Welcome back, class. This Tuesday, the House Warmers' word of the day is "fenestration," a handy catchall term for windows, doors and skylights — all the major holes in your house.

By fenestrating wisely, you can save big on energy bills by keeping warm air in and cold air out.

Sealing gaps around windows and doors with inexpensive caulk and weather stripping is one of the quickest, cheapest steps you can take to cut costs, according to weatherization experts at the Energy Coordinating Agency (ECA), a local nonprofit.

And the U.S. Department of Energy estimates that upgrading a houseful of single-pane windows to Energy Star-rated replacements will lop about $325 a year off the average Philadelphian's utility bills.

Our first House Warmers' installment (online at go.philly.com/house) offered detailed instructions from ECA on air-sealing around the house.

Today, we're presenting a master class in fenestration, starting with shopping and installation tips from Pennsauken's own Fenestration Consultants Inc.

The unsung local fenestrators, with offices here and on Long Island, N.Y., have done the replacement windows for the north façade of the U.S. Capitol building and the 2,500-window U.S. Treasury building — "It was a pretty big job," says co-owner John Graziano — among other noteworthy gigs.

We then move on to the nitty-gritty on the windows and doors that qualify for the 2009 and 2010 IRS energy-efficiency tax credit. Being an IRS production, this gets slightly complicated, but read along and the mysterious numbers that you see below on the World's Best Windows label will start to make sense.