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Retrofitting Rooftops: The Overcoat Method

NORTHERNSTAR BUILDING AMERICA Partnership has been investigating the use of an external insulation retrofit approach for existing houses under the “Project Overcoat” label. The overall design intent is to put thermal, air and moisture control layers over the exterior of an existing building, including the foundation, wall and rooftops. The approach uses a peel-and-stick membrane for an air barrier and layers of rigid foam for a thermal barrier to provide exceptional thermal and air barrier alignment and continuity in older homes with simple geometries. NorthernSTAR found the approach worked well for roof retrofits in 1-1/2-story homes, which are difficult to insulate under the roof due to lack of space, and often have serious air leakage problems that can lead to ice dams.

There are several strategies for applying a roof overcoat:

  • External thermal moisture management system (ETMMS)
  • Spray foam
  • One-sided structural insulated panel (SIP) overlay (often called a “retrofit” panel)
The ETMMS method is most common. A study by Building Science Corporation (BSC) for Building America on high R-value roofing assemblies for new construction promotes unvented, R-60 roof insulation achieved by combining two layers of two-inch rigid polyisocyanurate foam on the exterior of the roof sheathing with netted fibrous fill blown into the rafter bays (for cold climates). Optimization analysis of the strategy indicates source energy savings above 50 percent and whole-house leakage reductions of 20 to 60 percent when the roof alone was retrofitted.

General Recommendations
Venting: Researchers at BSC recommend that if the ground snow load is greater than 50 lb/ft², the roof should be vented.

When to Spec a Roof Overcoat

Existing homes do not always have the flexibility of design necessary for optimizing energy efficiency and durability because of barriers to effective air sealing, insulation and ventilation. In these cases, an overcoat strategy is the best choice:

- The truss structure cannot be changed to accommodate an appropriate insulation depth.

- Lack of industry consensus on roof venting may lead to the creation of unvented roof decks that should be vented.

- The presence of ceiling penetrations, such as those from recessed lighting, limits opportunities for effective air sealing.

- Existing homes where homeowners wish to live in the attic space limit interior strategies.

- Increased cost in either approach is driven by the complexity of the roof and the need to maintain architectural or historical features.

Venting the upper roof deck may also be necessary when overhangs are present. Wall claddings can absorb solar radiation and release heat that can get trapped under the overhang. This heat, even though produced on the exterior of the building, can melt snow and result in ice dam formation.

Alternative materials recommended to lower costs: A liquid applied membrane could be used in place of the typical peel-and-stick membrane.

Mineral wool could be used in place of rigid foam exterior insulation.

Because these materials are not typically used for roofing, constructability and durability would have to be researched before the materials could be recommended as replacements.

Furring strips: Some rigid foam board manufacturers make their products with built-in furring strips that could be customized to extend beyond the face of the foam board. This would result in a built-in ventilation area between foam board and roof sheathing.

SIPs: There is a growing use of SIPs for roof retrofit applications. SIP manufacturers are also modifying their products by removing one layer of sheathing so the foam face can be applied to existing roof sheathing. These revised panels are often called nailbase panels or retrofit panels. In response, classes are available to teach proper installation methods, including effective air sealing, along with post-construction air quality and combustion safety.

Resource: Project Overcoat—An Exploration of Exterior Insulation Strategies for 1½-Story Roof Applications in Cold Climates