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Community and Q&A

Insulation retrofit

Jeremy Kachejian | Posted in GBA Pro Help on

Looking for some advice on a smallish rehab project I am about to start. I’ve read many of the great articles and Q&A discussions here, so I think have bit of direction…but as I am novice, I am interested in some feedback before I moved forward.

I recently purchased a house in south eastern NH (zone 5) which has a 20 x 40 outbuilding that was used as an antique store back in the 50’s. The structure is sitting on a slab foundation and is 2×4 framed with very old fiberglass insulation sandwiched between exterior and interior ship lap, hung vertically. The ship lap is pretty beautiful, its true one inch, width’s vary between 8 and 24+. There are no interior walls. Ceiling, if you can call it that, that is really just planks nailed to the cross beams. The interior of the (newish) roof is ship lap as well, there is no sheathing or insulation as far as I can tell

My primary goal is turn this structure into a home office for myself. Secondary goal is to be able to utilize as a guest house down the road. I am going to remove the ceiling and frame a lofted space. My goal on this forum is to kindly seek out the best approach at insulating the space appropriately.

I am planning on having a demo crew take down the interior ship lap (carefully) and cleaning the disintegrating rug off the slab floor. If I had too, id be open to removing the exterior ship lap, but only if it was absolutely necessary to meet the goal.

I have read several of Martin’s articles that spoke to this scenario. I recall one article in particular that mentioned furring out the studs, creating an air channel with 1×1’s and then using a combination of rigid foam and roxul (I think). That approach might ultimately work, though i the idea of ripping that much foam seems daunting.

I’ve attached a couple of pictures in the event it helps.

Thanks for any feedback!


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  1. Jeremy Kachejian | | #1

    didnt see my pics come through so reattaching

  2. Brendan Albano | | #2

    Insulating "appropriately" covers a pretty wide range of solutions!

    Do you have specific goals regarding energy efficiency? Best you can do on a minimal budget? Bring it up to code? Bring it to "pretty good house" standards? Passive house? Net zero?

    That will probably direct the discussion in a useful way for you.

  3. Jeremy Kachejian | | #3

    Thanks for the quick reply Brendan. From a priority perspective, my number one goal/concern is air quality, which for me, means choosing building materials that conform to very high standards. Secondary priority would be to create a space that could lead me down a net zero path.

    From a project planning perspective, I will be hiring contractors for various pieces of the project and completing others myself. While I don't have the budget to hire a GC anyway, I really want to use this project to cut my teeth on several areas of building that are only theoretical to me at this point (my father in law has many years of carpentry/building under his belt, so I do have some experienced help available when i need it).

    Even if I wind up contracting a significant amount of work out, I figure that understanding some of the core principles and approaches will allow me to direct traffic and ensure I get what I really want out of this.

    thanks again,

  4. GBA Editor
    Martin Holladay | | #4

    Your questions are too general. You will be more likely to get help if you ask a focused question.

    I'm guessing that your slab is uninsulated, so the slab will need, at a minimum, vertical rigid foam at the perimeter of the slab that extends below grade to a depth of between 2 and 3 feet. If this were my building, I would install rigid foam that was between 2 and 4 inches thick in this location.

    It sounds from your description that the building has one layer of exterior boards -- in other words, it has no sheathing. This raises two problems: your walls lack an exterior air barrier, and your walls lack a water-resistant barrier (WRB). This article may give you a few ideas of how to proceed: Insulating Walls in an Old House With No Sheathing.

  5. Brendan Albano | | #5


    At a high level, you want your envelope to control the movement of 4 things between outside and inside:

    - Liquid water
    - Air
    - Water vapor
    - Heat

    The link that Martin posted about insulating an old house with no sheathing explains a few ways to add these control layers into your wall without removing the exterior cladding. If you are willing to remove the exterior cladding, there are also a variety of ways you could add sheathing and insulation to the outside of your building.

    For air quality concerns:

    - You want to avoid anything getting moldy (which involves making sure things don't get wet and can dry out when they do get wet, which is discussed in the article Martin posted),
    - You want an airtight envelope, so that you control where the air comes from.
    - And you probably want balanced ventilation, again to control where the air comes from.
    - If you're interested in energy efficiency you probably want to use an HRV or ERV for your ventilation.

    For net-zero:
    - You're most likely going to need (significantly) thicker walls to achieve adequate R-values for net-zero, unless you just want to go crazy with the solar panels.
    - One option would be to add 4"-8" of rigid insulation to the exterior of your roof and walls.
    - Another option would be to build a double stud wall by building a second wall inside your building and filling it with fluffy insulation, which would shrink the interior space.
    - You're probably going to need to do (or hire someone to do) some energy modelling if you want to make informed decisions about how much insulation vs how many solar panels. There is free software out there that you can use for this, for example, but be careful of the garbage-in-garbage-out problem!

