How to stop condensation under metal roof

Written by sean farmer
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How to stop condensation under metal roof
Corrosion of metal roofs usually starts with condensation under the roof. (sheet metal image by Karin Lau from

Leaky metal roofs are not always caused by gaps in the roof letting in water during a rainstorm. Metal roofs that do not have adequate airflow may have a build-up of condensation under the metal. This condensation can cause damage to your roof and to your insulation, and negatively impact your thermal conduction, causing your heating and air conditioning bills to increase. Adding polyurethane foam to the interior side of your metal roof will create an airtight seal that will stop condensation build-up and lower your heating and air-conditioning cost.

Skill level:

Things you need

  • Polyurethane foam application kit

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  1. 1

    Determine the number of kits you will need to appropriately apply the polyurethane foam to the metal roof. This can be determined by taking the length of the roof and multiplying it by the width of the roof, then taking this number and comparing it to the square footage that the foam kit will cover. For example, a 10- by 10-foot roof will require a kit that covers at least 100 square feet.

  2. 2

    Set up the polyurethane foam application kit per the manufacturer's directions. This kit will usually contain a canister containing the foam, and a device such as a sprayer head to spray the foam.

  3. 3

    Spray the polyurethane foam onto the edge of the roof's interior, where the roof meets the wall. Thoroughly cover the entire edge with roughly 1 inch of foam. Continue to spray at least 1 inch of foam onto the roof, until the entire roof is covered.

Tips and warnings

  • Adding a batting system to insulate the roof, in addition to the polyurethane foam, will greatly reduce the heating and air-condition costs of your building.
  • Spray the foam onto only the bar-metal interior side of the roof, to provide the best sealant from condensation.

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