The glut of vacant homes has helped send home prices on a downward spiral for years now. Most real estate experts blame record foreclosures for the excess inventory, but a new report says there are other factors contributing to the oversupply of housing units on the market.

Foreclosures may leave homes empty, but the owners need to go somewhere. If they end up renting elsewhere, that leaves the housing glut unchanged. The proportion of vacant units stood at 14.5% at the end of the second quarter of 2010, just below the 14.6% record high set in the first quarter of 2009. The vacancy rate hasn’t changed much, even though new home construction has plunged — housing starts have been running at an annual rate of under 1 million since June 2008. Historically, the market needed to build 1.5 million to 1.7 million per year just to meet demand.

So what’s the real culprit swelling the housing glut, if foreclosures aren’t to blame?

According to IHS, there are two, actually: The fall in immigration and the growing number of young people moving back in with their parents amid a frustratingly tough job market. These factors have contributed to slowest growth in the number of new households since the second World War.

The underlying reason why inventory remains unnervingly high is essentially the same however you see the troubled housing market: Overall economic weakness – in particular, unemployment, which federal officials last week reported was unchanged from August at 9.6%.

The economy is certainly recovering, but not fast enough to absorb the millions still unemployed. What the IHS report underscores is that, at this point, our economic troubles are perhaps far deeper than our housing troubles.

Admittedly, the trends in immigration and “doubling up” aren’t easy to track. Hard data on immigration does not exist, but IHS points out that households headed by those foreign born under the age of 35 dropped by 338,000 in 2009.

What’s more, it appears more young people are moving back in with their parents or doubling up with others to save money. The number of households headed by 15 to 24 year-olds fell by 124,000 in 2009 from the previous year, while the number of households with six or more people increased by 355,000 or 8% during the same period. In fact, the number of households headed by all younger age groups – those in the 15 to 24, 25 to 34 and 35 to 44 age brackets — fell in 2009, while the number in all the older age brackets increased.

Combined, these factors have overwhelmingly contributed to marked drops in the number of households across the nation. Between March 2009 and March 2010,the number of households rose by 357,000, the smallest increase since 1947. Between 2002 and 2007, households rose by an average of 1.3 million a year.

What the data reveals is that housing can’t rebound until the unemployment rate falls, which will attract more immigrants to the U.S. and help young people live on their own again.

Update: An earlier version of this story incorrectly cited the housing starts numbers as monthly figures. They are annual figures.