High-Performance Thermal and Moisture Protection Strategies  

The design of building enclosures continues to receive a great deal of attention by numerous design professions, building scientists, and product manufacturers. Some are motivated by meeting minimum building and energy codes in the most cost-effective manner. Others are seeking performance beyond code minimums to meet voluntary certification or rating programs, or simply to improve building performance over time. Some focus on thermal control in walls, roofs, and floors, while others are concerned with air and water barriers to protect the building and people. Still others look at the details of construction and how to incorporate appropriate continuity of barriers across some of those detail areas. The result of all of this attention has been an ongoing need for architects to stay abreast of advancements and improvements in this arena to keep up with the best options and assembly configurations for buildings under design or renovation. This includes being aware of some of the latest products and how they are intended to be used in order to create successful results. Based on all of the above, this course will examine some of the advanced, high-performance options for effective thermal and moisture control in building enclosures. It will also highlight why it’s important that manufacturer guidelines and best practices for installation are followed.

Drainable Barriers

The use of water-resistant barriers (WRBs) and air barriers (ABs), particularly in exterior walls, is not only required by codes but also critical to good performance of a building enclosure. The codes don’t dictate how to achieve these barriers, they simply provide the criteria for materials to qualify as either one and require them to be continuous. At the same time, the established best practice is to be sure that water or moisture can safely drain away from an assembly if it does penetrate. In exterior walls, the most common way this is achieved is to create a space or gap between the outermost cladding (i.e., siding, masonry, rainscreen panels, etc.) and the WRB surface (i.e., sheathing covered or treated with a water-resistant material). Also quite commonly, the WRB surface contains an air barrier against unwanted exterior air infiltration. Hence, the gap separates the cladding from these barriers and allows the assembly to be considered drainable if it does become wet for any reason. If the barriers also allow for vapor to pass from inside to outside while still resisting bulk water and air on the outside, it is considered dryable as well.

It has been noted that traditional construction techniques such as masonry walls manage water and moisture by using a space or gap between the sheathing layer and the masonry veneer (cladding) to allow any accumulated water to exit through weep holes. Similarly, rainscreen installations use a gap between the cladding and the sheathing to allow water to drain away harmlessly and ventilate the space between the cladding and the sheathing that is covered with a WRB/AB. In any wall construction, failure of water to drain away can damage the cladding, or worse, the rest of the wall assembly.

Thermally, there is also the increasing use of continuous insulation between the cladding and sheathing. This creates another set of surfaces that may require a gap for drainage and drying in case moisture or water find their way between the insulation and sheathing. Fortunately, the gap does not need to be very large (less than ¼ inch works) so there is little impact on the thermal properties of the insulation.

One of the challenges then in designing an exterior wall assembly can be in finding a way to simply and economically provide all of the needed barriers and gaps in the right places. When it comes to providing a WRB, a high-performance, nonwoven building wrap made from a synthetic sheeting material is often selected. The advantages of a modern synthetic wrap product over traditional products (such as building paper) include a WRB that is more durable, and more easily sealed along the seams to create a continuous barrier over a variety of materials or different configurations. Further, if the synthetic WRB is tested for air infiltration, then it can also double as a continuous exterior air barrier meeting the code requirements for both WRB and AB in a single layer.

When a high-performance building wrap is used as the WRB in a project, it takes advantage of several innovative advances in its development. First, as an engineered product, it creates a weather barrier behind exterior cladding to protect the sheathing and reduce water intrusion into the wall cavities. This is important in all wall systems since all cladding will likely allow some degree of water intrusion at some point. It is also particularly important in rainscreen assemblies where water is expected to enter behind a cladding material and is allowed to drain away. Second, as a vapor-permeable or breathable product, WRBs allow water or moisture trapped behind it to escape, thus allowing any damp or wet materials to dry in a relatively short amount of time. During this drying, WRBs maintain their water resistance because they are constructed with pores that are large enough to allow moisture as a vapor to pass through but too small for water as a liquid to pass. Third, as an air barrier, a WRB will be an energy-efficient means to stop air infiltration and exfiltration through walls.

A significant innovation in such high-performance building wraps has been the addition of integral spacers to very effectively create a manufactured drainage gap between the cladding and the wrap. The conventional means to create a gap in a framed wall system is to use furring channels or wood strips. This works, of course, but requires more labor, time, and cost to install that furring. The alternative that has become recognized as a best-practice solution is to use a drainable building wrap that provides its own integrated method of drainage. Specifically, at least one manufacturer creates this gap by bonding noncompressible propylene spacers that are1.5 mm thick (less than ¹⁄₁₆ inch) onto a high-performance WRB building wrap. A similar product uses 6.3-millimeter (less than ¹⁄₄-inch) spacers bonded in the same manner for higher water-risk conditions, such as marine environments or heavy-rain climates. This deep cavity also lends itself to reservoir cladding products—products that shed water but also absorb some of it—especially stucco and thin stone.

