binder.

        That all being said.... cellulose is still the 'greenest' insulation and would be perfect for your shallow, irregular walls cavities. Borate treated cellulose will not burn and pests hate the stuff. I've never heard of anybody having issues with borate. My wife has extreme sensitivities to spray foam but had no issues at all with the 3 tons of cellulose in our attic.

        1. user-7538235 | | #66


          Thanks for your reply and info. I really like the idea of the blown fiberglass product. I'm looking into whether there is anyone in proximity to NW FL who installs it but there may not be.

          As I mentioned before, the original plan was to use BIBS so I was hugely disappointed when that was no longer an option. I'll check out cellulose if I can't find a Spider installer.



  52. user-7538235 | | #67

    Hello... me again. An update and new conundrum on my historic reno project...

    We decided to go with the same open cell foam inside the remaining stud cavities after rigid foam and air gap. Turns out they are too shallow. Some test areas revealed that the foam peeled off during the trimming.

    Ok, so options are do nothing, another layer of rigid (not preferred) or perhaps use thin batt insulation separated down to 1-2" (most are about 1.5-1.75" deep but it varies). My preference is to use batt but mostly due to my concern over street noise over actual r value. It would be no worse than where the house was before removing plaster, with the exception of soundproofing which could be.
    That brings me to natural sheep wool. I have two suppliers - one uses borates and the other uses Thorlan IW for pest control. I'm okay with either, but the Thorlan may hold up better over time (from what I've read, with grain of salt on source). It's more expensive than borate treated.

    I am not finding much info on use of these products in the US. There are old articles and blog posts from 10 years ago. Are there any new thoughts?

    It's used extensively in Europe and there are nightmare stories about moth infestations but they were mostly about early providers that did not properly treat the material. Not much new there either.

    I can post a new thread for sheep wool insulation but thought I would begin here since it relates to the whole wall system for rebuilding a historic property.

    Comments are much appreciated!

    1. User avater GBA Editor
      Martin Holladay | | #68

      I've never used sheep wool insulation, but I'm skeptical about its usefulness. It's basically a gimmick. It costs more than fiberglass or mineral wool, it's hard to source, and it there's probably a long lead time if you order it. If you want a fluffy air-permeable batt, I suggest that you use fiberglass or mineral wool. If blown-in insulation is an option, cellulose is best.

      1. user-7538235 | | #69


        I can appreciate that claims on its usefulness may be coming from the manufacturers, but there are lots of green/sustainable building orgs in the UK that endorse it. It does seem widely used there and elsewhere. Similarly, there are lots of lawsuits and defunct companies that didn't treat with insecticide that caused infestations of "biblical proportions." Obviously, I want to avoid that!

        But, there also seem to be some compelling reasons for me to consider it - can standard fiberglass batts be easily separated down to a 1-2" ply and would that afford equal sound deadening?

        According to insulation sub, no blown or sprayed product is available that would work in such a shallow cavity at this point.

        One scientific article discussed the pest issue, that Thorlan salt seems to address. And the other is marketing info on one product available here in case you feel like revisiting the material.

        Thanks again,


        Ps - if you know of any colleagues that have used it with positive or negative results, I would be very interested in further input.

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