SHEFFIELD based insulation products maker SIG has bought for £38.9 million sterling a private company, WKT Group, which distributes insulation and fire protection products. Also, it plans to buy the Golanski Group in a deal worth up to £19.5 million. During 1995, pre tax profits rose to £24.1 million from £19.8 million in the previous year. The dividend total is up to 6.6p from 6p.