Closed-Cell Foam andMoisture Control

It is usually an air problem, not a water problem.

Homeowners tend to picture moisture damage as a leak, a roof, a pipe, a foundation crack. Some of it is. But the published research on building science points at a less obvious source: warm, humid indoor air finding its way to a cold surface and condensing there, night after night, all winter long. A rim joist, sitting right at the cold junction between framing and foundation, is one of the most common places this happens.

This page summarizes what the research and manufacturer technical data say about vapor retarders, perm ratings, and why closed-cell spray foam is the standard recommendation for this assembly in a cold climate. It is not a description of a specific Trust Works job.

If most moisture is riding along with leaking air, then sealing the air leak addresses the moisture problem at the same time, which is exactly what closed-cell foam is built to do in one application.

Sources: ORNL/TM-2011/437, Spray Foam in Accessible Spaces (Christian & Gant, Oct. 2011)

What a perm rating actually measures.

Building codes classify vapor retarders by how many perms of water vapor they let through, tested under ASTM E96. Class I materials are the tightest, Class III the most permeable. The International Residential Code and International Building Code require a Class I or Class II vapor retarder on the interior side of frame walls in climate zones 5 through 8 and Marine 4, a category that covers all of Minnesota.

IRC / IBC vapor retarder classifications.

Sources: Huntsman Building Solutions, BES-102 Design Note: Vapor Retarders and Vapor Permeance

How thick does closed-cell foam need to be?

Closed-cell spray foam is dense enough that its own perm rating drops as it gets thicker, and manufacturer testing shows it typically crosses into Class II territory, a perm rating at or below 1.0, somewhere between 1 and 2 inches, depending on the specific product. That means a rim joist cavity sprayed to a normal working thickness often ends up as its own vapor retarder, with no separate product needed.

Open-cell foam does not get there on its own. Tested as a Class III vapor retarder regardless of thickness, it needs a separate, approved Class II layer added toward the interior when it is used in a cold climate zone, which is one reason closed-cell is the more common recommendation for rim joists specifically.

Manufacturer test data for closed-cell foam.

One manufacturer's third-party test results for water vapor permeance at increasing thickness, illustrating why thickness matters for reaching Class II.

Figures are third-party test results for one manufacturer's closed-cell product, published as an illustrative example of how permeance drops with thickness. Different products cross the Class II threshold at slightly different thicknesses; the underlying pattern, tighter with more thickness, holds generally.

Sources: Huntsman Building Solutions, BES-102 Design Note: Vapor Retarders and Vapor Permeance

One cavity, three jobs done at once.

The rim joist sits directly on the foundation, one of the coldest surfaces in the house all winter, while warm, moist basement or living-space air presses against it from the inside. A fiberglass batt stuffed into that cavity does not stop the air movement or the vapor diffusion; it just sits there while both keep happening around it, which is why older homes so often show frost or dampness on the rim joist by midwinter.

Closed-cell spray foam, sprayed to a normal working thickness in that cavity, insulates, air-seals, and crosses into Class II vapor-retarder territory in the same application. That is the technical case, grounded in the sources above, for why it is the standard material recommendation for this specific assembly in a cold climate, not a claim about any particular Trust Works job.