The microclimate within a Neolithic passage grave

Microclimate measurements in a Neolithic passage grave in Denmark have shown that natural ventilation through the open entrance destabilizes the relative humidity (RH), whereas a
sealed entrance gives a much more stable RH, above 90%. Episodes of condensation occur on the stone surfaces in summer with too much ventilation and in winter with too little
ventilation. Soil moisture measurements above, below, and beside the grave mound indicate that rainfall on the mound is not a significant source of moisture to the chamber, whereas the ground below the sealed chamber is constantly moist. The chamber can be kept dry all year by putting a moisture barrier membrane over the floor. Apart from the more variable climate within the open chamber, there is also a significant penetration of ozone, which is absent in the sealed chamber. The ozone may have deteriorated the folds of birch bark put between the horizontal sandstone slabs within the chamber. The damage is supposed to have happened when the chamber was left open during the last 200 years. Visitors touching the fragile bark
may be another cause of damage. Keeping the entrance closed seems to be the best solution for preserving the remaining birch bark.