Unvented Flat Roof Insulation

Layers of rigid insulation have staggered joints to facilitate airtightness.

Unvented flat roof insulation. Most unvented attics incorporate spray polyurethane foam spf on the underside of the roof deck and attic walls. 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. R 46 unvented roof assembly two layers of r 6 5 inch rigid insulation yielding a total combined thickness of 2 5 inches. Four options for building an unvented roof assembly.

Sc historically typical construction of homes in canada and the us have vented attics between the ceiling of the interior space and the roof. An unvented roof assembly is constructed without any ventilation and is fully sealed on all sides and filled with insulation. If air gaps are present between insulation layers convection currents can significantly reduce the performance of the assembly and compromise durability. Low slope and steep slope unvented roof systems i e.

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. Minimum of felt paper should be installed as the primary water barrier under the roofing. When hvac equipment and ducts with significant air leakage inhabit the attic much greater energy savings are possible as that leakage is now contained. 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.

Two layers are preferable to one layer due to increase in air flow resistance. Control rain in any roof assembly an underlayment e g. 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. Two potential unvented roof assemblies.

Monolithic continuous exterior insulation ci above structural sheathing. In this post we will show that while the propink unvented insulation system may provide a safe roof in zones 2b 3b if we instead combine dense fiberglass insulation batts or densepacked with intello smart vapor retarder and airtight membrane we can make safe and robust unvented roofs in climate zone 6.