/ Field knowledge · IMC 604.11–604.12
R-value on paper is not a vapor retarder.
A cooling duct can meet R-value on paper and still fail in the field. IMC 604.11–604.12 are moisture rules, not thermal rules. External insulation on cooling ducts needs a vapor retarder (maximum 0.05 perm, or aluminum foil at least 2 mils thick) with joints and seams sealed. Exterior insulation also needs an approved weatherproof barrier. One does not replace the other.
Educational 2024 International Mechanical Code (IMC) 604.11 and 604.12 from Daily Code Talk. Confirm the adopted edition. Distinct from 604 location / R-value and 604.13 coil-discharge liner. This is not an insulation specification.
- 604.11: cooling ducts with external insulation — vapor retarder 0.05 perm maximum, or 2-mil aluminum foil. If the insulation itself is 0.05 perm or less, a separate covering is not required.
- Joints and seams sealed to keep vapor-retarder continuity. Do not leave seam sealing implied.
- Exception: spray polyurethane foam does not need a separate vapor retarder if water-vapor permeance is not greater than 3 perms per inch at the installed thickness.
- 604.12: insulated exterior ducts protected with an approved weatherproof barrier. Exterior wrap is not automatically a complete vapor-retarder system, and a vapor retarder is not automatically weather protection.
On the set: identify cooling ducts with external insulation and all exterior insulated runs. M-501: continuity, seam sealing, and weatherproof barrier. Schedule: permeance basis, foil basis where used, exterior protection.
R-value on the schedule. Open seams on the roof
Condensation, wet insulation, reduced performance, visible deterioration. In South Florida that is a field failure, not a paperwork miss.
Source: Daily Code Talk — IMC 604.11–604.12 · tool: Vapor / weather screen · 604: location / R-value · 604.13: coil discharge.
What to send
Whether cooling ducts are externally insulated, whether they run outdoors, and the insulation / jacket product if you have it.
If the wrap meets R-value and the seams are open
Send the product and a roof photo. Masterbuild will say whether 604.11 still has no 0.05-perm path — or 604.12 still has no weather barrier.
Send Project Background+1 (786) 398-6940 · osmany.portal@masterbuildconsulting.com
Masterbuild provides engineering services under a written scope. Nothing on this page is project-specific engineering advice, a code interpretation for your project, or a representation about permit approval. Confirm the adopted edition. The 0.05 perm, 2-mil foil, SPF 3-perms-per-inch exception, and weatherproof-barrier split are 2024 IMC 604.11–604.12 education from Daily Code Talk — not an insulation specification. This page is not legal advice.