Unvented Flat Roof Insulation

Two layers are preferable to one layer due to increase in air flow resistance.

Unvented flat roof insulation.

Low slope and steep slope unvented roof systems i e. R 46 unvented roof assembly two layers of r 6 5 inch rigid insulation yielding a total combined thickness of 2 5 inches. Control rain in any roof assembly an underlayment e g. An unvented roof assembly is constructed without any ventilation and is fully sealed on all sides and filled with 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. Two potential unvented roof assemblies. 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.

Most unvented attics incorporate spray polyurethane foam spf on the underside of the roof deck and attic walls. If you don t want to use sips or nailbase there are four basic approaches. Closed cell medium density spray polyurethane foam ccspf insulation can be used to create an unvented roof assembly for any climate zone. 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.

Monolithic continuous exterior insulation ci above structural sheathing. Insufficient fill below the impermeable insulation layer. Sc historically typical construction of homes in canada and the us have vented attics between the ceiling of the interior space and the roof. When hvac equipment and ducts with significant air leakage inhabit the attic much greater energy savings are possible as that leakage is now contained.

If air gaps are present between insulation layers convection currents can significantly reduce the performance of the assembly and compromise durability. Minimum of felt paper should be installed as the primary water barrier under the roofing.