  6. Expert Member
    Dana Dorsett | | #6

    The exterior looks like board & batten siding- is it over a horizontal or diagonal ship-lap sheathing? Or is it nothing but the boards of the board & batten siding what you are looking at when you remove an interior side plank? Is there any cut-in bracing stabilizing the structure from racking forces?

    With board & batten it's worth giving yourself at least a foot of roof overhang at both the eaves & rake to limit how much of the roof drainage gets behind the siding. It looks like you currently have about 3", 4" tops. With low density fiberglass and a not-so-air tight interior the siding dries pretty quickly, but just giving it an air gap with cut'n'cobbled foam inside the stud bays might not be sufficient. The discoloration of the siding between the windows compared to siding close to the windows indicates that it's not really drying all that well in the "before" picture, and that the extra heat transfer near the windows may have kept that part of the siding drier.

  7. Jeremy Kachejian | | #7

    Martin - agreed, ill refocus my questions. Re: the existing slab, yes, it appears to be just an uninsulated 4 inch thick slab of concrete. You mentioned insulating the perimeter with rigid foam. If I go that route, how do I protect the foam that is above grade? Would installing rigid foam over the slab be an acceptable alternative, or am I missing the point of why I should be insulating the perimeter of the slab?

    Brendan - thanks for the info, I've done a bunch of reading this weekend on GBA and feeling a bit more educated. I think I am going to take down both the exterior board and batten and interior verticals planking. My thought process is by getting down to the studs on both sides I can essentially start from scratch and give myself several options. I also have a feeling I'm going to need to address some rot in sections of the sill plate. All in all, removing the exterior b&b seems to be worth the effort. Anyway, thinking is to install 2-4 inches of rigid foam to the exterior (adding strapping for the rain screen between the board and batten). On the interior, thinking to fur out the 2x4 walls and fill with Roxul, then drywall. Does that approach sound reasonable? Assuming I take care with the rigid foam installation, does this sound like a reasonable approach to better control water and air?

    Dana - it's just board and batten on the exterior and board on the interior. Old r13 pink insulation is in the 2x4 wall cavity. No sheathing. Not sure about any bracing yet either. I took down one interior plank and it looked pretty dry, no idea what the exterior boards look like just yet. I had a mold contractor come by yesterday to look the structure over and he said it actually was in decent shape from his perspective. few spots of evidence of an older roof leak, but the new roof is only a few years old so previous leaks are now mitigated. Thanks for the feedback on interior foam install, i am now thinking of installing to exterior. However, you also mentioned concern with the roof overhang, does that concern remain if I install exterior foam and add in strapping between the b&b? If so, what are my options to mitigate? Also, Interesting observation with the discoloration difference between the windows. I had no idea of the reasoning. This whole topic is very interesting to me, though admittedly quite daunting.

    Thanks for all the feedback folks.

  8. Brendan Albano | | #8

    Jeremy, stripping everything down is great!

    You might take a look at this article on Building Science Corporation for some inspiration:

    It's a small structure completely sheathed in zip sheathing, 4" of exterior mineral wool, with some small overhangs tacked on. There's also a drawing of putting rigid insulation over the top of the slab that you may be interested in.

    Regarding adding exterior insulation and increasing the interior insulation, just make sure that the ratio of exterior insulation to total insulation is appropriate for your climate zone (27% in climate zone 5). Here's a GBA article talking about that:

    If you do go down the route of using your foam as an air barrier (rather than taped sheathing), you'll still probably need to address the lateral stability of the structure somehow.

  9. GBA Editor
    Martin Holladay | | #9

    If you simply install rigid foam on top of the slab, you'll still end up with a thermal bridge at the slab perimeter -- unless you make the walls significantly thicker on the interior (or unless you install interior rigid foam on your walls, and connect the wall foam with the slab foam).

    Here is a link to an article that discusses different ways to protect the above-grade portion of exterior foundation foam: How to Insulate a Basement Wall. (Scroll down to the section of the article under the heading that reads, "If I insulate on the outside, how should I protect the above-grade foam?")

  10. Jeremy Kachejian | | #10

    Hello again GBA, hopefully its ok that I add to this thread instead of starting a new one. Same project, sort of new question.

