who pays cost of roof insulation

We live on the ground floor of a self-managed 4 unit strata block. The owners upstairs owners want insulation installed in the roof. The two ground floor owners will get little/no benefit from this. The roof is common property so insulation must be approved by the body corporate. 

Is there a precedent or guideline on how much of the cost of insulation should be contributed from the strata funds?

Thanks