We ran across this photo on a flickr site in Seattle: http://www.flickr.com/photos/14361725@N07/2986359346/

Its a compelling photo visually, but not what you really want for a panel product if the end result is longevity and moisture control. And it's a great illustration of what happens when moisture isn't sufficiently controlled in a cladding system (especially with materials that aren't designed to be continually wet). And I'm sure its also probably an excellent example of someone being penny-wise but dollar foolish with the design of the system.

As part of any good cladding design, the end goal should be to control the forces that allow moisture to enter the cavity of the cladding and allow moisture to dry and exfiltrate the cavity. In the case above, it appears that edges of the panels are staying wet, causing the panels to absorb moisture and discolor.

Moisture attacks these surface edges through forces generated by gravity flow of water, capillary action and surface tension, or from an inward drive of moisture caused by negative pressure differentials in the cavity of the cladding. Also, if the cavity of the cladding is closed off or restricted from having sufficient ventilation, you may find yourself experiencing significant problems associated with condensation that occurs at the backside of your panel.

With wood and paper-based products where moisture can shorten the lifespan of your product, designing the system sufficiently will allow you to extend the serviceable life of the product. An accumulation of moisture at the backside of the panel can lead to panel bowing and warpage and ultimately failures at the fastening locations due to the stresses caused from the panel movement.

We find that one of the most underrated components of any exterior enclosure application are flashings and system terminations. You may address all of the factors mentioned above at the exterior face of the panel, but if there is a failure where it abuts a dissimilar material or where the system terminates, you could have significant moisture infiltration problems associated with the installation. That is why we feel it is critical to consider the use of products where the panel supplier is intimately involved with the design and supply of the attachment system. Who knows the product or the environmental forces that impact the performance of the panel better than the supplier of the panel product?

Other design factors to consider with rear ventilated cladding products are open vs. closed jointed designs and the cavity depth of the system. These will be discussed in future posts in more depth, but you should know that they can have a significant impact on the performance of the system with regards to the amount of moisture that is allowed to enter the cavity and hit the membrane as well as how designers can limit the UV degradation of the weatherproofing membrane at the panel joint of the cladding system.