Field Testing Unvented Roofs with Asphalt Shingles in Cold and Hot-Humid Climates

Stacey Rothgeb, Kohta Ueno, Joseph Lstiburek, Stacey Rothgeb (NREL Technical Monitor)

Research output: NRELSubcontract Report


Insulating roofs with dense-pack cellulose (instead of spray foam) has moisture risks, but is a lower cost approach. If moisture risks could be addressed, buildings could benefit from retrofit options, and the ability to bring HVAC systems within the conditioned space. Test houses with unvented roof assemblies were built to measure long-term moisture performance, in the Chicago area (5A) and the Houston area (2A). The Chicago-area test bed had seven experimental rafter bays, including a control vented compact roof, and six unvented roof variants with cellulose or fiberglass insulation. The interior was run at 50% RH. All roofs except the vented cathedral assembly experienced wood moisture contents and RH levels high enough to constitute failure. Disassembly at the end of the experiment showed that the unvented fiberglass roofs had wet sheathing and mold growth. In contrast, the cellulose roofs only had slight issues, such as rusted fasteners and sheathing grain raise. The Houston-area roof was an unvented attic insulated with spray-applied fiberglass. Most ridges and hips were built with a diffusion vent detail, capped with vapor permeable roof membrane. Some ridge sections were built as a conventional unvented roof, as a control. In the control unvented roofs, roof peak RHs reached high levels in the first winter; as exterior conditions warmed, RHs quickly fell. In contrast, the diffusion vent roofs had drier conditions at the roof peak in wintertime, but during the summer, RHs and MCs were higher than the unvented roof (albeit in the safe range).
Original languageAmerican English
Number of pages124
StatePublished - 2015

Bibliographical note

Work performed by Building Science Corporation, Westford, Massachusetts

NREL Publication Number

  • NREL/SR-5500-64543

Other Report Number

  • DOE/GO-102015-4705


  • ASHRAE 160
  • asphalt shingles
  • BSC
  • Building America
  • cathedralized roof
  • cellulose
  • conditioned attic
  • dense-packed cellulose wall insulation
  • insulation
  • inward drive
  • residential
  • residential buildings
  • roof insulation
  • roof retrofit
  • unvented attics


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