Measuring the Hemp Test Building: Sensor Locations


The construction of this project is an exciting demonstration of new materials and new construction principles being applied to an actual project. However, beyond providing a physical demonstration, this studio also provides the opportunities for assessing the hygrothermal performance of these new materials and techniques as they perform over time within the Irish climate. That is, it allows us to monitor how heat and vapour migrate through the building fabric.

To assess the hygrothermal performance of the HempLite and hemp lime as building fabric materials, Building Life Consultancy will be monitoring the temperature and humidity within the interior and exterior of the building as well as within the fabric itself. Sensors installed in the building fabric will allow us to test our thesis that using vapour permeable materials will reduce moisture problems when applied correctly. To assess the energy saving potential of these materials, we will also be measuring the U-value through the external walls, roof and floor. This value reflects the amount of heat lost through the building fabric and can be directly compared to other building materials in common usage.

Permanent temperature (T) and relative humidity (RH) sensors have been embedded at multiple locations within the walls, roof and floor in both the studio (HempLite) and Tool Store (hemp lime) sides of the building. This will allow for monitoring of the temperature and relative humidity at different depths within the build-ups. Additionally, because high moisture is a concern relating to the durability of timber, sensors have been installed to monitor the temperature and humidity within the timber frame studs in each wall type. In total, 16 permanent temperature and humidity sensors have been installed.

In addition to the permanent sensors, temporary sensors for measuring surface temperature (Tsurf) and heat flux (HX) will be mounted on interior and exterior surfaces. The readings from these sensors will allow us to perform in situ calculations of U-values for each build-up. These U-value calculations will be conducted in accordance with the procedure outlined by Dr. Paul Baker of Glasgow Caledonian University in his technical paper, In situ U-value measurements in traditional building-preliminary results.

In order to compare our measured values to those of other materials, we must ensure that our readings reflect one-dimensional heat flow through the component (i.e. heat flow in one direction perpendicular to the wall) similar to that obtained in laboratory testing. True idealised one-dimensional heat flow measurements would require large flat areas consisting of only the hemp materials under consideration. In our small test structure this is not possible due to intermittent timber framing and small surface areas.

To obtain accurate readings specific to the hemp materials, we therefore selected sensor locations that minimized the influence of the timber framing and building geometry. To determine these locations, the test studio was modelled using THERM software . The figures below are results from this THERM model. These figures display predicted isotherms and heat flow through the walls. One dimensional heat flow occurs where the isotherms are parallel to the wall surfaces.


Following the THERM assessments, the selected locations of the sensors within the build-ups are shown in the part-plans and part-sections below:

The results from these sensors will give us a real-life assessment of how these materials behave over time. We will be able to observe the rate at which they release moisture obtained during construction and how the thermal performance changes as the materials cure. In the long term, once the construction-related moisture has been evaporated and materials have cured, we will be able to assess the performance of these materials at maintaining healthy and comfortable conditions once occupied.



Wednesday, July 21, 2010