How to Plan for Continuous (Exterior) Insulation on an Existing Homes

This post first appeared on the Green Building Advisor website.

When I purchased my small 1950’s vintage Cape Cod home in 2018, I knew I had a lot of work to improve its performance and comfort.  I was planning a multi-year, self-performed (and solo), deep energy retrofit.  A case study where I could learn and share the difficulties of such an endeavor. 

One of my goals was to improve the insulation levels of both the walls and roof.  I went back and forth trying to decide if I should add a layer of continuous insulation to the exterior.  After all, I was going to remove the existing siding and replace all the windows.  In the end, I decided against it.  The gable ends of my roof have no eave (see photos below), adding exterior insulation would have required me to rebuild a portion of the recently re-shingled 12/12 pitched roof.  I now regret that decision, more on that later.

I recently wrote a two-part article covering continuous insulation (CI). 

Continuous Insulation, Part 1 – GreenBuildingAdvisor

Continuous Insulation, Part 2 – GreenBuildingAdvisor

Those two articles primarily concentrated on new construction, though much of the information also applies to existing homes.  The “why” to include CI has been well discussed both on the Green Building Advisor website along with other forums.  The reasons being comfort, durability and energy cost savings, I won’t rehash those conversations, instead we will be discussing the how to’s.

Start at the beginning. 

Before beginning any construction project, whether it’s new or a renovation, you need to have a plan.  Adding CI is no different.  My recommendation is to begin with figuring out how the existing wall is assembled.  What products and techniques were used when the wall was new?  Have there been any changes to the original design?  Some de-construction will be required to figure this all out.

Another good idea is to perform a blower door test.  There are opportunities to improve air sealing on the exterior of the home while adding CI.  If the home is already tight, you may only need the WRB to simply perform bulk water management duties.  A blower door test may also show other opportunities when developing the scope of work.

Let’s use my 1950’s Cape as an example.  All the exterior walls are framed with 2” x 4” studs.  Some walls have a double layer of drywall (someone’s attempt to repair or hide water damage from an interior roof condensation problem).  Most of the home only has one layer of drywall.  Some areas have polyethylene sheeting and newer R-13 fiberglass insulation (poorly installed) while others have the original craft paper encapsulated mineral wool R-7 insulation.  The home blower door tested at 12.34 ACH50, 2976 CFM50, very leaky.

The exterior is sheathed entirely with 1 x 6 board sheathing.  A layer of 15 lb. felt was acting as the water resistive barrier.  Cladding was the old asbestos shingle siding.  As we work through what products to use when rebuilding the walls, we’ll want to consider how the wall will handle any bulk or water vapor issues.  How will the wall dry if it becomes wet?  Do we need a vapor retarder?  Is an existing vapor retarder going to cause us problems?  Next, we want to look at ways to improve the air tightness.  Lastly, we choose the type and thickness of the insulation.  The R-Value of the continuous insulation will possibly affect any vapor retarder selections.  (Sound familiar, it’s the four control layers roughly in their order of importance.)  The reason for good planning is so changes aren’t needed on the fly.  There will be enough of those dealing with the hidden water damage that isn’t seen until the demolition phase progresses.

Choosing the assemblies

I usually base the choice of WRB on the existing sheathing of the home, along with any blower door test results.  If air sealing is one of the goals, my selection will be a WRB that performs well as an air barrier.  I will also choose the sheathing as the location for the water control layer.  This means the insulation will be outboard of the WRB, slightly complicating how windows and doors are integrated into the WRB.

If I don’t need to improve air sealing, my choice of WRB might be one that can work best outside the continuous insulation layer.  Most builders are going to be more familiar with this installation.  This assembly will at least include a drainable WRB.  How the cladding is attached may require a different approach, such as 1×4 boards creating a vented rainscreen.  This is based on the thickness of the CI and the type of cladding.

Another decision that will need to be made is with the windows and doors.  Are either going to be replaced?  Is the plan to remove and re-install the existing windows?  Is the continuous insulation going to simply be installed around the existing windows?  Innies (a window installed to the inner portion of the rough opening), outies (a window installed at the exterior plane of the cladding), and twinnies (windows that are installed in the middle 1/3 of the rough opening) all require different flashing details.  In my opinion, outies are the easiest to detail simply because most contractors are familiar with nail flange style windows.  Innies and twinnies will require flashing details, often these flashings are custom.

