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The heat insulating potential of straw, an agricultural residue, used as part of the envelope of unconditioned buildings is the focus of the current work. Using a CFD software with experimentally validated boundary conditions, transient thermal analysis is performed on a 3-D model room of dimensions 3 m × 3 m × 2.5 m with vents and heat source representing a naturally ventilated indoor space. The thermal performances of a reference uninsulated concrete envelope and a straw insulation-based envelope are compared in terms of quantitative parameters such as indoor air temperature, decrement factor, and time lag as well as a qualitative discussion on the need for occupant adaptation, namely closing or opening of inlet vent and requirement of additional heating. The assessment and subsequent recommendation of straw insulation utilization is done for representative summer and winter days of the five different climatic zones of India. A maximum reduction of up to 3.5 deg C in the peak indoor air temperature is achievable with inlet temporarily closed when straw insulation is included in the envelope in summer conditions. In winter conditions of cold climatic zone, with inlet always closed, use of straw insulation leads to the lessening of additional active heat source requirement by as much as 600 W for the considered model room. Further, the effects of increased straw insulation thickness, removal of heat source, varying inlet air velocity, and changing the position of the straw insulation to the outside are also analysed for the extreme warm and cold conditions.