Appeal 18033

Section R806

SECTION R806 ROOF VENTILATION (2017 ORSC) ((2014 OSSC - 1203.2 similar))

R806.1 Ventilation required. Enclosed attics and enclosed rafter spaces formed where ceilings are applied directly to the underside of roof rafters shall have cross ventilation for each separate space by ventilating openings protected against the entrance of rain or snow. Ventilation openings shall have a least dimension of 1/16 inch (1.6 mm) minimum and 1/4 inch (6.4 mm) maximum. Ventilation openings having a least dimension larger than 1/4 inch (6.4 mm) shall be provided with corrosion-resistant wire cloth screening, hardware cloth, or similar material with openings having a least dimension of 1/16 inch (1.6 mm) minimum and 1/4 inch (6.4 mm) maximum. Openings in roof framing members shall conform to the requirements of Section R802.7. Required ventilation openings shall open directly to the outside air.

R806.3 Vent and Insulation clearance. Where eave or cornice vents are installed, insulation shall not block the free flow of air. A minimum of 1-inch (25mm) space shall be provided between the insulation and the roof sheathing and at the location of the vent.

The preferred design {‘A’ in EXHIBIT #1} provides for more than the code required minimum dimension of ventilation in structured channels above a vapor permeable baffle that also serves as an air barrier layer in the roof assembly.
This baffle layer is specified as Plywood (perm rating ~10 per BSC Information (rev. April 2013) Sheet 312, Vapor Permeance of Some Building Materials, Building Science Corporation, {EXHIBIT #2}), which has sealed (caulked/taped) seams with a vapor permeable WRB layer atop.
Above this layer are 2x wood furring battens on flat attached down into the rafters – providing for a minimum of a structured 1.5” ventilation space - and a second layer of exterior grade plywood above the battens for the roof covering assembly.

The code requirements for insulation baffles, moisture control, vapor retarders and air barriers are met as noted:

The 2017 ORSC code requirements for baffles are found in:

N1104.2.5 Baffles. Baffles of a durable rigid material shall be provided to prevent obstruction of vent openings and to deflect incoming air above the surface of porous insulation so as to prevent wind-washing and blowing of loose material. Thermal insulation shall not be installed in a manner that would obstruct openings required for attic insulation.

Plywood meets the “durable rigid material” requirement – Plywood also has the benefit of being vapor permeable – meaning it can ‘dry outwards’ to the individual ventilation channels.

The 2017 ORSC code requirements for moisture/condensation control are found in:

Section R318 Moisture Control
R318.1 Vapor Retarders. In all framed walls, floors and roof/ceilings comprising elements of the building thermal envelope, a Class II vapor retarder shall be installed on the warm-in-winter side of the insulation.
Exceptions:

1. In construction where moisture or freezing will not damage materials.
2. Where the framed cavity or space is ventilated to allow moisture to escape.

and:

N1104.9 Moisture Control. To ensure the effectiveness of insulation materials and reduce the hazard of decay and other degradation due to condensation within the structure, moisture-control measures shall be included in all buildings and structures and portions thereof regulated by this chapter.

N1104.9.1 Vapor retarders. Class I vapor retarder shall be installed on the warm side (in winter) of all insulation.
Exceptions:

1. When insulation is installed in ceilings in an existing structure and ventilation is provided as specified in Section 806, a vapor retarder need not be installed.

Additionally the 2014 OSSC code requirements for vapor retarders are found in:

1405.3 Vapor retarders. Class I or II vapor retarders shall be provided on the interior side of frame walls in Zones 5, 6, 7, 8 and Marine 4.
1405.3.3 Class III vapor retarders. Class III vapor retarders shall be permitted where any one of the conditions in Table 1405.3.1 is met.

Table 1405.3.1 indicates that in the Marine 4 (4C) climate zone a vented cladding over wood structural panels meets this requirement. {EXHIBIT #3}

In the preferred design a Class III vapor retarder at the gypsum-board ceiling – a painted latex coating - provides the roof assembly moisture control.

The 2017 ORSC code requirements for air barriers are found in:

N1104.2.6 Air Barriers. An air barrier shall be provided on every vertical portion of air permeable insulation and on the warm side of horizontal, air permeable insulation.
Exceptions: Unvented attics, continuous insulation walls and similar conditions where an impermeable insulation layer forms an air barrier.

The gypsum-board ceiling layer can serve as the code required air barrier, however the preferred design also benefits from the plywood as a second durable air barrier – allowing the roof assembly to perform the way code compliant walls are built with air barriers on both faces of all enclosed framing bays.

