'Feel here," Jacob Hellman said.
He took his hand away from the windowsill in a Port Richmond rowhouse and stepped back.
Sure enough, cold air was streaming in.
He and his crew of weatherizers - Robert Wilson and Tee Spearman - were there to fix that and numerous other leaks that were driving the family's electric bills skyward.
Over the course of several hours on Friday, they did a lot of fancy things - such as installing a wall of insulation in the basement and filling gaps around joists with expandable foam. They put weather strips along doors and reflector board behind radiators.
But they also employed a cheap substance, simple to use, that no greenie should be without: caulk.
It's a secret weapon in the home energy arsenal.
Hellman and his crew should know. Every day, the city's Energy Coordinating Agency, a nonprofit organization that administers Philadelphia's energy-assistance programs, sends Hellman's crew, and 35 others like it, out to weatherize homes.
In 2008, the agency weatherized 1,743 low-income homes alone.
Hellman is amazed. He keeps thinking they'll run out of places, but they never do.
Philadelphia's old homes - and surely the distinction doesn't stop at the city's borders - are notoriously leaky. So perhaps not surprisingly, heat is a huge portion of the overall energy cost, expected to run more than $2,000 per household this year, according to the agency.
The federal government's Energy Star program estimates that residents can save up to 20 percent on heating and cooling costs by sealing leaks.
Unfortunately, they muddy the statistic by including both caulking and insulating.
Nevertheless, on many homes, caulk can be "a great first step," said the ECA's executive director, Liz Robinson. A tube, depending on type, can cost as little as $5 or $10. And other than being slightly icky and messy for the inexperienced, the work goes fast.
In the case of the Port Richmond home, caulk was the finishing touch.
Hellman was using siliconized acrylic caulk, which cleans up with water. But at least one manufacturer of silicone caulk claims that kind is better, even if more expensive, because it's waterproof and flexible, won't shrink or crack, won't freeze . . . and so on.
He used a special fan mounted in the doorway to "depressurize" the house so it was easy to detect leaks, but he said it's also fine to simply wait for a windy day and go around checking every seam by holding a hand up to it. (Others say it also works to wave lit incense in front of a seam. If the smoke blows sideways, it's leaking.)
That would include all around windows, doors and baseboards. It would take in electrical outlets on outside walls, places where pipes go through outside walls, wiring holes, and recessed lights.
Hellman is amazed at the variety of leaks he has seen, and sealed.
"Every place you stop up, the air can come through someplace else. It can seem endless."
Caulk can seal anything less than a quarter inch, and since those cracks are less visible, they're often the culprits many people overlook.
They'll put in a new $200 eco-window, Hellman said, but then they forget to caulk, or don't do it properly. The window will still prevent the loss of radiant heat, but drafts will enter around the sides.
It can make a difference in more than the energy savings. A room without all those drafts will actually feel more comfortable at a lower temperature.
"When people talk about being cold," Hellman said, "they're feeling a draft."
Hellman advises something unusual for most people: Read the directions before you start. Make particular note of the detail about cutting the caulk tube's tip on a 45-degree angle (makes for a neater job) and doing it only in warm areas (it may shrivel otherwise).
Fancy gizmos exist for getting a smooth seam. But Hellman prefers a simpler approach.
"Here's the best tool for caulk," he said.
And he held up his index finger.
For more information about caulk and caulking go to http://go.philly.com/earth