Insulating Ceiling in Basement Bathroom


  #1  
Old 07-13-03, 08:05 PM
dachrist
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Insulating Ceiling in Basement Bathroom

I am installing a new bathroom in my basement. I am going to put insulation with vapour barrier on the outside walls, but my question is with the ceiling. Half of the ceiling is under the front entryway (ie exposed to outside) and the other half is under the 1st floor bathroom. Should I put up a vapour barrier only under the part of the ceiling that is under the entryway, and just insulation under the bathroom upstairs, or should I continue the vb all the way across the ceiling? I know that you aren't supposed to use vb on interior walls, but this is a strange one.
 
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Old 07-14-03, 05:24 AM
Doug Aleshire's Avatar
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The kraft paper has a permeability rating of ~1.0 Poly has a permeability rating of .5 or less.
The vapor barrier is designed to keep moisture in the house and not lose it to an arrid exterior. Northern climates are very arrid in the winter. I don't think that the addition of poly will help any moisture migration problems that you will encounter in Seattle.

Building codes require a vapor barrier on ceilings to prevent moisture from moving through the sheet rock or plaster into the insulation. Some contractors have had problems installing vapors during the winter. When they do the drywall finishing, the drywall on the ceiling may sag from moisture. This is usually because they have not insulated the attic before beginning the drywall finishing. Moisture from the drywall finishing condenses against the plastic and dampens the drywall. If the ceiling had been insulated, the vapor barrier would have been warm and condensation would not have occurred.

Another argument used against vapor barriers is that the house needs to "breathe." Bear in mind that beathing is based on good ventilation practices, soffit and/or gable vents, good roof vents or ridge venting is important. The vapor barrier is intended primarily to deal with moisture movement. It only affects air movement to the extent that it makes the ceiling less leaky. Encouraging water vapor to get into the attic will lead to wet, ineffective insulation and possible damage to the house. It is much better and cheaper to provide some type of mechanical ventilation if you are concerned that the house will not "breathe" enough. This is critical in a bathroom regardless of location when it is going to be sheetrocked and if you are installing insulation. So your statement of "I know that you aren't supposed to use vb on interior walls, but this is a strange one." is contingent on the room in question. In this case the answer is yes, apply poly
to walls and ceiling. This includes insulation the ceiling of the bathroom, all of it as the cold coming in could cause problems.

Hope this helps!
 
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Old 07-15-03, 06:56 PM
dachrist
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Thanks for your reply... so as I understand it, I should apply the poly vb to the interior walls as well?
 
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Old 07-15-03, 07:01 PM
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dachrist,

You got it!
 
 

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