This research introduces methods to evaluate and improve the indoor thermal environment of existing single-family-detached housings built in Minnesota, United States, prior to 1975. The houses were primarily built with a wood frame construction with little or no insulation, and hence are more vulnerable to thermal transfer than contemporary houses built in accordance with current building codes. This study integrates several components; assessment of 63 wall assembly retrofit methods using computing simulation; evaluation of thermal comfort for occupants through retrofits; impact on durability related to moisture contents of wood materials; and cost analysis of each alternative. The research found 32 possible retrofit methods that meet minimum requirements on current code, thermal comfort, and moisture related durability. The major elements affecting the indoor environment of houses are thermal and moisture transfer through the envelope. The analysis of thermal and moisture performance between existing and improved wall assemblies was carried out using computer simulation program WUFI. R-value and moisture contents were measured to determine thermal performance and risk related to mold growth. THERM and Thermal Comfort Tool were used to determine the operative temperatures to meet thermal comfort in severe conditions in cold climate regions such as the upper Midwest (Department of Energy climate zone 6 and 7). Whole-building energy performance for this case study was calculated using the BEopt software to determine the energy savings and energy bills from retrofitted enclosures. The RSMeans was used to calculate the retrofit costs. The main retrofit methods explored fill uninsulated frame cavities and add continuous insulation to the exterior of the assembly. This research compares the performance of retrofit methods and provides the best envelope types for energy savings contributing to a cost-effective retrofit. The exterior insulation is a key factor to acquire high R-value of whole-wall since it is continuous; it encloses the overall wall and reduces heat loss. The high R-value of the exterior insulation also reduces the possibilities of having high initial moisture contents, while improving thermal comfort. The ultimate goal of this research is to achieve maximum efficiency at a low cost for thermal comfort and durability in existing houses. The research also provides retrofit alternatives to maintain the indoor thermal environment and to allow residents to select their own methods within their economic conditions. The methods and results of this research may also be applicable to new constructions.