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A homeowner planning to convert a 30-by-32-foot post-frame barn in climate Zone 4A into a workshop asked GreenBuildingAdvisor readers how to insulate its cathedral roof. The submitted question weighs an unvented roof with reclaimed XPS above the sheathing against adding ventilation, but provides no final design or expert resolution.
A homeowner planning to remodel a 30-by-32-foot post-frame barn in climate Zone 4A asked GreenBuildingAdvisor readers how to insulate its cathedral-style roof while managing moisture in a building that will not be heated or cooled all the time. The question lays out two possible approaches, but the supplied report does not include a final recommendation or evidence that construction has begun.
The barn is intended to serve as a weekend wood shop, initially heated with a wood stove and cooled with a window air conditioner. The owner says the building may be occupied more often after retirement, when a permanent HVAC system could be added. The stated insulation targets are R-38 to R-45 at the roof and R-19 in the walls, with the aim of reducing energy needs when the shop is in use.
The roof has 2-by-10 rafters spaced 24 inches on center, running to a central ridge beam. The rafters end flush with the exterior wall girts, leaving no existing soffit arrangement described for a conventional vent path. The owner says roof ventilation may require adding rafter tails for vented soffits and possibly moving conduit near the purlins to clear a route for airflow.
The proposed unvented option would add sheathing, tape and a water-resistive barrier, then place reclaimed 3-inch XPS foam above the roof sheathing and fiberglass batts between rafters. The owner estimates that this could reach the desired roof R-value while allowing for the possibility that the reclaimed foam does not provide its full stated R-15. The alternative under consideration is to create a ventilation channel beneath the sheathing and install soffit and ridge vents, or use shuttered gable vents and a fan.
Moisture Risk in an Intermittent Workshop
The question matters because the building’s use pattern is central to the proposed assembly: it may be unheated or uncooled for stretches, then heated or cooled when the owner is working. The owner explicitly asks whether a tightly sealed barn would have moisture problems without continuous climate control and whether a dehumidifier would be needed during idle periods. Those are questions, not reported findings; the source does not establish that either approach will cause condensation or that a dehumidifier is necessary.
The choice also affects the scale of the renovation. Adding a vent route could involve altering the eaves and dealing with conduit, while an unvented design depends on the complete roof assembly and the condition and performance of reclaimed insulation. Because the owner plans to remove the metal roofing and replace some water-damaged girts, the insulation decision is part of a broader envelope project rather than a standalone product choice. No construction advice should be treated as settled based on the question alone.
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Planned Barn Conversion Details
The submitted account describes a barn with a full foundation, rather than posts standing only on piers. The owner plans to excavate the existing dirt floor, install insulation and a vapor barrier, and pour a concrete slab. The walls and roof are also slated for work: the metal will be removed, some water-damaged girts replaced, and doors and windows addressed.
The building’s roof is described as cathedral-style, with a second-floor loft space and no trusses. That geometry informs the owner’s concern about ventilation: the existing rafter ends are flush with the wall girts, and conduit near the purlins may obstruct a clear path. The source records the owner’s planning questions and a local contractor response, but does not provide measurements, a complete roof-section specification, or a reviewed construction plan.
“I’m leaning toward an unvented roof with above sheathing insulation.”
— The homeowner, in the GreenBuildingAdvisor question
unvented roof insulation materials
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Design Questions Still Open
The source does not say which roof assembly the owner will choose, whether the reclaimed XPS has been tested or its condition assessed, or what its actual thermal performance would be. It also does not provide a detailed analysis of how the proposed layers would handle moisture in this specific roof, including the effects of intermittent heating and cooling.
There is no reported expert answer to the questions about sealing the barn, maintaining a dehumidifier, or installing ventilation. The extent of required work to create a vent channel, move conduit, or add soffit and ridge openings is also unknown. The supplied text gives no publication date, construction schedule, cost estimates, or confirmation that the planned renovation is underway.
reclaimed XPS foam board insulation
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Awaiting a Roof Assembly Recommendation
The next step described in the source is a decision by the homeowner among the possible roof strategies. A useful resolution would require evaluating the entire proposed assembly and the barn’s expected indoor conditions, rather than treating the choice as simply foam versus ventilation. The supplied material does not state whether GreenBuildingAdvisor or another professional later responded.
For readers following this project, the relevant developments would be a documented roof design, confirmation of the reclaimed foam’s condition and performance, and a plan for managing indoor moisture when the shop is not conditioned. Until such details or an expert assessment are available, the post remains a request for advice, not a report of a completed insulation system.
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Key Questions
What is the homeowner planning to do with the barn?
The owner plans to remodel the 30-by-32-foot post-frame barn as a weekend wood shop, initially using a wood stove and window air conditioner. More regular use and HVAC are possible after retirement.
Which roof insulation options are being considered?
The owner is considering an unvented roof with reclaimed XPS above the sheathing and fiberglass batts between rafters. Alternatives include building a ventilation channel with soffit and ridge vents, or using shuttered gable vents and a fan.
Has the source determined whether a dehumidifier is needed?
No. The homeowner asks whether a dehumidifier would be needed while the building is unconditioned, but the supplied report does not provide an answer.
Is spray foam the recommended solution?
The homeowner says local pole barn contractors have advised using spray foam. The source does not identify those contractors, explain their recommendations, or establish that spray foam is the final choice.
Source: rss
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