Case Studies

Interstitial Condensation

On behalf of the roof framing subcontractor, Steve Klingler and team were contacted to evaluate the cause and resulting damage of rust and lumber decay discovered throughout a panelized roof framing system in a large warehouse facility located in Phoenix. The exterior walls of the building consisted of concrete tilt-up panels with a roof framing system comprised of steel girders, purlins and wood sub-purlins which supported a low-slope plywood roof sheathing. A built-up-roof (BUR) was placed on the top side of the sheathing with a radiant barrier of foil insulation installed against the underside.

Upon completion of construction, the majority of the warehouse had remained unoccupied. Without occupancy, the facility would not require ventilation, though it is important to note that this building would have contained a fair amount of moisture within the framing lumber and concrete materials. Aside from materials such as metals, plastics, roof membranes and foil insulation, most building materials are to some extent permeable and do not obstruct the movement of moist air through the structure. As such, warm, moist air during the summer may eventually cool below its dew point within the fabric of the building, resulting in condensation.

This form of condensation that occurs within a building element is called interstitial condensation, and it can lead to timber rot, oxidation of metal and other problems. Interstitial condensation is more complex than surface condensation and presents a greater hazard, because the moisture content is hidden from view and can go undetected for long periods until serious damage has developed. It also renders any insulation within the component ineffective.

In the case of this warehouse, some time after the construction had been completed, a sub-purlin fell from the roof framing more than 30 feet to the concrete slab below. It was discovered that the mechanical fasteners which supported the 2x4 sub-purlin had rusted through. Further research uncovered extensive rust throughout the underside of the panelized system; in addition, rust was found in all metal braces, truss components and other metal features located above the foil insulation sheets.

Our research uncovered a variety of shortcomings, including building design issues, poor consideration of the intended purpose of the building and the lack of ventilation. Proper understanding of the building use should be considered before specifying this type of radiant (barrier) system.

We found other issues that may have contributed to the decay and rust phenomenon:

• While every construction project requires effective scheduling, in order to properly sequence the installation of membranes and barriers scheduling must also be responsible. It was discovered that the roof assembly was installed during reported rain storms, raising questions about moisture levels within the wood substrate materials.
• Project records indicate that the insulation was installed at the same time as the roof membrane, perhaps trapping added moisture within the substrate materials.
• Improperly placed radiant insulation did not provide appropriate air space.
• The integrity of galvanizing properties for the sub-purlin hangers was questionable.

Evaluations were conducted by a variety of professionals and repairs which would salvage the existing framing structure were recommended and implemented, avoiding a large scale catastrophe. Although no common conclusion was reached, the signs and symptoms suggest that the primary cause for this hidden decay was interstitial condensation, the consequence of moisture trapped in a poorly ventilated building.