Prague Basement Apartments: How to Tell Dangerous Damp from Common Mould

"Mould in the upper corners of a room" is almost always a ventilation problem — condensation due to thermal bridges, where warm humid air hits a cold surface. It is annoying but fixable cheaply.

"Saltpetre (white crystals) and peeling plaster at floor level" is structural rising damp. Ground moisture is migrating through the masonry. This is expensive and invasive to fix.

Key diagnostic: if the damage is at ceiling height → thermal bridge. If it is at skirting-board level → rising damp. If it is everywhere → you have both.

Old Prague brick apartment buildings were designed to "breathe" — gaps in original windows provided passive air exchange.

When PVC windows are installed without a ventilation (recuperation) system, the building becomes airtight. Humidity from cooking and bathing has nowhere to go. Result: condensation mould appears within 1–2 years of the renovation.

The fix is not to remove the new windows — it is to install a recuperation unit or at minimum HRV vents. Budget: 15,000–40,000 CZK depending on flat size.

Warning sign at viewing: new PVC windows in a pre-1950 brick building with no visible ventilation unit = high mould risk.

A visual inspection tells you there is a problem. Only calibrated moisture measurement (capacitive meter + speedy test) tells you how severe it is and whether it has stabilised or is progressing.

For a basement unit or ground-floor apartment, professional measurement adds ~3,000–5,000 CZK to the due diligence cost. Remediation of structural rising damp costs 100,000–400,000 CZK.

TUTEL recommendation: never purchase a ground-floor or basement unit in a pre-1960s Prague building without a dedicated moisture audit.