What You Need to Know About Continuous Insulation-Part 1

This post first appeared on the Green Building Advisor website.

Living in a very cold climate where wintertime temperatures colder than -20°F aren’t uncommon, you would think contractors and homeowners in my market would be excited to use continuous insulation (CI) on every new home under constructed along with every existing home having the exterior cladding replaced.  In reality, I rarely see either happening.  Part of the reasoning, my state and local building codes don’t require CI.  Cost is another driving factor.  As both homeowners and builders become better educated on the benefits of CI, this should start changing, I’m advocating for its inclusion on projects I’m involved in.

There are many reasons to use continuous insulation.  Reduced energy consumption is one benefit, though lowering heating and cooling costs are, in my opinion, less important and a secondary reason to add CI.  Improving comfort (thermal and sound) and expanding durability are more important.  (I’ll get into durability in detail later in this article.)  Building codes have begun to align with building science and we are seeing more areas across the country adopt continuous insulation as part of the energy code.  The scary part for most builders is the unfamiliarity of installing insulation on the exterior of a wall.  What insulation can be used?  What thickness and R-value are required?  How do I detail window and door openings?  How do I secure the insulation?  How do I attach the cladding?  Where does the house wrap go?  What about other penetrations such as exhaust vents and electrical penetrations?  What about the vapor retarder?  We will be discussing these topics and more over the next two articles.  In this post, we will be discussing the building code requirements along with the building science principles covering CI.

Continuous insulation requirements with regards to the energy code can be found in Chapter 11, Energy Efficiency of the International Building Code (IRC).  Most contractors will use the “prescriptive” code path to meet the continuous exterior insulation requirements (I will be referring to the 2021 edition of the IRC and the prescriptive code path for this article).  There are other compliance path choices such as the total building performance and energy rating index options, these allow for some tradeoffs.

Table N1102.1.3 (R402.1.3) Insulation Minimum R-Values and Fenestration Requirements by Component (2021 IRC)

The yellow highlighted area in the table above shows the different R-values possibilities for wood framed walls based on climate zone.  Climate zones 1, 2, and 3 are unique in that they have the option of cavity only insulation.  Climate zones 4 through 8 all are required to have some amount of continuous insulation.  The first set of numbers in the table for those climate zones are for walls containing cavity insulation, R-20 (typically a 2×6 framed wall) plus they are required to have R-5 continuous insulation.  The second number is framed walls with cavity insulation of R-13 (typically a 2×4 wall) plus continuous insulation of R-10.  Climate zones that have a third option would be if continuous insulation is the only insulation used, 0 means no cavity insulation plus the R-value for the specific climate zone’s continuous exterior insulation. 

As an example, climate zone 6 has an option to use R-20 cavity insulation with R-5 continuous insulation.  A second option would be to use R-13 cavity insulation with R-10 continuous insulation.  The third option is to leave the wall cavity void of insulation and install R-20 continuous insulation.

The second area of code pertaining to continuous exterior insulation is in Chapter 7, wall covering, specifically R702.7 Vapor Retarders.  There are four different tables of interest in this section.  The first is Table R702.7(1) Vapor Retarder Materials and Classes.  This table identifies the class of vapor retarder based on the materials perm rating. 

Before we get into these tables, we should look at the definition for perm or permeance with regards to building materials.  A perm or permeability of a material refers to the passage of one grain of vapor through 1 square foot of material in 1 hour with a pressure differential between the cold and warm sides of the material at 1 inch of mercury.  The higher the number, the easier water vapor can diffuse through the material.  Glass and metal move no or almost no water vapor through the material, whereas fibrous insulation will be vapor open, easily passing water vapor though the insulation.  A test used to verify a materials permeance is called ASTM E96.

Table R702.7(1) (not shown) classifies materials based on how easily vapor moves through the material.  A class I material will have a perm rating of less than or equal to 0.1.  The glass and metal discussed earlier are in this class, along with polyethylene sheeting.  A class II material will have a perm rating between 0.1 and 1.  Kraft faced insulation, vapor retarding paints and most smart (responsive) vapor retarders fall in this classification.  The class III materials are listed at 1.0 to 10.0 perms.  Latex or enamel painted drywall fall into a class III vapor retarder.  Any material more than 10 perms is considered vapor open.  The classification of perm ratings needs to be understood before we can discuss the next table.

Table R702.7(2) is about where you can and cannot use a specific class of vapor retarder in a wall assembly.  Class I and II vapor retarders should not be used in climate zones 1 and 2 (at least not on the interior side of the wall).  Why is this?  It has to do with vapor drive.  The direction vapor moves in areas with higher outdoor heat (and humidity) is inward, towards the cooler and less humid interior side of the building.  A low permeance vapor retarder on the interior (this even includes some wallpaper) will trap this inward drive of moisture inside the wall which may cause moisture related issues.

