Why we insulate roofs and walls with spray foam

Clients ask about insulation more than almost any other part of a house they will never see. On most of our new homes and many of our renovations, the answer is the same: closed-cell spray foam applied to the underside of the roof deck, and spray foam in the walls. This entry explains why, and is candid about what that choice asks of us in return.

What spray foam does that other insulation does not

Insulation slows heat. Air leakage is a separate problem, and in a cold climate it is often the larger one. Warm, moist indoor air that escapes through gaps in a ceiling or wall carries heat with it, and it can leave water behind on cold framing and sheathing. Most of the moisture trouble we find when we open up older houses traces back to air leaks rather than to anything that soaked through a wall.

Spray foam addresses both at once. It insulates, and because it is sprayed in place and expands into every gap and around every wire and pipe, it also seals. Closed-cell foam, the denser of the two common types, adds a third benefit: at a couple of inches thick it slows the movement of water vapor as well, so in a cold climate it can serve as the vapor control layer without a separate sheet of plastic. Building Science Corporation, whose research informs much of how we think about this, recommends closed-cell rather than open-cell foam against the roof deck in climate zones like ours for exactly that reason.

Why we put it at the roof

The traditional approach insulates the attic floor and ventilates the attic to the outdoors. That works well on a simple roof over a simple ceiling. Many of the houses we build have neither. Vaulted ceilings, dormers, valleys, recessed lights, and attic spaces that hold ductwork or mechanical equipment make a perfectly sealed ceiling very difficult to achieve in practice.

Moving the insulation up to the roof deck changes the problem. The attic becomes part of the house, conditioned and protected, so ducts and equipment up there are working inside the envelope instead of in a space that reaches well below zero in January. The complicated ceiling below no longer has to be the air barrier, because the roof itself is. It also removes the warm, leaky attic that drives most ice dams.

Michigan’s residential code, based on the International Residential Code, allows this kind of unvented roof with conditions. In our climate zone, the insulation against the deck must be the air-impermeable kind, and where other insulation is added below it, at least R-20 of it must be foam to keep the deck warm enough to avoid condensation. (Michigan is currently enforcing its 2015-based codes while an update is held up in court, so requirements are always confirmed with the local building department.)

The trade-offs, and how we handle them

No assembly is without compromise, and a few deserve to be stated plainly.

Roof leaks are harder to see. In a vented attic, a small roof leak can sometimes be spotted from inside. With foam on the deck, it may only show up later. Our answer is to treat the roof itself, with its underlayment, flashing and penetrations, as the part that has to be right, and to put the care there.

A conditioned attic needs a plan for humidity. A sealed attic is part of the house, and like the rest of the house it needs a modest amount of conditioned air or dehumidification so moisture does not accumulate. That belongs in the mechanical design from the start, not added later.

Studs still conduct heat. In a wall, the wood framing interrupts the insulation, so foam’s high rating per inch is partly lost at every stud. Building Science Corporation’s own estimate for a 2x6 wall filled with closed-cell foam is a whole-wall value well below the nominal number. We use foam in walls mainly for what it does for air sealing and moisture, not for the rating on the label. Some respected builders argue that other insulations perform nearly as well in a wall at lower cost, and on some projects they are right.

Fire protection and curing. Foam has to be covered by an approved thermal or ignition barrier, typically drywall in finished spaces, as its product evaluation and the inspector require. During installation the house is vacated for the period the manufacturer specifies, which matters most on renovations where people are living nearby.

Carbon. Foam is a petroleum-based product, and older formulations used blowing agents with a high climate impact. Federal rules since 2025 have moved the industry to agents with a small fraction of that impact. It is still not a low-carbon material, which is one reason we do not use it everywhere.

Where we would choose differently

Spray foam is our default, not our rule. A simple gable roof over a detached garage, an existing attic that is easy to seal at the ceiling, or a client who places a high priority on lower-carbon materials can all point to a vented attic with blown cellulose, or to mineral wool in the walls. The goal is the same in every case: a house that stays dry, comfortable and efficient for a long time. Spray foam is usually the most reliable way we know to get there in a cold, wet climate on the kind of houses we build.

Further reading: Building Science Corporation, BSI-116: Interior Spray Foam and BSD-149: Unvented Roof Assemblies for All Climates; U.S. Department of Energy Building America Solution Center, Unvented Conditioned Attic; and, for a different view on walls, Green Building Advisor, Installing Closed-Cell Spray Foam Between Studs Is a Waste.

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The Journal shares general information about home design and construction for educational purposes. It is not professional advice for any specific property and does not replace guidance from a licensed architect, engineer or your local building department. Codes, products and site conditions vary, and recommendations change over time. Views expressed are those of the author. Reading this article does not create a client relationship with Mosher Dolan; any work we perform is governed solely by a written contract. Full Journal disclaimer.