3355 -- Solar-Driven Moisture Transfer through Absorbent Roofing Materials PDF

A laboratory investigation into solar-driven moisture transfer through absorbent roofing materials is described. It is believed that this mechanism has not previously been investigated systematically. This work arises as a response to moisture problems which have been observed in New Zealand with "skillion" or "cathedral" roofs (roofs where the ceiling shares a common pitch with the roof, separated by a shallow cavity, also called chapel roofs) clad with cellulose-fibre-reinforced cement shingles. The moisture is known not to be due to leaks, thus it was speculated that the moisture transfer through the roofing was caused by solar heating of the rain-wetted cladding material. A laboratory test rig, built to simulate the effects of rain followed by sun on different types of absorbent roofing materials, is described. The rig allows capture and measurement of any moisture that is driven through the cladding into the closed roof cavity below. Results with various absorbent claddings are presented, both alone and with various combinations of permeable and impermeable membranes placed under the cladding or interleaved between shingle layers. Cellulose-fibre-reinforced cement shingles transmitted considerable amounts of moisture under solar heat gain. Of the other absorbent roof claddings, wooden shingles transmitted the most solar-driven moisture. Permeable building paper placed under the cement shingles did not greatly impede the transfer of moisture, but an impermeable membrane interleaved between the shingles, as has become standard practice, reduced the solar-driven moisture transfer to nearly zero.

KEYWORDS: moisture, roofing, membranes, laboratory testing, roofs, New zealand, solar radiation, measuring, materials.