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Discussion Starter · #1 ·
We are remodeling our kitchen and ended up removing insulation on about 14 feet of exterior wall and of course need an insulation inspection.

The inspector was not sure if we needed a vapor barrier....which leads me to all kinds of question!

House was built in 1981, we live in the Denver area.

This is the construction:

cedar siding -> 1" thick closed cell foil backed board -> studs with unfaced mineral wool (new) -> drywall

The question was whether we wanted/needed a vapor barrier on the INSIDE (something like MemBrain). Inspector was wondering whether it was needed since we had the foil backed board.

The fact that the inspector was unsure makes me doubt I will get a straight answer...anyone here want to weigh in on whether it is needed?

Mineral wool is being used to get approx R-21 (assuming closed cell foil backed board is about R-5-6 and new mineral wool is R-15).
 

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if the rigid insulation board is air impermeable, it can form a thermal break and prevent condensation from forming in cold weather, eliminating the need for a vapor barrier. It will also stop the air leakage.

it need to have a high enough r-value -> if it's too low the batts will make that surface cold enough for condensation to form.

1" may not be thick enough.
 

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Discussion Starter · #3 ·
Am I at risk of causing problems if I install the additional vapor barrier at the inside, so it goes foil backed foam -> mineral wool -> vapor barrier?

The concern that was floated was having 2 vapor barriers...and I can't seem to get a clear answer from people who should know.
 

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Condensation will only happen in that wall if warn moist air from the house can get to where the wall is cold. Or you leave spaces in the insulation where air can circulate in the wall and bring cold to the back side of the warm wall.
So make sure you fit the insulation in every nook and cranny and you want to seal the wall with drywall or vapour barrier.
Holes for pipes and wires going up and down out of that wall want to air sealed and the outlets and switches all want to be sealed.
Having 2 barriers is only a problem if you spring a leak in the wall and it cannot dry,
If you spring a leak in that wall, you will be opening it.
 

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When you stick a vapour barrier on one side, you must have drying potential on the other unless the surfaces don't get cold enough for condensation to form. No barrier is perfect, there's always leakage.
 
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