Each sensor location is precisely determined in advance, and each sensor group measures the moisture content of wood at eight different depths. In the top layer, with a vertical grain, measurements are taken from a depth of 8 and 18 millimetres with 30- and 40-millimetre-long screws.
Determining the sensor locations, conducting the measurements and documenting sensor locations and serial numbers took the same amount of time as the actual installation.
Each measuring site has four sensors and 16 screws, whose varying length (30–240 mm), locations and predrilling depth follow a precise plan. The function of the calibrated moisture content sensors is based on electrically conductive screws of 10 different lengths. The longest sensor screws extend all the way to the surface layer of the interior wall. To avoid any of the longest screws breaking, each hole was drilled to a precisely defined depth. Image: Anssi Yli-Jyrä
Monitoring the exterior surface of the building’s CLT frame is crucial, as laminated timber weakens and ages under intense fluctuation in the outdoor environment. Most of the sensors are installed in the exterior of the building, with only ventilation laths and cladding panels installed on top of them.
The interior surface of the frame has a multi-layered flame-retardant coating, which is why it is good that as few holes as possible were drilled in the interior wall during the construction phase. To measure the moisture content in the top layer of the interior of the building, the lamella was penetrated from the exterior of the wall using 240-millimetre-long electrodes.
Following the installation, the sensors have operated entirely independently, transmitting data through the Internet of Things (IoT) network to the data clouds of both the University of Helsinki and the sensor manufacturer. The data make it possible, among other things, to observe the moisture content of wall elements at various layer depths.
The sensors have been modelled in the floor plan. In the example image, the moisture content of the surface layer of the wall element is over 13%, while the mid-layers retain a lower moisture content of roughly 10%, which prevailed in the material under factory conditions.
Only a fraction of the sensors enabling its infrastructure have been installed in the new building. While as many as 108 different measurements can be obtained from these alone, the entire sensor system designed for the frame encompasses nearly 10 different sensor types and hundreds of individual sensors. The aim is to expand the Living Lab in Hyytiälä so that the variation of indoor conditions and the effect of the built environment on user experience can be investigated.
The expected lifespan of the structural sensors is 15 to 30 years. The goal is for the monitoring of the frame to continue for decades. The longitudinal data generated can be combined with extensive data from the SMEAR II station. The resulting multidimensional data are needed, for example, to model the impact of local conditions and the atmosphere on the sustainability and user experience of timber building.