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!

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