This integral gap design provides an effective drainage space or capillary break between the sheathing and cladding material. The depths are large enough to provide true drainage between the sheathing and cladding material but not enough to compromise thermal performance if insulation is installed over it. In essence, drainable building wrap acts as a full rainscreen system in miniature, without the added labor or cost. Further, it will work with all types of cladding systems, particularly those that can be moisture sensitive, such as wood or fiber cement siding. The economical beauty of it is that the cladding can be applied directly over this drainable building wrap, eliminating the labor step of installing the spacers as a separate component.

When specifying drainable building wraps, it is important to recognize that there are literally dozens of building wrap products available with wide variations in performance and cost. While most are made from polyethylene or polypropylene plastic, they can vary noticeably in terms of water resistance, drainage efficiency, water-vapor transmission, or breathability. They can also be different in their ability to impede air flow, in their overall durability including tear resistance, and in cold-weather flexibility. In many exterior walls, flammability and smoke developed ratings are important too and need to be considered. Selecting a drainable building wrap that excels in all of these areas will assure best results overall.

Beyond the product itself, sealing the edges and seams properly and fully without undue penetrations from staples or nails is critical for good performance and code compliance. Some manufacturers offer full sealing systems that use a compatible adhesive, sealant, or tape. This allows the edges to be held permanently in place, usually by using a hard roller or hand applicator to press the wrap and the tape together. Such products may also use fasteners with plastic heads that minimize the impact of the fastener penetrations and help maintain the barrier continuity.

Equally important are the details of how the drainable building wrap deals with openings in the wall, such as doors and windows. Being able to flash and seal the wrap properly with window and door flashing materials will assure that water draining down the face of a drainable WRB will flow away properly and not enter behind other building elements into the wall. It will also maintain the continuity of the air barrier in the overall system.

Installed properly, drainable building wrap effectively eliminates excess moisture and mitigates the damaging effects of mold and rot. It is a cost-effective product, particularly when it is manufactured to be installed in any position, horizontally, vertically, or diagonally. Overall, this approach has been shown to have the same drying capability of a ³⁄₈-inch rainscreen wall. This is important because there is growing recognition among building scientists and building codes that exterior walls need to drain and dry. That means, going forward, building wrap products will increasingly be judged by how effectively they provide positive drainage of water from the wall.

Choosing the right building wrap, then, requires an understanding of the product’s key attributes, including things like water resistance, durability, vapor permeability, and drainage. Brian Keith, an architect at JHP Architectural Designs in Dallas, says, “Critical to the success of any design is the detailing and construction of it. We have been impressed with the drainable building products we have specified due to their complete system approach.”

High-Performance Insulation in Thinner Assemblies

Incorporating insulation into walls, floors, roofs, and other areas of a building enclosure is required by code and needed in order to control heat loss or gain in a buildings. The design challenge has often been balancing the need to meet R-value targets for the insulation with the thickness of the construction assembly (i.e., walls, roofs, etc.) required to incorporate that level of insulation. The default approach for many is to use some type of fibrous insulation in batt, blanket, or loose form installed between wood or metal framing members. Since such insulation is generally available with R-values in the range of R-3 to R-4 per inch, framing depth has increased in many projects to accommodate the need for more inches of insulation to meet the targeted total R-value. For places where still more insulation is needed, then continuous insulation is added outside of the framing or in some other manner (as in continuous roof insulation over a roof deck) that is similarly built up by the inch to achieve the needed total R-value. Of course, part of the struggle is the recognition that increasing thicknesses also increase construction costs, not just of the construction assembly itself but also the details of things that integrate with them, such as windows, doors, skylights, etc. This issue is particularly exaggerated on existing buildings that are being renovated.

Hearing the outcry from designers and contractors for ways to achieve higher thermal values in less thickness, product manufacturers offer a number of options. There are various types of insulation on the market, but it is important to note that they have different circumstances where they are best fit to be used. In that light, we look at three types of advanced products in the following sections.

