Steel building, R20 rolled Insulation and more Insulation

embo

I have a steel building, spray foam insulation of about 1", R20 rolled fibreglass insulation with a white kraft paper attached. I am wanting to frame another 2x6 wall along with R20 batts insulation. My question is do I add a vapour barrier to the inside surface of the new 2x6 wall. Or asked a different way, does the rolled R20 insulation that can with the steel building already take care off the vapour barrier. By adding another VB (if that is what is on the rolled insulation???) will I create some sort of vapour lock?

thanks

Windows on Wash

Where is the building located?

The PSK fiberglass is a class I vapor retarder so you do not want to stack them and create a trap.

Are you drywalling the ceiling?

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jklingel

On a steel building, would it not be wise to have NO vapor restriction on the inside (the kraft paper will do some retarding)? With the outside zipped tight (the steel) I would think as vapor open on the inside as one could get would be wise. Yes?

Windows on Wash

Yes. I think you are correct on this one.

A vapor retarder on the wall would be problematic but I would like to know where the building. While they are supposed to be vapor tight on the exterior (i.e. metal) I would be worried about vapor drive from the outside and it not being able to dry if you put two vapor barrier up like you are referencing.

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jklingel

Roger that. Thanks for verifying.

AGWhitehouse

Despite the "laboratory" logic, vapor retarders are required on the insulation here in CT. That "white kraft paper" is the vapor retarder that is treated to meet the flame and smoke index so that it can be left exposed.

The actuality of metal buildings is that they are quite leaky at the base and top plates. The fluting is rarely sealed tight and even the vertical ribbs are sometimes left un-caulked or gasketed and allow for moisture migration. The same goes for the interior paper. So while on paper you have a double barrier system, in reality it is a double "kinda" retarder system. Both sides allow for moisture to come and go but don't let "bulk" amounts through that cause damage/mold/etc.

To answer the OP, if you're going to add more insulation, you'll need to breach the existing vapor retarder and add a new one on the interior side of the insulation PROVIDING you are in the proper climate zone for a warm-side in winter vapor retarder.

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