The rainscreen concept, now almost 50 years old, has only recently gained traction in current building practices. As we explore here, simply placing a capillary break behind your cladding may not provide the protection you expect.
Moisture management has always depended on a balance between the entry of water and the capability to remove it. Strategies have largely relied on dual-barrier systems where the cladding provides the first line of defense and a second is provided by a drainage plane consisting of a continuous weather resistive barrier and provisions for moisture release. For over a century, this approach has provided quality performance attributed primarily to the fact that: a) wall assemblies accommodated bi-directional drying aided by natural air leakage; and b) the employed cladding systems, and the substrates to which they were installed, possessed adequate moisture storage and drying capabilities.
But with the advent of tighter wall construction in the 1980’s, the dual-barrier approach was not enough, and further attention was refocused on vented and ventilated rainscreens. Vented rainscreens have an air cavity (capillary break) between the cladding and weather-resistive barrier. This cavity is open at the bottom of the assembly for drainage and limited air exchange. A ventilated system goes one step further by providing vent openings at the top and bottom of the air cavity.
A number of products have been introduced to achieve cavity depths of 1/4” (6.4 mm) to 1” (25.4 mm). Many of these products accommodate the Canadian building code requirement for a 10 mm (~3/8”) drainable capillary break. Municipalities in wetter regions, such as Vancouver, BC require a minimum 3/4”cavity. Our experience here in the United States indicates that the 1/4” dimension is widely used behind wood, fiber cement, and stucco. But is the 1/4” rainscreen sufficient?
We used hygrothermal simulations to predict the 30-day running average relative humidity for conventional wood-framed assemblies with various rainscreen configurations. The 30-day average relative humidity of <80% reflects one of three criteria established by ASHRAE 160 for the purpose of preventing mold growth. This study also incorporated the ASHRAE 160 requirement for 1% moisture infiltration against the weather resistive barrier. Our modeled climate was Minneapolis, Minnesota. We used a typical wet month (June) with a north-facing orientation. Air change rates were derived from reviewed literature; they are expressed here as Air Changes per Hour (ACH).
- No rainscreen: stucco fails; fiber cement marginally meets ASHRAE 160 criteria
- 0.25” capillary break (0 ACH): unchanged
- 0.25” with 15 ACH: marginal improvements indicated, but stucco still fails criteria
- 0.375” with 30 ACH: both systems meet criteria; improved performance indicated for fiber cement
- 0.5” with 50 ACH: improved performance
- 0.75” with 100 ACH: improved performance
Conclusions
Our results agree with published findings that show significant drying improvements with well-ventilated rainscreens (i.e. 8mm to 19mm; >50 ACH). The design challenge stems from the 1% moisture infiltration load, which reflects 1% of wind driven rain to be deposited directly against the weather-resistive barrier. This can be a rigorous burden, especially for assemblies that do not accommodate bi-directional drying. The minimum dimension for your rainscreen depends on a number of factors, the most important being the type of cladding, the hygric properties of the overall assembly, the expected moisture burden, and expected ventilation capacity (e.g. air pressure, building exposure, vent occlusion). Rainscreen performance is highly dependent on achieved ventilation rates, which are reduced significantly by even small occlusions at vent openings or cavities. Clearly, the effect of occlusion is greater for smaller vent spaces. Ventilated drainage cavities less than 3/8″ are not adequate for the tested climate conditions. Rainscreen depths of 0.75″ would better accomodate incidental occlusion while still achieving air changes in excess of 30 ACH.
Resources
- Salonvarra, M., A.N. Karagiozis, M. Pazera, and W.A. Miller, 2007. Air Cavities Behind Claddings-What Have We Learned? In: Thermal Performance of the Exterior Envelopes of Buildings X, proceedings of ASHRAE THERM X, Clearwater, FL, Dec. 2007.
- Ge, H. and Y. Ying. 2007. Investigation of Ventilation Drying of Rainscreen Walls in the Coastal Climate of British Columbia. In: Thermal Performance of the Exterior Envelopes of Buildings X, proceedings of ASHRAE THERM X, Clearwater, FL, Dec. 2007.
- Straube, J. and G. Finch. 2009. Ventilated Wall Claddings: Review, Field Performance, and Hygrothermal Modeling. Research Report #0906.
- CMHC. 2002. The Rain Screen Wall System. The Canadian Mortgage and Housing Association and The Ontario Association of Architects.
The Building Enclosure 