Two terms are used interchangeably in the industrial insulation literature and they should not be. A moisture barrier blocks liquid water at the metal-to-insulation interface. A vapor retarder slows the diffusion of water vapor through a system. The Enerpro Tank Panel System uses a moisture barrier. The distinction matters because the failure mode each prevents is different.
Definitions
Moisture barrier
A physical film or membrane that prevents bulk liquid water from contacting a substrate. In tank jacketing applications, the moisture barrier sits between the inner face of the metal jacket and the outer face of the insulation. Its job is to prevent any liquid water that has entered the system from sitting in contact with the metal substrate — the condition that drives corrosion under insulation (CUI).
Vapor retarder
A material that slows the diffusion of water vapor through an assembly. Vapor retarders are rated by perm (US perm or metric perm) — a measure of how much water vapor can pass through per unit area per unit time per unit pressure differential. Vapor retarders do not block liquid water; they slow vapor migration that would otherwise condense on a cold surface.
What the Enerpro system uses
Aluminum jacketing supplied with the Enerpro Tank Panel System carries a DuPont Surlyn moisture barrier, heat-laminated to the interior face of every panel. The construction:
- Material: Surlyn ionomer resin, manufactured by DuPont.
- Construction: Three-layer co-extruded polyethylene.
- Thickness: 3 mil (0.003″).
- Bonding: Heat-laminated to the interior face of the aluminum jacket during fabrication, not field-applied.
The Surlyn layer is a moisture barrier in the precise sense: it is a continuous polymeric film that prevents liquid water from contacting the aluminum substrate.
Why this prevents CUI
Corrosion under insulation initiates when liquid water sits against an unprotected metal substrate for long enough to drive an electrochemical reaction. In aluminum jacketing, this manifests as pitting and chalking on the interior face. In carbon steel substrates, it manifests as wall-thickness loss in the tank or vessel itself.
The standing seam closes the exterior to bulk water entry. The Surlyn moisture barrier handles the case where moisture does enter the system — via condensation, vapor migration on cold service, or any geometric imperfection that allows a small ingress. With the barrier laminated to the jacket interior, any moisture present in the insulation core cannot bridge to the metal.
Steel jacketing
The DuPont Surlyn moisture barrier is supplied on aluminum jacketing only. Steel jacketing — galvanized (ASTM A653), galvalume (ASTM A792), aluminized (ASTM A463), or pre-painted — relies on the substrate’s own corrosion-resistant coating system for the protection that Surlyn provides on aluminum jacketing. Steel substrate selection therefore drives the long-term corrosion protection of a steel-jacketed installation; the choice is project-specific and typically follows the client’s specification.
What this means in procurement
When evaluating any standing-seam tank insulation system, the substantive specification question is not “does it have a vapor retarder.” The question is:
- Is there a continuous moisture barrier at the jacket interior?
- What material is it — named, by manufacturer?
- How is it bonded to the jacket — factory-laminated or field-applied?
- What thickness?
These four data points distinguish a real CUI-resistant system from a marketing claim. Specifications that reference “vapor retarder” without naming a moisture barrier are conflating two different functions.
Standards referenced
| Standard | Relevance |
|---|---|
| NACE SP0198 / AMPP | Control of corrosion under thermal insulation and fireproofing materials |
| ASTM E96 / E96M | Standard test methods for water vapor transmission of materials (the test used to rate vapor retarders by perm) |
| ASTM B209 | Aluminum jacketing substrate; underlies the laminated Surlyn moisture barrier specification |