Federally insured savings and loan associations suffered a $1.5 billion loss in the first six months of this year, the Federal Home Loan Bank Board reported yesterday, confirming industry projections of a steeper slide from the poor performance in last year's second half.

Nearly 70 percent of the federally insured S&Ls experienced losses in the first half, doubling the percentage of those reporting losses in the preceeding six months.

As a result of losses reported today, net worth of the savings and loan industry declined as a percent of savings. Aggregate net worth of the industry, however, remained "sharply higher" than the level required by the bank board, the regulatory agency said.

The industry reported a profit of $302 million for the second half of 1980, down sharply from net income of $480 million in last year's first half.

The loss in the January-June period this year represents a minus .49 percent annualized return on assets, a sharp contrast to the positive .10 percent return on assets in the last half of 1980, the bank board said.

The primary reason for the sharp decline from a modest profit to the $1.5 billion loss was a rise in interest rates paid by S&Ls to savers and lenders for funds, the agency added. While associations were paying higher rates, the return on their principal investments -- long-term mortgages -- rose only moderately, the board said.

The average cost of funds to S&Ls rose from 9.11 percent in the second half of 1980 to 10.31 percent during the first six months of this year, bank board calculations show.

The dismal earnings performance of the thrift industry will be a major focus of hearings to be held later in the week by a House Banking, Finance and Urban Affairs subcommittee.

The subcommittee will take up legislation designed to allow federal regulatory agencies more flexibility to aid problem financial institutions. Originally drafted by federal regulators, the proposed bill would authorize interstate and cross-industry mergers to save failing institutions.

Meanwhile, the savings and loan industry is counting heavily on funds from the widely publicized All Savers certificates to stem the flow of red ink over the next year.

Interest on the one-year certificates, which institutions will begin offering Thursday, will be tax free, up to $1,000 on a single savings account and $2,000 for joint accounts.

While some S&L officials expect the certificates to generate considerable funds for mortgage loans, others in the industry are concerned that a new interest ceiling authorized for passbook savings accounts might result in a further drain on earnings.

The maximum allowable rate S&Ls may pay on passbook accounts was increased last week from 5 1/2 percent to 6 percent beginning Nov. 1. A panel of federal regulators approved the hike despite arguments by bank board Chairman Richard T. Pratt that it could adversely affect the earnings of financially strapped S&Ls.