You'll have to excuse Dan Spaeth, an engineer for Fenestration Consultants Inc., in Pennsauken, if he seems to have windows on the brain.

"I look at windows constantly," says Spaeth, whose company has done window retrofits at the U.S. Capitol, among other government buildings.

"Even just walking around, I'll say, 'That's not right.' " Shoddy installation jumps out at him.

Recently, Spaeth has been working on window-consulting jobs for people who live in glass apartments in Manhattan, and for the architects and developers who construct the swank buildings.

He took a break from that rarefied clientele to poke around with the Daily News in the replacement-windows aisle at Buy Rite Lumber, a hardware store on Marlton Pike near his office. He offered these shopping tips for the general window-buying public:

  • "Low-E coatings" and "argon gas" aren't just hype. They are the best of the best in thermal insulation for windows and are features you're likely to find in most replacement windows that qualify for the new federal tax cut.

    Spaeth said he was impressed to see a Low-E coating called Solarban 60 on a line of mid-range replacement windows. "That's actually the same coating we use in New York."
  • Look beyond the glass. Buyers should pay attention not only to the insulating properties of the window panes but also to what's around them, Sapeth said. "The frame matters."

    High-end commercial windows use a "thermal strut" — a rib of insulating material embedded in the frame — to help keep warmth inside and chills out. In some ordinary residential windows at Buy Rite, Spaeth noticed that a layer of hard foam nested inside the frame seemed to serve the same purpose, a nice plus.
  • Gaskets matter, too. Commercial-grade windows use a type of rubber called EPDM in the seals around their edges. Spaeth noticed the same material for sale as weather-stripping in a nearby aisle at the Buy Rite. He recommended it for anyone looking to insulate drafty old doors and windows.

    Although EPDM is more expensive than foam weather-stripping, it won't degrade in sunlight, Spaeth said. Foam will.
  • Caulk is still key. Regardless of what you spend on new windows, you'll still have drafts — or even water leaks — if the windows are poorly installed. "They've got to be properly flashed for waterproofing and caulked. That's huge," Spaeth said.

    A well-insulated window that's properly installed shouldn't feel icy cold to the touch on the inside, even on an icy-cold day, he said. You should not be able to feel drafts around the edges.