Although details may vary slightly from home to home, the average mobile home built up to the mid 80's is constructed with 2 x 4 in. exterior walls and RSI 2.1 (R-12) batt insulation. From the mid 80's to present day: exterior walls are 2 x 6 in. walls with R-20 batt insulation. Ceiling insulation is R-24 to R-34.