A batt of insulation is sold with a number on the bag. The effective resistance of the assembly differs from the batt rating: framing, other layers and junctions all affect the result.
Look at the framing layerOr watch it happen. Turn on the heat overlay and the escape routes are the framing, the header over the window, and the rim joist at the floor — every one of them a piece of solid wood spanning the insulated layer.
Watch heat take the easy way outNominal versus effective
Insulation is rated in isolation — a slab of material, heat flowing straight through it. A real wall is not a slab. Every 406 mm there is a stud, and wood conducts heat several times better than the insulation it sits next to. Around every window there is a header, a pair of king studs, jack studs, and a rough sill: a lot of solid wood in the exact places you would least like it.
Toggle to the Lifespan lens and you can see something else: the insulation is Skin, but the studs bridging it are Structure. One is straightforward to improve. The other is not.
Switch to the Lifespan lensWhere the heat actually leaves
Framing is only part of it. Other locations that can contribute to heat loss include:
- The rim joist at the floor edge, often uninsulated and unsealed
- The joint where the bottom plate meets the subfloor
- Penetrations — every box, pipe and duct through the exterior wall
The renovation that does not work
Insulation and air sealing need to be considered together. Batt insulation slows heat flow but does not form an air barrier. A wall that leaks air performs well below its label whatever you stuff into it — and in some assemblies, adding insulation without addressing air movement makes the wall wetter, because you have made the outer part of it colder.
What to take from this
When you read an insulation figure in a listing, it is describing a material, not a wall. The questions that actually determine what a house costs to heat are whether the assembly is continuous, whether it is sealed, and how much wood is bridging it — and the first two are usually cheaper to fix than the third.
Check yourself
Two identical walls are insulated to the same nominal value. One has studs at 406 mm on centre, the other at 610 mm. Which performs better, and why?