How insulation slows heat through walls

Heat always flows from warm to cold, and a wall can only slow it down. Insulation traps still air in fibers or foam, which conducts heat poorly, and its resistance is rated as an R-value. The heat flow through a wall is its area times the indoor to outdoor temperature difference, divided by the R-value, so doubling R halves the heat loss.

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Wall R-value

US units: ft²·°F·h/Btu. An uninsulated wall is about R-4.

Indoor minus outdoor temperature

Wall area

Step through it

Warm sideHeat produced inside the room moves toward the colder outdoors. Every layer of the wall slows it a little.
Insulation layerFiberglass, mineral wool or foam fills the space between studs. Its trapped air resists heat flow far better than wood or drywall.
A barely insulated wallWith R-4, about what an empty wall cavity gives, heat races through and the heater has to run constantly.
Thicker insulationAt R-30 the same wall leaks less than a seventh as much heat. Adding R gives the biggest savings when starting from a poorly insulated wall.
Colder weatherHeat flow scales with the temperature difference, so the same wall loses more on a frigid night.

The heat flow equation

Steady heat flow through a wall is Q = A x delta T / R. A is area in square feet, delta T is the temperature difference in °F and R is the total thermal resistance in ft²·°F·h/Btu. The U-factor is simply 1/R, the heat flow per square foot per degree.

This is a simplified model of one uniform wall. Real walls lose extra heat through wood studs (thermal bridging), windows and air leaks, which is why energy codes now often call for continuous insulation outside the studs.

Try it: start at R-1 and step up to R-5, R-10, R-20, then R-40. The first few steps cut heat flow dramatically, while the last doubling saves far fewer Btu per hour.

Diminishing returns

Because R sits in the denominator, each added R saves less than the one before. Going from R-1 to R-5 saves far more energy than going from R-20 to R-40, which is why recommended R-values depend on climate and on what is already in the wall.

Sources

The Importance of Residential and Commercial Building InsulationOklahoma State University Extension
Wall Upgrades for Residential Deep Energy Retrofits: A Literature Review (PNNL-28690)U.S. DOE Office of Scientific and Technical Information
Guide to Home InsulationU.S. Department of Energy

How to cite this model

Avid Solutions International. (2026, November 11). How insulation slows heat through walls [Interactive 3D model]. Avid Field Lab. https://avidsolutionsintl.com/explore/wall-insulation

Last updated November 11, 2026

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