This post first appeared on the Green Building Advisor website.
In the first part of this continuous insulation blog, we covered building codes and some building science and how they relate to CI. This second part will discuss some of the difficulties many builders have with how the details work.
I remember my first time using continuous insulation in a new home. I had many questions; where to put the WRB, how to attach the cladding, how to address the windows and doors, and how much insulation to use. That home is now more than a dozen years old without any complaints from the homeowner. Would I change some of the details I implemented on that first CI home? Absolutely! But the design and execution all those years ago seems to be performing.
What I would choose to do today would be based on whether I was dealing with new construction or a renovation. Other factors include budget, the experience level of the construction crews, along with how the homeowner will use the building. An example, maybe a family member has a health concern that requires the need for higher than recommended indoor humidity, or maybe the homeowner is planning on an indoor swimming pool. I would approach those projects differently than simply building to the minimum code requirements for CI outlined in Chapter 11 of the IRC.
Let’s start the discussion with what are the common CI insulations. There are four main types with one new product on the horizon. Most installations will include some sort of rigid insulation, but fibrous insulations may be an option with certain designs and assemblies (I won’t be covering the fibrous options in this post). In no particular order, extruded polystyrene (XPS), expanded polystyrene (EPS), polyisocyanurate (Polyiso), and rigid mineral wool (Rockkwool’s ComfortBoard 80). Another product which has been around for a while in Europe but is just beginning to be manufactured in North America is the TimberHP wood fiber insulation.
Each of the insulations has its advantages and disadvantages. For example, some versions of XPS have been shown to contribute to global warming. Polyiso has the highest R-value of all the listed insulations but, depending on the facer used, can have a low permeability. Rockwool’s ComfortBoard is recommended in areas where wildfires are prevalent, but I’ve heard complaints of how it compresses during installation. The most environmentally friendly insulation is wood fiber, but availability is currently limited.
One of the best resources I’ve found with regards to insulation is the BuildingGreen Guide to Thermal Insulation. You do have to purchase the more than 100 page guide, but it can answer almost all questions anyone should have on any type of insulation used in construction. I reference my copy often.
In addition to the basic insulation, there are products available that combine insulation with other building materials. Huber’s Zip-R sheathing, for instance, combines Polyiso insulation with their Zip Systems wall sheathing. When the system is installed per the manufacturer’s instruction, you have your water and air control layers along with the CI in one product. ZIP System® Insulated R-Sheathing | Huber Engineered Woods (huberwood.com) A basic nail base would be another option. A nail base typically includes EPS insulation (other types can also be used) and a standard OSB panel bonded together. Nail base panels are installed over an existing wall sheathing. Usually, the installer will also need to provide the WRB and address the air sealing details. A third product that is available is called structural insulated sheathing. This type of sheathing includes a thinner hard board sheet that supplies the structural requirements bonded to some sort of insulation. Structural Insulated Sheathing – OX Engineered Products
Other products might include insulation and WRB manufactured as one. An example is ThermalTight, produced by the makers of ThermalBuck. This product combines graphite polystyrene insulation (GPS, a variant of EPS) and a WRB in a single panel. Each panel is installed over wall sheathing and the integrated WRB is designed to shingle lap the previously installed panel. A double-sided tape secures the lap and provides a level of air tightness. Graphite Polystyrene GPS Insulation Experts | ThermalTight
When choosing the type of insulation, a question that needs to be asked is how does the wall dry if it gets wet? All walls should have the potential to dry in at least one direction, inward or outward. Analyzing the wall section can help to determine how a wall will dry. What product in the wall assembly has the lowest permeance? If that product happens to be polyethylene sheeting (a class I vapor retarder) installed on the interior side of the wall cavity, the only drying potential is outward. Choosing a low permeance continuous exterior insulation in combination with a polyethylene product installed on the interior could be a poor choice and possibly a code violation. The same is true for a low permeance product on the exterior, such as a foil faced exterior insulation. This could prevent outward drying making inward drying the only pathway. In this case, careful attention will need to be given to the interior wall finish, low permeance wallpaper for example might be risky.
The Control Layers
Water control. How will we address bulk water concerns when using continuous insulation? We want to protect the moisture sensitive products in the wall assembly. Where we see issues with moisture entering walls is typically around planned openings and penetrations through the wall. Windows and doors along with vents, pipes and wires that are not effectively installed to shed water are all potential leak points. There are many articles here on Green Building Advisor that cover methods of flashing and sealing these openings. It all comes down to the location of the WRB, is it at the sheathing or outboard of the CI? Water management will require flashings to be integrated and continuous with the WRB.
