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A homeowner designing a house in northeast Wisconsin asked GreenBuildingAdvisor whether a ventilated over-roof is appropriate above parallel-chord roof trusses filled with dense-pack cellulose. The supplied report documents the design question, not a final expert answer or approved assembly; key details about moisture control and insulation installation remain unresolved.
A homeowner planning a house in northeast Wisconsin has asked whether a ventilated over-roof can manage moisture above dense-pack cellulose in a cathedral ceiling built with parallel-chord trusses. The question, published by GreenBuildingAdvisor, describes a proposed assembly rather than a completed design, and the supplied report does not include an expert answer or confirmation that the assembly is suitable.
The homeowner places the project in IECC climate zone 6 and proposes a 12:12 roof slope with trusses spanning 26 feet. The intended insulation target is R-65, with the truss depth expected to provide space for dense-packed cellulose and second-floor bedrooms beneath a cathedral ceiling. The homeowner says the steep slope is also intended to help shed snow from rooftop photovoltaic panels.
In the proposed roof build-up, a raised-seam metal roof would sit above roofing membrane and 5/8-inch plywood sheathing. Below that, the plan lists 2-by-4s set on edge to form a 1.5-inch ventilation channel, another 5/8-inch sheathing layer, and an 18-to-20-inch parallel-chord truss. The interior side would include a gypsum ceiling with Class II vapor-retarder paint, described by the homeowner as an air barrier, a 1.5-inch service cavity and another gypsum ceiling.
The homeowner’s central concern is whether moisture entering the cellulose from indoors could dry toward the exterior if ventilation baffles are not vapor-open. The question also asks whether an insulation contractor could dense-pack the trusses by injecting from the eaves, avoiding holes through the interior gypsum ceiling. Those are questions raised by the homeowner; the source material does not establish that the proposed ventilation path, vapor-control strategy or installation method will work.
Moisture Control in a Cold-Climate Roof
The design choice matters because a cathedral roof has limited room for error if air leakage or other moisture movement wets insulation or wood components. The homeowner is weighing a high insulation target against the need to manage moisture in a cold-climate assembly. Whether an over-roof ventilation channel provides useful drying cannot be determined from the question alone; it depends on the complete design and its execution.
The proposal also combines several functions: insulation, a ventilated space, two sheathing layers, a metal roof, an interior vapor-control layer, and a service cavity. Readers considering a similar roof should treat the listed materials and dimensions as a concept under review, not as a tested prescription. The photovoltaic array and snow-shedding goal add project-specific considerations, but the report supplies no performance assessment for either.
The installation question has practical consequences, too. Avoiding penetrations in the finished ceiling may reduce patching, but the source does not confirm that eave-side injection can reliably fill these particular trusses. An installer would need to assess access and the intended filling method against the actual truss layout and insulation specification.
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The Homeowner’s Proposed Roof Layers
The GreenBuildingAdvisor report is framed as a request for design feedback, not as a report of construction, a code ruling or a professional approval. The homeowner identifies the location, climate zone, span, roof pitch and target insulation level, then lays out the intended layers and asks for comments. No reply, engineering documentation or final construction detail appears in the supplied source.
The question specifically distinguishes between ventilation baffles and a separate ventilation space above the upper sheathing. The homeowner is concerned that baffles may restrict outward drying and proposes a channel formed by 2-by-4s on edge. The source records that concern as the homeowner’s understanding; it does not verify the vapor properties of a particular product or say how the channel should be designed.
The ceiling description also warrants care in interpretation. The homeowner calls the gypsum ceiling with Class II vapor paint an air barrier, but the report does not provide details showing how seams, edges, penetrations or connections would be sealed. The question therefore does not establish that the interior layer will function as a continuous air barrier in the built roof.
“My target is R 65.”
— The homeowner, in the GreenBuildingAdvisor question
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Design Questions Still Open
The supplied report does not say whether the assembly was later reviewed, revised or built. It provides no expert assessment of the proposed 1.5-inch ventilation channel, no confirmation that the full roof design meets applicable requirements, and no measured or modeled moisture-performance results.
Other unresolved points include how the ventilation path would connect and remain clear, how the proposed interior air-control layer would be made continuous, and whether the specified cellulose can be installed evenly from the eaves in these trusses. The source also gives no installation instructions, contractor assessment, structural review or permit information. The homeowner’s concern about baffles and outward drying remains a question, not a confirmed finding about this assembly.
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Review Before Construction Begins
The immediate next step described by the source is feedback on the homeowner’s questions; no response or project milestone is included. Before construction, the proposed layers and details would need review by qualified building-design and insulation professionals familiar with the project’s location and applicable requirements. The homeowner would also need a clear answer on how the ventilation route, moisture-control layers and truss filling method are intended to work together.
Until that review is documented, the roof should be described as a proposal rather than an established solution. Any later expert response, revised drawings or construction details could clarify whether eave-side dense packing is feasible and what changes, if any, are needed for the roof design.
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Key Questions
Is the proposed roof assembly confirmed to be suitable?
No. The supplied source contains a homeowner’s question and proposed layer list, but no final expert assessment or approval.
What insulation target does the homeowner want?
The homeowner states a target of R-65 for a house in northeast Wisconsin’s IECC climate zone 6.
Does the source confirm that cellulose can be installed from the eaves?
No. It asks whether contractors could dense-pack the trusses from the eave ends without making holes in the gypsum ceiling; it does not provide an answer.
Why is the homeowner considering an over-roof vent?
The homeowner is concerned about whether moisture entering the cellulose from indoors could dry outward through ventilation baffles, and proposes a separate ventilation channel above the sheathing. The source does not establish that this design will provide the intended drying.
Source: rss
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