Exterior Insulation ONLY Information
I’m having trouble finding much information on using only exterior insulation on a house. I am designing a house that I will be building primarily myself in zone 6b – long, cold, snowy winters but pretty dry and low humidity. I was a carpenter for several years with a green building company in another state and participated in several net zero and passive house builds with a lot of sealing, interior and exterior insulation. I’m not trying to build net zero – I will be building very small (not tiny, but probably 500-700 sq ft max) and on a very tight budget but I want my home comfortable and durable, especially in this area with super long winters. One insulation technique i’ve come across in a few youtube videos that I wasn’t really familiar with is exterior insulation only. I am having trouble finding more info but I find the interior aesthetic is attractive – I also hate drywalling and I like the idea of more natural or completely unfinished wood interior with no paint off-gassing. It would seem that this would be a great benefit for condensation issues as well as walls that are completely exposed to the inside should be drying extremely well? My plan is to be heating primarily with wood with some kind of backup system. How is electrical being run though? How much exterior insulation do you need to achieve a relatively high efficiency wall this way? Other issues? Anyone build this way that can help lay out advantages/disadvantages? Thank you.
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Jacob,
I suggest that you start by reading this article: "Getting Insulation Out of Your Walls and Ceilings."
Thank you!
https://architizer.com/projects/perfect-wall-house/
Where ever you put your insulation you still need the same R40 walls.
The stuff you can put on the outside tends to cost more and make siding installs difficult. The fluffy stuff on the inside tends to be cheap and easy.
Will you have a problem getting a permit without drywall? Some may consider it a fire hazard without drywall.
If you plan is to heat with wood be sure to think about where all the air that will go up the flue is coming from. Wood stoves and tight houses tend not to work together.
You may want to read this article when setting your insulation goals
https://www.greenbuildingadvisor.com/article/pretty-good-house-2-0
You may enjoy this Matt Risinger video.
https://www.youtube.com/watch?v=4mNTHP-Y_GE
Walta
I would take a look at this free ebook at Foursevenfive.com, if you haven't already seen it. Their "tier 3" framing has all of the insulation on the exterior.
https://foursevenfive.com/2x-framing-smart-enclosure-ebook/
HI Jacob - Martin cites a great GBA article on PERSIST and REMOTE systems; I just want to point out that he wrote this article in 2009 and ten years later, still pretty much up to date...
Peter
Joe Lstiburek designed a house (and practiced on a garage for photos, etc) that seems to me to be a more buildable version of PERSIST, using ZIP instead. (NB: it’s easy to understand from a conceptual standpoint. I’m not a builder, so others may see some difficulties in building it.) See BSI-096 and attached photo. In the iteration in the article, he used mineral wool for the exterior insulation on the walls and roof. He did use interior insulation as well, but you could do thicken the exterior insulation and eliminate the interior insulation.
As an aside, I think that designing a house with a simple exterior form will end up saving more money than changing some of the wall and roof cross-sectional details.
I remember a discussion of the Risinger house with no exterior insulation. As I recall, it took a lot of craftsmanship to achieve that look inside. I have to say I do like that house, even with no overhangs.
Hi Jacob,
I've been designing on a similar project for a while now: only exterior insulation, exposed studs, similar hatred of drywall & distrust of paints, 576 sq ft footprint (~1000 sq ft with the second floor), climate zone 5. These helpful fellows have already linked the bulk of the resources I have used (be sure to check out REMOTE: A Manual in Martin's link), but I think I can help you with some of the more specific questions you have.
>It would seem that this would be a great benefit for condensation issues as well as walls that are completely exposed to the inside should be drying extremely well?
While it is true that exposing the studs will promote drying, you should not be having condensation issues with the walls. Get the foam thickness right and make sure you have a solid air barrier. I'm sure you already know to use multiple layers and stagger the seams for the foam. For the roof, have an air barrier on both sides of the foam and/or tape the seams in each layer, to really make sure that moisture-laden air cannot travel through the thermal layer and condense on either side. It doesn't hurt to do the same for the walls. In my design, I have a layer of continuous plywood structural sheathing which I intend to tape at the seams, followed by a few layers of untaped foam and a peel and stick membrane.
