Construction Design-Continuous Exterior Insulation

You would think that in my very cold climate of Northern Minnesota, we would be putting as much insulation in our walls and roofs as possible.  Not the case.  The majority of new homes I blower door test have the code minimum R-21 of wall cavity insulation, usually fiberglass batts, and blown fiberglass in vented attics, R-49.  I have friends who are building homes in Missouri who install higher wall R-values.  Exterior wall insulation would be a great way to improve the thermal performance along with comfort in our homes, but it is rare in my area.  There’s a reason why, albeit not a very good one, that I’ll touch on in a bit, first, lets look at what the 2018 International Residential Code (IRC) has to say on the subject.

N1102.1.3 (R402.1.3) R-value Computation.  Insulation materials used in layers, such as framing cavity insulation or continuous insulation, shall be summed to compute the corresponding component R-value.  The manufacturer’s settled R-value shall be used for blown-in insulation.  Computed R-values shall not include an R-value for other building materials or air films.  Where insulated siding is used for the purpose of complying with the continuous insulation requirements of Table N1102.1.2, the manufacturer’s labeled R-value for insulated siding shall be reduced by R-0.6.

The code goes on to allow an alternative to the above code, using U-values, but we’ll keep it simple here and just use the R-value requirements.  Simply, the code wants us the add the manufacturer’s listed R-values together to determine the wall or ceiling’s R-value and not include the R-value of drywall or wall sheeting for example.  The code talks about Table N1102.1.2 which includes the requirements of climate zones 1-8, I’m just going to discuss the cold climate wall insulation requirements of climate zones 6, 7 and 8.  Table N1102.1.2, wood framed wall R-value says: 20 + 5 or 13 + 10.  What does that mean?  The first values, 20 + 5 are saying that if you build a 2 x 6 wall, R-20 cavity insulation is required along with R-5 of continuous exterior insulation.  If you are building a 2 x 4 wall, R-13 cavity insulation along with R-10 in exterior insulation would be the requirement.  So, if you live in climate zones 6, 7, or 8, you need to include continuous exterior insulation, according to the 2018 International Residential Code.  There is another section of the code that says R-5 on a 2 x 6 wall is not enough, but that’s a discussion for another time.

Now lets look at Minnesota’s 2020 Energy Code, Table R402.1.1, the code currently in force.  Climate Zone 6 wood frame wall R-value, 20, 13+5.  Climate Zone 7, 21.  If you are building in the State of Minnesota, the only time you are required to use continuous insulation is when building a 2 x 4 exterior wall in climate zone 6.  Apparently 2 x 4 exterior walls are not allowed in the northern half of the state.  Of course you can always build above code and add exterior insulation, but there are a few other things that need to be addressed, like the interior vapor control requirements.  Do I agree with the Minnesota wall insulation codes, not really, but I do get why they eliminated the exterior insulation requirement.  It has to do with double vapor barriers.  Extruded Polystyrene insulation (XPS) is very common in my area, and I would bet that most builders would use this product as the continuous exterior insulation because they are familiar with it, and because it’s fairly cheap.  XPS can be a vapor barrier if its more than 1 inch thick, two inches of XPS has a perm rating of around .5, making it a Class II vapor retarder.  Combine that with polyethylene sheeting as the air/vapor control layer, perm rating of .06, on the warm in winter side of our walls, which is also very common in my area, we now have drastically diminished the drying potential of the wall.

The topic of this blog is Construction Design-Exterior Insulation.  I’m going to discuss two different ways I’ve installed exterior insulation along with a new product that just hit the market that I think would work well in my area.  First, a project of mine in 2013.

Unfortunately, I do not have any photos of how we installed the exterior insulation on this home.  Instead I’ve drawn the detail using Sketchup.  First, the home is a seasonal vacation property used very little during the winter months.  The homeowner wanted us to use exterior insulation.  I didn’t know then what I know now and made some okay decisions and some of the details I wish I could change.

First, I bucked out all the windows and doors using 1 x 3’s attached directly to the sheeting.  I then added the first layer of insulation, 3/4 inch, which was cut around all the window and door bucked openings.  I installed a second layer of XPS over the first layer of insulation and the bucks, giving us an R-7.5 worth the continuous insulations.  I figured I would be more confident installing the flanged windows and doors over 3/4 inch of insulation rather than the full 1.5 inch.  I was also able to offset the seams.  I had heard of XPS shrinking over time, the offset would hopefully reduce the cold temps from contacting the wall sheeting.  After all the insulation was installed, we installed a house wrap.  Window and door rough openings were water sealed, windows installed and the house wrap detailed to the manufacturer’s instructions.

