Energy-efficient buildings are vulnerable to moisture

Modern buildings are energy-efficient. But they are also vulnerable to moisture. When natural ventilation disappears, mechanical ventilation becomes critical — and even small faults have large consequences.

Introduction

Buildings today are built to be highly airtight in order to reduce heat loss. Ventilation is typically mechanical and controlled. That brings considerable energy benefits, but it also means that small faults in the ventilation can quickly lead to mould growth and indoor-climate problems.

In older buildings, leaks in the building envelope provided a constant, if uncontrolled, air change. It was energy-inefficient, but it often ensured that enough moisture was removed.

Natural ventilation was a safety valve that compensated for faults and shortcomings elsewhere in the system.

When a tighter building envelope creates new problems

We see it again and again after renovations: new windows, better insulation, a lower heating bill — and then damp problems that were not there before. The reason is that natural ventilation is drastically reduced while moisture production is unchanged.

Before the renovation, air was exchanged continuously through leaks. After the renovation the home is airtight. The new windows seal perfectly, the façade has been sealed. But if mechanical ventilation is not established at the same time, the moisture has no way out. It accumulates in the home, and the moisture level rises.

In older solid masonry the area around the window is often a thermal bridge, where the surface temperature can fall so low that surface moisture rises to a level that creates a risk of mould growth under normal use of the home. The problem is made worse if curtains prevent warm room air from passing across the surface.

The problem is not the renovation. The problem is when the ventilation is not upgraded at the same time.

Small deviations — large consequences

In an airtight building, small faults have consequences faster than in a leaky one. When there is no natural ventilation to compensate, the mechanical ventilation becomes critical. The four typical problems are well known:

1. 1Residents close off vents because of draughts or noise, without knowing that they are eliminating the home's air change.
2. 2Fans are not running at the correct speed or have broken down, without anyone noticing.
3. 3Filters are not changed and drastically reduce the air volume.
4. 4Ventilation systems have never been commissioned correctly.

Experience from Danish housing renovations shows that residents often lack basic information about their ventilation system. There is uncertainty about whether windows may be opened while mechanical ventilation is running (yes, they may), how filters are maintained, and what the different settings mean. This uncertainty leads to suboptimal use.

Data shows whether the ventilation works

With IoT measurements we can see directly whether the ventilation is working:

• Does the moisture level fall after a load?
• How quickly?
• Does the pattern vary between homes in the same stairwell?

If several homes in the same block have persistently high humidity, that points to a shared problem — typically the ventilation system. If only one home stands out, the cause is more likely to be local. That distinction is only possible with data.

In airtight buildings, ventilation is not a comfort feature — it is an operations-critical parameter. And what is not measured cannot be managed.

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