After the energy efficiency class requirements in order to get all necessary house construction documents has changed, from 2018.01.01 all new buildings that has been or will be built must comply (A+), and from 2020.01.01 (A++), energy efficiency class.
According to STR 2.01.02:2016 construction technical regulations for dwelling house energy efficiency.
| Energy efficiency class | A | A+ | A++ |
| Doors | U=1,4 W/m²K | U=1,3 W/m²K | U=1,2 W/m²K |
When choosing an exterior door, it is very important to evaluate the properties of all materials used in the production. For example, in order to obtain a heat transfer coefficient U = 1 W / m²K, it is necessary:
about 3cm polyurethane foam filling;
or about 10cm of wood;
or about 12cm of PVC profile;
or about 5000cm of metal.
Polyurethane foam is rigid, hydrofobic material, which is perfect in “warm” exterior door production. However, to fit all door mechanisms in, a solid door leaf and door frame construction made of wood, plastic or metal is required. In order to compensate all weak thermal insulation properties of “colder” materials such as lock cylinders, handles, hinges, etc., the layer of polyurethane foam filler should be at least 5 cm thick. It is not easy to compensate for the mentioned heat loss in this way, therefore the door structure must be correct and have only the inevitable cold bridges (locks or handle bolts).
Wooden doors that were previously made from solid wood are “sensitive” to moisture and temperature. Nowadays glued wood is being used, so it does not have the mentioned drawbacks, but just wooden doors would become very massive and heavy, therefore a “warm” filler in the structure is needed.
Plastic (PVC) doors are sufficiently “warm”, but due to temperature changes tend to deform, because the PVC thermal linear expansion is about 10 times higher than wood. To improve tightness and stability of such doors multi-point locking system must be used.
Metal can not hold the house heat. Consequently the door construction interior and exterior metal parts must be separated by sufficient insulating material layer. The internal door leaf or door frame metal parts must also be correctly insulated. In the long term if the metal parts are insulated poorly, despite huge heat loss, the mold will start to grow on the door opening edges.
In conclusion the most rational solution is to combine all the best properties of different materials.
In many cases, glued wood should be used for the rigidity of the door frame and door leaf, interior and exterior side covered with some nature-factors resistant coatings, and the door leaf filled internally with polyurethane foam.
