For residential construction, Canadian Building Experts recommend a minimum cavity depth of approximately 1/2inch. This is the rule of thumb minimum depth that has been determined will still allow the free flow of air in the cavity (and encourage cavity drainage away from the sheathing and membrane - assuming adequate flashing and guttering members have been designed). But this depth is precipitated upon the fact that the cladding application can utilize vertical battens attached directly to the sheathing. This type of approach is what we may see as a solution for low windload applications, single family and type V residential construction. This is what is commonly termed a "simple rainscreen" or "residential rainscreen" approach.
For Commercial construction, higher performing residential construction, and anything over 3 stories, we would anticipate the use of a deeper cavity. For steel stud construction, typical stud spacings occur at 16 inches OC. So a system designed to a 40 psf windload per ICC, should be sufficient for most project windload designs (that's a design windload in excess of 120 psf). That means that the subframing system must attach directly to the steel studs, and be sufficient to support the project design loads. In that case, we would expect to see a cavity depths in excess of 3 inches.
The deeper the cavity, the greater the potential for resistance to the forces of wind driven rain (negative pressure differential). But its not that simple, yet something that needs to be considered as part of a quality wall design.
For a pressure equalized rainscreen wall, the depth of the cavity is instrumental in the cavitation simulation and compartmentalized design. Cavity depth affects the volume of the wall compartments, which in turn impacts the sizing of venturi slots (ventilation baffles). Its not merely a function of drilling holes into perimeter extrusions or encouraging the free flow of air through panel joint constructions. Rather critical analysis of the how air flows into the PER cavity (and out it, i.e. the permeability of the membrane) will determine the effectiveness of this wall type.
For the rear ventilated rainscreen systems, several factors impact the design approach for addressing negative pressure differential in the cladding cavity. These systems require sufficient airflow from the base of the cavity to the top of the cavity. When you also increase the cavity depth, you reduce the potential for moisture transfer across this plane. Also, air and water testing has shown that if you also integrate a closed jointed design, you will limit the intrusion of water into the cladding cavity. This design feature can also act as a great deterrent to UV degredation of the waterproof membrane.
With modifications to regional energy codes and the move towards improved thermal envelope design, we are seeing the introduction of exterior grade insulations (min
eral wool) in the wall cavity to offer improved R values for the enclosure system. The system to the right for the Artesian Water Company in Delaware integrated an 11-1/2 inch deep adjustable system to support a ceramic tile rainscreen system. Over 4 inches of mineral wool insulation was integrated into the design to improve energy and acoustic performance of the wall. This is an added benefit that a rainscreen system offers over conventional construction (yet doesn't adversely impact the installed cost of the cladding). Groups like Lawrence Berkeley National Labs, Oak Ridge National Labs and the DOE are involved in assessing the use of high performance wall cladding with regards to the energy saving payback period that their use assumes. For projects where an integrated design approach is taken for the design of the enclosure systems and MEP systems we would anciticipate a very short payback period for the use of such systems.
No comments:
Post a Comment