Oak barrel overflowing as weather gets warmer

We have a barrique of pinot, vintage March 2006, and approaching our southern summer the temperature is rising. We have a solid silicon bung on the barrel, and the option of a waterlock (hasn't been through malo yet). The problem is, as we get a warm spell of three or four days, the ambient temperature is rising and the wine is expanding; when we take off the bung the wine spills out. Then when the weather cools down for a few days, the wine contracts again and we get ullage. I guess this is why people age their wine in cellars, where the temperature changes are smaller. The barrel is kept in a brick room adjoining the house, but is subject to fairly strong late afternoon heat in our (oncoming) summer. Any ideas as to how we can protect the barrel from extreme heat changes? Worried about oxidation, bacterial contamination etc.