Insulate Unvented Roof

Four options for building an unvented roof assembly.

Insulate unvented roof.

Exterior insulation solves a lot of thermal nosebleeds but the amount of foam you put on the outside needs to be proportional with the amount of insulation on the inside within the rafter cavity. In the most recent issue of fine homebuilding martin holladay wrote a piece on insulating unvented roof assemblies. 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.

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. We re big fans of fine homebuilding. 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. 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.

We agree that avoiding sheathing rot is critical and that air permeable insulation such as fiberglass or cellulose cannot be used alone. If you don t want to use sips or nailbase there are four basic approaches. 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 soffits etc which make it difficult to achieve an airtight ceiling plane. Insulating the underside of roofs making the case for the unvented attic assembly background.

Don t stay stuck in the sci fi nightmare of foam foam foam. When hvac equipment is in the attic insulation and. Insulating unvented roof assemblies.