Unvented Roof Insulation

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Unvented roof insulation.

In the most recent issue of fine homebuilding martin holladay wrote a piece on insulating unvented roof assemblies. Unvented roofs vented roof. However designers of compact roof assemblies must consider vapor permeance and arrangement of the roofing materials to produce acceptable hygrothermal performance and low. Unvented roofs have a number of advantages.

As a result unvented roofs have become popular in homes with feature dense roofs think multiple skylights dormers and other aesthetic flourishes. Many local building codes do not account for non standard construction alternatives such as unvented attic assemblies. With rigid insulation in unvented roof assembly there is no potential for condensation until interior moisture levels exceed 45 rh at 70 f during the coldest month of the year if r 16 rigid insulation is installed oever a rafter cavity insulation to r 30. An unvented assembly can perform well but it s important to get the details right to avoid sheathing rot.

To provide an unvented conditioned space for locating hvac equipment in the attic and or to provide a continuous thermal barrier for designs that have complex coffered ceiling planes and or numerous penetrations for lights speakers vents. Install insulation along the underside of the roof deck of an unvented attic rather than on the ceiling deck of a vented attic for either of two reasons. Enhancing roofing material life span by reducing sheathing temperature. Potential for condensation in washington d c.

Controlling ice damming by keeping the roof cold. Removing moisture from roof cavities structural members sheathing and insulation. Ventilation becomes more difficult the more complicated a roof s construction is. We agree that avoiding sheathing rot is critical and that air permeable insulation such as fiberglass or cellulose cannot be used alone.

Insulating unvented roof assemblies. Senior engineer manager at icynene john broniek makes the case for the unvented attic assembly and why insulating the underside of the roof makes sense. Also unvented roof assembles make complicated roof geometries more viable as they are difficult to ventilate effectively. These assemblies are created by eliminating ventilation openings and moving the thermal moisture and air control boundaries to the plane of the roof deck.

The quest for greater energy efficiency in buildings has led to increasing interest and need for insulation improvements in hvac equipment more airtight construction new materials and assemblies. 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. There are traditionally four main reasons for venting a roof. 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.