When shopping, consider the aesthetics and the strenght and durability of exterior doors, but do not forget their insulation-value and their airtightness.
Most people are aware of small drafts and air leakage around the doors, but they often ignore or do not know that the biggest problem with exterior doors is their low insulation value - a fraction of the insulation of well built walls and attics.
Low insulation value
The fact that an exterior door is made of solid wood, or steel with a layer of polyurethane, does not mean that the door has a good insulation value.
Even the best entry doors have relatively modest insulation levels. The insulation of the best doors is in the range of U-0.10 - 0.20 (Metric: 0.56 to 1.13 W/m2K), which is high compared to other doors, but low compared to well insulated walls and attics, with many inches of foam or mineral wool or other insulation material in their assemblies.
Obviously, this is in part unavoidable. Doors cannot offer the same insulation of attics or walls. But you must be aware of the issue. You should look for the best doors, especially in homes designed with energy-efficiency goals in mind.
Pay also attention to the size and to number of exterior doors, and their type. Likewise with windows (which are also potential weak thermal points in the home's envelope) it's important not to oversize the doors, or to limit their number.
Metal entry doors
Pay special attention to cheap metal doors.
They are common in many parts of the world, and contrary to other steel entry doors, they haven't a thick core of foam. Cheap aluminum and other metal doors have very little or no insulation at all, and are responsible for serious thermal imbalances.
Patio and glass doors
Patio and other glass doors can also have a very negative impact on thermal comfort. See: Patio and glass doors energy efficiency. They should be considered as windows, and sized and located following the same rules.
vinyl, fiberglass, steel, Wood entry doors
Consider a few basics:
- Vinyl, fiberglass and steel-polyurethane doors can be a lot less expensive than solid wood doors, while providing better insulation value (which doesn't mean that they are necessarily better than wood or wood-composite doors, which vary a lot in their thickness and U-value). Many of these doors are remarkably like real wood and may have a thick polyurethane or other foam core - a feature that boosts their insulating value.
- Look for a door with a good insulation value, say, a U-value of at least 0.2, that is, R-5 (Metric: 1.13 W/m2K). In new homes, especially in cold climates, in very energy efficient buildings, look for the best-performance doors possible. Select doors with a very low U-value (the lower the best), that is, a U-value close to 0.10 (Metric: 0.56 W/m2K). A few German manufacturers have such doors.
- The prices of the doors mentioned in the previous point are high, but these doors worth their price in very energy-efficient buildings, in harsh cold and hot climates. For less expensive doors, also with a very good insulation value, look for other Central European door manufacturers.
- Pay also attention to the air-sealing ability of the doors, especially their weatherstripping - and do not forget the glass door lites issue. Special lites can provide a U-value of 0.2 (R-5) or more, which is higher than the insulation provided by standard solid wood doors, but the opposite is more common. Lites often reduce the insulation of the doors.
Top exterior doors manufacturers