The weather is getting cold and a lot of people do not understand the way a fireplace is designed. In order to allow the smoke to draft out of the fireplace and up the chimney, the fireplace must have a continuous flow that take the smoke out of your room. This is called an updraft. This updraft, tends to go on all of the time, even when not burning a fire. Most dampers are warped or broken, so they are not closing off the air gap. In the case of gas fireplaces, the damper must be in the open position all of the time, to allow for any gas fumes to go out and up the chimney.
To remedy the problem, I covered the fireplace with an insulated, magnetic fireplace cover. I discovered this solution 6 years ago and have saved a lot of wasted energy going up the chimney. My room is warm in the winter and much cooler in the summer. The humidity was entering the chimney and causing the air conditioner to run more often than necessary.
There are many energy solutions out there, but none to address the fireplace. I know everyone does not have a fireplace, but those of us who do, are cold and tired of paying for the heat going up the chimney.
I'm surprised no one has mentioned the external air source; in many parts of the country it is a code requirement for new construction. As a retrofit, a damper-like plate and chute can be purchased through a brick supplier, let into the hearth and piped outside. Three other points: 1. A fire needs to burn hot to keep the chimney from becoming so cool that combustion byproducts do not condense on the flue wall. It might be worthwhile to insulate the chimney through an unconditioned attic, and most certainly the gap between the chimney and siding. 2. Even an air tight wood stove or insert requires minimal air sources to support combustion. 3. Glass doors prevent sparks from starting fires and supply the only damping of what is otherwise unrestricted combustion, besides the amount of wood added to the blaze.
In existing homes I have not seen the outside air intakes executed very well. Most times they are without dampers. Worse yet, in prefab fireplace configurations they terminate inside chimney chases instead of outside. So they just end up being another penetration that has to be dealt with.
I presume that you are referring to rooms with a fireplace and chimney which is left without modification after central heating has been installed, to allow for the occasional use of the fireplace on high days and holidays?
I think that the main points to make about this are, for damp climates when closing the chimney off from the room:
a) Leave a small vent permanently open in the room
b) Don't concrete over the top of the chimney at the top.
We bought a house almost 10 years ago, in the UK, where a chimney had been blocked off at the top and there was no longer a chimney pot. The previous owner was finding it hard to sell, due to dampness in the bedroom below the chimney. We were able to get the price down due to that, but all it took to solve the problem permanently was to open up the flue again and replace the original chimney pot! There was already a vent in the fireplace opening at the bottom. It only took about 2 hours of work. No drafts.
Steve, Chimney dampness is a much bigger problem in the UK than it is in the US. It is because of the weather difference and the chimney structural differences combined. But you are correct that you do not want to trap moisture inside of a chimney structure, or you could have a mold problem.
In your UK home example I would bet the chimney pot removal resulted in precipitation seeping in at the crown of the chimney despite the plug at the top. Then there was no way for that seeping moisture to evaporate out, so it grew mold inside the chimney. That is exactly why a functional chimney pot (or cap, as they are known in the US) is important especially if the fireplace is not used.
In USA 83% of houses have AC so there is a way to get some water vapor out. I think its more about A) keeping bulk water out of building, roof and flashing does this B) draining water away from building C) vapor barrier to keep RH out of building D) some type way of getting RH% down like AC.