My roof has two layers of 4-inch foam insulation with a 2-inch gap between them. Do I need to vent this gap to stop condensation?

Question

My roof has two layers of 4-inch foam insulation with a 2-inch gap between them. Do I need to vent this gap to stop condensation?

Asked by Kerry Fenn

If I vent it, am I not effectively doing away with the benefit of the top four inches of insulation? If I need to vent it, can I put the vents inside the rooms so I am not circulating cold air through?

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Answers

Bill Bradbury's picture

No, do not vent this! Condensation occurs when moisture-laden air moves across a surface with a temperature below its dewpoint. You must make sure this gap is completely sealed from air and water. No moisture = no problems.