As we have learned to insulate and tighten up our buildings to conserve heat in the winter and coolness in the summer, we have created another problem. On a cold winter day your house is usually warmer inside. This warm air holds more moisture than cold air. Warm air rises...and often infiltrates the roof construction. As it gets closer to the outside layers it is cooled and drops its moisture. Often this moisture has no way to escape...and is trapped. Having water trapped above your ceiling sheetrock and below your roof plywood is bad. The rafters will rot and mold might form on the sheet rock.

Another common moisture problem that I have observed is often found in older stucco houses. If your stucco was not installed with a sufficient air space behind it and an opening at the top and bottom, water can get trapped behind the stucco and this is bad. This frequently happens around windows, where the stucco is cracked and very frequently in houses with stuccoed parapets and no eaves. Modern details call for metal flashing & ventilation on top of these parapets. It might not look as traditional...but it will work way better. Modern details also call for a good drainage layer behind the stucco, (way better than chicken wire) to space the stucco away from the plywood sheathing. Vents at the bottom and the top to let out any moisture that gets behind the stucco are also very important. It is important to stop the stucco 6" or more above the grade to allow for adequate ventilation. There are also new elastomeric stucco products that will not crack so easily, but there is another school that is into more natural, breathable products (oftern called earth plasters) that don't attempt to seal out water.

The latest thing in "water managed wall systems" is the rainscreen. This is a system where the siding only provides a first level of protection and deflection of water. Underneath is a ventilation space and a waterproof membrane.

This site explains the difference between the traditional way and the rainscreen.

It is quite common for people with old houses to tear out the ceilings and use the attic space for vaulted ceilings. This can create wonderful spaces, but it is important to do this the right way so as not to create a moisture problem and so that you can properly insulate the roof.

If you replace all your old single paned leaky windows with modern double glazed windows and insulate thoroughly, you might want to consider a system for bringing in fresh air and exhausting the stale air. These Air RecoupAerators might solve the problem and also transfer much of the heat (in winter) from the exhaust air to the supply air, saving you additional heating costs. These sorts of systems also can filter the air. This is of couse a vast improvement on the old way of getting fresh air through all sorts of cracks and dusty crevices in your house.