Hutt at Heart
a ground moisture barrier underneath a house

Eco Design Tips: Subfloor moisture

Moisture from damp soil under the house can contribute significantly to moisture levels inside the house – and can be the source of mould inside.

Moisture can be present even without drainage or plumbing problems, or runoff entering the subfloor. BRANZ tells us that 40 – 60 litres a day evaporate from the ground under an average house causing elevated humidity. If the subfloor ventilation is inadequate then much of this moisture will pass through the flooring and into the atmosphere inside the house. Often the subfloor ventilation will not be enough to fully get rid of this moisture.

The local clay soils of Lower Hutt hold moisture particularly well. If the ground under the house is obviously damp or dark then it will indicate the presence of moisture. If it has a white or yellow ‘dusting’ there is mould, which will eventually find its way into your home.

The soil will usually be damp even over summer, even if the lawn and garden are dry.

A simple test is to lay a piece of plastic on the ground, about a metre square, with something to hold it down. Check it a week or so later and see if there is condensation on the bottom side of it, or if the ground is damp beneath it.

A Ground Vapour Barrier(polythene sheet) is the simplest solution. If subfloor ventilation is absent, then restore it if it has been removed or covered. Some houses even have fans fitted to help stop this damp air accumulation and becoming a permanent problem. You might even be eligible for subsidized insulation. Check the EECA website to see if you qualify. There are still houses that have missed out!

Hutt City Council offers a free home assessment service by our in-house Eco Design Advisor.

For a full house assessment and advice,

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Subscribe to Hutt at Heart – Te Manawa, your monthly email newsletter where you can find out about fantastic things going on across our city and at Hutt City Council.