How long does a new house take to dry out?

A new Australian house arrives with a large volume of water built into it, most of it in the concrete slab. That moisture leaves gradually over the first year and beyond, and it is the reason a brand-new house often has streaming windows, mould in the wardrobes and doors that stick through its first winter. In most cases nothing is wrong. The house is drying out.

This catches people badly, because a new home is the last place anyone expects a damp problem. Owners assume a defect, builders field complaints about something that is largely physics, and the genuine defects that do exist get lost in the noise.

Almost everything written about this describes British and American houses — wet plaster, screed floors, mechanical ventilation with heat recovery, trickle vents in the windows. Australian houses have none of those. The moisture is in a different place, and it behaves differently.

Where the water actually is

The slab, overwhelmingly. Concrete needs water to cure, and it uses only part of what it contains. The rest has to leave, and it leaves slowly — through the top surface, into the rooms above. A slab-on-ground house sits on a large volume of concrete that is still releasing moisture long after handover.

Timber framing. Kiln-dried framing arrives at a moisture content suited to service conditions, but it absorbs water during construction — from rain on an open frame, from the wet slab beneath it, and from the trades working around it.

Wet trades. Render, bedding mortar, tile adhesive, grout, screeds and joint compound all carry water in and release it out.

Paint. Less than the rest, but not nothing across an entire house.

The key point is that in Australia the slab dominates, whereas in the northern hemisphere the linings do. That is why so much overseas advice — about wet plaster drying times, about running a dehumidifier against the walls — does not quite fit.

Why the first winter is the worst

Three things line up.

The house is still releasing construction moisture. The occupants have started adding their own — showering, cooking, drying washing, breathing. And the weather has turned cold, so surfaces are colder and the dew point is being reached far more easily.

Then, most significantly, people close the house up. Windows stay shut, heating goes on, and the ventilation that was carrying moisture away all through summer stops.

A house that felt fine for six months can turn in a fortnight.

What it looks like

• Condensation on windows, worst in bedrooms in the morning. See winter condensation and mould for what that is telling you.
• Mould in wardrobes and behind furniture on external walls — the coldest, least ventilated surfaces in the house.
• A musty smell in rooms that are closed up.
• Doors binding as timber takes up moisture and swells, particularly in wet areas.
• Timber flooring cupping or gapping, if it was laid before the slab was dry enough.
• Dark marks appearing on the ceiling along the joist lines — see what dark marks on your ceiling mean.
• Efflorescence on brickwork, the white bloom that appears as construction water carries salts out through the masonry.

How long it takes

Longer than most people expect, and the honest answer is that it varies too much to put a single number on.

It depends on the slab thickness, the climate zone, the season the house was finished in, how much rain fell during construction, how the house is ventilated and how it is heated. A house completed in autumn in a cool climate zone faces its first winter with most of its construction moisture still present. One completed in spring in a warm zone has months of favourable drying weather first.

What is consistent is the shape of it. The first year is where most of it happens, the first winter is when it is most visible, and the second winter is markedly better than the first in almost every case. If your second winter is as bad as the first, that is worth investigating as something other than drying out.

What to do

1. Ventilate deliberately, especially in winter. Windows and doors wide open for a few minutes, ideally on opposite sides of the house, once or twice a day. Short and sharp moves far more air and loses less heat than a window cracked open all day.
2. Run the exhaust fans, and run them longer. During the shower and well after. Most of the moisture leaves the room once the water stops. And check where they discharge — a fan venting into the roof cavity makes all of this worse rather than better.
3. Do not dry washing indoors through the first winter. A single load releases litres of water into the house.
4. Heat evenly rather than intensely. One hot room surrounded by cold ones drives moisture into the cold rooms, where it condenses. Bedrooms left unheated all winter are where the mould appears.
5. Keep furniture off external walls, with a gap for air to move. Wardrobes on external walls are the classic first casualty.
6. Wipe condensation up rather than letting it dry. Wiping and removing the cloth takes the water out of the house; letting it evaporate returns it to the air, where it condenses again the next night.
7. Use a dehumidifier in a problem room if you need to, but treat it as a stopgap rather than a solution.

What not to do

Do not seal the house up. The instinct in winter is to close everything and turn the heating up. That is precisely the wrong response while a house is drying, because you are trapping the very thing that needs to leave.

Do not lay moisture-sensitive floor coverings too early. Timber, engineered board and resilient flooring all have moisture limits for the slab beneath them, and there are standard test methods for confirming a slab is dry enough. Laying early is one of the more expensive mistakes available in a new house, and the damage appears months later as cupping, gapping or adhesive failure. If flooring is going in soon after handover, ask whether the slab has been tested and what the result was.

Do not assume every symptom is drying out. Which brings us to the distinction that matters most.

Drying out, or a defect?

Construction moisture is diffuse, seasonal and improving. A defect is localised, persistent and often worsening.

Points toward drying out: condensation on windows across the whole house; mould appearing in the coldest, least ventilated spots; symptoms concentrated in the first winter and easing in warmer months; efflorescence on new brickwork; everything improving in the second year.

Points toward a defect: damp in one specific place regardless of season; a stain that grows after rain; water appearing on a wall or ceiling with no cold-surface explanation; mould in a room that is well ventilated and heated; anything that is worse in the second year than the first.

If it looks like the second list, raise it in writing with the builder while you are still inside the defects liability period, and check the statutory warranty period that applies in your state — they are two different clocks and the statutory one runs considerably longer.

Common questions

Is condensation in a new house a defect?

Generally not by itself, and most builders will say so. What can be a defect is the thing making it worse — an exhaust fan discharging into the roof space, missing or badly installed insulation, or inadequate ventilation provision. The condensation is the symptom; the question is whether something built into the house is amplifying it.

Should I run the heating harder to dry it out?

Heating alone raises the air's capacity to hold moisture but does not remove any. Without ventilation you end up with warmer, wetter air that condenses the moment the heating goes off overnight. Heat and ventilate together.

My builder says it is normal. Is that fair?

Largely, for the first winter, and it is a reasonable answer to whole-house condensation in a newly completed home. It is not a reasonable answer to a specific persistent damp patch, or to a fan that does not vent outside. Distinguish between the two before deciding whether to push.

What about the smell in the wardrobes?

Closed, unheated, against an external wall, packed with fabric — that is every condition mould needs. Leave the doors ajar occasionally, do not pack them solid, and pull them away from external walls where you can.

Does this happen in an apartment too?

Yes, and often more noticeably. Apartments have more concrete per square metre of floor area and are generally tighter than houses, so there is more moisture and less accidental ventilation to remove it.

Where to go next

Most of what a new house does in its first winter is temporary. The value is in being able to tell which part is not.

Read next: winter condensation and mould, or what practical completion means if you are still working out what is covered and for how long.

A new house spends its first year giving back the water it was built with. Ventilating through that year is the whole job.