Foam plastic insulation: the “Swiss Army Knife” of building enclosures
December 8, 2016 - Continuous insulation is not a new idea; in fact, we put this concept into practise every time we decide to put on a coat to stay warm. Sure, a sweater gives us that warm, cozy feeling on a cold day but, when it comes down to it, would you stand outside in gusty January weather wearing just that?
December 8, 2016 By Jay Saldana PE
Our inclination is to choose the coat because it separates our bodies from ugly outside conditions. Similarly, the building enclosure can be considered an environmental separator and, just as we dress in layers for those cold January days, the building enclosure, too, has its layers.
A high-performance building enclosure will have continuous insulation on the outside of the structure and insulation in the stud cavity—the coat and sweater, respectively.
But insulation is not the only consideration; a high-performance building enclosure must control rain, air, temperature (heat/cold) and vapour. Traditionally, each of these functions were performed by individual products, or layers, comprising the enclosure. Given the growing complexity of having to draw high-performance wall details, architects would prefer these functions be combined into as few product layers as possible.
And who could blame them?
Again, think of these layers like the differences in jacket types we wear. The function of a soft fleece jacket keeps us warm but doesn’t stop cold wind from blowing through. We could add a nylon jacket over top to stop the wind, but those jackets are typically thin and don’t stop rain. We could then add a raincoat over top of that to keep us dry. While this system of three jackets may work, it is not ideal for everyday use. We would prefer one jacket layer that can perform multiple functions: keep us warm and dry, and stop the wind.
Wraps, membranes and barriers
For many years now, the most common product type to combine air and water control layers into one “jacket” has been building wraps, seen as the minimum baseline of performance. However, the growing segment of self-adhered membranes and liquid-applied barriers have shown wraps are not up to the task of high-performance buildings. The many penetrations required to nail the wrap into place undermine the product’s air and water barrier performance.
Self-adhered membranes and liquid-applied barriers don’t require penetrating the product to install it, which allows them to perform as intended. Advancements have pushed their performance to achieve water, air and vapour control in one layer, yet these products still don’t have a thermal control element, so they require an additional layer for that purpose.
Rigid foam plastic insulation boards have shown they can achieve water, air and vapour control in one, thick layer, as well, making them seem like a one-stop solution for dealing with all four control layers. However, rigid insulation boards are not a magic solution, as they do have a few drawbacks.
These boards require significant time and additional materials (e.g. screws and washers) for installation, while curved or oddly shaped buildings increase the job’s complexity and time onsite for installers to fit the boards as needed. Penetrations ranging from brick ties and piping/conduits to windows further increase the challenge of maintaining unbroken water and air barriers.
Moreover, rigid foam boards have a lot of joints. Depending on the size of the building, there could be 10,000 or more insulation joints, which may have been sealed at the time of construction but became unsealed over the life of the building, thereby allowing air or possibly water leakage into the structure.
One high-performance alternative to rigid foam boards is medium-density, closed-cell spray foam insulation, which performs remarkably well as an all-in-one product.
Spray foam as air and water barrier
Spray foam insulation is actually a two-part product. In general terms, manufacturers make and sell their product as two liquids (Part A and Part B). At the jobsite, they are pumped in fixed proportion by specialized equipment through hoses up to the applicator’s hand-held nozzle where they are mixed together as they’re sprayed into place. The liquid hits and adheres to the surface at which the installer is aiming, then expands as a foam. The product grows in thickness as Parts A and Part B react with one another.
Two main spray foam insulation types are typically used in construction. Low-density (“half-pound density”) foam typically has an open cell structure, expanding rapidly upon application and is soft to the touch. Low-density foam is best used inside wall and roof cavities (i.e. inside the building exterior sheathing). The other foam, medium-density (“two-pound density”), typically has a closed cell structure and is ideal for exterior applications, such as continuous insulation.
While expanding, spray foam adheres to and seals just about any clean, dry surface with which it comes into contact. Medium-density spray foam can be an air barrier material at 1-in. thick (possibly thinner, depending on the manufacturer). However, to maximize the benefits of continuous insulation as a thermal break, spray foam is almost always applied in thicknesses exceeding 1 inch.
With its ability to seal to adjacent materials and itself, it is ideal for exterior wall air barrier systems. While rigid foam boards can also be an air barrier material, every board joint must be sealed to make it a continuous air barrier across the wall. This is typically done with tapes, liquid flashings or fully adhered membranes over top. Not only can this be labour intensive, it can also increase the cost of construction due to additional material.