Air control is the next concern. The air barrier location can be at the WRB, the exterior insulation, the wall sheathing, cavity insulation (closed cell spray foam) or an interior membrane. If you’ve done a good job detailing the water control membrane, often this also results in an effective air barrier, but not always. Choosing the product and location will dictate how the CI details work out.
We discussed the code requirements for vapor control in the previous posting on CI. Installing enough continuous exterior insulation and creating an effective air barrier will greatly reduce the concern of any vapor related issues. If a sufficient level of CI cannot be installed to reduce the risk of a condensing surface inside the wall assembly, the next option (and my recommendation) is to use a class II (responsive or smart) vapor retarder. Avoid class I vapor control products whenever possible.
The last control layer is thermal, which is what this blog is about. We covered the code requirements in the previous post. I thought it might be a good idea to include one more table. This one comes from Building Science Corporation and shows the recommended cavity to exterior insulation ratios for each climate zone.
This table is the recommended exterior insulation levels for condensation control inside a wall assembly. Something to remember with regards to condensation control, the more cavity insulation present, the cooler the outer portion of the wall will be. This will require more continuous exterior insulation to warm the wall to a safe level where condensation is no longer a risk (the reason we need more CI in a 2×6 wall than a 2×4 wall).
A few of the details
I could easily write a book trying to cover all the options and details for CI. Here are a few questions that I get asked often.
How do I attach the CI to the home?
It depends (Sorry). What’s the type of insulation? What’s the thickness? What is the cladding? All this comes into play. Most of the time a cap nail or large plastic washer will be sufficient, but if there is a wood rainscreen planned, a 1x can be used to secure the insulation. I usually use a combination of a few large plastic washers to temporarily hold the insulation in place, then add the 1x rainscreen to permanently secure the insulation to the structure.
How do I install windows and doors?
Again, it depends. Are the windows being installed as outies (windows with nail flanges installed to the outside of the window rough opening), innies (windows without nail flanges installed all the way to the interior of the window rough opening), or twinnies (windows with or without nail flanges installed towards the middle of the window rough opening)? What is the thickness of insulation? Where are the water and air control layers with relation to the insulation? Most builders are familiar and prefer outie windows, or windows that have a nailing flange. The window will need to be connected to both the water and air control layers. This is simplest if these layers are in plane with the window. If these layers are at the sheathing, you will need a way to connect the sheathing to the window around or through the insulation. This is often done with a window buck which is typically just a wood extension that extends the window opening flush with the added thickness of the exterior continuous insulation. Other products, such as ThermalBuck can also be used to flush the window with the CI. If the Insulation is relatively thin, you may have the option to simply use a longer fastener to secure directly to the wall sheathing and framing.
How do I address other planned penetrations?
Electricians, plumbers and HVAC crews will need to penetrate both the water control layer and CI (and possibly the air control layer as well). How these openings are sealed will depend on the location of both the air control and water control. Sequencing can be difficult, such as for this electrical box. It needed to be installed after the insulation. It’s best to have most of these details planned before the CI installation begins. Personally, I prefer using a gasket material whenever possible to seal these penetrations, this will allow the penetration to move some without reducing the seal. Tapes and sealants can also also effective.
How do I attach the cladding?
This will depend on the cladding weight and thickness of the insulation. When a thin insulation is used, it may be possible to simply use a longer fastener to secure the cladding through the insulation and into the building structure. If a 1x rainscreen is used, most claddings can be attached directly to the rainscreen, the weight of the cladding will be the determining factor. The attachment of the rainscreen may require long, heavy gauge fasteners. Most cladding manufacturers and a few of the insulation manufacturers have fastening schedules depending on insulation thickness. It’s best to refer to their installation instructions, or have the design engineered.
How about decks?
My preference with decks is, whenever possible, not to attach a deck directly to the home. This isn’t always feasible. There are a few products and assemblies that can tie a deck to the home through the insulation layer. This detail shows one way. There are also manufactured products designed for this purpose. One manufacturer I’m aware of is called Maine Deck Bracket. Maine Building Materials; The Maine Deck Bracket Company
Continuous insulation has been in the residential building codes for more than a decade now, though many markets are just beginning to adopt the requirement. I have more and more homeowners asking about its benefits, and then asking who I can recommend to install CI correctly. Right now, my list of contractors in my market is small, but hopefully the increased interest in the use of CI will lead to more builders willing to include continuous insulation in their standard assemblies.