We were hired to construct the shell of the home, the homeowner then took over with hiring of the subcontractors for heating, electrical and plumbing.  They self-performed the insulting and interior air sealing.  I remember getting a phone call from the homeowner asking if they should install polyethylene sheeting on the interior.  I told them yes, which turns out was an ok decision, code did require a class one or two vapor control layer on the warm in winter side of the assembly.  My climate now requires R-15 exterior insulation before you can move to a class III vapor retarder, the poly is helping to keep the back side of the wall sheeting dry during the winter months.  A better decision would have been to use a smart vapor control product, like Intello, Majrex or Membrain, but I’m not sure if any of these existed in 2013.  The cabin also has no air conditioning, which helps to keep the polyethylene sheeting from becoming a condensing surface during the summer.  If I were building this cabin today, the details would be much different.

The second way I’ve installed exterior insulation was on a recent project, the concreteless slab on grade home from 2019.  You can read all about the wall assemblies used on that home here.

I mentioned a new exterior insulation product I’m excited about, the photo below is a teaser.  I will be writing a dedicated blog for this product, stay tuned!

2 Replies to “Construction Design-Continuous Exterior Insulation”

  1. For our new home build, starting this spring, I have decided to do this from outside to in. Hardie siding, Benjamin Obdyke Slicker rain screen, Zip R-6 or R-12, closed cell spray foam into the 2×4 wall cavities and drywall with no vapor barrier. This assembly will seal air intrusion (assuming all joints properly taped with Zip tape and moisture intrusion protection. The R-Value for the wall will be either 25.5 or 31.5 depending on which Zip-R I use. It is recommended to use at least R-12 for exterior insulation with a breathable cavity insulation like fiberglass, mineral wool or blown in cellulose (dries to inside) but with closed cell foam in the cavities, I’ll have no worries of moisture condensing in the 2×4 cavities by using closed cell spray foam.

    In any wall insulation project using Zip-R, you should not use a vapor barrier behind the drywall due Zip-R being a moisture barrier. Water needs to dry and it cannot to the exterior with Zip-R.

    The issues using Zip-R12 is the thickness and 4″ fasteners are required plus the windows need to be trimmed out. Special nail guns must be used due to the 4″ fasteners and a lot are needed for shear strength. Zip-R6 need more standard 3″ fasteners. but not certain if windows need to be trimmed out for nailing if 3″ nails are OK without compressing the foam backer too much, prior to window installation.

    1. Hi John, Where are you located and do you know which code cycle your area is on?

      In my area of climate zone 7, and when using fluffy cavity insulations like fiberglass, cellulose or mineral wool, building codes will require the use of a class I or II vapor retarder in a 2 x 4 wall with anything less than R-10 exterior insulation. This moves up to R-15 exterior insulation in a 2 x 6 wall, but you are right, I wouldn’t recommend a vapor barrier like polyethylene sheeting, which is a class I vapor retarder. If a vapor retarder is required, I would use one of the smart vapor control products. Certainteed’s Membrane is a good, affordable option and it can also be installed as a secondary air control layer. The exterior insulation requirements and vapor retarders are found in R702.7.1 of the ICC codes.

      All that being said, closed cell spray foam, if applied at a specific thickness, is a vapor retarder. You could just hang your wall coverings without the need for any other vapor retarding products.

      You’ve designed a nice wall assembly and have considered water, air, vapor and thermal properties, now you just need to figure out how to build it. What I do when trying to figure out how everything is going to go together is build a mock-up. A 4 x 8 stud frame with a window opening. This will give you a chance at working with the Zip-R and seeing how different fasteners affect the product. It will also give you a chance to work out the window buck details and work with the tapes before actually building your home. If you have questions on a specific products, like fastening Zip-R, I recommend contacting the manufacturer. I know the Huber factory rep in my area are more than happy to help out. Social Media is also a resource, I’ve reached out to a few different manufacturers on Instagram, I’ve always been directed to someone with answers.

      One last observation, Zip makes it really easy to air seal the seams and around windows. The tougher transitions are between the foundation and wall and wall to interior ceiling. It’s recommended that these transitions be continuous. That wall to roof is, in my opinion, one of the hardest details to nail. (Pun intended). Here’s an article I wrote for Green Building Advisor on how I like to detail that transition.

      Hope this helps and good luck with your project!

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