CalcBuiltAll calculators

Insulation & HVAC Calculators

Insulation, BTU sizing and ventilation.

Heating, cooling and insulation are sized from area, climate and how much heat leaks through the envelope. The calculators here cover the two sides of that problem: how much insulation to buy for a given R-value, and how many BTUs of heating or cooling a space needs once it is insulated.

The sizing tools use the rules of thumb contractors quote (20 to 35 BTU per square foot for cooling, 30 to 60 for heating by climate zone) plus a simple heat-loss calculation for a room. They are for planning and comparing quotes; a Manual J load calculation by an HVAC contractor is how equipment is actually sized.

All insulation & hvac calculators

Insulation CalculatorCalculate how many packages and batts of fiberglass insulation you need for a wall, floor or attic from the area, framing spacing, R-value and coverage.BTU CalculatorCalculate how many BTUs of air conditioning a room needs from its size, climate, sun exposure and occupants to size a window unit or mini-split.Spray Foam CalculatorCalculate how many board feet of spray foam you need for a wall, ceiling or crawlspace, the R-value at that thickness, and how many 600 or 200 board foot kits to buy.Blown-In Insulation CalculatorCalculate how many bags of blown-in cellulose or fiberglass you need to bring an attic to R-38, R-49 or R-60, plus the depth to blow and the coverage per bag.Furnace Size CalculatorCalculate what size furnace a house needs in BTU per hour from its square footage, climate zone, insulation and ceiling height, matched to standard 80% or 95% AFUE sizes.Mini Split CalculatorCalculate what size ductless mini split head a room needs in BTU from its area, climate, sun, insulation and occupants, with sizing for winter heating and multi-zone use.Heat Loss CalculatorCalculate a room's heat loss in BTU per hour through walls, ceiling, floor, windows and air leakage from R-values, U-factor and design temperatures to size a heater.Attic Ventilation CalculatorCalculate the attic ventilation you need under the IRC 1/300 or 1/150 rule in square inches of net free area, split into soffit intake and ridge exhaust vents.Dehumidifier Size CalculatorFind what size dehumidifier you need in pints per day from the room size and how damp it is, using the Energy Star sizing guidance and the 2019 DOE test ratings.Water Heater Size CalculatorSize a water heater by first hour rating from the showers, baths, dishwasher and laundry loads in your busiest hour, with gas, electric and tankless recommendations.

A typical project, in order

Carry each result into the next tool.

  1. 1Insulation CalculatorBatts for walls and floors
  2. 2Blown-In Insulation CalculatorBags of cellulose or fiberglass for the attic
  3. 3Attic Ventilation CalculatorNet free area so the attic breathes
  4. 4Heat Loss CalculatorRoom heat loss once insulated
  5. 5BTU CalculatorCooling load for a room
  6. 6Furnace Size CalculatorHeating output for the whole house
  7. 7Mini Split CalculatorDuctless head size per zone

Insulation & HVAC questions

How many BTUs do I need per square foot?

About 20 BTU per square foot for cooling in a mild climate, 25 in a moderate one and 30 to 35 in a hot one. Heating runs 30 to 60 BTU per square foot depending on the climate zone.

What R-value should my attic have?

R-38 to R-60 for most of the US under DOE guidance. Blown cellulose at R-3.5 per inch needs about 11 inches for R-38.

Is a bigger furnace or AC better?

No. Oversized equipment short-cycles, wears faster and controls humidity poorly. Size to the load with a margin of 10 to 20%, not double.