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A Massachusetts homeowner asked whether 2 inches of closed-cell foam plus 8 inches of open-cell foam on a roof deck would be adequate for an unvented attic in climate zone 5A. The source is a homeowner’s question, not a published technical ruling: the proposed closed-cell thickness, moisture performance and code compliance have not been independently verified.
A Massachusetts homeowner planning an unvented attic retrofit has asked whether a proposed roof-deck system of 2 inches of closed-cell foam beneath 8 inches of open-cell foam can limit moisture problems in climate zone 5A. The question, published by GreenBuildingAdvisor, describes a proposed project and the homeowner’s concerns; it does not report a completed installation, inspection or expert determination that the design meets code.
The homeowner says the 1976 house suffered interior water damage from ice dams last winter. Its asphalt-shingle roof has since been replaced and includes a ridge vent and soffit vents. The attic remains vented, with old fiberglass and cellulose insulation on its floor, and contains an air handler and ductwork. The homeowner says the equipment in the attic was a main reason for considering an unvented design.
Under the contractor’s plan, the existing attic-floor insulation would be removed, and no new insulation would be installed there. The roof deck would receive 2 inches of closed-cell foam, then 8 inches of open-cell foam, for a stated total of R-49. The homeowner says the rafters are less than 10 inches deep, so the open-cell foam would extend past them and would not be trimmed. Other listed work includes sealing the soffits, extending a bathroom exhaust vent through the foam and applying an ignition-barrier coating.
The homeowner questions whether the closed-cell layer is thick enough. They cite their reading of IRC R806.5 and Table R806.5, which they understand to require at least R-20 of air-impermeable insulation in a hybrid assembly in zone 5. They estimate that 2 inches of closed-cell foam provides roughly R-13 to R-15 and ask whether the project should use 3 inches or more, or all closed-cell foam. Those figures and the code interpretation are reported as the homeowner’s understanding, not as a verified code review.
Moisture Risk at the Roof Deck
The choice affects more than the attic’s stated insulation value. In an unvented assembly, the foam’s location and properties influence how moisture from indoor air, leaks or other sources can reach and dry from the roof sheathing. The homeowner specifically raises concerns about damp sheathing beneath open-cell foam, including at ridges and on north-facing roof slopes. The source does not establish that moisture damage will occur, or that the proposed closed-cell layer would prevent it.
The project would also change the attic’s current arrangement: insulation on the attic floor would be removed, vents would be sealed, and the air handler and ducts would be inside the insulated enclosure. That makes the design, ventilation and moisture-control details relevant to the condition of both the roof deck and the equipment. The past ice-dam damage makes the retrofit consequential for this homeowner, but the source does not establish that the proposed foam assembly will prevent future ice dams.
The homeowner says an insurance-company estimator raised moisture buildup as a concern. That is an attributed concern, not a documented finding about this roof. A general contractor consulted by the homeowner instead favored keeping the attic vented, air-sealing the ceiling and adding blown cellulose on the floor. The source offers no comparative assessment showing which approach is better for this particular house.
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A Retrofit Before Installation
The house is in Massachusetts climate zone 5A, according to the homeowner. The planned work is a retrofit rather than new construction: the attic is currently vented, and its existing insulation is on the floor. The homeowner describes a hip roof, new asphalt shingles, ridge and soffit vents, and an old powered roof fan. The contractor’s proposed conversion would seal the soffits and create an unvented attic around the roof deck.
The homeowner said the installer was expected to begin in early December, but the source does not give the year or confirm that work began. They were seeking feedback before installation and said they could provide photographs. Their questions include whether to increase the closed-cell layer, pay about $6,000 more for an all-closed-cell option, add a supply duct and return path for attic air circulation, or retain a vented attic with insulation at the ceiling plane.
“The air handler and its ductwork are in the attic, which is one of the main reasons we went with an unvented attic.”
— The homeowner, in the GreenBuildingAdvisor question
Code and Assembly Details Unverified
The source is a homeowner’s request for a technical “gut check,” not a building-code determination or a report on completed work. It does not establish the exact R-value of the selected foam products, whether the installer’s design satisfies applicable code, or what local officials would require. The homeowner’s reading of the IRC and estimates for foam R-value have not been independently checked in the source.
It is also unclear how the proposed assembly would manage moisture under the home’s specific conditions, including at the roof ridge and north-facing slopes. The source provides no roof-deck inspection results, indoor humidity measurements, detailed product specifications or analysis of the air handler’s operation. It does not report whether a supply duct or return path is needed, whether the bathroom exhaust extension is properly detailed, or whether the powered attic fan will remain in use. No response from a building official, foam manufacturer or building-science professional is included.
Review Before Foam Installation
The next reported milestone was the installer’s planned start in early December, although the year and current project status are not given. The homeowner’s immediate questions are whether to alter the thickness or type of foam, keep the attic vented instead, and change how air is supplied to the attic. The source does not say which option was selected.
Before work proceeds, the open questions include the applicable code requirements, the specified products and their tested properties, and how the roof deck and attic will be managed for moisture. The source does not document a final design or approval. Any later update would need to establish whether the work went ahead and provide evidence of the completed assembly and any professional or code review.
Key Questions
Has the proposed hybrid insulation system been confirmed as code-compliant?
No. The source reports the homeowner’s interpretation of IRC R806.5 and Table R806.5, but includes no code-official review or confirmed compliance determination.
Is 2 inches of closed-cell foam enough for this roof?
The source does not answer that question. The homeowner estimates that 2 inches provides roughly R-13 to R-15 and questions whether the cited zone 5 requirement calls for R-20. Product specifications and a project-specific code review are not provided.
Does open-cell foam mean the roof sheathing will get wet?
The source records a concern about damp sheathing, including at ridges and on north slopes, but does not establish that moisture damage will occur in this assembly. No inspection or moisture analysis is reported.
What alternative did the homeowner consider?
A general contractor the homeowner consulted preferred a vented attic, with air sealing and blown cellulose on the attic floor. The source does not compare the two designs or determine which is appropriate for this house.
Source: greenbuildingadvisor.com