    I demo'd the interior of the aforementioned 'cottage' and am getting ready to start exterior work. My plan was to remove the exterior board and batten siding (which is also serving as sheathing). From outside it, my thinking was: 1x10 shiplap > horizontal cor-a-vent battens for my rain screen > 7/16 zip sheathing > 2x4 studs (which I am planning on furring out with 2x2) > RoxulR23 > Gypsum

    Lot of work, but I figured it was the best long term approach and I am not under time constraints as its going to serve as a home office eventually. I felt good about the approach until a conversation today with someone who cautioned against removing the board and batten from a structural perspective. I mentioned I was going to do a wall at a time, so I wouldnt be tearing down the entire exterior at once. He suggested tearing off the existing batten, leave the board, and use that as my sheathing. Definitely made me pause. I love the idea of giving materials a second life, but I am neither skilled nor experienced enough to understand viability of this approach.

    So my question: is there any way to make that approach work and still wind up with a properly insulated structure? My rain-screen approach could still be the same I suppose, but I dont see how I would make it airtight. If I didn't mention earlier, I am on the sea coast of southern New Hampshire, zone 5

    Attaching an internal picture, not sure its needed, but figured it couldn't hurt. (You'll notice some rotted sill plate section, that's first up on my list before I begin exterior.)


  11. GBA Editor
    Martin Holladay | | #11

    Unless the boards are rotten, there is no need to remove the exterior boards. If you want the advantages of the Zip sheathing -- better bracing and better airtightness -- just remove the battens, and install the Zip sheathing on the exterior side of the boards, with long fasteners back to the studs.

    If you plan to thicken your 2x4 wall on the interior, why not just add a continuous layer of interior rigid foam? That way, you'll address thermal bridging through the studs. Here is a link to an article with more information: Walls With Interior Rigid Foam.

    -- Martin Holladay

  12. Jeremy Kachejian | | #12

    I'm back with a few more questions on this project and figured I'd add to this thread rather than create a new one. My question is around air sealing and I'll kick it off with progress pics and a description of what I have done so far.

    Wound up removing both the interior and exterior board sheathing and removed old crusty pink batt insulation leaving just the full dimension stud framing and the roof. This is sitting on a 4 inch slab on grade. I taped the bottom plate to the slab using Tescon primer and tape (found through a thread on this site, from 475 building supply). I used zip sheathing and tape. Added 1x3 strapping to create a rainscreen. Used a coravent product for top and bottom of wall. Exterior siding is 1 inch ship lap which I back primed. I didn't think to seal the seam between the sheathing and bottom plate.

    I am looking for feedback on air sealing details now that I have started the interior. Keep in mind there are no interior walls at this point, the structure is a 34x 24 shell and about 70 years old. Slab is in perfect shape somehow. Note that I am the homeowner and this is a DIY project for the most part, I am not in the trade professionally. I've just spend lots of time reading this site and watching high performance building videos. I've made lots of mistakes but it's been a great learning experience thus far.

    My plan is:

    - seal interior bottom plate to slab with Tescon tape. There is no gasket or mudsill except in the few spots I had to replace some sections that rotted.

    - seal seam at top plate with tape

    - seal stud bays with combination of tape and Contega caulk

    Questions: Am I on the right path? Is anything overkill? Am I missing air sealing detail?

    Plan for the floor: 6 mil poly taped or caulked at plate, 1.5 inches of floated EPS, 3/4 inch floated advantech sub floor, 3/4 inch hardwood floor

    Questions: slab doesn't seem to take on water from outside, I've done several tape tests and not seeing
    Evidence of water intrusion. Is my plan sound? Am I ok to float the whole thing? Do I need to eps and subfloor seams given they are both T&G?

    Thank you for any feedback or advice or comments. This site is an awesome resource

  13. GBA Editor
    Martin Holladay | | #13

    It sounds like you have already sealed the leaks in the sheathing layer, with one exception: you forgot to seal the sheathing to the bottom plate. So if I were you, I would run a bead of caulk (on the interior side of the sheathing) along the crack between the wall sheathing and the bottom plate.

    If you do this, there is no need to "seal the stud bays with a combination of tape and Contega caulk."

    When you install your new subfloor on top of the rigid foam, the best way to proceed is to fasten the subfloor to the concrete with long TapCon fasteners. If you want to try the "floating plywood" approach, you should either use tongue-and-groove plywood (or OSB), or you should install two layers of 1/2-inch plywood (or OSB) with staggered seams, screwed together. (This approach prevents the "potato chipping" problem.)

