How to Size Heating, Cooling and Insulation

Comfort at home is three separate questions — how much cooling, how much insulation, how much fuel — and each has one counter-intuitive fact sitting in the middle of it.

Air conditioning: bigger is worse

The Air Conditioner BTU Calculator starts from roughly 20 BTU per square foot and applies the ENERGY STAR adjustments for sun, occupancy and room type. It also carries a warning, and the warning is the important part: the mistake everyone makes is buying too big.

This surprises people, because with most equipment oversizing is merely wasteful. With air conditioning it actively works worse. An oversized unit cools the air to the thermostat's target very quickly and shuts off — before it has run long enough to pull moisture out. You get a room that is cold and clammy, cycling on and off, wearing the compressor.

Size it to the room. "A bit extra to be safe" is the failure mode here, not the safety margin.

Insulation: does it physically fit?

The Insulation R-Value Calculator answers how many inches your target R needs — and, more usefully, whether it fits.

The concrete example it carries: a 2×4 cavity is 3.5 inches, and standard batt cannot reach R-13 in it. No amount of pushing changes that, and compressing insulation to fit reduces its R-value, so forcing it in makes the problem worse rather than better.

If the target does not fit the cavity, that is a real answer. It means a different product, or a different assembly — continuous exterior insulation, a deeper stud, something structural — not more of the same material squeezed harder.

Firewood: one word, three amounts

The Firewood Cord Calculator exists because of a genuine ambiguity in how firewood is sold.

A cord is 128 cubic feet. A "face cord" of 16-inch logs is exactly a third of that. So the same word — cord — can mean three times the wood depending on who is saying it, and "half a cord" from one seller and another may be completely different quantities.

Establish the stack dimensions and the log length before agreeing a price. This is the tool to have open during the phone call, not after the delivery.

What it costs to run: watts, hours, your rate

Sizing tells you what to buy. The Appliance Running Cost Calculator tells you what it costs once it is in, and the arithmetic is small enough to check by hand: watts ÷ 1000 × hours = kilowatt-hours, then × your rate.

Two things it deliberately refuses to assume. The rate is yours — take the electricity portion of your bill and divide by the kilowatt-hours it covers, which is usually higher than the tariff's headline unit price. And the label wattage is a maximum, not an average: anything thermostatted cycles, so a 1,500 W heater holding a warm room may draw full power only a third of the time. Enter the hours it is actually heating.

⚠️ It costs electricity only. A gas boiler or gas dryer burns fuel priced entirely differently and its electrical draw is just the fan and controls, so this will make it look nearly free to run.

The short version

1. Size the AC to the room. Oversizing gives you a cold, damp, short-cycling room.
2. Check your R-value fits the cavity before buying — a 2×4 bay will not take R-13 in standard batt.
3. Never compress insulation to make it fit.
4. Agree firewood by dimensions, not by the word "cord".

Each tool has a printable carrying its own assumptions — the firewood one is worth having in front of you when a supplier quotes a price per "cord".