Table R702.7(2) 2021 IRC

There is a footnote that pertains to class I and II vapor retarders.  Footnote a states:

 “Class I and II vapor retarder with vapor permeance greater than 1 perm when measured by ASTM E96 water method (Procedure B) shall be allowed on the interior side of any framed wall in all climate zones.”  

The code language in this footnote can be confusing, a class I or II vapor retarder could be used if it has a vapor permeance greater than 1 perm?  This is contradictory to the definition of the perm rating for a class I or II vapor retarder.  I contacted Kohta Ueno from Building Science Corporation for some help, his reply was, “Vapor retarders are measured under ASTM E96 Procedure A (“dry cup”) conditions.  So this clause is saying if you have a Class I/II vapor retarder, and it’s 1 perm or greater under Procedure B (wet cup), it is allowed—i.e., a variable perm or smart vapor retarder.”  In other words, if we use a class I or II smart (responsive or variable) vapor retarder that achieves a perm rating greater than 1 perm, it can be installed in any climate zone.  (I think the codes should be a little clearer with identifying smart vapor control products.  Labeling them as an ASTM E96 water method (procedure B) doesn’t compute with many people.)

We are permitted to use class I and II vapor retarders in the climate zones of marine 4 through 8 with a couple of qualifiers.  If you are using a class I interior vapor retarder in conjunction with a continuous exterior insulation product that is also considered a class I vapor retarder, you’ll need to provide the code officials with some sort of engineering or analysis showing the design will not trap moisture between the two vapor retarders, in other words, you’ll need approval (footnote b). 

Another qualifier, footnote c says, “where a class II vapor retarder is used in combination with foam plastic insulation sheathing on the exterior side of framed walls, the continuous insulation shall comply with Table R702.7(4) and the class II vapor retarder shall have a vapor permeance greater than 1 perm when measured by ASTM E96 water method (procedure B)”. 

Again, the ASTM E96 water method (procedure B) refers to a “smart” vapor retarder.  This footnote will eventually get us to Table R702.7(4), but first, let’s look at Table R702.7(3) which covers class III vapor retarders and when they can be used in marine zone 4 through zone 8.  Because we are discussing continuous insulation, we are most interested in the yellow highlighted areas.  This table shows us how much continuous exterior insulation is needed to move from the required class I or II vapor retarder to a class III product, such as painted drywall.

R702.7(3) 2021 IRC

If we use climate zone 6 as our example again, a 2×6 wall is going to require R-11.25 continuous insulation IF you want to move to a class III vapor retarder.  A 2×4 wall will require R-7.5.  A question I get asked often is why does a 2×6 wall, insulated to R-20 need more continuous exterior insulation than a 2×4 wall insulated to R-13?  It comes down to the movement of heat through the wall assembly and the temperature of the exterior sheathing.  More cavity insulation slows heat, resulting in colder sheathing.  More continuous exterior insulation will be needed to assure the sheathing stays above the dew point temperature.

The final table, R702.7(4) covers class II vapor retarders and continuous insulation.  If we use a class II vapor retarder (smart vapor retarder or kraft faced insulation), the code will allow us to use less CI, the R-value depends on framing thickness and climate zone.  There is one footnote that states: “The requirements in this table apply only to insulation used to control moisture in order to permit the use of Class II vapor retarders.  The insulation material used to satisfy this option also contribute to but do not supersede the thermal envelope requirements of Chapter 11.”

R702.7(4) 2021 IRC

Back to our climate zone 6 example.  If we build a 2×6 wall and fill it using insulation of R-20, install a kraft faced or smart vapor retarder, we can get by with R-5 CI and still be confident the wall assembly will be safe.  But what if we use R-19 as the cavity insulation?  We then have a code violation, Chapter 11, Table R402.1.3 requires a minimum of R-20 + 5 for climate zone 6. 

Something thing we notice looking at vapor retarders and continuous exterior insulation in chapter 7 compared to the energy code requirements of chapter 11 is we may need more insulation depending on the vapor retarder you are looking to use.  This doesn’t mean that chapter 7 supersedes chapter 11.  If you want to use R-5 continuous exterior insulation with R-20 cavity insulation in climate zone 6, you can, but you will be required to use a class I or II vapor retarder (I would choose a class II product) on the interior side of the wall assembly.  That being said, you will be required to pay attention to the type of insulation you are using so that you do not end up with a class I interior vapor retarder and a low perm continuous exterior insulation where drying potential of a wall is diminished.