Extruded Polystyrene Insulation

There are a number of different types of rigid foam insulation made from different types of plastic foam and with different facings on them, or no facing as the case may be. One that has been in common use for some time is extruded polystyrene insulation board, commonly abbreviated as XPS. It is made by extruding thermoplastic polystyrene foam through a machine to form it into continuous boards that are cut to length (just like some metals are extruded for other purposes). This produces a comparatively dense insulation product with closed cells compared to the somewhat lighter expanded polystyrene (EPS), which can have an open-cell structure. Because of this makeup and the extrusion process, XPS offers superior cold-temperature performance when compared to many other insulation types. Specifically, it typically provides R-values on the order of R-5 per inch, meaning that it can achieve higher overall total R-values in less assembly thickness than fiber-based batts, blankets, or loose-fill.

XPS insulation also offers superior moisture resistance compared to fibrous insulation, meaning that it does not absorb water nor lose its R-value rating when wet. Because of this, it is often considered to be a superior choice for masonry cavity walls, below-grade walls and floors, and in inverted roofing applications. These are all moisture-prevalent applications, and XPS is often chosen to be specified in these circumstances. There are other rigid foam insulations, such as polyisocyanurate insulation, that may claim a higher R-value per inch, but those are not as well-suited for situations where moisture is a concern. Many designers find XPS to be a reliable and reasonably economical insulation solution when the total costs of different assemblies are compared.

Phenolic Insulation

In some cases, even higher R-values per inch are needed for code compliance or performance specifications. An alternative is rigid thermoset phenolic insulation, which is produced by mixing high-performance solids and phenolic resin with a surface acting agent. It is manufactured by a process in which a plastic foam forms an insulating core between two facers with a fiber-free closed-cell content. European formulations of phenolic insulation have been in use for several decades in both residential and commercial construction.

Phenolic insulation offers an extensive range of premium-performance insulation products for wall, floor, soffit/structural ceiling, rainscreen, and concrete sandwich wall-system applications. With an R-value of up to 17 per 2 inches of insulation (i.e., approximately 8.5 per inch), it is the thinnest among commonly used insulation products on the market. In addition to its superior insulation properties, phenolic insulation is based on a fiber-free, rigid, thermoset phenolic insulation core that resists moisture as well as water-vapor ingress. It also exhibits excellent fire performance with very low flame spread and smoke developed ratings when tested in accordance with ASTM E84/UL 723: Standard Test Method for Surface Burning Characteristics of Building Materials.

In terms of environmental impact, phenolic insulation boards are available that are manufactured with a blowing agent that has zero ozone depletion potential (ODP) and low global warming potential (GWP). By resisting moisture and water-vapor ingress, it also eliminates problems that can be associated with open-cell materials that absorb water and can result in reduced thermal performance. Similarly, phenolic insulation thermal properties are unaffected by air infiltration. The products are safe and easy to install, with no fibers that can irritate human airways and harm health.

The high R-values of phenolic insulation can help to reduce the build-out depth of residential and commercial wall systems, potentially leading to added rentable or sellable space on the interior. It is designed to offer a thin solution for common exterior wall continuous insulation applications. It will also help to reduce the length of fasteners and bracketry in commercial wall assemblies. This product has many applications for cavity wall, rainscreen, and soffit applications. When installed correctly, phenolic insulation is known for providing reliable long-term thermal performance over the lifetime of a building.

Vacuum Insulated Panels

A new and very innovative insulation product is known as vacuum insulated panels (often called a VIP). This is a next-generation insulation comprising rigid panels with a microporous core, which is evacuated, encased, and sealed to form a thin, gas-tight envelope. It provides outstanding R-values by virtue of the literal vacuum formed in the panel all in an ultra-thin insulation solution. This product is designated as being applicable specifically to commercial roofing applications, offering thermal resistance values up to five times greater than XPS. For example, a 1.6-inch VIP product has an amazingly high R-value of R-46. Clearly, this is a solution for those times when space is tight and reduced thickness is critical to success.

As a means to substantiate these performance claims, manufacturers use ASTM C1667: Standard Test Method for Using Heat Flow Meter Apparatus to Measure the Center-of-Panel Thermal Transmission Properties of Vacuum Insulation Panels, which is the only test method designated by ASTM to be used specifically for testing center panel thermal resistance of VIPs. ASTM C1667 further states that VIPs fall outside the scope of the more commonly known test method ASTM C518: Standard Test Method for Steady-State Thermal Transmission Properties by Means of the Heat Flow Meter Apparatus. All vacuum insulated panels should be tested according to these protocols, and results of those tests should be made available from the manufacturers as part of the normal project submittal process to confirm the tested R-value results. Note that testing is based on the center of the panels. For edge conditions, calculations can be performed per the ASTM protocols to determine results.

