boric acid. Although this is effective, it does slowly bleed out of the insulation into the surrounding earth, so loses its effectiveness over the years. Another alternative is to treat the ground around the house with insecticides. Although this is required by certain regions' building codes, it's not a very efficient or environmentally friendly solution.

If your region suffers from termite or ant invasions, you may want to consider using a rigid insulation made of basalt rock and slag (or similar materials), such as Roxul Drain Board. Such products are good insulators, provide water resistance and are very hard for insects to breach.