So you've tightened up unwanted/unplanned ventilation, such as leaky windows and doors, cold draughts and bigger issues to avoid heat loss and save energy? You may find that you have more problems with condensation than before, which can lead to mould and other unpleasantness. For example, it could lead to rot and damage in your building structure, and health problems for occupants of the house in bad cases.
We found that condensation on the bedroom windows in particular was getting bad, requiring an occasional wipe down of the windows two or three times per week with a towel to remove the water and wipe away spots of mould.
(We've also had problems with consendation between the panes of our (old) double glazing, not just on the inside of the inside pane, which I think is due to failed seals, and is annoyingly unsightly.)
We had a rucksack jammed beside a wardrobe against an ouside wall go mouldy and leave a worrying mark on the wall; the bag was rescued with a thorough wash and we've improved ventilation where it was. This incident forced me to consider the whole issue more carefully...
In the very cold weather of the 2009/10 winter (several degrees below freezing) we have all been sleeping with our windows shut, so to improve general ventilation all round during the day we make sure that the bedroom radiators are off and doors shut and the windows open for somewhere up to around an hour, ie we force a change of air in the room without cooling the rest of the house too much.
Another important means to avoid condensation is to keep the doors closed of the the kitchen when cooking (and a keep window open) and the bathroom while bathing or showering and then open the bathroom windows afterwards (with the door shut). This avoids the bulk of the moisture getting into the rest of the house, but rather vents it to the outside, again not cooling the rest of the house.
We avoid using the tumble dryer if we possibly can (maybe once or twice November to February when outdoor drying is impossible) and so we bought a washing machine with the fastest spin we could leaving little residual dampness in the clothes after the wash. We then dry the clothes in the kitchen on a clothes horse (with a few stubborn things on the radiator too) but as we keep the temperature up a bit ("1" rather than frost TRV setting) and the kitchen has direct external ventilation because of the gas cooker, this does not seem to cause a dampness problem itself, eg the kitchen window is rarely signifcantly misted up or worse.
If air-tightness gets very good, for example anywhere near Passivhaus levels, then it will probably be necessary to introduce forced ventilation, in particular MHRV (Mechanical Heat-Recovery Ventilation) to get excess moisture out of the house without losing too much heat (or 'cool' in a cooling season/climate).
I'm keeping an eye on Chris Benson's installation...
As a test 2010/12/24 I bought the Argos 425/0777 (MDT-10DMN3) portable dehumidifier nominally able to extract 10l of water per 24h, and rated at 220W but actually measured at 183W. For our smallest bedroom it reduced the relative humidity from about 73% to 50% in a couple of hours and the air felt distinctly drier.
The aim in the first instance is to reduce or remove visible condensation on windows (and any out of sight elsewhere), but also should act as a small heat pump with a CoP (coefficient of performance) of better than 1 by extracting heat from the moisture that was in the air.
2012/01/13: given that our MHRV was having trouble preventing condensation one windows upstairs with low temperatures (<5°C) outside, possibly because of laundry drying and cooking in the kitchen, I set the dehumidifier up in the kitchen late morning and 9 hours later the kitchen was warm (~20°C), humidity was low (~57%RH) even after some more cooking, etc, and the laundry was reasonably dry. The dehumidifier had removed ~1.5l of water from the air. Running ~9h at 180W (as previously measured) ie ~1.7kWh is a big chunk of our usual ~4kWh/d energy budget, but probably less than tumble drying, provides heat, and removes other kitchen/house humidity.
Removing 1l of water from the air with that device uses ~1kWh and takes about 6h and possibly has a heating CoP of ~1.6.
2017/07/04: 'Greenfish' suggested in this thread on GBF
Re internal condensation, have you tried simply getting the air moving more locally e.g. a fan, in those rooms far from the MVHR. So not venting the energy filled air air out, but just moving it about.
Getting the RH down would of course be best, by dehumidifying and/or lifestyle changes, but in my old house static air, e.g. in the "furniture shadows" was the real contributor to where condensation appeared.
Be careful that you don't just disguise other problems associated with poor ventilation by using a dehumdifier. In particular if you have something like a gas fire or gas oven it will put humidity into the air along with combustion products such as CO2, which may make you tired or give you a headache. Even without a combustion device you put out about 1kg of CO2 each day breathing; you need to let it out and oxygen in one way or another!
In severe cases with gas fires and the like, poor ventilation may result in build up of CO (Carbon Monoxide) not just CO2, which can be deadly.
Yes, you want to conserve heat, but "make it tight, ventilate right" still needs the ventilation part.