Design and efficiency of houses can affect the amount of peak load reduction available from a residential demand response program. Twenty-four houses were simulated with varying thermal integrity and air conditioner size during the summer cooling season with and without a demand response program. Improved house thermal integrity reduced the effectiveness of the demand response program in limiting peak demand. Air conditioner size had a less significant effect but still changed the demand reduction available. Both provided significant long-term reductions in demand and energy consumption. These results should be considered in the design of demand response programs, and the simulations should be expanded to include other days, locations, and home designs.