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: 2x4 studs, 16 inches
         o.c., with inset-stapled kraft-faced fiberglass batts;
  • Wall 3: 2x4 studs, 16 inches
         o.c., with face-stapled kraft-faced fiberglass batts;
  • Wall 4: 2x4 studs, 16 inches
         o.c., with damp-sprayed cellulose;
  • Wall 6: 2x4 studs, 16 inches
         o.c., with R-13 open-cell spray polyurethane foam;
  • Wall 6: 2x4 studs, 16 inches
         o.c., with R-13 closed-cell spray polyurethane foam;
  • Wall 7: 2x4 studs, 16 inches
         o.c., with R-13 friction-fit fiberglass batts plus 1-inch-thick exterior
         XPSExtruded polystyrene.
  • Wall 8: 2x6 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.

 

The findings are shown below.

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 any air flow. The heat flow through the open-cell spray foam
wall was only 16% higher, and the heat flow through the closed-cell spray foam
wall was only 23% higher, than when the same walls were tested without any air
flow.

The findings are shown below.

Summary:

  • 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: http://www.buildingscienceconsulting.com/presentations/documents/TM%20Research%20Project%20Summary_rev.pdf