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false wall to add insulation

terryb's picture

false wall to add insulation (post #185256)

terryb on Wed, 02/24/2010 - 11:05
in

If I add a 3 1/2" thickness to the inside of a bedroom I'm remodeling and put in additional R12 batts, do I need to remove the old sheetrock first? Same question for a vaulted ceiling. I'm in the Seattle area so it's damp here. Thanks

-A +A

If I recall correctly, the (post #185256, reply #1 of 4)

by rjw in reply to terryb [original] on Wed, 02/24/2010 - 12:07

If I recall correctly, the rule of thumb is that a vapor barrier should not be more than 2/3 deep in an insulated wall. (From conditioned to unconditioned side.)

Considering the existing drywall as a possible vapor barrier, and assuming that the existing wall has about the same amount of insulation, it would probably be OK.

OTOH, I'd be a bit concerned about the existing penetrations in the wall: e.g., receptacles, as a potential concentration point for water vapor migration through the "drywall vapor barrier."

 Hopefully someone who really knows will jump in.

======================================== "Man's capacity for justice makes democracy possible, but man's inclination to injustice makes democracy necessary." Reinhold Niebuhr: 'The Children of Light and the Children of Darkness' http://rjw-progressive.blogspot.com/ ========================================

This book is a good source (post #185256, reply #2 of 4)

by milesd777 in reply to terryb [original] on Fri, 03/12/2010 - 17:33

This book is a good source for your answer.

"Builder's guide to Mixed Climates" by Joseph Lstiburek; Taunton Press 1997

I think a second wall is fine here, but would not put a vapor barrier in or use batts with a vapor barrier in that second wall.

Also, think sound barrier here.  To do that , thesecond wall should not touch the first, and you can use foam strips or rubber on the top and bottom plates as isolaters.  http://www.conservationtechnology.com/building_gaskets.html

 

I am not an expert here, though and the pros might be more informative.

Sounds good (post #185256, reply #3 of 4)

by Clewless1 in reply to terryb [original] on Thu, 03/25/2010 - 06:15

to me ... If you have any moisture problems now, you want to take more care, maybe. your climate is mild, temps usually not severe. You could determine your condensation point/condition given your new wall and your general climate/indoor condition if you are concerned about it. 

Have you considered other options? Why are you adding the second wall? What about e.g. rigid insulation w/ new drwall screwed to the existing studs?

As the other poster mentioned ... may be issues w/ e.g. outlet boxes. BTW ... how ARE you handling outlet boxes in your renovation?

Concepts often are simple and easy ... it's the details that are often messy.

There ain't NO free lunch. Not no how, not no where!

If Rez were around............ (post #185256, reply #4 of 4)

by calvin in reply to Clewless1 [original] on Fri, 03/26/2010 - 03:29

This quote would have been certainly entered into the hall of fame.

Concepts often are simple and easy ... it's the details that are often messy.

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