Screed Relative Humidity Testing

The most used moisture testing method for screed and concrete is the surface humidity box or a plug. 

The use is detailed in BS 5353, BS 8203, BS8201, and BS 8425 

When conducting the moisture test, a humidity box traps air above the slab/screed.  

The box is sealed to the floor with butyl sealant tape and left for not less than 72 hours.  This will allow an equilibrium to be reached. 

The building should be fully watertight and ideally have good ventilation. 

Any underfloor heating or other artificial drying methods in the base should be switched off 96 hours before any testing is carried out. 

An alternative to the box method is a humidity sleeve or the in-situ method, as it is referred to.  The relative humidity probe is inserted into the slab/screed, and the small pocket of trapped air reaches equilibrium as with the box, which is then read with the probe. 

This testing method does require the slab/screed to be drilled. However, it can give more stable results.  The airspace is situated inside the slab, and this can act as a better insulator against temperature change. 

After drilling the holes for the sleeve, testing should not be carried out for at least 72 hours. 

The probe is inserted into the sleeve and left for 30 minutes.  After a minimum of 24 hours, the process is repeated until two consecutive readings are identical, meaning the trapped air is in equilibrium. 

The sleeves can be left in situ, and repeated tests made until the required reading is achieved. 

The slab/screed is deemed to be dry at: 

  • 75% RH or less 
  • 65% RH or less for some timbers