HVAC & insulation calculators

Heating, cooling and insulation

The BTU calculator estimates cooling or heating capacity for a single room or small space from its floor area, ceiling height, insulation, sun exposure and occupants, and converts the result to tons of cooling (one ton is 12,000 BTU per hour). The insulation calculator turns an area and a target R-value into batt packages or bags of blown-in insulation.

A common starting point for a room air conditioner is about 20 BTU per hour per square foot of floor for larger spaces; small rooms need more (ENERGY STAR lists 5,000 BTU per hour for 100 to 150 square feet). Adjust up for sunny rooms, kitchens and extra people and down for shaded rooms. That is a room-level estimate. Whole-house heating and cooling equipment should be sized with an ACCA Manual J load calculation, which the residential code calls for when new equipment is installed; oversized equipment short-cycles and removes humidity poorly.

Insulation targets depend on climate zone. The R-value by climate zone chart lists the 2021 IECC minimums for ceilings, walls and floors; for attics that means R-30 in zone 1, R-49 in zones 2 and 3, and R-60 in zones 4 through 8. Allow 5 to 10% extra batts for cutting around framing, wiring and blocking. For blown-in insulation, follow the coverage chart on the bag, which gives bags per 1,000 square feet for each R-value.

Installing or recharging a split system involves refrigerant, which requires EPA Section 608 certification, so that work belongs with a licensed HVAC contractor. Insulation and air sealing are good DIY projects as long as you keep clearances around recessed lights, flues and chimneys.

How each calculator is specified, tested and corrected is described on our methodology page.