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Unit: Inspecting and testing subfloors

LMFFL2004A: Moisture test timber and concrete floors
LMFFL3101A: Inspect sub-floors

Section 4: Measuring moisture and pH

Insulated hood test

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The surface-mounted insulated hood test works on the basic idea that moisture will rise to the surface of a concrete slab and evaporate into the air - so if you trap the vapour above the surface, you can measure the air's relative humidity.

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There are several types of hygrometer designed to measure RH in a contained atmosphere.

The traditional synthetic hair hygrometer has a hair that changes length in response to the RH level. The length is measured by a gauge on top of the unit.

A more modern variation uses a moisture-sensitive probe inside the sealed box. This is attached to an electrical cable that can be connected to an external unit with a digital readout.

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Using a relative humidity probe and insulated hood

For slabs at ground level or below, it's best to place one hygrometer in the centre of the room and others within one metre of all exterior walls where floor coverings will be laid.

Carry out as many tests as needed to meet the requirements of the Standard.

In particular, choose areas that might be susceptible to high readings, such as entryways and walls near garden beds.

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At each test location, clean the concrete surface back to bare porous concrete.

Remove all traces of curing compound, adhesive residue, and anything else that might impede the free flow of vapour.

You can check that there's no burnished or non-porous surface material by doing the 'water drop' test - see. Inspecting concrete subfloors.

Once it's clean and free from dust, leave the test area exposed for at least 24 hours.

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Place the insulated hood on the test area and seal it off from the surrounding air.

Some installers use plumbers putty, others use several layers of insulation tape. Either way, it's important that the seal is completely airtight.

Leave the hood in place for at least 72 hours.

Note that if the slab is 200 mm thick or more, or if it has a low porosity, you might need to wait a lot longer than 72 hours for the air chamber inside the hood to reach moisture equilibrium.

In these situations it might be better to use the in-situ probe test.

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The way the reading is taken will depend on the model of instrument. The 'Tramex Hygrohood', for example, has a built-in digital readout.

Be careful of 'drift' in the readings - the meter reading must not drift by more than 1% RH over a 20 minute period.

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Learning activity

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There are lots of YouTube video clips produced by the manufacturers of moisture testing equipment. These videos are designed to promote the company's own products, so naturally they show them in the best possible light. However, as long as you keep this in mind, the videos are very helpful in demonstrating how particular systems work.

The link below will take you to a clip produced by Tramex demonstrating their 'Hygrohood'.

Tramex Hygrohood

Have you used the hood method to measure RH in a slab? What brand of hygrometer did you use? If you haven't used one before, do you know of any other manufacturers of hygrometers? Find out as much information as you can about the hygrometer you'll be using in preparation for the practical assessment demonstrations.

Share your answers with your trainer and other learners in your group.

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