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Insulation retrofit

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 2x4 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 1x1'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!


Asked by Jeremy Kachejian
Posted Jan 12, 2017 5:18 PM ET


11 Answers

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didnt see my pics come through so reattaching

ceiling.JPG exterior.JPG wall.JPG
Answered by Jeremy Kachejian
Posted Jan 12, 2017 5:19 PM ET


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.

Answered by Brendan Albano
Posted Jan 12, 2017 5:41 PM ET
Edited Jan 12, 2017 5:43 PM ET.


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,

Answered by Jeremy Kachejian
Posted Jan 12, 2017 6:55 PM ET


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.

Answered by Martin Holladay
Posted Jan 13, 2017 5:43 AM ET



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, https://beopt.nrel.gov/ for example, but be careful of the garbage-in-garbage-out problem!

Answered by Brendan Albano
Posted Jan 13, 2017 11:14 AM ET


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.

Answered by Dana Dorsett
Posted Jan 13, 2017 4:41 PM ET


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.

Answered by Jeremy Kachejian
Posted Jan 15, 2017 3:51 PM ET


Jeremy, stripping everything down is great!

You might take a look at this article on Building Science Corporation for some inspiration: https://buildingscience.com/documents/building-science-insights/bsi-096-...

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: http://www.greenbuildingadvisor.com/blogs/dept/musings/calculating-minim...

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.

Answered by Brendan Albano
Posted Jan 16, 2017 10:59 AM ET
Edited Jan 16, 2017 11:41 AM ET.


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?")

Answered by Martin Holladay
Posted Jan 23, 2017 12:20 PM ET


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.)


Answered by Jeremy Kachejian
Posted Mar 15, 2017 9:35 PM ET


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

Answered by Martin Holladay
Posted Mar 16, 2017 4:23 AM ET

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