The U.S. home resale market seems to be running low on houses.

The National Association of Realtors reported Thursday that there were 1.74 million homes listed for sale but not yet sold in the United States in January – the lowest in 13 years.

Based on the current pace of resales, that’s just 4.2 months of inventory – an eight-year low, and less than half that of just 18 months ago.

The thin supply comes amid stronger demand, as the U.S. housing market re-awakens from the nightmare of the late-2000s mortgage crisis.

January sales were up 9 per cent from a year earlier, and prices are up 12 per cent. The NAR cited the lack of inventory of unsold homes as a key driver in continuing price increases. It even argued that sales growth would be even higher, if only there were more homes to sell.

But here’s the thing: There are. They’re just not on the market. Yet.

The NAR’s numbers ignore the so-called “shadow inventory,” the vast number of homes on which lenders have foreclosed, but have been unwilling to put up for sale due to still relatively weak prices and fears of flooding the market with cheap supplies that would hurt pricing power.

(Indeed, the NAR’s survey of realtors last month found that foreclosed properties – which accounted for 14 per cent of January’s home resales – sold for an average of 20 per cent below prevailing market prices.)

The shadow inventory at the end of 2012 was estimated at 3.53 million. It has been shrinking gradually from its peak of 5.4 million at the end of 2009, but the number is nevertheless still substantial.

It suggests the supply of unsold homes in the U.S. market, more realistically, is about triple the multi-year-low number the NAR cited in Thursday’s report. (Compare that with 1999, the last time the inventory of home listings was as low as it is now – the shadow inventory was about 650,000 homes.)

Indeed, the thinning supply of homes officially listed for sale could well trigger the release of more of this shadow inventory into the market.

Lenders sitting on the properties have been waiting for better returns on their underwater investments and more favourable supply-and-demand conditions in which to unload homes that, let’s face it, are essentially unwanted headaches. They’re getting both.

But this release of shadow inventory could also have a cooling effect on the U.S. home market in the coming months. The market for home sellers in the U.S. is certainly improving, but this inventory overhang still means it may not be nearly as hot this year as the realtors would like to believe.