Hi Folks,

I'm finishing out a 3rd space (attic area and have several places where the walls back up to the existing attic. Can I put R19 insulation here or do I need to put some time of barrier in the back? The inspector came be and mentioned to put some barrier in the back but he wasn't clear if this was code or not (I will clarify with him tomorrow). He said I could put some plastic sheathing in the back and then put the insulation in. I want to be able to get thicker insulation in these areas (r19) but the plastic sheathing is going to limit me to r15.

also I noticed they sell some insulation boards at Lowe's... would it be better if I added tehse in the back instead of the plastic to give me some extra r value?

thanks

Best to get a full anser from the inspector but any building grade vapor barrier should be fine, not paper backed insulation. You can but fiberglas insulation that is encapsulated in a plastic sheet barrier, easy to install and not issue with it touching your skin. But tell me which area is heated, the new space or the old attic? BTW, those insulation boards at Lowes need a barrier too.

The new space I'm finishing out will be conditioned. Some of the walls are external and some are to existing attic space. I guess one question is why does it need a vapor barrieron the wall if the ceiling below doesn't need oNe? It's a common space. I.e. ceiling from 2nd floor and wall from 3rd floor

Vapor barrier goes on the cold side of the insulation. That stops condensation from forming which would defeat the insulation properties. Ceiling below does not need a barrier because the attic is conditioned. The ceiling of the attic needs abarrier if it is to be insulated.

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Attaching a picture - maybe it will clarify. The cross hatch I've drawn is existing insulation and this is blown in - there is no vapor barrier as far as I can tell. And the "???" is where I need to put new insulation. So why would I need a vapor barrier in the "???" section but there is no vapor barrier in the cross hatch insulation? They are basically sharing the same space?

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Originally Posted by ballengerb1

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The inspector confirmed I need a barrier between conditioned and unconditioned space. Can I use rigid foam for this? I was reading that the boards can be flammable. Can I use these? One side will back uo to attic and thefe will be drywall ob the other side

Yes you can use that board for insulation but you still need a vapor barrier. The boards are not vapor tight around the edges so condensation can still form. Install your insulation first, cover with barrier and then dry wall.