What does it mean when we say that a log house breathes?

It’s a proven fact that a log house is good to live in. Your stress levels fall, your mind is at rest and your lungs feel great. When indoor air changes sufficiently and in a controlled way, the air humidity is pleasant, the indoor temperature is even and no draughts can be felt, residents can enjoy a sense of wellbeing.

Logs are a special construction material because their natural properties support these factors, thereby contributing to better indoor air. We interviewed Jukka Rintamäki, our Product Development Manager, who told us what makes a tight but breathable log wall, and why it provides an ideal indoor environment.

So is a log house sealed?

Honkarakenne’s log houses are very airtight. “Based on years of product development, we have managed to make all building joints tighter,” says Rintamäki. “If construction is done properly, using the best sealing materials, the best airtightness category (A) is easily achieved. Indeed, many Honkarakenne houses have impressive figures: q50 = 0.2 m3/(m2h).”

When the building envelope is airtight, ventilation is under control. “Controlled ventilation means exactly the right amount of indoor air being changed mechanically and incoming air being of high quality because it passes through filters in the ventilation unit, with no air bringing in impurities, small particles or odours through uncontrolled and unfiltered penetration of structures. Controlled ventilation also saves energy and increases ecology, because the ventilation unit recovers some heat from outgoing air to warm up incoming air,” Rintamäki says.

If construction is done properly, using the best sealing materials, the best airtightness category (A) is easily achieved.

Jukka Rintamäki

A solid log wall also stores thermal energy. “This is based on the logs’ high thermal capacity, density and favourable thermal conductivity,” says Rintamäki. “A log house utilises solar energy in the spring and autumn in particular, with solar heat being stored in the log structures when the sun is low. The log wall also releases heat slowly during the night, which translates into lower heating costs.”

How can a wall be both tightly sealed and breathable?

If a structure is breathable, this does not mean that air flows freely through it. “Breathability refers to wood’s natural ability to bind and release moisture. Wood is a hygroscopic material, meaning that it always strives to be in balance with the surrounding air. During the summer, the log absorbs moisture from indoor air and releases it during the drier winter season. The same happens every day and night: for example, a breathable log surface in bedrooms binds humidity from indoor air at night and releases it during the day. ”

Breathable log walls even out the spikes in humidity variation, which is important to both health and living comfort.

Jukka Rintamäki

“A log wall built from a single material is a safe choice in terms of humidity, and its durability is not affected by the fact that it breathes. Nor has it any effect on energy efficiency. Breathable log walls even out the spikes in humidity variation, which is important to both health and living comfort.”

How does even air humidity contribute to human health?

We know from experience that people suffering from allergies and asthma enjoy a sense of wellbeing in log homes. Rintamäki has a scientific explanation for this phenomenon: the harmful factors in indoor air – bacteria and mould spores – are at their lowest when the relative air humidity is between 30 and 55%. “The more a building has breathable log walls to even out variations in relative air humidity, the easier it is to stay within this optimal air humidity range,” says Rintamäki.

He adds that, according to a 2018 indoor climate classification, pine and spruce logs are considered low-emission construction materials in the M1 class.

How does gluing or surface treatment affect breathing properties?

“The glue lines of all Honkarakenne lamellar logs allow permeation by water vapor. This means that they are not a cause of humidity being trapped inside logs. Research findings indicate that the water vapor resistance of one glue line is equal to that of wood 25 mm in thickness,” says Rintamäki.

Rintamäki gives advice on the interior treatment of log walls. “You can leave the interior surfaces of the logs untreated, if you prefer. However, treating them protects them from dirt and darkening as a result of solar UV radiation. Because treatment of wood surfaces affects the wood’s humidity behaviour, to maintain the logs’ natural ability to maintain a stable humidity level you need a surface treatment agent that allows permeation by humidity, and you must always follow the manufacturer’s instructions. When selecting a surface treatment product, you should focus on low emissions as well as humidity permeability, so an M1 Emission Classification is always a positive choice.