A musty smell under the floorboards may have one of several causes . It may indicate a problem that could affect the health of the occupants, so it is important to identify the source and deal with it as quickly as possible. The solution may be very simple, or might require more extensive remedial work. The important thing is not to ignore it and hope it will go away -- the earlier a problem is addressed, the cheaper the cost of putting it right.
Dust and debris
An accumulation of dust under the floorboards can be the source of a musty smell. Dust comprises particles of dead skin, food, fibres from fabric and tumble dryers, dirt, pollen, insects and pet hair. It's not surprising a build-up will start to smell over the years, especially when bacteria get to work on it. If it is impossible to lift the floorboards and clean out the dust, then sealing the floorboards may prevent further dust becoming trapped and stop the smell.
If the smell began after a recent spell of rain or occurs at spring and autumn, it is likely that the ground beneath the dwelling has become saturated due to the water table rising. This may well be a temporary one-off event and the smell (usually of damp soil) will go by itself. However, if this becomes a frequent occurrence, then work will be necessary to protect the building's foundations. You will need to seek advice from a structural engineer. Another cause of flooding beneath a property is when a water mains pipe fractures. If you suspect this may be the case, contact your water supplier.
Mustiness can be caused by ventilation bricks becoming blocked by debris outside. Clear away the build up of soil and leaves so that air can circulate. If the smell began after new flooring was laid, then it's possible that air circulation has been interrupted and new ventilation is required.
Rising and penetrative damp
If the musty smell is accompanied by a smell of damp and the appearance of damp patches on the walls, mould growth or peeling paint, then it's a good idea to call in a damp proof specialist, unless the cause is visible and you can fix it yourself. For example a cracked downpipe can lead to rainwater seeping into the brickwork and down into the foundations. Rising and penetrative damp is usually curable quite easily. However, if left, it can cause dry rot, which is much harder and more expensive to eradicate.
The underfloor area can become infested with vermin, such as rats and mice. This will cause a musty smell as their droppings accumulate. Contact your Local Authority who will advise you on the appropriate course of action. If there are children in the house, they will often send a team out to deal with the infestation.
Your landlord has a legal obligation to ensure the property is sound and habitable. You should draw his attention to the problem and ask that he investigate. You must also keep a record of conversations and take photographs if there are visible signs of damp or mould. If he fails to act, then contact the Environmental Health department of your Local Authority.