Dissimilar Metals Touch at an Exterior Duct Support

Fast answer

Verify the specified isolation material and support detail before allowing incompatible metals to remain in direct contact, especially where moisture can collect.

What to check

1. Identify both metals and their coatings.
2. Inspect for existing staining, coating damage, and trapped water.
3. Check the support detail for pads, sleeves, washers, or barriers.
4. Verify compatible fasteners and sealants.

Do not

Do not improvise isolation with cardboard, random tape, or material that absorbs water or degrades outdoors.

Ask the foreman

“This aluminum duct is bearing directly on a galvanized support and the detail shows an isolation layer that is missing. Can we verify the approved pad and fastener arrangement before final tightening?”

Why it matters

Moisture and direct metal contact can accelerate galvanic corrosion and damage coatings at the support.