Insulation, Radiant and Vapor Barriers - Adding insulation around bath tub wall

I am replacing an existing bath tub with a new air tub and wanted to add more insulation to two exterior walls (in the area from the top of the tub to the floor). Currently, this area is unfinished but is insulated and has a vapor barrier. A new platform made with 2 x 6 lumber has been installed for the new tub. I have some fibreglass batt insulation and am thinking of adding this on top of the existing vapor barrier. This will be friction fitted and will remain unfinished without another vapor barrier on top. Is this the correct procedure? The blower motor will suck air from around the tub but will be located more that 3 1/2 feet away from any exposed insulation. Is this an issue? Any suggestions would be appreciated.

Hi Chris and welcome to the forum. I'm not familiar with air tubs, but can explain vapor barriers, air movement and condensation.

The basic rule on vapor barriers is that they should be on the warm side of the insulation and in contact with the air barrier. Having a vapor barrier somewhere in the middle of the insulation would risk a cool enough temperature to cause moisture to condense inside that insulation and bathrooms are the worst area when it comes to moist air.

There is another element of insulation that is often overlooked, it needs access to a source of heat. Kitchen cabinets, walk-in closets, and pantries that have little or no source of heat, will go to the outside temperature or close to it regardless of the amount of insulation. Keep this is mind as you don't want a cold tub or cold surfaces to cause a moisture problem from condensation.

Now, drawing air from around the tub sounds problematic to me. Depressurizing any area will cause air to seep in from all sources, so it would need to be sealed in all unwanted directions. A plastic vapor barrier that can breathe in and out will cause similar air movement on the other side and it has been shown that plastic that is allowed to move will not last long, it self destructs.

So, some rigid (pink or blue) could be fitted into that space as it would provide insulation and an air barrier. I would also be sure the air for the blower can not come from the cavity above.

There are still details I can't see, so here is some additional reading:
Energy Savers: Vapor Barriers or Vapor Diffusion Retarders (http://www.energysavers.gov/your_home/insulation_airsealing/index.cfm/mytopic=11810)

enjoy the tub
Bud

Bud,
This is very helpful. Thank you for your time and your comments/suggestions.
Chris