Exterior Roof Insulation Retrofit (Cathedral Ceiling) Rigid Foam

The toughest details are those that have to match up with someone else's work, or those done — even done well — when energy was really cheap. These details are a collection of some common — and tough — dovetails of existing work with retrofits or additions. Bear in mind that green remodeling means creating a new operating regime that is better, not worse, than the one that may well have been working just fine before. Integration of energy efficiency, moisture management, and indoor air quality is much more important and challenging in remodeling than in new construction.

Where Roofs Meet Walls is a Critical Connection

Corners and connections are where insulation and air barriers can have trouble. Compressed or insufficient insulation can cause cold spots, which lead to condensation, mold, and rot. Air leaks at this connection can cut the effectiveness of the insulation substantially. In cold climates, this is where ice dams begin.

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
Moisture from both outside and inside a house can thwart your best efforts at keeping the building dry. Moisture in roof and wall assemblies is inevitable, so it's a good idea to design them so that they can dry. Roofs and walls that can dry to either the outside or inside are good, but those that can dry both directions are even better.

  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.

Unvented roofs can perform well as long as they are properly detailed to limit moisture transfer from the interior. Construction details vary depending on climate, but closed-cell spray polyurethane foam (specifically allowed by Section R806.4 of the International Residential Code) can be used anywhere.

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

DRAWING DETAIL

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2.
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.


1.
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.