Two unloved commodity ETFs during the first half of the year tracking natural gas and coal paced the gainers this week as investors add money to energy sector funds.

U.S. Natural Gas Fund (NYSEArca: UNG) and Market Vectors Coal (NYSEArca: KOL) gained about 5% this week. [Natural Gas ETF at 2012 High]

Meanwhile, Energy Select Sector SPDR (NYSEArca: XLE) was the top-selling ETF this week with about $500 million of inflows.

In the major U.S. equity indices, the S&P 500 reversed last week’s gain with a 2.3% loss as third-quarter earnings season gets underway. The Dow was also on track for a 2.3% weekly setback while the Nasdaq Composite Index lost 3%.

U.S. Treasury ETFs were among this week’s top advancers on the risk-off action, while volatility-linked ETFs such as ProShares VIX Short Term Futurse (NYSEArca: VIXY) also moved higher.

Conversely, the steepest decliners included ETFs indexed to sugar, homebuilders, semiconductors and base metals.

The top three unleveraged ETFs this week were U.S. Natural Gas Fund, Market Vectors Coal and PIMCO 25+ Year Zero Coupon U.S. Treasury (NYSEArca: ZROZ) with gains of more than 4%.

The bottom three unleveraged ETFs this week were iPath Sugar (NYSEArca: SGG), iShares DJ US Home Construction (NYSEArca: ITB) and SPDR S&P Homebuilders (NYSEArca: XHB) with setbacks of more than 5%.

In next week’s economic data, look for reports on retail sales, business inventories, consumer prices, industrial production, housing starts and existing home sales.