XPS thermally insulated sheathing placed over OSB sheathing
As Figure 3 shows, moisture content in the OSB increases in the fall and winter, and dries during the spring and summer, although not completely. The result is the OSB’s moisture content percentage increases over the three-year period due to the restricted drying potential. While the moisture accumulation is not drastic based on the timeframe, it can still potentially compound over the coming years and lead to future problems.
Foil-faced polyiso thermally insulated sheathing placed over OSB
As shown in Figure 4, the moisture content in the OSB increases in the fall and winter, and dries during the spring and summer months, but not completely. The red line indicates moisture accumulates throughout the period of the model due to the restricted drying potential. As was the case for the XPS thermally insulated sheathing over OSB, the increase in the strandboard’s moisture content is not drastic based on the three-year period. However, again, it can potentially compound over the coming years and lead to future problems. Moisture content in the polyiso sheathing also increased due to the presence of bulk water.
Dimensional stability
To design buildings that function and perform as intended, understanding sheathing materials characteristics at these dynamic temperatures is crucial. Using materials with a high co-efficient of linear expansion, (Materials can either expand or contract based on this co-efficient and at a temperature differential. Several foams’ co-efficient of linear expansion are greater than 10 times that of steel). especially in cold climates, can have a negative effect on the building envelope’s thermal efficiency.
Due to expansion and contraction, gaps between butt joints in the insulating materials can substantially increase heat flow through the thermal envelope. Even materials installed in two lifts or with overlapping joints can have similar heat loss profiles.
As part of the thermal envelope design process, and to reduce heat
loss potential, it is necessary to review the manufacturer’s technical data sheet for the listed coefficients of linear expansion, specifically ASTM D2126, Standard Test Method for Response of Rigid Cellular Plastics to Thermal and Humid Aging. In colder climates, it can be ideal to use a material with a very low co-efficient of linear expansion (e.g. mineral wool) to reduce the potential for thermal bridging.
When an exterior sheathing product is installed in the summer at a temperature of 24 C (75 F), it will fit tightly. During the winter, when the temperatures drop to –10 C (14 F), a gap between materials can form from the shrinking due to extremes in temperature. This differential can allow for some foam plastic insulations to reduce in size by up to two per cent (i.e. curing process). This shrinkage in foam plastics must be taken into consideration when designing the wall assembly’s thermal envelope, as this void acts as a heating system for the exterior façade.
Gaps located between joints of insulation boards have significant increased heat flow through the building envelope. Figure 5 shows the heat loss due to fasteners and 25-mm (1-in.) gaps based on heating degree days (HDDs), which are a common metric for reflecting the demand for energy needed to heat a building. Using materials with the potential for gapping should be accounted for in designing the thermal envelope . (See “Effects of Mechanical Fasteners and Gaps between Insulation Boards on Thermal Performance of Low-slope Roofs,” by T.W. Petrie et al in the April 2000 edition of Journal Thermal Envelope and Building Science. Rockford Boyer, B. Arch. Sc., is the North American manager of the Energy Design Centre (EDC) for Roxul. He has been with the company for more than five years. Boyer is a registered Building Science Specialist Ontario (BSSO). He can be reached via e-mail at rockford.boyer@roxul.com).
Determining the durability of an insulating sheathing product is essential when specifying a material to use for a project, as it will last as long as the building itself. Stone wool, XPS, and polyiso are all durable options; however, fire performance, ability to resist ultraviolet (UV) rays, moisture resistance, and ability to hold shape should be looked at before selecting a sheathing product. (Foam products are classified as ‘combustible.’)
To assist in determining the time frame for the thermal envelope, one must review durability standards that identify typical design lifecycles on standard building classifications and components. This author suggests contacting the manufacturer’s technical department for generic lifecycle information.
Conclusion
Exterior insulated sheathing can be the most effective way to insulate a building. With varying products on the market, each has positive and negative performance characteristics. With the increasing pressure to create energy-efficient buildings, ‘breathability’ may become more of an issue in the future.
It is an ideal situation to design a thermal envelope that can meet the needs of cold climate, including locations with high relative humidity (RH), with higher perm rating products. Materials with high perm ratings will not limit the wall’s drying potential. It is basic physics that materials either expand or contract due to temperature fluctuations; to decrease heat loss through thermal bridging, the best option is to use a material with minimal risk of dimensional instability. It is imperative to design materials located within the thermal envelope to be as durable as possible, as there are limitations for repair or retrofit once the envelope is enclosed.
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