  14. Jeremy Kachejian | | #14

    Thanks Martin. For the floor, is the 6 mil poly needed? I am not quite sure I understand the benefit to adding the poly vs 1.5inches of xps directly on the slab. It's T&G foam as well so I am imagine if I taped the seams and sealed the perimeter into the bottom plate it would be pretty well sealed. this is probably a silly question but if the poly is applied to the concrete, what happens to any water that wicks through the concrete?

  15. GBA Editor
    Martin Holladay | | #15

    You're right that 1 1/2 inch of XPS is a pretty good vapor barrier, so it's possible to omit the polyethylene if you want. Most builders would say, "Poly is cheap, and it's good insurance." It's also true that polyethylene has a lower vapor permeance than XPS, so if you want the lowest possible rate of vapor transmission, install the poly.

    If the concrete ever gets damp from below, it will stay damp. Installing polyethylene above the concrete will prevent the concrete from drying upward. But that's OK. Damp concrete stays strong. The concrete won't rot.

    What you want is a good vapor barrier to separate the damp concrete below from the vulnerable building materials above.

  16. Jon R | | #16

    See below for discussion of plastic over a slab causing mold odor problems.

    Walking the Plank

  17. GBA Editor
    Martin Holladay | | #17

    I have never been involved with a job that had an odor due to polyethylene above a concrete slab. But Joe Lstiburek says that it happens. To be safe, you can use one of the products he recommends instead of polyethylene.

  18. Jeremy Kachejian | | #18

    Point taken, I have actually been wondering how to apply the poly with a true seal. I have found basically no literature on how to seal poly to a concrete floor for slab on grade retrofits. If I may, are you aware of any product or articles referencing products that could fit here? I'd rather not use something that will off-gass.

    Would I be better off just applying the xps to the concrete and sealing all seams and sides to the sill with tape or caulk? I know I asked this before, but given the concern I won't avoid small holes on the poly, I'm not sure what direction I should take.

    Thanks again for the time,


  19. Jeremy Kachejian | | #19

    Looking into that Jon, thanks

    Does putting down a sleeper frame make this issue go away?

  20. Jon R | | #20

    My guess is that sleepers would only help to distribute any air that leaks out from under interior side poly.

  21. Jon R | | #21

    You could use zero VOC epoxy instead of poly. And then tape any cracks in the concrete.

  22. Jeremy Kachejian | | #22

    I'm back with another two part question

    1 - though I am not taking in any water (confirmed during the storms in NH this week), I understand vapor drive will be at play through the concrete slab since there is no vapor barrier below the four inch slab. Do I need to be concerned with vapor drive in the stud bays? The mud plate is older full dimension 2 x 4 with no sill seal. I will have a fairly air tight wall assembly, my hope is any vapor will diffuse internally/externally through the walls and not create an environment for mold. But I don't know if that is a wish or a reality.

    2 - i snapped the idea of sealing the concrete slab because I don't think it will penetrate the remainder of the carpet glue that is imbedded. I tried scraping with bean e doo and there is just way to remove it all - very messy as well. So, I am either going to use a dimple mat product like DMX-1 (the air space sounds like it makes sense to me) or 6 mil poly before I put the rigid foam down on the floor. Understanding there will be vapor drive here as well, is this a breeding ground for mold as well? Am I overthinking it?

    I attached a new photo for reference. Thanks for any comments/thoughts

  23. GBA Editor
    Martin Holladay | | #23

    1. I wouldn't worry about vapor drive below the bottom plates of your walls. The amount of vapor that might diffuse through those areas is quite small.

    2. I think that either the dimple mat approach or the polyethylene approach will work well. Even though Joe Lstiburek worries about odors when polyethylene is used, I think the problem he describes is quite rare.

  24. Jeremy Kachejian | | #24

    Thank you both for the responses. Did several rounds of taped poly tests, left it over a week at one point. No evidence of moisture at all. It's almost hard to believe since the slab is only 4 inches. I bought a calcium chloride test kit, we'll see what that comes with.

    I found an Tescon double sided tape that I may be able to use at the slab/sill perimeter to create a seal, in lieu of mastic or glue. After trying to clean the existing mastic mess,I've had my lifetime fill of the stuff

  25. Jon R | | #25

    For the slab floor and mold question, it might be useful to use a test similar to the better one used for slab moisture - tape polyethylene sheet to the floor and stick a relative humidity probe behind it and look for 70+% RH. Of course a test now doesn't mean there wouldn't be a problem at other times/seasons.

    The best option for above slab poly might be fresh carpet glue/mastic under the poly to create the "fully adhered vapor barrier" that Lstiburek (and below) recommends.

    Possibly a mildewcide would be an effective alternative.

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