Unvented Flat Roof

These assemblies are created by eliminating ventilation openings and moving the thermal moisture and air control boundaries to the plane of the roof deck.

Unvented flat roof.

In this case you can learn a lot from foam tech s article vented vs. Unvented roof assemblies such as conditioned attics and unvented cathedral ceilings are becoming common in north american construction. Basically the term cold roof refers to a traditional roof or vented roof while hot roof refers to a roof that is not ventilated and which has foam insulation attached directly to the roof. Controlling ice damming by keeping the roof cold.

This brings us full circle back to flat or low slope roofs being difficult to ventilate particularly if they re used as occupiable space such as a rooftop patio. An unvented assembly can perform well but it s important to get the details right to avoid sheathing rot. The unvented attic space is completely contained within the building thermal envelope. Condensing surface temperature controlled the thickness of the rigid insulation at both the roof deck and perimeter is determined by a calculation procedure similar to figure 5.

The higher the interior moisture level the thicker the. However designers of compact roof assemblies must consider vapor permeance and arrangement of the roofing materials to produce acceptable hygrothermal performance and low. 2009 irc section r806 4. There are traditionally four main reasons for venting a roof.

Unvented flat roofs a technical discussion flat roofs have often been built with the insulation all on top of the vapor closed drainage plane as this avoids dew point condensation issues inboard of the roofing. Unvented roofs vented roof. Removing moisture from roof cavities structural members sheathing and insulation. Unvented roof systems where the insulation and air barrier are applied directly to the underside of the roof deck are common in applications where designers are seeking cost effective and energy efficient roof assemblies.

Unvented flat roof assembly. In some of the above areas where vented attics can be a challenge an alternative solution is an unvented roof assembly. 2012 irc section r806 5 unvented attic assemblies unvented attic assemblies spaces between the ceiling joists of the top story of the roof rafters shall be permitted if all of the following conditions are met. 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.

However in many situations the insulation may need to be primarily or entirely below the flat roof membrane and between the roof joists if so what are the best options.