47.5 MB - The Whole Building Design Guide

Secondary line of protection: Almost all wall coverings permit thepassage of some water past the exterior surface of the covering,particularly when the rain is wind-driven. For this reason, mostwall coverings should be considered water-shedding, rather thanwaterproofing coverings. To avoid moisture-related problems,it is recommended that a secondary line of protection with amoisture barrier (such as housewrap or asphalt-saturated felt)and flashings around door and window openings be provided.Designers should specify that horizontal laps of the moisturebarrier be installed so that water is allowed to drain from thewall (i.e., the top sheet should lap over the bottom sheet so thatwater running down the sheets remains on their outer surface).The bottom of the moisture barrier needs to be designed toallow drainage. Had the metal wall panels shown in Figure 3-58been applied over a moisture barrier and sheathing, the amountof water entering the building would have likely been eliminatedor greatly reduced.In areas that experience frequent wind-driven rain, incorporatinga rain screen design, by installing vertical furring stripsbetween the moisture barrier and siding materials, will facilitatedrainage of water from the space between the moisture barrierand backside of the siding. In areas that frequently experiencestrong winds, enhanced flashing is recommended. Enhancementsinclude use of flashings that have extra-long flanges,and the use of sealant and tapes. Flashing design should recognizethat wind-driven water could be pushed up vertically.The height to which water can be pushed increases with windspeed. Water can also migrate vertically and horizontally by capillaryaction between layers of materials (e.g., between a flashingflange and housewrap). Use of a rain screen design, in conjunctionwith enhanced flashing design, is recommended in areasthat frequently experience wind-driven rain or strong winds. Itis recommended that designers attempt to determine what typeof flashing details have successfully been used in the area wherethe facility will be constructed.Underside of Elevated FloorsIf sheathing is applied to the underside of joists or trusses elevatedon piles (e.g., to protect insulation installed between thejoists/trusses), its attachment should be specified in order to3-70 MAKING CRITICAL FACILITIES SAFE FROM High Wind