Mould in a new building comes as a surprise, but it is logical: the walls still hold construction water while there is next to no air exchange.
Construction is a wet process. Concrete, floor levelling screed, mortar, plaster and filler are all worked with water, and a large share of that water stays in the material long after the surface is hard and dry to the touch. A single square metre of floor screed can hold several tens of litres of water, and all of it ends up in the indoor air sooner or later.
The rate of drying depends on the thickness of the layer, the material, and how much air exchange there is in the room. In practice we are talking about months rather than weeks, and for a thick floor layer often the whole of the first year. That is not a shortcoming and it is nobody's mistake: it is simply a property of the material.
The problem starts when that water has nowhere to go. If moisture leaves the structure into the room but the room does not take it outside, the relative humidity of the air rises and settles at a high level. How that works is described in more detail in the section on indoor humidity.
In the first year in a new build the moisture regime has to be managed deliberately — it does not sort itself out if you simply close the place up and switch the heating on.
A modern new build is designed to be airtight. That is deliberate and correct — an airtight envelope makes a building cheap to heat. But an airtight building fundamentally cannot work without functioning ventilation, and this is exactly where the gap between design and reality opens up.
The typical situations we see at inspections:
On top of that come the refurbishment and moving-in works: filling, painting, tiling and wet cleaning all generate further moisture at exactly the period when the room is least able to get rid of it.
Each of these situations on its own would not be critical in an old, leaky building. In a new build with construction moisture in the walls, they mean the moisture stays inside. How air exchange ought to work is described in the section on ventilation.
In summer, high humidity in a new build often goes unnoticed: the surfaces are warm and no condensation forms. In autumn and winter the inner face of external walls, window frames, wall corners and the areas at the floor slab cool down. Moisture from the air starts to settle on these cooler surfaces, and within a few weeks the first patches appear.
Another reason it happens specifically in winter is the heating being switched on. Warm air can hold more water vapour, so heating speeds up the release of construction moisture from the structure into the room. If that moisture is not carried outside at the same time, switching the heating on makes the problem worse rather than better.
So the typical call about mould in a new build comes in December or January — in the first or second winter after moving in. The most commonly affected spots are external wall corners behind furniture, the bedroom's external wall, sill edges, and an unheated storeroom or dressing room with no extract.
| Location in a new build | Most common reason |
|---|---|
| External wall corner in the bedroom | The coolest room, high humidity at night, air that does not move |
| Edges of window frames and sills | The coldest surface in the room, where condensation appears first |
| Dressing room or storeroom | No extract, door shut, weak heating |
| The wall behind a wardrobe | Air behind the furniture does not move, the surface is cooler |
| The floor edge at an external wall | The screed is still giving off construction moisture, the edge is cooler |
| Bathroom with no working extract | Steam stays in the room after every shower |
Another frequent factor is that in the first winter in a new home the heating is often kept lower than it should be. Here it is worth saying something that sounds contradictory: in the first winter in a new build it pays to keep the heating slightly higher and air more often, rather than the other way round.
This is the question an inspection has to answer, because what happens next depends on it. Normal drying of construction moisture shows up as evenly raised air humidity throughout the home, falling slowly month by month provided the ventilation works. A real defect shows up locally and does not fall.
We compare readings between rooms. Evenly high humidity points to the drying process, locally high humidity to a specific source.
We measure in walls, floors and slabs. This shows whether water is still in the structure and how deep.
We check whether the system works at all, whether the filters are clean and whether air reaches every room.
We look for places where the surface temperature is lower than it should be — these point to a break in the insulation.
We set out what is normal drying and what is a defect, with specific readings and locations.
If the readings point to a structural problem — a break in the insulation, a cold bridge, an unsealed vapour barrier, a leak or unbalanced ventilation — we set it out in writing and specifically: which spot, which sign, which reading. That is the document to take to the developer or the builder during the defects liability period. Our inspection is not a structural survey and does not replace one, but it is a concrete, measurement-based basis for the conversation.
We also say when there is no problem. If humidity in a new build is evenly raised, the ventilation is not switched on and there are no defects in the structure, then the answer is not treatment but air exchange and time. Selling treatment in that situation would mean taking money for work that changes nothing.
Inspection with moisture readings from €40. Treatment from €1.50/m², minimum order from €80. In new builds an inspection and a written finding on the moisture source is often enough, because the affected area is still small — that is also the cheapest moment at which to deal with the problem. If the rooms have to be dried with equipment, we price by area. For the overall order of work, see the section on mould removal.
It can, especially with thick floor layers and heavy concrete structures. In the first year it is to be expected. If humidity does not fall in the second year either, the reason is usually no longer construction moisture, and another source has to be found.
During the drying period it can be useful, especially if the ventilation is not yet fully commissioned. But it is a temporary measure. Permanently working air exchange matters more than any piece of equipment.
Order an inspection with readings and get a written finding on the moisture source and its location. You can take that to the developer or the builder during the defects liability period. Our finding is not a structural survey, but it is a measurement-based basis for the conversation.
Describe the situation — we will tell you whether treatment is needed at all, and which one.
Fill in the form — we will call you on the number you provide, ask a few clarifying questions and propose a solution.
Thank you! We will contact you on the phone number you provided shortly.
