I am putting an addition on to an existing home. I am putting 2 foam board on the outside and 6" fiberglass in the studded area. Can I or should I put a plastic moisture barrier on the inside sealing all seams and joints. Also If I do that should I vent the 2" foam board with holes at the top and bottom. Thanks in advance for your answer.
The quick answer is yes to all! In your climate we need to prevent as much moisture from being trapped in the wall as we can. Your home interior has a much higher absolute moisture level that the outdoors. the moisture tries to get out to the to the area of low moisture. when it hits the dew-point temperature in the wall it will condense, wetting insulation and other building materials. That causes loss of insulation value, mold, mildew and dry rot. Many foam board products are already perforated to allow moisture vapor to escape. But, yes some type of venting is needed, just like for brick veneer siding. Remember that the last touch is to use the foam gaskets behind switch and outlet plates.