Sealed and Insulated Attic Hygrothermal Performance in New California Homes Using Vapor and Air Permeable Insulation - Field Study and Simulations

This project investigated the thermal and moisture performance of a low-cost approach to sealing and insulating attics using glass fiber insulation. The work included a combination of: (1) field measurements of attic and heating, ventilation, and air conditioning (HVAC) system performance in two new, high performance homes in California’s Central Valley (Fresno and Clovis), and (2) hygrothermal simulations of attic performance. Each field study attic was continuously monitored at multiple locations for more than a year for wood moisture content, air humidity, condensation, temperature, and heat flux, together with on-site weather and solar conditions. The Fresno test home showed periodic condensation and high surface wood moisture content, but no surface mold or degradation upon visual inspection at the end of the test period. The Clovis test home showed less indication of high moisture levels but did have visible suspected mold growth on the inside of the north sheathing. These results show the limitations of current moisture measurement techniques focused on wood moisture content, rather than potential for mold growth. From a thermal/energy perspective the attics were close to indoor conditions thereby realizing the design intent for reducing duct system losses. Simulated site-HVAC energy savings for sealed versus vented attics averaged 18 percent across California climate regions (8 percent time dependent valuation energy savings). Savings were dominantly from heating energy reductions; cooling savings were substantially less. Simulations investigated moisture issues in greater detail. The field and simulation results indicated that using air and vapor permeable insulation can be acceptable from a thermal/energy point of view, but additional measures need to be taken to reduce moisture risks, primarily from mold growth.