I cannot imagine a scenario that would result in an unoccupied home not drying out over a six-year time frame.
In my opinion, an unoccupied home would dry out quicker than an occupied home since the occupied home would be adding moisture daily based on occupant lifestyle.
I have been involved in the evaluation of an occupied home about this age that still had moisture in the bottom half of exterior wall cavities (wet blown cellulose). In this case the house was extremely tight, had no mechanical ventilation and was occupied by a family of four. The wall cavities had a vapor barrier on both sides of the cavity reducing moisture transmission to allow the insulation to dry out.
You mentioned a relative humidity level of 30%. We can work with this number to explain what might be happening in your home.
Moisture on the windows is a sign that the glass is at a dew point temperature.
- This is the temperature at which the air becomes 100% saturated allowing for condensation.
- When warm moist air reaches dew point it gives up a portion of its moisture.
You are familiar with this condition when you have a cold beverage in a glass during the warm summer days. As warm moist air comes into contact with the exterior of the beverage container the air reaches dew point and moisture condenses on the glass.
Your windows are like that beverage glass. Outdoor air temperatures act like the cold beverage in the drinking glass.
- When warm moist air within the home comes into contact with the cold surface of your windows it gives up its moisture in the form of condensation.
- Since the thermal performance of a window is best at center of the window the perimeter of the glass is the first area to reach dew point.
If the indoor air temperature is 68 degree Fahrenheit and the relative humidity is 30% dew point is reached when a surface reaches 35.41 degree Fahrenheit. If you raise the air temperature to 70 degree the dew point rises to 37.13 degree .
In your climate zone I suspect that reaching dew point on your windows isn't all that difficult.
Reducing condensation on your windows
I recommend that you have a blower door test done on your home to determine the air change rate per hour, natural, to determine if your air exchanger is operating optimally.
The only way to reduce window condensation is to reduce the relative humidity or raise the surface temperature of your exterior windows (not an easy task).
- If the condensation is relatively universal on all windows in the home the relative humidity level might be lowered by judicious use of the mechanical ventilation system.
- If the window condensation is in or near rooms with water (kitchen, spa or bathrooms) it might be best to use spot ventilation during periods of moisture generation (cooking, cleaning, showering, dishwashing, etc.) to reduce the overall relative humidity levels in the home.
A very tight home can hold moisture generated from cooking, cleaning, plants, pets, perspiration and respiration so test and adjust ventilation accordingly.
For more information:
Read "Our aluminum-frame windows are harboring condensation and mold. Can you recommend window solutions for a damp environment?" a Q&A answered by Steve Saunders.