Heating & Insulation calculators
BTU sizing, R-value and how much insulation a space needs.
What is in this category
Two calculators for comfort rather than surfaces: the BTU calculator sizes heating and cooling for a room, and the insulation calculator works out how much insulation an attic or wall needs to reach a target R-value.
They belong together because they are two halves of the same problem. Insulation decides how much heating and cooling a space needs; the BTU figure tells you what to buy once you have decided.
Bigger is not better
The instinct with air conditioning is to oversize, and it backfires specifically. An oversized unit cools the air quickly, hits the thermostat setting and shuts off — before it has run long enough to pull moisture out of the air. The room ends up cold and clammy, the compressor short-cycles, and both comfort and equipment life suffer.
Undersizing has the obvious failure mode of never quite getting there. Sizing it correctly is worth the ten minutes, and a rough figure to sanity-check against is roughly 20 BTU per square foot for cooling in an average, reasonably insulated room — adjusted up for sun, high ceilings, a kitchen or a lot of glass.
R-value is per inch, and it adds up
R-value measures resistance to heat flow, and it is cumulative: four inches of something rated R-3.5 per inch gives you R-14. That is why the insulation calculator asks what is already up there before telling you how much to add — topping up existing insulation is almost always cheaper than replacing it.
| Material | R per inch |
|---|---|
| Blown cellulose | 3.2 – 3.8 |
| Blown fibreglass | 2.2 – 2.7 |
| Fibreglass batt | 3.1 – 3.4 |
| Rigid foam (XPS) | 5.0 |
| Closed-cell spray foam | 6.0 – 7.0 |
Recommended attic levels run from about R-30 in the far south to R-60 in the north. Your local energy office publishes the figure for your zone.
Air sealing comes first
This is the part most people skip, and it is worth more per dollar than the insulation on top of it. Insulation slows heat moving through materials. It does nothing about air moving through gaps — around recessed lights, at the top plates of walls, around the attic hatch, up the chase behind a chimney.
Blowing insulation over unsealed gaps buries the problem without fixing it, and in a cold climate the warm moist air escaping through them condenses in the insulation and wets it, which makes it perform worse. Seal first, insulate second.