What Insulation Actually Does

Heat leaves a house three ways

Conduction - heat moving through a solid. Warm drywall passes heat to the stud behind it, the stud passes it to the sheathing, and so on to the outside. This is the one R-value measures.

Convection - heat carried by moving air. Warm air escapes through a gap and cold air is drawn in behind it. R-value does not measure this at all, and in an old Maine house it is often the larger loss.

Radiation - heat moving as infrared, no contact required. Matters most in summer, from a hot roof deck downward.

The clearest example is a bob house

Every winter there are whole villages of them out on the New Hampshire lakes. A bob house is a plywood box with a propane bottle outside it and nothing at all in the walls. Light the heater and it is warm almost immediately. Shut it off and it is cold again almost as fast. Nobody insulates one, because nobody needs it to hold heat until morning.

That is what makes it such a clean demonstration. All three losses above are running at once, none of them is being slowed by anything, and you can feel the result in minutes rather than reading it off a bill.

A house is the same problem stretched out. It is bigger, it has more mass to hold heat, and the burner runs off a thermostat rather than a knob, so the losses are harder to notice. The heating system hides them by replacing heat at roughly the rate it leaves. The fuel bill is where the difference shows up instead.

Where spray foam is different

Foam goes in liquid and expands into every gap before it hardens, so it handles conduction and convection in the same pass. There is no separate air-sealing step and no seam for air to find.

That is the whole idea. Everything else on this site is a detail of it. Here is how the job runs, and here is which foam goes where.