I live in south Georgia and am enclosing a back deck. I am using metal roofing. Will styrofoam insulation prevent condensation from dripping from roof? Would the silver side go up or down?

  • The best way to prevent condensation of course is by having appropriate ventilation. – Ben Welborn Apr 20 '16 at 14:29
  • What if the ventilation air is as humid as the humid air that caused the condensation? Condensation is a phenomenon of temperature, primarily. – iLikeDirt Apr 20 '16 at 14:33
  • @iLikeDirt ventilation in humid conditions are done with attic ventilation dehumidifiers. If it is enclosed space. – spicetraders Sep 18 '16 at 15:21
  • It sounds as if your metal roof is exposed on the underside. Did you plan to leave this open once your deck enclosure is done? Or is it your intention to install a ceiling inside the room. – spicetraders Sep 18 '16 at 15:25

Condensation is caused by the underside of the roofing getting colder than the dew point. To prevent this, you would need to insulate the underside and achieve a perfect air seal so that no humid air can bypass the insulation and touch the cold roof. Otherwise you'll still get condensation on the underside of the roof, but it will drip onto the insulation, negating its insulating power and maybe rotting the wooden structure (ain't wood great?).

Typically spray foam is used for this, to ensure the necessary perfect air seal. Unfortunately, because your roof has no sheathing, you would need to spray foam right under your exposed roof, meaning the foam would stick to the metal panels and make future maintenance or replacement a nightmare. This will work, it's just that it will create problems for anybody who needs to replace panels in the future.

Here's an alternative if you're willing to remove the metal roof and increase the height of the roofing by a few inches:

  • Remove the metal roof
  • Install plywood as roof decking over where the roofing used to be
  • Install 4-6" of inexpensive foam boards above the plywood, with any foil-faced sides facing up
  • Install standard roofing underlayment over that
  • Fasten them down with 2x4 purlins
  • Re-install the metal roofing over the purlins
  • Insulate between the rafters on the inside with fiberglass or mineral wool batts

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