On the flip side, spray foam insulation is applied in one continuous layer, meaning joints are no longer a consideration. Confidence in the air barrier performance for the life of the building is improved with the removal of insulation joints that could one day leak air. Through-wall penetrations are easily sealed with spray foam as it adheres to their surfaces. In contrast, rigid foam boards could require tricky detailing and have sequencing issues, depending on the size and shape of the penetration. Complicated air barrier detailing of interior and exterior corners, dissimilar materials and tricky building shapes is a thing of the past with exterior, continuous spray foam insulation.
Medium-density spray foam insulation used as an air barrier on the exterior of the wall can also perform as the building’s water-resistive barrier (WRB). The foam develops a hardened, almost glossy, exterior shell once cured. The surface of this shell is an excellent drainage plane for repelling liquid water. Since the spray foam eliminates joints altogether, water is unable to find its way into the building. Rigid foam can also serve as the building’s WRB, with the foil-faced variety being the most common type. They can perform quite well in this capacity but, again, there are those pesky joints.
Cheaper solutions for sealing board joints involve foil tape. As a material, foil tape should withstand the elements just as they do on ductwork in attics that can experience extreme temperatures. No, the real concern is with those installing the joint tape—the insulation installers. They are not waterproofing professionals trained to overlap tape joints, to use shingle-style taping, and to ensure all tape surfaces are tightly adhered. They are trained to install the foam and make sure the tape is applied. Liquid flashing has proved to be a more foolproof way to seal board joints, but can increase the cost significantly when you are considering 10,000 or more joints.
Condensation occurs when warm, humid air touches a cold surface. To prevent interior humid air from coming into contact with a cold surface inside of the wall cavity, we’ve historically used vapour retarders as a separation layer. By relying on vapour retarders to reduce the likelihood that vapour-filled air will touch a cold surface inside of the stud cavity, the retarding layer would go on the interior, warm side of the wall. When the proper amount of continuous insulation is used on the building enclosure, the cost of the vapour retarding material, the labour to install it, and the time it takes can all be removed from the project.
This is possible because the proper amount of continuous insulation creates a warm interior wall, which dramatically reduces the likelihood that material surfaces inside of the continuous insulation get cold enough to create condensation. Building science tells us the proper amount of continuous insulation is defined by a ratio of exterior to interior insulation, the climate zone in question, and general conditions and use of the building while operating.
Dew point modelling can be performed to show performance with interior vapour retarders being equal to continuous insulation. However, when comparing modelling with real-world performance, building enclosures with the warm wall design perform more consistently and have greater cushion for unforeseen occurrences. After all, all it would take is one interior building remodel creating holes in the walls for the vapour retarder to become compromised.
When the design relies solely on this retarder to prevent condensation, then there could be a problem. Were it a warm wall, humidity in the cavity would not matter. When a window breaks down and leaks water into a warm wall cavity, the fact that there is no interior vapour retarder facilitates drying to the interior of the building.
While continuous insulation creates a warm wall for the northern climates, it is also a vapour retarder, which is great news for warm, humid climates. Depending on the product and thickness, both rigid foam boards and spray foam insulation can act as a vapour retarder on the exterior side of the enclosure. In climates where the vapour drive is from the exterior to the interior most of the year, having a vapour retarder on the outside of the building makes sense. By preventing some of this humidity from migrating into the building by air infiltration and now vapour diffusion, it should be easier for the mechanical system to maintain interior comfort.
The Swiss Army Knife of building enclosures
Over the years, architects and engineers have seen building codes slowly increase requirements for continuous insulation. If they haven’t designed for it yet, they soon will. Choosing a continuous insulation product may seem complicated at first, as there are many options but, conceptually, it just makes sense to go with options that make the overall design and details easier.
By first narrowing the choices to those that can serve multiple purposes in a building enclosure (ability to control rain, air, temperature and vapour), we find we are considering foam plastics. R-value may play a key role in this decision. The more R-per-inch, the thinner the wall’s profile. R-values for rigid foam are typically R-4/in. for expanded polystyrene (EPS), R-5/in. for extruded polystyrene (XPS), and R-6 to R-6.7/in. for polyisocyanurate (polyiso). Medium-density closed-cell spray foams can vary in R-values from 6.2/in. to R-7.1/in.
Were R-value the only deciding factor, then there would be a clear winner with the continuous insulation selection process. However, this is rarely, if ever, the case; many factors, including cladding selection, building shape, above or below grade, roof or wall, time of year, product availability and, most commonly, material price may end up affecting the insulation choice.
That said, the notion that foam plastics can reduce the number of layers, plus the amount of labour and installation time required, from the project—all of which save money—often make the insulation material cost less relevant.
ABOUT THE AUTHOR • Jay Saldana, PE, is a senior engineer specializing in commercial construction for Icynene, a manufacturer of open- and closed-cell spray foam insulation. Jay holds a Bachelor of Science in Civil Engineering and possesses a Professional Engineer licence.
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