lnsulation or housewrap?

Sefl builders both

fS HOUSEWRAP necessary? Can Ibuilders construct energy efficient homes with just standard insulation? Can housewrap be used instead of typical insulation?

According to tests conducted for DuPont Tyvek by an independent laboratory, an energy-efficient wall system must have both insulation and housewrap to protect and maintain installed R-values.

Air moving inside and through walls reportedly can reduce the installed R-value up to 63Vo. During tests with an average wind pressure of 9 mph, the installed R-value of the wall system plummeted from R-14 to R-5 without housewrap. Under the same conditions, wall systems with housewrap maintained 94Vo of the installed R-value at R-13.

Air or wind that moves through seams or cracks into a wall cavity (windwashing) or through the wall

Story at a Glance

Tests show true energy efficiency requires both insulation and a housewrap.

itself (air infiltration) are two main causes of energy loss. Air flowing through seams and cracks to the inside of a home can contribute up to 40Vo of a home's energy loss.

Windwashing raises or lowers the inside temperature of walls, even if the wind never enters the home. When wall temperatures are low, occupants feel less comfortable and use more energy for heating. The reverse is true in cooling climates - more energy is used to keep walls cooler when warm air moves inside a wall cavity.

Help builders to see housewrap as an extension of the insulation.

Housewrap helps keep out wind, preserving both the heating and cooling performance of a wall's insulation. In a lost 48Vo of its installed R-value. At 9 mph wind pressure, the installed Rvalue dropped from R-18 to R-9. Under the same conditions with housewrap, the installed R-value remained at R-15.4. l4-home field study for Tyvek, the housewrap reduced air infiltration an average of29Vo.

An unwrapped 314" tongue-andgroove extruded polystyrene sheathing configuration lost 77% of its installed R-value, plunging from R-18 to R-4. With housewrap, the installed R-value was maintained at R-16.

Housewrap even improved the performance of an airtight drywall installation. An unwrapped wall assembly with l/2" extruded polystyrene sheathing sealed with an airtight drywall approach lost l87o of is installed R-value at a lO-mph wind pressure from windwashing. An unwrapped wall assembly with l/2" oriented strand board sheathing and airtight drywall lost 22Vo at l5-mph wind pressure. Under the same conditions, wrapping the home virtually eliminated any measurable loss of installed Rvalue from windwashing.

All tess were conducted with standard2x4 or 2x6 walls, 16" o.c., representing standard construction practices. The warm side was covered with l/2" gypsum board containing an electrical receptacle and light switch. The wall cavities were insulated with R-13 or R-19 inset stapled, Kraftfaced fiberglass bats.

The housewrap also showed a significant improvement in preventing loss of installed R-value compared to caulking. When the sole plate/subfloor interface was caulked, the wall still lost SlVo of the installed R-value falling from R-18 to R-9 with a 9 mph wind pressure. With housewrap, the wall system lost only 9% of its installed R-value, slipping from R-18 to R-16 under the same conditions.

Other results of the laboratory tests showed that housewrap improves wall system performance, regardless of sheathing type or the use of air-tight drywall.

An unwrapped wall system with a l/4" extruded polystyrene fanfold leveling board

"Even caulking the most commonly recognized leakage areas did little to prevent air from getting through," said DuPont research associate Dave Jones. "It is virtually impossible to seal every crack, crevice or seam with caulk. The air will always follow a path of least resistance and find a way to penetrate."

The impact of stopping air flow into a wall by using a housewrap could outweigh the advantage of adding more insulation to improve a wall's performance. And, with nearly a half mile of exterior seams on a typical 2,000-sq. ft. home, housewrap could eliminate excessive caulking during construction."