Exterior Roof Insulation Retrofit (Cathedral Ceiling) Rigid Foam


Where Roofs Meet Walls is a Critical Connection


To keep the air barrier continuous, span the wall sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. over the framing connection and use adhesive or sealants at framing connections as shown.

Roofs and walls need to dry

  1. Designing to dry out means doing two things well:
  2. 1. Choosing materials carefully—each layer affects the vapor profileA vapor profile is an assessment of the relative vapor permeabilities of each individual component in a building assembly and a determination of the assembly's overall drying potential and drying direction based on vapor permeabilities of all of the components. The vapor profile addresses not only how the building's enclosure assembly protects itself from getting wet, but also how it dries when it gets wet. For a detailed treatment of this subject, see Building Science Corporation's article Understanding Vapor Barriers. of the assembly.
  3. 2. Planning the construction to be forgiving—flashing keeps water out, and ventilation removes water vapor.


Exterior insulation keeps the framing warm and dry
By moving the insulation outside the framing, the chances of condensation are almost eliminated. Another benefit is that you can get a superinsulated roof without increasing the size of the rafters, or furring the framing down and encroaching on the living space.

For detailed information on this topic, read Unvented Roof Systems at BuildingScience.com

Learn more in the Green Building Encyclopedia

Enclosure overview
Exterior walls
Roofs: Attics, Structure, Claddings


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Aug 5, 2011 10:52 AM ET

Edited Aug 5, 2011 10:53 AM ET.

Vent below sheathing?
by Peter Yost

If we allow air to outside (unconditioned) air to circulate between the two insulation systems (the cavity fill and the exterior rigid foam), then we decouple them and significantly reduce the R-value of the assembly. If there is enough air flow from the venting to be effective for drying, then there is enough air flow to counter much of the R-value of the topside rigid insulation.

In this assembly, we would need to ensure that the cavity insulation and interior sheathing of the cathedral ceiling allowed/continued drying to the interior; the vapor profile of this assembly requires drying to the interior.

Aug 5, 2011 4:10 AM ET

Why not vent below the sheathing?
by Vincent Alvarez

Soffit vents allowing air to flow under the sheathing to ridge vents will reduce R-values some, but will help to dry and cool the roof.