Closed-Cell Insulation Thickness in Oregon

How Thick Should Closed-Cell Spray Foam Be?

Premier specifies 2 inches of closed-cell spray foam in walls and a minimum of 3 inches under a roof deck or in an attic assembly. In the Pacific Northwest the roof deck is the number that matters, and it is the one most often cut short.

That recommendation comes from heat-flow-reduction data published by The Energy Modeling Agency. Two inches of closed-cell delivers a 94% reduction in heat flow. Three inches delivers 96%. Doubling the thickness again buys very little beyond that, which is why we do not specify more.

The reason for the extra inch is not the last 2% of heat flow. It is moisture. A roof assembly takes a large temperature swing from solar gain on the roofing material above it. Large swings drive vapor. The third inch is what moves the dew point far enough into the foam that condensation does not form on the underside of the deck.

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Closed-Cell R-Value by Thickness

Closed-cell spray foam runs roughly R-6.0 to R-7.0 per inch, which is the highest of any commonly installed insulation material. The figures below are the values Premier publishes for its own installed assemblies.

These are installed values for a continuous, correctly applied layer. R-value alone does not describe an assembly: a high number with gaps in it performs worse than a lower number that is genuinely continuous and air-sealed.

Why 3 Inches and Not 2 on an Oregon Roof Deck

Two inches is not wrong everywhere. It is wrong here, on a roof deck, for a specific reason.

Western Oregon has a long, damp heating season and the coast is wetter still. Attics and crawl spaces in this climate are prone to forming a dew point inside the assembly rather than outside it. When the dew point lands on the underside of a cold roof deck, moisture condenses there, and the result is mildew, rot and eventually structural repair.

Three inches of closed-cell moves that dew point into the foam itself. The foam does not care about being at that temperature; the sheathing does. That is the whole argument, and it is why we will not quote two inches on a roof deck in this climate even when a competitor has.

Closed-Cell Spray Foam as a Vapor Barrier: What Thickness?

Closed-cell spray foam becomes a vapor retarder at around 1.5 to 2 inches, depending on the specific product. Below that thickness it insulates but should not be relied on to control vapor.

At the 3 inches we specify for a Pacific Northwest roof deck, the layer is doing both jobs at once: it is the insulation and it is the vapor control layer. That is the practical advantage of closed-cell over open-cell in this application, and it is why open-cell, which is vapor-open by design, belongs in different parts of the building.

Always confirm the perm rating for the specific product being installed. Manufacturers differ, and the number that matters is the one on the data sheet for the foam actually going into your building.

What This Data Does and Does Not Tell You

The heat-flow figures above describe heat flow reduction through a continuous layer of closed-cell foam. They are not a whole-building energy model and they are not a promise about a heating bill, which depends on the rest of the envelope, the equipment, the weather and how the building is used.

Local building code governs minimum insulation values and code does not always align with what the heat-flow data suggests is efficient. Where they differ, code wins on the permit and we will tell you so during the estimate.

Every assembly is different. The right specification for your roof deck depends on the framing, the existing sheathing, ventilation, and whether the assembly is vented or unvented. We confirm all of it on site before quoting.