Adds wall vented field

[shorowit]

Adds a Wall/Vented (boolean) field with associated documentation:

Indicates whether the walls are vented or not. For example, the walls of a mobile home may be intentionally ventilated to remove accumulated moisture by having corrugated metal siding open at the bottom to provide space for air to flow between the exterior and interior wall materials.

[shorowit]

Need to clarify relative to a rain screen. Will dig up some additional information on this. Include as a sub-element under the appropriate wall type (WoodStud)?

[shorowit]

Not that important, not worth additional effort.