Unvented Flat Roof Insulation

Two potential unvented roof assemblies.

Unvented flat roof insulation.

However architects of these compact roof assemblies must consider vapor permeance and arrangement of the materials to produce acceptable hygrothermal performance and low risk of condensation in the resulting conditioned attic space. Insufficient fill below the impermeable insulation layer. Minimum of felt paper should be installed as the primary water barrier under the roofing. Two layers are preferable to one layer due to increase in air flow resistance.

Control rain in any roof assembly an underlayment e g. If you don t want to use sips or nailbase there are four basic approaches. When hvac equipment and ducts with significant air leakage inhabit the attic much greater energy savings are possible as that leakage is now contained. 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.

Layers of rigid insulation have staggered joints to facilitate airtightness. R 46 unvented roof assembly two layers of r 6 5 inch rigid insulation yielding a total combined thickness of 2 5 inches. Low slope and steep slope unvented roof systems i e. Four options for building an unvented roof assembly.

If air gaps are present between insulation layers convection currents can significantly reduce the performance of the assembly and compromise durability. 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 roof assemblies should be insulated either with air impermeable insulation rigid foam or spray polyurethane foam or with a combination of foam and air permeable insulation. Monolithic continuous exterior insulation ci above structural sheathing.

Most unvented attics incorporate spray polyurethane foam spf on the underside of the roof deck and attic walls. An unvented roof assembly is constructed without any ventilation and is fully sealed on all sides and filled with insulation.