As a leading provider of forensic investigation services for local businesses, insurance companies, and homeowners, Orata Engineering has the distinct advantage of being able to identify common trends and problem areas that affect property owners. One such area that we find consistent issues with are exterior stucco cladding systems. Stucco tends to suffer performance failures in areas prone to high amounts of moisture and precipitation. Regardless of whether the system is traditional or acrylic modified, failures seem to occur in similar manners.
Traditional vs Acrylic Modified Stucco
Traditional systems include the following:
- Interior finish (gypsum)
- Timber Frame filled with
- Cavity insulation (generally R-12 Batt insulation)
- Sheathing (usually plywood)
- A single layer of asphalt impregnated tar paper (building paper)
- Exterior siding system
- Scratch Coat
- Brown coat
- Finish Coat
Modern traditional stucco systems finishes include:
- Interior Finish (gypsum)
- Timber Frame filled in with
- Batt Insulation
- Spray Foam
- Other
- Sheathing (usually OSB)
- Two layers of tar paper (building paper)
- Exterior siding system
- Scratch Coat
- Finish Coat
- Interior Finish (gypsum)
- Timber Frame filled in with
- Batt Insulation
- Spray Foam
- Other
- Sheathing (usually OSB)
- Two layers of tar paper (building paper)
- Exterior siding system
- Scratch Coat
- Acrylic Finish Coat
Since the start of the use of these traditional systems, the characteristics and qualities of the building product have changed. This has resulted in some modifications to the system described above. Including having two layers of building paper instead of one, because it was noticed that the first layer of building paper bonds with the scratch coat of stucco, rendering the drainage plane that the building paper is supposed to create non-existent. Additional modifications include the use of water resistant barriers with high water permeability, which allows for the absorption of a large amount of water. A traditional stucco system is not capable of drying this amount of water within a reasonable time frame, allowing for water accumulation.
Acrylic finish coats contain acrylic resins and polymers that given this finish a little more colour and definition that homeowners prefer over traditional stucco finishes. These finish coats are however limited in the types of textures and aggregate sizes that can be used. The ranges on these features vary between manufacturers.
The most important characteristic of acrylic stucco finishes is that it repels water and is usually described as water resistant. This is a drastic change from traditional stucco, as traditional stucco finish coats are water absorbent, but the water will eventually either drain or evaporate out of the building envelope. This difference completely changes the direction in which a building envelope will dry.
The direction a building envelope will dry is dependent on whether it’s in a heating climate or a cooling climate and the location of the vapour barrier(s) within the building assembly. If the vapour barrier is closer to the interior, the system will tend to dry to the exterior of the system, where the vapour barrier isn’t blocking the vapour path. This is the common design for buildings built in Alberta, and when an acrylic finish is applied to the exterior siding system, it is like creating a second moisture barrier. This restricts the pathways in which the water vapour can exit the building’s envelope and as such increases the drying time of the building envelope. Over time this will gradually increase the amount of moisture accumulated in the wall (over seasons and cyclic moisture loading), and result in serious water damage to the surrounding materials should enough water enter the assembly.
Both systems have pros and cons, but with proper construction, use of materials, and application, most potential drawbacks can be minimized or avoided.
Still stuck on stucco?
Still, want the stucco appearance, but are worried about the concerns addressed above? Exterior Insulation and Finish Systems (EIFS) may be your perfect solution. This product is a non-load bearing building cladding system that is assembled onsite and has not been seen to fail in the same ways as the stucco systems addressed above. This is because OSB does not become vapour permeable when wet, as plywood does. This severely decreases OSB’s drying ability in comparison to plywood. When plywood becomes vapour permeable when wet, it is allowing the re-distribution of moisture over the sheet, drastically increasing the surface area, and reducing the drying time. OSB does not allow for this transfer of water vapour within itself, limiting the surface area and increasing the drying time. As a result, OSB/building paper interfaces become concentrated with moisture, which increases the building envelope’s exposure to moisture.
Stay tuned to our blog to learn more about EIFS systems and how they work, along with other sources of building failures. To learn more about our services, contact us today!