Moisture storing insulation such as cellulose has gained a large segment of the residential market, both in new and retrofit construction. Moisture deposited by exfiltration and adsorbed within the envelope reduces the thermal performance of the envelope and favors fungus growth leading to deterioration of the materials and contamination of the indoor air. Roof assemblies thus insulated are particularly vulnerable to the mechanism of moisture accumulation.  The objective of this study is to investigate the pattern of moisture accumulation in single cavity flat roofs fully insulated with cellulose insulation. Through an extensive experimental program using two test huts in an environmental chamber, patterns of moisture movement and accumulation for a full wetting-drying cycle have been developed for the first time. Using these results for validation, a model using the water potential concept as the driving force for moisture movement was developed to simulate the complete wetting and drying of wood components within the envelope.  Within an Environmental Chamber, which allows for testing full scale specimens, a 190-day test with daily cyclic conditions was designed and performed during which moisture transfer was monitored with electronic sensors and gravimetry. The test consisted of eight flat roof assemblies fully insulated with cellulose subjected to one complete, quasi real-time wetting-drying cycle. The research presents a methodology to evaluate the performance of retrofitting energy efficiency measures using moisture storing insulation in residential buildings.  An hygrothermal model using the concept of water potential gradient as the driving force for moisture movement in wood has been developed for flat roofs insulated with cellulose and integrated into an existing finite element computer model. The model includes the geometrical representation of the wood component, moisture content-water potential relationships, effective water conductivity and mass and heat transfer coefficients. The results from the computer model compare favorably with the experimental results and validate the use of the water potential approach in predicting moisture movement through building envelope assemblies.  The experimental and modeling data document the moisture condition of the assemblies. This moisture condition is an indicator of durability in terms of fungus growth. Design guidelines have been derived from this study for flat roof assemblies incorporating moisture storing materials. This information can be used by the designer to evaluate the hygrothermal performance and the durability of his/her design.