We’ve discussed the codes, now let’s look at the science (and some math) of why we want to add insulation to the exterior of our buildings.  The biggest reason to add CI, we are able to keep building materials closer to the interior temperatures, reducing the risk of wetting of those materials.  For example, I live in climate zone 7.  The thermal image above is of my personal home.  The home was built in the early 1950’s.  I’ve replaced the original R-7 craft encapsulated mineral wool batt with a modern R-15 Rockwool batt and a smart vapor retarder.  There is no exterior insulation.  There was a temperature difference of 50°F between inside and out when this thermal image was taken.  There’s only about a 5°F temperature difference between the 2×4 framing and insulated cavity, not terrible, but enough to show up in the thermal image.  The temperature at the sheathing layer is going to be much closer to the outdoor temperature.  If there were an air leak and some of the conditioned inside air, which contains some level of moisture, should come in contact with the exterior sheathing, the moisture would condense inside the wall.  (The process is a little more complicated, but hopefully you get the point).

Another issue is the surface temperature of the wall is 10 degrees colder than the air temp.  Our skin temperature is around 98 degrees, the wall temperature is 60 degrees.  I lose heat from my body to the walls in my home which creates a comfort issue.  It’s not uncommon for us to see a difference in indoor and outdoor temperatures of 100 degrees at some point during the heating season.  That wall temperature becomes even colder drawing more heat away from my body.

Any issue that may be present in the thermal image had to do with the colder framing becoming cold enough to condense water out of the air (again, the process is more complicated), possibly causing mold to form on the drywall.  I see this from time to time inside closets that have an exterior wall and behind larger pieces of furniture placed in front of an exterior wall.  The limited air moving across these walls usually makes the wall colder and wetter.  Wetness, a food source, oxygen, and eventually, warmer temperatures are perfect conditions for mold growth.

One final topic in this post, how continuous insulation can reduce heating and cooling needs.  A wall with cavity only insulation will have an effective R-value less than that of the rated R-value of the insulation.  Let’s look at a conventionally framed 2×6 wall with R-20 cavity insulation as an example.  The framing has a much lower R-value than the cavity insulation.  That R-20 wall might be closer to R-15 depending on the percentage of the framing members compared to the insulated cavity.  The most effective way to increase the overall wall performance is by adding continuous insulation.  You simply add the CI to the effective R-value figure to calculate the overall wall insulation level.  So, if you had R-15 as an effective R-value, and you added R-10 to the wall, the new value would be R-25.  (Now windows drastically change the wall’s performance, but that’s a discussion for another time.)

CI part 2 will cover types of insulation, fastening the insulation and cladding, where to put the house wrap, dealing with penetrations through the insulation and more.

Want to learn more about continuous insulation, some of the best resources are on the Building Science Corporation’s website.  I suggest starting with BSI-001.

7 Replies to “What You Need to Know About Continuous Insulation-Part 1”

  1. Thanks Randy!

    We certainly need a lot more like us. I’m the only one adding 4-12″ to exteriors in NE Wisconsin and the calls flood in. No way I can do this alone… Trying to work with NWTC & the Menominee Nation College to change that.

    1. Paul,

      12″ of CI, wow, that’s great!

      Minnesota is adopting the 2021 energy code, but we do not know yet in what form. I know they are currently debating moving the air tightness requirement to 2 ACH50. I’m not sure about the CI. Having that code required would be a game changer for MN.

      Thanks for the comment!


  2. Randy,

    I hope you include a section on retrofitting CI which I know is tricky to write about given all the variables but it’s a topic that I believe is becoming more popular. I’m trying to figure out the best way to do it to our 1920s bungalow that for now will not be a gut reno.

    1. Hi Will,

      The second part of the CI article discusses the basics of how to install insulation outboard the wall sheathing. I like the idea of a dedicated piece on existing homes. I’ll try to do something on that in the coming months. Thanks for the suggestions.


  3. We seriously explored having an exterior renovation company resheath our old house and add 3″ of exterior insulation in the process. The exterior insulation added around $30k-$40k to the bid between materials and labor, which seemed extremely high, given the resheathing + basic LP SmartSide came in at a similar price. The company has not done exterior insulation previously, and they had a hard time finding a sub who was willing to do the work, so I think the sub who was willing felt they could go super high.

    I’ll instead be doing the work myself at a small fraction of the overall cost the full bid came in. At least I’ll be more confident I’m doing it right as well, because I was bringing them up to speed on the detailing. (I’m a DIYer who has been following folks like Risinger and now you for the last ~5 years, and I’ve finally got an old house to upgrade!) Hopefully more companies will get into this and help bring some of the costs down as more folks get familiar with it.

    1. Hi David,

      Thanks for the kind words, to be on the same list as Matt, wow, thanks.

      I have a couple more blogs I’m hoping to write yet this year, probably for Green Building Advisor but they will also appear on the Northern Built site. One will be on indoor air quality monitoring and the other on continuous insulation for existing homes. I believe homeowners like you will be how builders evolve. Change is slow in our industry, thanks for demanding it!


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