How to insulate ceiling of a room built under an open porch to prevent condensation?

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Posted by: from Bouchette
1/29/2017 at 2:00:25 PM

I built a room under an existing open porch to expand living space in the basement, and to add structural stability to the porch itself. The new room faces south and has been fitted with windows and doors to take advantage of the passive solar heat. The porch floor of the open porch above consists of a plywood base covered by a Ditran waterproof membrane, then porcelain tile. The ceiling of the room under this is insulated with Roxul.

Will a vapour barrier installed over the warm side of the Roxul be enough to prevent condensation from forming on the cold side or should additional prevention be added? If so, what?

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Date/Time1/29/2017 at 4:15:34 PM

I think you're on the right track. It sounds like all you need to do is put on the vapour barrier and then get your drywall over top of that. I guess another question we should've asked was how good is the vinyl membrane up top as long as it's not leaking You should be pretty safe with what you're doing.

Good luck.

Jim Stewart

Stewart Construction

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Date/Time1/29/2017 at 4:53:22 PM

The idea is sound. However, there is a little more to it. You need to have some form of ventilation for the space between the upper side of the insulation and the sheathing above. Otherwise there is no place for the build up of moisture to go ... but down. It can collect and find the lowest spot on the vapour barrier then possibly leaks through the seams or any holes. Talk to a flat roof specialist and get a separate opinion.

Good luck.

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Date/Time1/29/2017 at 6:20:02 PM

Tyvek is my choice. Depending on the season the warm side Varies. Tyvek is a Breathable AIR barrier Allowing Condensation to dissipate. Vapour will cause and trap Condensation.



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Date/Time1/31/2017 at 5:00:43 PM


I can see some problems with your porch. Ditra, no n, is a decoupler not really intended to be roofing material, so you will get leaks, simply a ? of when. I would fix the problem first, close in above or remove your tile and use a recommended water proof membrane. Sorry its a ? of pay now or pay more later.

John Miller

Superior Handyman

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