The Globe impressively adds up all the recent construction projects announced or under way in Boston, and concludes we’re seeing an apartment construction boom.

The numbers back up the claim: 850 units now under construction. Another 295 as of Monday when developers start yet another project. Those numbers don’t include the approximate 1,000 housing units, mostly apartments, that were under way before this fall season.

The main driver of the trend: a 4.2 percent apartment vacancy rate in the city and among the highest rents in the country.

This is all great for the city, renters and the hard-hit construction industry in particular.

File under: Build, and They Will Come

Boston’s apartment rent prices have hit an all-time high, so good luck finding an affordable unit before the traditional September 1 lease-transition date. The usual-suspect neighborhoods top the list of most expensive areas in the city to rent.

One is tempted to say these rent spikes will sooner of later send people flocking to buy condos or homes. But this housing market has defied all common sense and past trends. Many people remain highly nervous about buying during turbulent economic times.

File under: More jobs, please

The Herald has a good piece on the apartment rental market in Boston.

One interesting item: Those moving in from out-of-state for jobs are preferring to rent. We guess that makes them generally no different from others these days, but it does put added pressure on the rental market here.

Remember: The state’s economy is generally performing better than other parts of the country, keeping workers here and attracting others from elsewhere. It all adds up.

Rents in Boston (and many other places) are climbing, while typical home values — thanks to foreclosure sales — are down.

Take a look at two of the implications:

–Affordability problems for renters. About 24.8 percent of renter householders within the Boston region were spending more than half their pre-tax income on housing and utilities in 2009, up from 19.7 percent in 2000, according to a new report by Harvard’s Joint Center for Housing Studies. And rents have only increased since, according to Hotpads.com

That’s a severe cost burden, but plenty of others are feeling pinched. Nationwide, just 50 % of renter households were spending less than 30 percent of their income on housing and utilities in 2009, the center said.

“In the last decade, rental housing affordability problems went through the roof,” co-author Eric S. Belsky of the joint center stated in the report. “And these affordability problems are marching up the income scale. In real terms, it means more people have less money to spend on household necessities such as food, health care, and savings.”

–New ammo for the buy vs. rent debate. HotPads.com, a real estate site that shows both rental and for-sale listings, states that rents in Boston have jumped in the last year while asking prices for Boston condos on the market have slumped. That’s lowered the “rent ratio” — its buy vs. rent calculation — to the point that buying is becoming more favorable than renting.

Of course, Boston condos (foreclosures) in need of fixing up — substantially, in some cases — are helping to drag down the median asking price. Another cost consideration for the mix if you’re crunching the numbers.

Patrick Killelea, of the housing-bubble-and-bust site Patrick.net, has his own buying rule of thumb:

The only true sign of a bottom is a price low enough so that you could rent out the house and make a profit. Then you’ll know it’s pretty safe to buy for yourself because then rent could cover the mortgage and ownership expenses if necessary, eliminating most of your risk. The basic buying safety rule is to divide annual rent by the purchase price for the house:

Annual rent / purchase price = 3% means: do not buy, prices are too high

Annual rent / purchase price = 6% means: borderline

Annual rent / purchase price = 9% means: OK to buy, prices are reasonable

This could be an interesting exercise for Boston condo owners, how much could you rent your Boston condominium for, and does the annual amount divided by your purchase price work out to 9 percent or more? Renters, would the purchase price on your home — or a home comparable to your apartment — meet Killelea’s test?

If you’re doing the math, remember that the result will be a decimal. Multiply by 100 (that is, move the decimal two places to the right) if you want to see it expressed as a percent.

Okay, I’m going to take a stab at this for a typical priced Boston condo, using HotPad’s figures based on the median asking price ($309,990.00) and the median rent ($1,750.00), works out to about 6.8 percent. Borderline, by Killelea’s reckoning. I’m sure that calculation varies a lot from neighborhood to neighborhood.

I’m interested to hear what your thoughts are.

Area home prices may be still falling in places, but Boston’s apartment rents are expected to rise about 6.2 percent this year, on top of last year’s increase of 7 percent.

Says Scott: “Renters, are you feeling the heat yet? And are we headed for another real estate bubble, this time with apartments?”

A true bubble probably won’t happen until we see contractors scrambling to build new units, flooding the market with surplus supply. But we get the picture.