Owens-Corning Fiberglas Corp. reported a 28 percent drop in second-quarter earnings Thursday, saying high interest rates have hurt its major markets. The supplier of construction and industrial materials said earnings for the quarter ended June 30 totaled $39 million or 92 cents per share, vs. $54 million or $1.28 per share during the year-earlier period. Second-quarter sales totaled $771 million vs. $721 million. The 1990 figures, unlike those of 1989, include results from Fiberglas Canada. ``These results reflect weakness in our major markets, including new housing construction, pleasure boats and automobiles,'' said William W. Boeschenstein, chairman and chief executive officer. ``They are all being hurt by continued high interest rates and cautious consumer buying.'' Owens-Corning said most areas of its Construction Products business were down during the quarter, thanks to declining new housing and commercial construction. But sales of insulation and roofing products for home improvements and remodeling held steady. Sales for the company's Industrial Materials Group were up slightly during the quarter because of the addition of the results from Fiberglas Canada. However, Owens-Corning said demand for industrial materials appeared to be weakening in North America and in Europe. For the first six months, Owens-Corning said earnings totaled $71 million, or $1.68 per share, compared with $89 million, or $2.10 per share, during the first six months of 1989. Sales totaled $1.5 billion vs. $1.4 billion.