Air Leakage Degrades Thermal Performance of Walls

A Building Science Corporation (BSC) report compares the insulation performance of a wall when air is leaking through it with the performance of the wall without air leakage.

Insulation performs differently at different temperatures

Researchers have long known that insulation materials behave differently at different temperatures. Fiberglass batt performs better at low temperatures than at high temperatures.  Since there is more radiation at high temperatures, the performance is degraded; the same thing happens with expanded polystyrene, but not all materials.

Description of the wall assemblies

BSC tested the performance of seven wall assemblies.

  • Wall 2: 2×4 studs, 16 inches      o.c., with inset-stapled kraft-faced fiberglass batts;
  • Wall 3: 2×4 studs, 16 inches      o.c., with face-stapled kraft-faced fiberglass batts;
  • Wall 4: 2×4 studs, 16 inches      o.c., with damp-sprayed cellulose;
  • Wall 6: 2×4 studs, 16 inches      o.c., with R-13 open-cell spray polyurethane foam;
  • Wall 6: 2×4 studs, 16 inches      o.c., with R-13 closed-cell spray polyurethane foam;
  • Wall 7: 2×4 studs, 16 inches      o.c., with R-13 friction-fit fiberglass batts plus 1-inch-thick exterior      XPSExtruded polystyrene.
  • Wall 8: 2×6 studs, 16 inches      o.c., with R-21 friction-fit fiberglass batts.

The walls were built with a few deliberate air leaks. Each tested wall had three electrical boxes with Romex wiring between the boxes. Shims were used to introduce a 1/32 inch gap between the OSB sheathing. While this decision may sound odd, the deliberate gaps were included so that the walls would leak about as much air as “typical” unsealed walls seen in the field.

Each of the seven wall assemblies was tested at five outdoor temperatures (-18°F, 0°F, 35°F, 108°F, and 144°F) without any air leakage through the walls. Then they were re-tested at two outdoor temperatures (0°F and 108°F) with two types of air leakage at each temperature (first with 10 Pascal of infiltration, and then with 10 Pascal of exfiltration).

Researchers’ findings

When the walls were sealed and tested without air flow, they had approximately the same heat flow (within 4%).

When the walls were tested at a 10 Pascal air pressure difference, they no longer behaved the same.  The heat flow through the fiberglass insulated walls was about 35% higher than when it was tested without air flow. The heat flow through the open-cell spray foam was only 16% higher, and the heat flow through the closed-cell spray foam wall was only 23% higher.


  • Stopping air leaks is more      important than adding insulation. Unless builders prevent air from leaking      through walls and ceilings, insulation alone won’t do much good.
  • If you choose an insulation      that doesn’t stop air flow, it’s important to install an adjacent air      barrier.
  • Fiberglass-insulated homes      are the leakiest.

For BSC report:

For more information and charts:



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