You said you used earthen plaster on the exterior. Did you seal with 4-6 coats of limewash? Also, have you maintained your earthen plaster? Earthen plaster needs regular maintenance unlike other plaster options. Limewash does as well, and acts as a deterrent for insects as well as rodents. Aside from those issues, what kind of "tight seal" are you talking? Insulated windows, and such are great but their purpose is defeated if there are unaccounted for weaknesses in the areas where the windows, doors, vents, and such meet the walls. Leakage is way more common than most people realize, particularly when it comes to strawbales.