There are three retrofit assemblies in the guide for the attic/roof: vented attic with insulation and air sealing at the attic floor (Attic 1), unvented attic with exterior roof insulation (Roof 1 or Roof 1 Alternate), and unvented attic with interior-only roof insulation. (Roof 2 or Roof 2 Alternate)

Before selecting the retrofit assembly to use, one must decide whether the attic will be vented or unvented. In cold climates, the primary purpose of a vented attic is to prevent ice dams caused by heat loss from conditioned space and to ventilate moisture out of the attic space. With careful detailing of the air, vapor and thermal control layers for the attic, these two potential problems can be minimized so that the decision for a vented or unvented attic can be made for functional reasons instead.

If the attic will be used for habitable space, contains mechanical equipment or ductwork, or the roof structure is complex, an unvented attic assembly should be considered. Another consideration is that the roof structure may not provide sufficient space for required insulation needed for the attic retrofit assembly, especially at the eaves, where insufficient insulation can contribute to ice damming.

The best performing attic/roof retrofit assembly is the unvented attic assembly with exterior roof insulation - Roof 1. This assembly locates most of the control functions to the exterior so that it is easy to correctly establish continuous air, vapor and thermal control layers. However, since this is applied over the existing roof sheathing, this assembly is only appropriate for DER projects that include replacement of the existing roofing. The other unvented attic retrofit assembly, Roof 2, applies all of the insulation to the interior side of the roof. For this assembly, as well as for the vented attic assembly, replacement of the roofing is not required.

If the DER project includes, or will include in the future, an upgrade to the exterior walls, an extension of the overhangs at the eaves and/or at the rakes may be needed because retrofit walls are thicker than standard construction. If this extension lowers the elevation of the soffit at the eaves, it may conflict with the existing window head heights or head trim. This needs to be taken into account during planning for the overall DER. Typically, this situation can be avoided if the roof is raised several inches, as is the case when the retrofit Roof 1 assembly is used.

In the drawings, the roof cladding is represented as shingles, but other roof claddings would be acceptable provided that the attachment of the roof cladding does not result in horizontal obstructions on the water control layer beneath the cladding.

ATTIC 1: VENTED ATTIC WITH INSULATION AND AIR SEALING AT ATTIC FLOOR

• This assembly should not be used if there is ductwork or mechanical equipment in the attic.
• This assembly requires that there be sufficient height (typically at least 81/2") between the underside of the roof deck and the top plate at the eave to maintain the air gap for ventilation and insulation with an R-value of at least R-40.
• If there is any history or evidence of leakage at the roof, this must be corrected since the assembly depends on the existing roof for the water control function.
• If existing bath fans vent into the attic, these must be vented to the outside.
• Provide attic ventilation per the applicable building code. If the vent area cannot be split evenly between the low and high vents, it is better to have greater vent area low than high.

ROOF 1: UNVENTED ATTIC WITH INSULATION EXTERIOR TO ROOF SHEATHING

• Existing roofing and roof underlayment should be removed.
• If the roof rafters bear directly on the exterior walls, a "chainsaw" retrofit approach is recommended for this assembly.
• If the exterior walls are part of the DER project now or in the future and the eave and/ or rake overhangs do not extend far enough to protect a DER wall, overhangs should be extended as part of the DER roof assembly.
• If the existing rafter cavity includes closed-cell spray foam insulation, the existing roof sheathing should have a moisture content of 12% or lower before the exterior insulating sheathing is applied.
• If closed-cell spray foam is to be applied in the rafter cavities, it should be applied after the exterior insulation and water control is in place and the roof sheathing must have a moisture content of 12% or lower.
• In locations with ground snow load of 50 psf or greater, Roof 1 Alternate assembly should be used instead of the Roof 1 assembly.

ROOF 1 ALTERNATE: ROOF 1 ASSEMBLY WITH VENTED ROOF

• This is the same as Roof 1 except that roof ventilation is provided by placing "vertical purlins" (parallel to the rafters) between the exterior insulation and the new roof sheathing.
• This assembly can be used if vented roof cladding is desired.
• To prevent ice damming, this alternate assembly is required in locations for which the ground snow load is 50 psf or greater.
• Also see notes for Roof 1

ROOF 2: UNVENTED ATTIC WITH INSULATION INTERIOR TO ROOF SHEATHING

• This assembly requires that there be sufficient height (typically at least 61/2") between the underside of the roof deck and the top plate at the eave for closed-cell spray foam insulation with a total R-value of at least R-40.
• If there is any history or evidence of leakage at the roof, this must be corrected since the water control function is provided by the existing roofing and roof underlayment.
• In order to achieve R-60, the rafters will need to be encapsulated by the spray foam for most existing homes.

ROOF 2 ALTERNATE: ROOF 2 WITH STRAPPING FOR CEILING BOARD ATTACHMENT

• This is similar to Roof 2 except that it supports a sloped ceiling or use of a gypsum board as the thermal/ignition barrier.
• Also see notes for Roof 2.