Unvented Roof With Fiberglass

Mapping out the roof assembly for a new house in climate zone 6b steve mackay has settled on long i joists insulated with a mix of closed cell spray polyurethane foam and blown in fiberglass.

Unvented roof with fiberglass.

He doesn t plan on venting the roof and he wants to be certain his design will be problem free. Unvented roof assemblies such as conditioned attics and unvented cathedral ceilings are becoming common in north american construction. Wood roofing panels endorsed by the apa usually contain an in service water content of between 4 and 16. Owens corning a leading manufacturer of fiberglass insulation is now promoting a package of products basically blown in fiberglass insulation and a new type of insulation netting to create unvented conditioned attics.

Air permeable insulation such as fiberglass batts dense packed cellulose or blown in fiberglass can t be used alone for an unvented assembly since these types of insulation can allow moist indoor air to reach the cold roof sheathing leading to condensation or moisture accumulation in the sheathing. Unvented roofs have a number of advantages. If you want to put air permeable insulation like fiberglass or cellulose against the roof deck between the rafters rigid insulation board must be placed on the exterior of the roof deck according to the provided r value ratio tables. Unvented attic assemblies spaces between the ceiling joists of the top story and the roof rafters and unvented enclosed rafter assemblies spaces between ceilings that are applied directly to the underside of roof framing members rafters and the structural roof sheathing at the top of the roof framing members rafters shall be permitted if all the following conditions are met.

These assemblies are created by eliminating ventilation openings and moving the thermal moisture and air control boundaries to the plane of. 14 inch i joists with 5 inches of closed cell foam sprayed on the underside of the roof desk. However if moisture is able to enter the unvented attic space it may lead to roofing panel issues. The amount of exterior insulation needed depends on the climate zone.

Ventilation becomes more difficult the more complicated a roof s construction is. Here s what it looks like. At these low levels the moisture has a negligible effect on the strength and structural integrity of the wood panels. Code pushes the envelope iecc vs irc ibc.

Owens corning calls this proprietary insulation method the propink high performance conditioned attic system. Cellulose is a good choice for the especially environmentally conscious as it is made from. Gutex thermofiber a blow in wood fiber insulation is a very ecological choice which is ideal. Insulating unvented roof assemblies.

As a result unvented roofs have become popular in homes with feature dense roofs think multiple skylights dormers and other aesthetic flourishes.