These high R-values allow the roof insulation buildup height to be drastically reduced. For renovations or reroofing projects with height-restricted situations, such as parapet height and door threshold limitations, VIP insulation allows designers to meet code while eliminating the need for architectural renovations to accommodate the roof insulation system. It is worth noting that the products are also more than 90 percent recyclable by weight.

Manufacturers of insulation often offer multiple products. Therefore, many offer thermal calculation support for many products, and with some roofing insulation, they also offer a design service and installation support. This helps the architect be quite up to date on the innovative insulation board technologies that offer a higher R-value in minimal thicknesses, allowing for thinner wall system profiles. Andrew Wilson, commercial manager for Kooltherm & OPTIM-R with Kingspan Insulation LLC, sums it up this way: “High-performing insulation can help cut energy bills and makes for a very efficient building. Using a high-performance insulation product will allow for a building to go well above the energy code, and for someone who owns a building, the payback will be great over the life of the building.”

Addressing Details: Expansion Joints

Controlling thermal energy and moisture in buildings is clearly a multifaceted task, although it is fairly straightforward to comprehend when we are talking about the middle or primary areas of walls, roofs, etc. It is critically important, however, to pay attention to the detail areas. On larger buildings, expansion joints are one of those crucial details since they represent an intentional break in the structure to allow for the movement of different parts of the building due to thermal expansion and contraction, seismic loads, or other conditions. Since those expansion joints typically interrupt the thermal, moisture, and air barriers in a building, how they are treated and addressed will determine the continuity, or not, of these barriers across the expansion joints.

Different types of buildings will utilize expansion joints in different locations and in different ways. Some may use them on exterior roof or pedestrian decks in a horizontal location. Others may incorporate them into walls in a vertical fashion. Some may be connected to concrete structures, others to steel, or some to a hybrid system. Either way, they all will incorporate a gap that needs to be filled with some sort of expandable material that is appropriately secured to each side of the gap. Depending on the durability and the appearance of that filler material, they may then be left exposed or covered over with a metal cover system. From the standpoint of thermal and moisture protection, the key to successfully maintaining the needed barriers often comes down to the selection of the material or type of system used to fill the gap in the expansion joint. We will look at some of the more common choices below and comment on their suitability for different building applications.

Closed-cell foams. Closed-cell foams are very watertight and do not allow the moisture to enter the body of the foam. This is the best application for horizontal runs where water could pool. These are tougher to compress but can be placed under tension or pulled to expand quite well. The other key advantage of closed-cell foams is that they take well to heat-welding of seams. This renders a monolithic installation that reduces the risk of water infiltration.
A good rule of thumb/best practice when using closed-cell foam seals is to limit their application with a joint width of no more than 8 inches (200 millimeters) or smaller. Use of foams for expansion joints larger than 8 inches leads to two things:

1. Exceeding the foam’s performance characteristics. Plus, the weight of “super-wide” foam seals can lead to sagging in vertical applications.
2. Dramatically higher costs compared to other expansion joint cover solutions, such as a four-component system with face seal, rails, and back seal.

Open-cell foams. Yes, these products do allow for some flow-through of water vapor. Like many exterior veneer systems, if moisture becomes trapped in a wall cavity, building systems allow the moisture to wick out. This is a good quality and a major focus to eliminate potential mold issues in vertical applications. Hence, open-cell foams should be employed only in vertical installations, where gravity can wick any absorbed water downward and away from the building enclosure.

Architects should also be aware that open-cell foams for expansion joints come in a maximum lengths of 5 feet, and because they are not heat-weldable, caulk must be used at the seams. This can introduce a future failure point as well as higher periodic inspection and maintenance costs should the seams need to be repeatedly re-caulked to prevent leakage. It’s also important to know that while in a compressed state all foams may look the same, they are not. Specifications calling for foam seals made with “monolithic manufacturing methods” will help avoid product failures and claims down the road. Architects should look closely at the seal’s construction and ask questions of the manufacturer as to the seal’s makeup and the watertightness of seams.

Waxed vs. wax-free foam. Heavy wax-impregnated foams that help keep joints watertight have been in use for about 50 years. However, some consider the addition of copious amounts of wax old fashioned, which may be true up to a point. Today, a 2–3 percent wax impregnation is generally viewed as the best alternative since it drastically increases the hydrophobic properties of the foam and extends the seal’s lifespan.

