Good external doors are quite hard to source in the UK because we have had a poor tradition of insulating and draught proofing them. This is why eco-house builders often use the imported Scandinavian and German ones which are available here.
There are four main considerations
- The materials should be as sustainable as possible and with low embodied energythe total amount of energy it takes to make a material (or a building). See more on embodied energy – Woodmarked timber for instance.
- They should be well insulated (few UK manufacturers quote U values for doors)
- They should be well draught proofed
- They should be protected from weather damage.
Obviously there are other non-ecological considerations such as appearance and security.
Doors are a great opportunity to get creative and use native, locally sourced timbers. Check out Andy’s front door.
Check that timber door suppliers are using timber with the woodmark. Some manufacturers, particularly the Scandinavian ones use a small amount of aluminium to clad the outer surface of the timber or in the form of a drip section to protect the bottoms of glazed areas. This works very well.
The U valuemeasurement of how much heat escapes (or gets in). The units are W/sq.m./°c. see Insulation properties of a solid timber unglazed doorset is about 3.0 W/m²K – quite poor, whereas composite panel doorsets can achieve values of around 0.25W/m²K. Unfortunately most door manufacturers seem unwilling to publish their U values. The building regulationsThese are the legal regulations which govern how a house is constructed. (not to be confused with Planning Permission which is about whether you are allowed to build the house at all or what it might look like) see Building Regulations) cover the U values of doors and this now includes replacement external doors. Part 1LA of the Building Regulations cover this and can be downloaded as a PDF file.
Good draught proofing is essential and is now a factor in the Building Regulations section on air permeability. The problems associated with draught proofing are around endurance. They all work well at the beginning but it is very common (especially with PVC doors) to see draught proofing strips which have been accidentally torn out of place and are impossible to get back or replace with new.