In an arid climate, should we place the (plastic) vapor barrier inside the frame rather than Tyvek at the exterior?
I was told that using plastic sheeting stapled to the inside frame is preferred as a vapor barrier in our region. Is this accurate advice? We are building a green home in Boise, Idaho. The problem being considered is our higher relative humidity indoors: could mold-preventive chemicals in the insulation tend to offgas to the inside dwelling if an exterior vapor barrier (Tyvek) is used? We do want a vapor barrier to avoid mold problems over time. The insulation needs to breathe/offgas and we do not want the offgassing being trapped or pushed indoors.
Short answer: the advice that was given to you is correct. In your climate, a plastic sheeting vapor barrier on the inside of the framing is recommended, even without the offgassing concern. This is, however, in addition to the use of Tyvek Homewrap or building paper on the outside of framing, which is not a vapor barrier but protects from wind and water.
Long answer: water vapors travel through the wall from the warm to the cool side, cooling off along the way. At some point they will condense to water, depending on the outside and inside temperatures, but typically within the wall. This reduces the effectiveness of the insulation as it gets wet and is a cause of rot. Due to the long, hot summers you experience in Boise, the rot problem is limited as the walls dry out completely in summer.
It is still far better to eliminate the problem in the first place to get better insulation performance and a longer lifespan of your house. The plastic sheeting does just that as it prevents the vapors from traveling into the wall. If you have the sheeting installed properly in the whole house, your house will be fairly airtight (depending on how well windows and doors are installed and sealed) which is great for energy efficiency, but you must remember to let the water vapors you produce out somehow. Letting fresh air in by opening the windows for a couple minutes each day is the simplest way, or you can get an energy recovery ventilator from UltimateAir or others that constantly extracts air from the house and uses its warmth to preheat the fresh outside air it pumps into the house.
Your offgassing concerns will depend on the insulation product you select, but if you use plastic sheeting on the inside of framing it will not affect your indoor air quality anyway.
You will still need to use building paper or Tyvek, on the outside of framing; the choice will depend on your facade system. These are permeable barriers however, not vapor barriers. Think of them as the Gore-Tex of construction. They block out water and wind but let vapor barriers pass. If you would use a vapor barrier like plastic sheeting on the outside, you would experience heavy rotting as the vapors coming from the inside would be trapped inside the wall. Imagine walking around in 90 degree weather with a thick old-style plastic rainjacket: you would get very sweaty very quickly. The same happens to a house with an outside vapor barrier in your climate.
Note that all this advice only applies to non-humid climates that do not use air-conditioning the majority of the year. The described effects get reversed in those climates.
I hope this answer is helpful to you; if you have any further questions feel free to contact me.