The use of wood in construction continues to increase, with larger and taller buildings now being constructed from wood. At the same time, sceptics continue to highlight the disadvantages and challenges of wooden construction. So, what is the reality? What is true, and what is based on a lack of knowledge?
In this series of reports, Wood Talks, we ask experts to share their insights on some of the most common misconceptions about timber construction. In this article, we interview Per Karnehed, a moisture and energy expert from Karnehed Design & Construction AB, who shares his insights on moisture and timber construction.
Not at all. If this were to happen, it would be due to a fault in the construction. Even if the wood has been wet, it will dry out in a well-sealed house and will not become mouldy. If wood had a clear tendency to mould, Sweden would be full of old, mouldy houses by now — and that is simply not the case.
First and foremost, let us be clear about the three interacting conditions required for mould growth in a finished construction:
Wooden buildings provide us with nutrition — wood is undoubtedly an organic material. We also often have an indoor temperature of around 20 degrees, which fulfils two out of three conditions. However, it is extremely rare for buildings to have a humidity level above 75 per cent.
Of course, there are exceptions such as irrigation systems and greenhouses, but most residential buildings, public premises and swimming pools operate with a relative humidity (RH) between 30 and 60 per cent, which is well below the 75 per cent required for mould growth. Since only one of these three conditions needs to be absent for mould growth to be prevented, there is rarely any risk at all.
CLT is particularly easy and safe to build with because the insulation is on the outside. In Sweden, homes and offices are kept at around 20 °C, meaning that walls and joists are always warm and dry. In a stud wall with plastic sheeting on the inside, however, the construction only works as long as the plastic is completely sealed. If the sheeting is accidentally punctured, the risk of moisture problems increases. With CLT, however, there is no such problem – you can nail and drill as much as you want without risking damaging any sealant.
"I would say that it is a misconception that wood is particularly sensitive to moisture. In fact, wood is a robust and forgiving material with very predictable moisture properties. It is a homogeneous material that is easy to understand."
Per Karnehed, moisture- and energy expert, Karnehed Design & Construction AB
How could that be? Timber has been shipped to construction sites since time immemorial; the trees come from forests where it rains heavily. Who could possibly believe that wood cannot withstand water? In fact, wood is highly resistant to microbiological activity and rot. Of course, a roof will eventually be needed, and the wood must be able to dry in a well-sealed house. Wood will eventually break down, but I would say it can withstand four to six months without any problems, which is reasonable for a construction process.
The wood becomes weathered, but once it has dried, it poses no risk to hygiene, health or the environment.
I'm willing to stick my neck out and say that modern moisture protection and discussions about weather damage to wooden houses are based on economic interests. My experience – and my firm opinion – is that these measures are unnecessary. It is completely unnecessary when we refer to interior structures, where the wood is heated indoors.
For those who claim this, I would like to reiterate that if you ensure the structure is kept warm and dry, there will be no deterioration whatsoever. Taking extra care to ensure that each floor joist is as tight as possible, for example by taping the joints, will minimise the risk of both moisture and difficult or expensive maintenance.
Exterior structures made of CLT are another matter. Here, chemical or structural wood protection is needed, such as paint. Joints exposed to high loads can have reduced drainage capacity, meaning water can accumulate.
Therefore, it is important to protect beams close to ground level, as even metal joints can cause condensation, which can lead to damage to the wooden structure.
However, I conclude that moisture does not increase maintenance costs compared to other materials. Wooden structures protected behind a facade material on a warm, dry side, where wood should be placed, have very low maintenance costs.
Designing with wood is no more difficult or expensive, but all design work requires knowledge, particularly about the chosen building material. A clear production strategy is also needed; for example, the construction process for a wooden house should be vertical, not horizontal. It is important to understand how wood works, its suitability and its best uses.
Wood is not always very flexible, and certain limitations in terms of geometry and design must be acknowledged. However, knowledgeable architects, committed designers and clients can ensure high-quality craftsmanship.
"Many Swedish construction processes are prolonged and made more expensive by unnecessary measures. Wood that is allowed to dry can withstand exposure to water. Moisture is not the alarming danger that so many people believe it to be – perhaps especially when it comes to wood that can withstand weathering. As long as you make sure the wood remains dry, there is no cause for concern. We can coexist with mould spores as long as the relative humidity (RH) does not exceed 75 per cent.
I would say that it is a misconception that wood is particularly sensitive to moisture. In fact, wood is a robust and forgiving material with very predictable moisture properties. It is a homogeneous material that is easy to understand. Damp wood can easily be dried with a little air circulation and heat.
However, I agree that the consequences of water damage are likely to be more severe in a wooden structure, but that is not the material's fault. Instead, we should take wood's properties into account when creating structures and take measures to prevent damage.
Wood is a fantastic material to build with; humans have been doing so throughout history. It provides buildings that are fresh, climate-smart and healthy, and that can withstand exposure to moisture."
Further reading on timber construction and moisture can be found in the Swedish Wood Association's publication; Moisture-proof CLT construction without a full temporary shelter
