RH (Relative Humidify) Hood is an insulated impermeable box/hood manufactured from rigid thermal insulation type material. The hood’s design includes a recessed pocket that creates an isolated air chamber directly above the concrete surface. The hood’s design allows it to be sealed to the concrete surface during testing. This is achieved by lining the bottom edge using double sided Butyl – this product is designed to form a permanent moisture/vapour seal between two surfaces. The air chamber is lined with a PVC vapour barrier so as to isolate the air pocket from the humidity and fluctuations in temperature of the air outside the hood.
Based upon Standards, the hood’s insulated air chamber shall have a minimum area between 200 and 260cm2 with a minimum depth of 6.3mm directly above and exposed to the surface. The central area of the air chamber shall allow unrestricted movement of the entrapped air around the sensor end of the RH probe when it is on position within the hood. Hood is designed that when the RH probe is inserted it seals the chamber.
Underneath a hood
As a minimum, the number of tests performed are three for the first 93m2 and at least one additional test for each additional 93m2. Test locations are selected to provide information about moisture distributed across the entire concrete floor slab especially areas of potential high moisture. Test locations should include within one metre of external edge, centre and any other location that may be susceptible to high moisture.
Prior to placement of the RH hoods, the actual test area shall be clean and free of any foreign substance. Only clean bare concrete surfaces should be exposed to the air chamber underneath the hood. Concrete test locations at times need to be grinded before placement of hood to ensure adhesive residue, curing compound, sealers, paint etc. are removed which should occur at least 24 hours before starting the test.
Concrete floor slabs should be at service temperature and occupied air space above the floor at service temperature/RH under normal conditions. If artificial aids have been used to accelerate drying, these should be turned off at least 96 hours before hoods can be sealed on the concrete surface.
Placement of hoods
Seal the insulated hood in the selected location by peeling off the white doubled side tape, place the hood on the concrete floor and then apply firm pressure to ensure an air tight seal is achieved. The sealant used should be of a type that does not give off any emissions that could affect the relative humidity readings in the hood (Butyl meets this requirement). After the hood is located in place, insert the probe into the hood using the insertion/retrieval tool and place cap over the probe hole.
Allow 72 hours to elapse after sealing the hood to the concrete floor so as to achieve moisture equilibrium in the air pocket under the hood before taking the reading. Remove cap and connect the probe to moisture meter, record first reading after allowing a short period of time for the meter to stabilise and then take the final reading after 5 minutes where the final result must be within 1% drift of the original. Record ambient conditions per hole.
Complete clients report as per appropriate Standard. Concrete subfloor is considered sufficiently dry when measurements taken do not exceed 70% RH.
Photo of the results can be taken if requested by the client as well as pH testing if specified.
Taking readings for new slabs is normally done as late as possible in order to allow the slab to dry. Consequently, a lot of building activity is occurring when the hood is located into position. Hence, protecting the hood, which has a sensor inside, over the minimum 24 hours required is recommended so as to prevent any damage occurring.