Give heat a temperature difference
In winter, a warm inner wall face and cool outer face produce outward heat flow. In summer, a hotter outside reverses the direction. If both faces have the same temperature, the modeled net flow stops.
How can insulation keep warmth in—and heat out?
Open a house wall. Swap its added layer, make it thicker, and reverse the temperatures to see why insulation helps in winter and summer.
Enable JavaScript to change the conditions and run the interactive experiment.
Make a discovery
Ordinary passive insulation resists heat transfer. It supplies no heat of its own. Heat still moves from warmer to cooler places, but a more resistant wall transfers less for the same temperature difference.
Make a prediction
If both wall faces are 20°C, what does adding insulation do to net heat flow here?
- It makes heat flow outward
- It keeps the net heat flow at zero
- It generates warmth
Read the explanation
With no temperature difference, this model has no net conduction. Passive insulation does not generate heat.
Understand it
Add a layer that resists transfer
A material’s thermal conductivity describes how readily it conducts heat. Making a layer thicker increases its thermal resistance. The same thickness of generic insulation resists conduction more than the brick comparison here.
See a familiar application
House walls, insulated bags and thermos flasks all limit heat transfer, but their constructions differ. A real thermos also limits radiation and convection. This wall experiment isolates conduction through flat layers.
Look closer at the science
Layer resistances add
Area-normalized resistance is R=L/k in m²·K/W. The 10 cm base brick layer and added layer are in series, so their resistances add. For the 1 m² calculation area, Q̇=A(T_inner−T_outer)/R_total.
Know the boundary temperatures
Controls set wall-face temperatures, not indoor/outdoor air temperatures. The model omits surface convection, radiation, framing, windows and drafts. It describes a settled one-dimensional heat flow, not how quickly a room cools.
Metric and US R-values differ
One US R unit equals 0.1761102 m²·K/W. The metric display gives RSI; imperial gives US R. The representative conductivities, 0.7 W/(m·K) for brick and 0.07 for generic insulation, are textbook examples, not product specifications.
Try it yourself: Keep cold water cold for longer
Supplies
- Two matching cups and equal covers
- Dry cloth or bubble wrap
- Cool water from one mixed batch
- Thermometers, timer, tray and towel
- Change only the wrapping
Wrap one cup and leave the other bare. Give both the same cover and place them side by side away from sun and drafts.
- Start together
Add equal amounts of cool water without ice. Measure each starting temperature promptly. Keep thermometers and lids arranged the same way.
- Compare changes
Record both temperatures every two minutes for about 20 minutes. Compare changes from each cup’s own start. Repeat with thicker wrapping if the difference is hard to distinguish.
Will wrapping a cup slow its warming?
Cold water only; no boiling water or building-insulation fibers. Small differences, probe accuracy and room conditions can obscure the trend. Touch alone is not a reliable thermometer; this does not measure k or a whole house.
Sources and model limits
- Steady one-dimensional conduction through equal-area homogeneous layers with constant properties and perfect contact.
- The house is context; watts apply only to the stated 1 m² wall-face section. No whole-building energy, heating-bill or construction-code advice.
- Arrows show net heat-transfer direction and comparative magnitude, not physical heat particles or a transient cooling simulation. Electrical insulation is a separate property.
Series conduction resistances add; thickness and conductivity determine each resistance.
Thermal Resistance Circuits gives representative brick k=0.7 and insulation k=0.07 W/(m·K).
MIT · Thermal resistanceA slab model requires specified boundary temperatures and assumptions.
Steady, one-dimensional, constant-property conduction without internal generation.
MIT · Steady conductionUS thermal insulance converts to SI by 0.1761102.
SI Appendix B.9, thermal insulance.
NIST · Unit conversionInsulated and control cups can be compared by water-temperature change.
Published activity allows a cold-water-only investigation. The simplified adaptation below has not been trialed.
NASA/JPL · Mars ThermosIndependent subject review is pending.
Read the sources and model assumptions
