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Similar to How to properly insulate a garage? and https://diy.stackexchange.com/questions/31028/do-i-need-a-vapor-barrier-between-the-insulation-and-finish-wall-in-a-new-garage both of which do not have answers pertitent to my one particular question...

My garage is attached to my home. It sits next to a long vestibule area and underneath the bedrooms. All of the walls of the garage are cinder block, including the one which is common to the vestibule. I am planning to hang furring strips, insulation board, and drywall in the garage. I live in northeastern PA which has generally mild humidity apart from a few weeks each summer. As part of this project I will be installing a new windowless insulated garage door.

Do I need to, or should I, install vapor barrier behind the furring strips and insulation?

  • In Pennsylvania the critical season is the heating season. During heating season the hot humid side is the interior, and that is the side that the vapor barrier should face. If the garage is heated, the same criteria hold. If the garage is not heated then if it stays warm it is sucking heat from the rest of the house. – ben rudgers Sep 9 '14 at 16:27
  • @benrudgers Thanks - so vapor barrier would only need to be installed on that side of the garage against the cinder block? – Yuck Sep 9 '14 at 16:29
  • If the home is relatively recent, then there is [ought to be] an existing vapor barrier between the interior space and the garage. It is [ought to be] facing the interior heated space. The same logic would apply to a heated garage at the exterior walls. – ben rudgers Sep 9 '14 at 16:32
  • @benrudgers The house is early 1970s construction. The garage has raw cinder block exposed with no vapor barrier. I'll install it following your advice. Thanks - you should write it as an answer. – Yuck Sep 9 '14 at 16:36
  • Don't forget the "non-insulation" reason for a vapor barrier: the code requirements for blocking any possible flow of carbon monoxide from the garage to the living areas... – DJohnM Sep 10 '14 at 1:44
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As there will be a temperature differential between the garage and house it is a good idea as you could get some condensation here in winter even with mild humidity. Where I live is almost desert but vapor barrier is still recommended due to cold climate. Moisture reduses the effectiveness of insulation so for a small extra investment of time and money you ensure that your work stays effective for years.

  • There is already a temperature differential between the garage and the home whenever the interior home space is conditioned for heating or cooling. – ben rudgers Sep 9 '14 at 16:25
  • My home has both central air and, obviously, heating. I guess I'm worried that moisture could become trapped between the barrier and the cinder block. Is this a legitimate concern? – Yuck Sep 9 '14 at 16:28
  • Cinder blocks are vapor permeable, so they will allow outward drying if there isn't a vapor barrier on the other side. You probably don't want a vapor barrier on the house side, since if the house side has one you'll have a two vapor barrier sandwich that doesn't allow drying. – Zhentar Sep 9 '14 at 16:53

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