How an outie window is secured a home using CI will depend on the thickness of the insulation layer.  If the insulation layer is thin enough, a window buck may not be needed.  The illustration above shows an outie window with thicker insulation, requiring a window buck.  When using a window buck, the bucking material will need to be continuous with water control product along with the air control layer if air control is located at the sheathing layer.  Continuity of both is key.  The second illustration shows a twinnie installation where the window is installed at the sheathing and the insulation is installed around the window.  My biggest issue with the illustration is window replacement.  The window is behind a nailer and the cladding.  Parts of the building need to come apart to gain access to the nailing flange.  A flangeless window or the addition of a trim piece around the window might be a better plan.

We also have to choose the type of insulation.  Rigid foams (Extruded Polystyrene (XPS), Expanded Polystyrene (EPS), and Polyisocyanurate (Polyiso)) are all choices.  These are all easy to work with and in some cases, may be able to be used as the WRB.  The drawback, they have lower permeance meaning they are slower to allow moisture to move through the insulation should an event that wets the wall occur.  Higher permeance products would include rigid mineral wool and rigid wood fiber insulations.  These two are nonpetroleum-based insulations.  Another choice might be to use nail-base type insulation.  This rigid insulation is bonded to plywood or OSB and is designed to be installed over an existing wall sheathing.  The plywood or OSB is typically covered by a WRB and allows many cladding to be directly fastened without the need for a 1x nailer.  Zip-R, which, in most cases can be used as structural sheathing, falls under the category of a nail base panel.

Opportunities with New Cladding

Anytime cladding is removed from a home, we get to learn of any changes made over the years to the original structure along with any construction errors.  Water finding pathways through the cladding, around any water resistive barrier and into the wall are common in older homes.  Sometimes, the damage is limited to discoloration and staining on the sheathing or inside walls.  Other times the rot is bad enough that sheathing, and even framing might need to be replaced.  Repairs need to be made before any other work can be done.

Once any repairs are made, we can then start following the plan.  We might be bucking out window rough openings, installing a WRB, or installing continuous insulation, all based on the plan we developed.  Let’s use my house and assume I had installed CI.  I would have installed a window buck the thickness of the insulation plus ¾”.  (The extra ¾” would allow for the window to plane out with the CI plus a ¾” vented rainscreen board used to attach the new clapboard.)  I would follow the required ratio of CI to fibrous cavity insulation that is published by Building Science Corporation (and in Chapter 7 of the 2021 IRC) so that I can move to a class III vapor retarder, painted drywall.  (Rember, I’m performing a deep energy retrofit, my interior walls will also be gutted).  My climate (CZ-7) requires a ratio of 43% exterior to 57% interior, or R-10 CI when standard fiberglass insulation (R-13) is used in the cavity.

Once the window bucks are in place, I would begin installing the water resistive barrier.  Because I want to increase the overall building tightness, and I have board sheathing, I would choose a self-adhered product.  The WRB would connect from the foundation (in my case, a concrete block basement) to the wall using either a tape suitable for the purpose or some type of fluid applied membrane.  Once the WRB is in place with the window and door bucks also tied to the WRB, I then can begin installing the insulation.  My preference is to use a vapor open insulation product.  Rigid mineral wool or rigid wood fiber insulation would be my first choices.  Once the insulation is in place, I can then install the 1×4 vented rainscreen boards and install the new windows.

There are many other small but important details that need to be planned.  For instance, how should the metal head flashing over windows and doors be detailed into the CI and WRB?  Should the window buck be thermally isolated?  Or details that require engineering.  What happens if you have heavy cladding, such as natural stone or brick?  What happens if the insulation is thick, say 6” or more?  Sometimes a manufacturer may have answers.  The manufacturer might be the one making the insulation, or they could be manufacturing the cladding.  Don’t be afraid to reach out for help.

In the beginning of this piece, I mentioned that I regret not installing the CI on my home.  This thermal image of my family room shows why.  I improved the cavity insulation from R-7 (the original insulation installed in the early 1950’s) to R-15, more than doubling the R-value.  On top of that, I installed a smart vapor retarder which is well air sealed.  This work did improve the overall performance of the home, but the structure still isn’t as comfortable as I was hoping during cold weather.  The thermal image (the darker areas on the studs are the fasteners used to secure the drywall) was taken with a difference between indoor and outdoor temperatures of 90°F, the outdoor temp was -15°F.  Cold, but it can be worse in my area.  The average temperature of this west facing wall was in the low 50’s, cold enough to feel the heat leaving your body and moving to the cooler wall, that comfort thing.  That extra R-10 sure would be nice…