At least three options come to mind:
- refinish a salvaged antique door and provide new weatherproofing;
- make a new insulated door using salvaged wood; or
- buy a manufactured door made with eco-friendly materials and/or with excellent energy-conserving features.
Salvage and restore an antique door
The most eco-friendly option is to find an antique salvaged door, suitable to the style of the house, and refinish it using green stripping and finish techniques and materials. You should remember that an antique door probably has multiple layers of paint, some of them perhaps containing toxic lead. (This might be a refinishing project for a specialty contractor instead of a do-it-yourselfer.) Installing modern weather-stripping to the threshold and door frame will ensure a good air seal. The lack of insulation in the door itself might well be insignificant in the overall thermal envelope of the house, but selecting a thicker door with little or no glass will obviously improve the insulating value.
One supplier of salvaged doors is Old Wood Workshop in Connecticut. There is also a large, well-respected salvage shop in Astoria called Build It Green. Chances are, there is a local salvage yard with old doors within ten miles of your location. Historically, doors were made by local mills, so if you shop locally there is a chance you might find a perfect match for your old house.
Custom-build a door from salvaged lumber
Another option would be a custom-built door made from salvaged lumber. You might want to look at the websites of Alternative Timber Structures or Avision, both based in Idaho, or Liberty Valley Doors in California.
You could also hire a local craftsman to create a custom door.
For insulation value, I would suggest you consider building a sandwich-panel door with wood faces and edges and an expanded polystyrene or polyisocyanurate core. I would definitely recommend you find a specialty door fabricator, not just a good carpenter. Doors are subject to a lot of structural and thermodynamic stresses and their construction needs special expertise.
Buy a manufactured door made with sustainably harvested lumber or Energy Star-rated insulating materials
Also, take a look at doors that are made for energy conservation. Fiberglass doors with highly insulating foam-plastic interiors look good, take finishes well, and hold up better than wood or steel doors (durability being an ecological virtue, too).
One brand that is well-insulated and Energy Star-rated is Therma-Tru. Marvin and Pella also make Energy Star-rated products. The advantage of buying from a large manufacturer is engineering reliability backed by company warranties-but you may have to compromise on style.