Assessment of technologies for constructing self-drying low-slope roofs

摘要

Issues associated with removing excessive moisture from low-slope roofs have been assessed. The economic costs associated with moisture trapped in existing roofs have been estimated. The evidence suggests that existing moisture levels cause approximately a 40% overall reduction in the R-value of installed roofing insulation in the United States. Excess operating costs are further increased by a summertime heat transfer mode unique to wet insulation, caused by the daily migration of water within the roof. By itself, this effect can increase peak electrical demand for air conditioning by roughly 15 W/m(sup 2) of roofing, depending on the type of insulation. This effect will increase peak demand capacity required of utilities in any geographic region (e.g., 900 MW in the South). A simple formula has been derived for predicting the effect that self-drying roofs can have upon time-averaged construction costs. It is presumed that time-averaged costs depend predominantly upon (1) actual service life and (2) the likelihood that the less expensive recover membranes can be installed safely over old roofs. For example, an increase in service life from 15 to 20 years should reduce the current cost of roofing ($12 billion/year) by 21%. Another simple formula for predicting the reroofing waste volume indicates that an increase in service life from 15 to 20 years might reduce the current estimated 0.4 billion ft(sup 3)/year of waste by 25%. A finite-difference computer program has been used to study the flow of heat and moisture within typical existing roofs for a variety of US climates. Nearly all publicly available experimental drying data have been consulted. The drying times for most existing low-slope roofs in the United States are controlled largely climate and the permeability of the structural deck to water vapor.