Go back to article: A sustainable storage solution for the Science Museum Group
To date, the hemp-lime walls have been shown to be very effective moisture buffers. Even when the mechanical system was malfunctioning, introducing moisture-laden air into the store rooms instead of dry air, the interior RH levels never rose close to the extent of the environmental conditions in the hangar. Once stabilised at an acceptable RH level and with the mechanical system switched off, the building has demonstrated consistently excellent RH stability.
But as the hemp-lime functions as a buffer not as a controller of RH, some method of moisture removal from the hempcrete continues to be required in order to bring the RH back down to preferred values. Monitoring data from D2 hangar shows RH levels outside the store to be generally between 70–100 per cent. As the RH inside the store is lower than that in the hangar, moisture cannot be transported out through the hempcrete walls and into the hangar; the vapour-permeable membrane fitted between the hempcrete walls and the external wood fibreboard also prevents moisture movement from the hangar back into the hempcrete material. In order to remove moisture, the air handling plant runs cycles of heating and air flow, introducing drier air into the storerooms and extracting damp air. This does gradually reduce RH levels but creates continuous fluctuations which could contribute to the physical deterioration of organic materials over time, although the fluctuations are of short duration thanks to the buffering effect of the hygroscopic materials.
As collections are now stored in the rooms, the RH must be controlled within the group’s set parameters and should not be allowed to rise above seventy per cent for any extended period of time. So far, the air handling system has only reduced the RH to 55–60 per cent during the winter months, with the RH then rising to the maximum level over six to eight months of passive operation. If the moisture removal process is followed as originally intended, cycling dry air and heat through the winter season to reduce the RH, then allowing a passive drift for the remainder of the year, this should provide an acceptable level of control. However, at present the air handling cycling must be run by an operator who monitors external conditions; the building management system software is not sophisticated enough to check or predict drier external conditions in order to turn on the air handling units. As drier conditions generally occur at night outside normal working hours, this means the system does not fully run as designed. Using the heating to reduce RH rather than external dry air has been shown by the monitoring to cause greater fluctuations in RH. Because of the successful (lower RH level and fewer fluctuations) trial with the current dehumidification equipment, discussion is underway regarding the possible installation of dehumidification units in the ducting. These could be operated during working hours, drying external air before introducing it into the storerooms, mimicking the original concept but adding a little more engineering.
When the mechanical system has been running, its operation has been very low-cost. Currently it runs with electrical consumption at 20,397.5kWh against a predicted 24,320kWh, which is less than a third of the running costs and emissions of a conventional environmentally controlled museum store of the same size (Moore, 2013). When in passive mode, the running costs are negligible; costs of running the dehumidifier have yet to be calculated but it will not consume even close to the same power as the fully operating system.
The hempcrete museum store took a longer time to dry out than anticipated by either the contractors or the museum project team. But, because the hempcrete has demonstrated such excellent hygrothermal characteristics, objects stored in the building were not put at risk during the stabilisation period, as shown by the monitoring data. RH fluctuations have stayed within established parameters for the materials stored; RH levels have been slightly high but never in the range to support mould growth or promote corrosion.
Continued monitoring of the hempcrete store may show that, as the building materials continue to cure over time, RH levels do not rise quite as high so that operation of the air handling system will be required less frequently. Or the hemp-lime panels may now be ‘pre-conditioned’, with a memory of an RH level of 55–70 per cent and it may never be possible to prevent the eventual drift back up to 70 per cent. However, operating the building as is done presently still has given the Science Museum Group a very effective and efficient environmentally controlled store at a minimal running cost and an excellent prototype for future storage building projects.
A compilation of images showing (from top to bottom): HMS in use; Storage of horse-drawn carriages on ground floor; Art storage on second floor
Component DOI: http://dx.doi.org/10.15180/150405/013