So what if the specifier chooses to forego wax impregnation? Plain foam can act just like a sponge. In addition, plain foam assumes an unrealistic expectation of perfect installation of the silicone face in manufacturing and field perimeter caulk seals to keep the foam protected. If the face silicone seal itself is damaged—say, by the tip of a caulk gun jammed between the foam and wall or deck material—leaks will likely occur. With wax impregnation, the foam seal will remain watertight even if the silicone face seal is compromised, primarily because wax doesn’t dry out.

Compression seals. As their name implies, compression seal joint systems are installed into a blocked-out joint area and absorb movement and flexing through compression and expansion of the seal. The rubber-looking, corrugated joint material is also an excellent option for exterior application where waterproofing is required. These seals are best employed for heavy pedestrian and moderate vehicle loading. Proper use of two-part epoxies ensure solid adhesion to the deck, and heat-welded seams ensure watertight performance. The nominal joint width for these systems should not exceed 3½–43⁄8 inches (89–111 millimeters) since the material can’t really handle wider applications than that. Since these are often left exposed, building aesthetics can be enhanced through the use of colored compression seals.

Hybrid compression seal systems. It is important to note that expansion joint systems with tied-in waterproofing is critical to avoid water infiltration into adjacent spaces. A new hybrid design of compression seal system is delivering a greater level of waterproofing in split slab construction. The key benefit of this system is in the integrated counterflashing that is employed, which is engineered to channel water away from the joint opening. Note that it is vital that the counterflashing be compatible with the adjacent materials and adhesives being used. Failures in waterproofing can occur if the flashing fails to adhere or reacts to the adhesive. Where load factors require it, metal cover plates can be added over the top of the seal.

Reinforced vapor barrier seals. One solution that can be used in certain applications is to employ a reinforced vapor barrier (RVB) to prevent water infiltration or to channel water to drain locations via an integrated drain tube. The critical factor in installation of an RVB expansion joint system for waterproofing is to apply a bed of manufacturer-approved butyl sealant in the concrete block-out area or along the frame along the entire length of the expansion joint. This will aid in securing the moisture barrier to the concrete block-out and provide a watertight seal to prevent seepage around the perimeter of the joint. In order to work properly, always leave enough drape in the moisture barrier to ensure the system will be able to fully open to its maximum distance without interference from the expansion cover components.

Roof bellows systems. Roofing expansion joint systems use either an EPDM or a neoprene seal that flexes in the manner of a bellows to accommodate seismic movement. As with counterflashing, the seal must run under the metal flanges to allow water to be shed away from the joint opening. Also, a compatible, nonreacting mastic should be used to ensure watertight adhesion of the seal.

When detailing roofing systems, don’t forget the transitions. Tying in horizontal and vertical joint systems requires transition covers to help maintain watertightness. Transition areas include intersections where different roof bellows meet or cross and joints that turn up or down as they transition along horizontal to vertical surfaces. While we would like to think that architectural drawings and details always cover this, the reality is that sometimes transition covers and tie-ins are missed.

Thermal migration. Expansion joint thermal performance is often overlooked in building design, but is no less important than air and moisture performance. Most designers consider the RVB as the go-to-standard within expansion joints. RVBs are a durable membrane that resides within the joint. They accommodate movement but also prevent the penetration of air, debris, and pests from entering through the joint. There is some minor insulating benefit from RVB joint systems, but an insulted vapor barrier (IVB) should be considered wherever thermal performance is important. The addition of insulation within the dual-walled vapor provides a higher R-value—and the benefit, of course, is that the R Value works in both directions—heat or cold don’t penetrate the joint, and interior occupant comfort and HVAC performance are better shielded from the outside conditions along the entire length of the joint.

There are clearly numerous systems and products available when considering maintaining water, air, and thermal resistance along expansion joints. While some may say only foam, recognize that foam can be one of the most expensive products to use in some situations. And as we’ve seen, there are other better alternatives in many applications.

Conclusion

Balancing thermal and moisture protection in buildings with other design criteria and overall cost continues to be a prime consideration in the creation of effective wall, roof, and floor assemblies. Finding ways to include WRB and AB systems that include an appropriate, cost-effective drainage gap is needed in most wall systems. Effective insulation that can provide better performance in less thickness provides more affordable, high-performance solutions. Paying attention to the details of enclosures such as expansion joints helps assure that the entire building is performing as intended throughout. Architects who use these strategies and incorporate some of the latest products to achieve them should find very favorable end results in the constructed buildings.

Peter J. Arsenault, FAIA, NCARB, LEED AP, is a nationally known architect, consultant, continuing education presenter, and prolific author advancing building performance through better design. www.pjaarch.com, www.linkedin.com/in/pjaarch