I have learned a lot from this forum and have now started to have concerns regarding my home. Our 1900 sq ft ranch style home was built in 2006 and we had little-to-no awareness of the importance of energy-efficiency and green building. We live in a moderate climate, about 35 miles south of St. Louis, MO.
Our home was built with 2x4 construction, insulated with 3.5" unfaced fiberglass insulation and a poly vapor barrier on the interior (between studs and drywall). The home was not house-wrapped, and does not have any exterior insulation. There is simple plywood sheathing on the exterior, covered by vinyl siding.
My current understanding of green building design leads me to several concerns in the home. Additionally, our air-source heat pump was oversized for air conditioning and initially lead to high RH values in the summer (~55-65%) due to short-cycling and a high fan speed. Two years ago we purchased a Santa Fe Max Dry dehumidifier (for both basement and main floor) which has worked well to maintain interior RH levels at acceptable levels (~35/~45%), depending on which of the 4 hygrometers we read. Two of the hygrometers read about 10% higher than the other two with similar temperature values when all are next to each other and allowed to reach stable temps.
My most recent concern is that I would like to add exterior foam sheathing to increase the R-values of our walls, but am hesitant due to the interior poly vapor barrier as I realize there would be no area for any moisture that may accumulate to dry. This situation is compounded by the fact that we installed a high-efficiency fireplace to heat our home and decrease heating costs. This has worked great, but has lead to higher interior temperatures and therefore, more condensation on the windows. Although the condensation is not excessive, I am somewhat concerned about moisture in the walls. I am considering utilizing the dehumidifier even in the winter to decrease interior RH levels, but 1) I am not sure how low is too low and 2) I do not know which of my hygrometer values I should believe. Currently, our interior levels without the use of the dehumidifier are ~38% or ~48%, depending on which values I believe.
I should note that our house does not have a ventilation system, but needs one based on a blower door test. We are using our fireplace in the winter, which has the ability to bring in fresh outside air (auxiliary air control) which I have done somewhat to reduce interior RH levels. Additionally, I plan to install a fresh air duct to the return side of our dehumidifier to introduce fresh air in the summer. I should also add that the construction of our home is not atypical in our area, due to the lack of energy efficiency requirements in the state of Missouri and the very low electric rates with out electric utility.
I am considering removing all sheet rock on the interior walls and then removing the poly vapor barrier, although I would like to avoid this unless it would benefit the long-term durability of the home. By doing this, it would allow me to add the house-wrap and/or ~2 inches XPS foam insulation on the exterior of the home at a later date, which would involve removing the exterior siding. However, I am not sure if this is worth the effort or expense in our climate and could use some guidance.
Is my consideration of removing the interior poly (and perhaps adding exterior foam sheathing) worth the expense in reducing the risk of condensation in winter and/or summer? Are there other issues that may arise based on the building design of the home? Are there alternate options (such as using the dehumidifier) to reduce the risk of condensation? What interior RH level should I shoot for in summer and winter, given that I have conflicting readings of my hygrometers?