BUILDING SCIENCE

Incorporating Foamed Plastic Insulating Sheathings on Exterior Wall Assemblies

By Salvatore Ciarlo

As Provincial Energy Codes mandate higher insulation levels in new houses, builders face the challenge of constructing high performance wall systems that are cost effective, easy to build, and have no hidden consequences. Increased insulation plus improved air tightness are an effective way of reducing heat loss while addressing occupant comfort. To this end, builders can incorporate high performance batt insulation (R-19, R-22 or R-24) in wall cavities with conventional sheathings (OSB or plywood). Alternatively, a combination of foamed plastic insulated sheathing and high performance batt insulation can be used.


Schematic of wall assembly configuration showing different component layers and assumed
path of air flow through assembly; well assembly includes structural sheathing.

A major benefit of foamed plastic insulating sheathings (over wood sheathings or directly on wood studs) is a significant reduction of heat loss from thermal bridging. In a typical two-storey house, the exterior wood framing represents approximately 16 to 20 per cent of the wall area, and is poorly insulated. Exterior foamed plastic insulating sheathing addresses this thermal bridging and reduces heat lost through the structure. It will also minimize the risk of condensation by warming up the cavity.
Some builders are concerned that a double vapour barrier results if exterior foamed plastic insulating sheathings is used in combination with a 6 Mil polyethylene vapour retarder on the inside. A recent study at Canada's National Research Council (Report on Properties & Position of Materials in the Building Envelope for Houses and Small Buildings, no. A1 004615, December 2014) looked at the risk of moisture condensing in wall assemblies with exterior foamed plastic insulating sheathings compared to reference wall assemblies using OSB exterior sheathing. Stud cavity insulations were fibrous batt type (R-19 and R-24). Both exterior foamed sheathing over OSB or attached directly to the exterior face of wood studs were examined. The R-values of the exterior foamed insulations used were R-4, R-5 & R-6 and the water vapour permeance of the insulations covered a range of values from 2 (very low) to 1800 (very high) ng/(Pa.sm2).

The key observations included the following:
There is less risk of moisture condensation in walls with R-19 compared to R-24 fibrous batt insulations in the stud cavity because the temperature of the interior face of the exterior sheathing (OSB or foamed insulating sheathing) is colder with R-24 batts. Thus, adding exterior insulation in combination with R-19 batts is a good choice.
All wall systems incorporating foamed insulating sheathings, with or without an OSB interlayer, result in lower risk of moisture condensation than the reference walls sheathed only with OSB and no exterior insulation. Walls with foamed insulating sheathing installed over the OSB showed a slightly lower risk of moisture condensation than walls without the OSB interlayer. For cold, dry zones such as Ottawa, the risk of condensation was low regardless of whether OSB was used as an interlayer and regardless of the vapour permeance of the foamed insulating sheathing.
For the coldest and driest zones such as Edmonton, the risk of condensation was low for walls with OSB plus foam sheathing regardless of the vapour permeance of the insulating sheathing. Walls without the OSB interlayer showed the risk of condensation was slightly lower with a more vapour permeable insulating sheathing, but all those in the vapour permeance range examined performed better than the reference wall that used only OSB.
For the mild, humid, coastal zones such as Vancouver and St. John's, for walls with and without an OSB interlayer, the risk of condensation increases as the vapour permeance of the foamed insulating sheathing increases. This is because the higher outdoor water vapour pressure (humidity) is pushing the water vapour into the wall cavity. Less vapour permeable foamed insulating sheathings will reduce the risk of condensation inside the wall.
This research has demonstrated the benefit of adding exterior insulating sheathings to minimize the risk of condensation in exterior wall assemblies. Accordingly, section 9.25.5 of the NBC 2015 edition, Properties and Position of Materials in the Building Envelope, will be modified to reflect the added benefit of adding exterior insulation.
This work clearly shows that the fears of a double vapour barrier are unfounded in most of Canada-even when low vapour permeance exterior insulating sheathing is used-as long as the sheathing has a minimum thermal resistance of R-4. Builders using OSB structural sheathing can also rest assured that adding foamed insulating sheathings on the exterior side of the wall assembly will not result in increased risk of condensation; in fact the risk is reduced. Exterior insulating sheathings help keep wall cavities and critical surfaces warmer thus reducing the risk of condensation. As added benefits, this also makes homes more energy efficient, increases durability and improves occupant comfort.

Salvatore Ciarlo, P.Eng, is the National Codes/Standards and Technical Services Director at Owens Corning Canada. He has 23 years experience in the building materials industry and holds a bachelors degree in Civil Engineering specializing in building science and has a professional degree in Architectural Technology. Sal sits on several Provincial and National Code committees. He can be reached at salvatore.ciarlo@owenscorning.com.

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