>How is electrical being run though?
This is one of the trickier aspects, as is plumbing (and ducts, if you have them), but there are a few options. The easiest is the interior walls: those studs won't be open, and so routing pipes and wires through them is easy. I have one wall running right up the middle of my assembly; it touches every room, and has been useful for these things. The space behind cabinets, in closets, or under stairs is also useful. For the exterior walls, you can run wires at the rim joist and cover the cavity. It is also possible to run wires and pipes through a slab, you can even install outlets in it if you want.
If you want switches or outlets on the exterior wall, two options come to mind. If you don't mind the industrial look, you can use conduit. Alternately, you can select certain locations and inset drywall panels (or cement board, if you really hate drywall) to create a service cavity. My design uses 2x6s as the framing members, so in locations I need service cavities I can put 2x4s between the studs, and cut the drywall to fit. This also works for hiding plumbing or ducts. I can even throw in a horizontal 2x6 if I don't need a full height cavity; following it up with shelves will allow it to blend right in.
My design also has a bathroom on the second floor. I have little choice but to cover the joists below it. The space below is just a mudroom and half-bath, so it's no great loss.
>How much exterior insulation do you need to achieve a relatively high efficiency wall this way?
For your climate zone, the usual recommendation is ~R40 for the walls and ~R60 for the ceiling/roof. With all exterior foam and good air sealing, you can get away with a little less without losing comfort, but you will of course use more energy. I'd say about 6-8" of foam for the walls and 9-10" for the roof would be safe. You'll need long screws. Get epoxy coated, avoid stainless steel (that means no treated wood).
>Other issues?
Pay close attention to wall bracing requirements. The IRC expects interior drywall when calculating the length of braced wall lines, without it minimum bracing requirements are increased. If you are not in a high wind or seismic area, you can use diagonal wood boards as the structural sheathing, which will look pretty on the inside.
Usually people don't care much about stud placement when framing, but it matters a lot aesthetically when the studs are exposed. Plan out stud and window placement in a CAD program ahead of time, and make sure you know where any beams are going to fall as well. Plan out any plumbing, ducts, and electrical runs while you are at it. I'm using hydronic heating, but still have ventilation ducts.
Windows are fun. Figure out how you are going to attach them, as it will affect your rough opening sizes, and thus your framing. I'll be attaching mine to the furring strips of the rainscreen. The JLC had details online, but I cannot seem to find them. (Edit: It was not the JLC, it was this website: https://www.drjbestpractices.org/installation-guide/windows-fpis-wood-rainscreen) This approach makes your rough opening sizes more flexible, which is helpful for framing. I'll be over-sizing my rough openings by a few inches, and slanting the window wells to meet the window. I think it will look pretty this way, and will provide slightly more light and slightly better viewing angles than traditional outie windows.
Also know where your drainage layer is going to be. Outside the foam makes the most sense, but you can do inside as well so long as you leave a gap. This will affect your windows as well.
>Anyone build this way that can help lay out advantages/disadvantages?
I haven't built this way yet, but it is usually considered more expensive. Use cheap recycled foam, and you might reach parity. Check out REMOTE, using blown insulation in the attic is cheaper, and dealing with 10" of foam can be a pain.
Overall, the design process has been challenging, but it has also been fun and informative. If you enjoy designing down to the smallest detail, this will be a good build for you. However, if you prefer just to build it and figure things out on site, then you should find a different wall and roof assembly.
>I have a layer of continuous plywood structural sheathing which I intend to tape at the seams, followed by a few layers of untaped foam and a peel and stick membrane.
What was the intention with using the peel and stick membrane on the foam instead of applying it to the sheathing? Was it to prevent moisture laden air from traveling through the foam and condensing on the exterior of the sheathing?
If so, would applying the peel and stick membrane directly to the plywood present a surface that could accept condensation with no ill effects?