October 15, 2006

Abstract: 

Providing rigid insulating sheathing to the exterior of a wall assembly is a technique that has been used in cold climates for more than 40 years. Recently it has begun to be integrated into enclosure designs in all climates. As with any newly adopted technology, there can be concerns for its proper application. This paper examines methods of incorporating insulating sheathing into the thermal and moisture management systems of the building enclosure in a variety of climate zones across North America. This is done through examining the material properties of the various products and how these properties can be used to achieve an energy efficient and durable building enclosure design, while avoiding problems relating moisture accumulation and degradation of materials.

Introduction and Background

The desire to design more sustainable buildings through increasing the energy efficiency of the enclosure can result in an increase in problems with moisture accumulation within building enclosure assemblies. These moisture problems (from issues such as an increase in the condensation potential within the assembly, or a reduction in the drying potential of the assemblies) lead to premature material degradation and large costs for renovations. Many enclosure failures occur due to the lack of understanding of energy and mass transfer through assemblies and through a lack of appreciation that products and materials have other properties than the ones that they are principally known for.

Though these lessons were hard learned, we can now use this knowledge for our benefit. Through examining and understanding materials based on all of their properties (not just what they were initially created for) and how they integrate to become a system, we can eliminate redundancies in enclosure design, making the systems simpler and more cost effective.

Insulating sheathing has been shown to be an effective method of reducing material use in a building, while increasing the energy efficiency of the thermal envelope, and if properly incorporated into the design of the moisture management system, can help in increasing the overall durability of the structure. In order to understand how to incorporate the materials into the design of the building enclosure, an understanding of the material properties themselves is important.

Material Properties

There are three main types of insulating sheathing currently being used in the industry: Expanded Polystyrene (EPS), Extruded Polystyrene (XPS), and Polyisocyanurate (Polyiso). Each of these products all has a different set of physical properties (Table 1) that will affect the dynamics of the wall assemblies in regards to the transmission and management of heat and moisture.

Table 1: Iinsulating Sheathing Material Properties

R-value

The thermal resistance of each of the products is different. In general, EPS foam has the lowest R-value per inch, with XPS being slightly more efficient, and with Polyisocyanurate having the best R-value per inch. The R-value of EPS foams can be increased by increasing the density of the product, however, the more dense expanded foams are less common in the market. Typically EPS foam has a rated value of approximately R-4 per inch. XPS foams are pretty consistent with an R-value of approximately R-5 per inch.

The thermal resistance of these EPS and XPS foams are generally stable over the long term and therefore the initial R-value at the time of manufacturing will not change over time. Polyisocyanurate foams are rated with a Long Term Thermal Resistance (LTTR) R-value representing a 15 year weighted R-value. This is in response to issues of thermal drift of the polyisocyanurate products. Thermal drift occurs due to the gasses produced during the forming of the foam. These gasses slowly diffuse out of the product over time and are replaced by air.

Since these gasses also have more thermal resistance than air, the R-value of polyisocyanurate diminishes over time as the gasses diffuse out of the product. Facings on the insulation board, such as aluminum foil, will slow this process down as the diffusion can only occur out the edges of the product and not through the front and back faces. Most polyisocyanurate products have an LTTR R-value of R-6.5 per inch.

Permeance

For unfaced insulating sheathing, the permeability is a function of the material thickness. In general most product manufacturers list the permeance of the material based on a thickness of 1 inch. Increasing or decreasing the thickness of the material will affect the permeance. As an example, 1 inch of XPS has a permeance of 1.1 perms. Increasing the thickness to 2 inches decreases the permeance to 0.55 perms.

For faced insulating sheathing boards (such as foil faced polyiso, glass fiber faced polyiso, and plastic film faced XPS), the permeance of the facing is often much lower than the permeance of the polyisocyanurate and will govern the overall permeability of the sheathing board. For these products, the permeance will not change with increasing thickness.

Rain Water Management

The choice of the how to integrate insulating sheathing into the enclosure water management system is based predominantly on the rainfall, exposure, and wind load potential of the area in which the house is being built. In areas of low rainfall, less than 20 inches per year, the risk of water damage due to exterior rain penetration is lower than in other areas where the rainfall is much higher, more than 40 inches per year. The risk also increases in areas that are more prone to short intense rainfall and high winds (such as coastal and hurricane zones) or areas that have little protection (such as areas with no trees or other structures close by) or are elevated (such as hilltops). How the water management system is designed and where the insulating sheathing is placed in the assembly will be affected by these considerations.

System Design Incorporating Insulating Sheathing

Since insulating sheathings are resistant to degradation due to moisture they can be placed exterior of the drainage plane of the assembly or in some cases can be used as the drainage plane of the assembly. This allows for several possible configurations of the rain water management system.

Figure 1: Rain Water Management Options

Wall Section 1 - Insulating Sheathing and Housewrap installed over Plywood or OSB. The first strategy involves installing the insulating sheathing over top of a layer of building paper or housewrap and wood sheathing. In this assembly, the insulating sheathing protects the housewrap drainage plane from exposure to wind and excessive heat. In addition, while it is not designed to be the drainage plane of the assembly, it will shed most of the water that penetrates past the cladding, minimizing the amount of water that actually penetrates back to the housewrap. In addition the backer layer of plywood sheathing also protects the wall from wind blown debris and projectiles during storm events. All water management details (flashing and window installation details) should be tied back to the plane of the housewrap. This type of assembly would be recommended in areas of high wind and rainfall exposure (hurricane and tornado prone zones).

Wall Section 2 - Insulation Sheathing and Housewrap installed over Wood Studs. The next proposed strategy is to install the insulating sheathing outside a housewrap that is stretched over wood studs. In this configuration, the housewrap drainage plane is protected from exterior elements (excessive wind loading, and rain exposure). All water management details (flashing and window installation details) should be tied back to the plane of the housewrap. The type of assembly would work effectively in most rainfall zones, though potentially not in high exposure locations. With the lack of wood sheathing support on the exterior of the framing more care is required during the installation of the housewrap and insulating sheathing.

Wall Section 3 - Housewrap installed over Insulating Sheathing and Wood Studs. The third strategy would be to install the housewrap to the exterior of the insulating sheathing, essentially replacing the plywood or OSB in a traditional wall assembly with insulating sheathing. The housewrap is more exposed to exterior elements such as wind loading and moisture and may not be as durable as the other approaches. In addition the fasteners used to install the housewrap must be able to penetrate all the way through the insulating sheathing and into the wood studs beyond. In this configuration the water management and window installation details are integrated into the housewrap at the exterior face of the insulating sheathing. Water management details would be the same as normal details of recommended good practice for wood sheathed house design. This wall approach would function adequately in most rainfall zones.

Wall Section 4 - Insulating Sheathing installed as the Drainage Plane. The final approach would be to use the insulating sheathing as the primary sheathing and drainage plane of the assembly. In order for the insulating sheathing to be used as a water resistive barrier, the vertical plane of the exterior face of the sheathing must be as continuous as possible. This is to prevent . . .

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