The 2014 OSSC lists approved air barrier materials:

502.4.1 Air barriers. A continuous air barrier shall be provided throughout the building thermal envelope. The air barriers shall be permitted to be located on the inside or outside of the building envelope, located within assemblies composing the envelope, or any combination thereof.
The air barrier shall comply with Sections 502.4.1.1 and 502.4.1.2.

502.4.1.2.1 Materials….

1. Plywood with a thickness of not less than 3/8 inch (10mm).

So, the preferred assembly meets the code intent for baffle material requirements, moisture control, vapor retarders, and air barriers outright. The remaining question is does the assembly provide cross ventilation ‘equivalent’ to that described in 2014 ORSC R806.1 and 806.3?

Code requires a 1” minimum airspace for ventilation for each rafter bay; with nominal 2x rafters/trusses these sections would calculate as:
22.5 square inches @ 24” o.c.
14.5 square inches @ 16” o.c.

By calculation the preferred Option ‘A’ configuration provides for more net free area than code minimum:
30.75 square inches @ 24” o.c.
18.75 square inches @ 16” o.c.

Meanwhile some of the typically installed commercially available plastic baffle products meet the code intent for area and some do not:

ADO Durovent
18.7 square inches @ 24” o.c.

ADO Provent
26 square inches @ 24” o.c.
15 square inches @ 16” o.c.

BRENTWOOD AccuVent
25.7 square inches @ 24” o.c.
15.5 square inches @ 16” o.c.

OWENS CORNING Raft-R-Mate & AtticMate
22.3 square inches @ 24” o.c.
11.15 square inches @ 16” o.c.

Also plastic baffles do not provide ventilation exposure for the full width of the sheathing due to nailing flange and reinforcement rib placements etc. {EXHIBIT #4}

The design provides for a more durable and resilient roof ventilation system. It uses common vapor permeable construction materials, and affords a working ‘platform’ for subsequent layers of roofing construction activities from above – avoiding the piece-work of baffle installation from below into each rafter bay overhead inducing associated air sealing issues, and increased labor costs. It is also a layering of construction conventions that builders can understand and sequence accordingly.

By integrating the robust air tightness layer into the assembly above the rafters allows for better dense-packing of blown-in insulation, resulting in an incrementally greater insulation R-value per given structural rafter depth (R-7.4 to R-8.6 +/-), and prevents wind-washing of the insulation - thus increasing energy conservation performance.

The roof assembly is designed to dry in both directions - which supports durability and resilience.
Please refer to cross sectional drawing {Exhibit #1} illustrating the ‘stack-up’ of the preferred design (‘A’) in comparison to a conventional (‘C’) and an intermediate version (‘B’). This graphically demonstrates that there is no inherent greater risk given the use of the same physical materials in these similar configurations. An additional benefit for building durability is that the vapor permeable WRB at the air barrier is protected by the ‘vented cladding’ of the roof covering assembly – similar to a rain-screen for wall construction.

The 2014 OSSC Table 1405.3.1 indicates that a vented cladding provides an equivalent level of protection against moisture/condensation risk (allowing use of Class III vapor retarders in exterior walls) as using insulated sheathing for 2x4 and 2x6 construction @ R-2.5 and R-3.75 respectively in Climate Zone 4C.

Adding up the R-values of the vented cladding material layers, multiple interior and exterior air films, and the air gap (even with downgrading for convective air movement) results in a similar R-value. This increase in exterior R-value also effectively reduces moisture risk to the plywood layers in addition to benefit provided by the ventilation channel.

The design of the preferred roof assembly is similar to a structured rain-screen design for wood framed walls; these most typically will have 1x or 2x furring battens (or sometimes battens and counter battens) to space the wall covering material(s) out from the exterior sheathing and WRB for increased durability and drying potential. A 1/8” min. gap dimension version of this system is required by the building code in Oregon for walls, and 1/4” min. gap for wood shingles and shakes above roof decks with air-impermeable (Class I vapor retardant) insulation below to promote outward drying:

R806.5.4. Where wood shingles or shakes are used, a minimum ¼” (6.4mm) vented airspace separates the shingles or shakes and the roofing underlayment above the structural sheathing.

As an additional confirmation of inherent resiliency due to the capacity of the assembly materials note that plywood has an increase of vapor permeability during any extreme RH (relative humidity) events – which provides for a greater outward drying rate. (Lstiburek, Joseph (May 2017) Insight 061: Inward Drive – Outward Drying, Building Science Corp. {EXHIBIT #5})