Stand in the average British hallway in January, and you’ll immediately feel where the exterior door stops sealing. It’s not the center of the door. It’s the bottom and the sides where the door frame has pulled away from the wall.
Understanding this should help you spend your money more wisely. Thermally insulated doors only provide energy savings if the rest of the entrance system improves as well.
What do the numbers mean?
When discussing energy efficiency of doors, the term U-value is used. U-value is measured in (W/m²K) and the lower the number the better.
A solid wood door has a U-value around 3. A warped solid wood door has a much worse energy efficiency. An insulated steel or composite door has a U-value between 1.0 and 1.4.
From 2022 onward, under the requirements of Building Regulations (Part L), England has required all new (or replacement) external doors of a dwelling to have a U-value of 1.4 or less. The requirement in Scotland and Wales may also differ. Your door fitter must be registered with a Competent Persons Scheme, or you must notify Building Control. Keep the documentation, as your solicitor will request it when you sell your home.
The U-value is something to check on quotes for each and every job. The leaf of a door always looks better than the doorset. The frame and glass of the doorset make up the bottom of the figure so the doorset U-value is better.
Core, skin and frame.
Doorsets are usually of a similar construction to an insulated garage door. Most are of a skin of aluminum or steel with a PU core. A timber frame is usually incorporated in the inner skin for added rigidity. For doorsets that are required to be of fire resistant construction, timber doorsets perform just as well as aluminum clad or steel doorsets. However, timber doorsets are more prone to distortion and if the doorset is to seal properly it is important the doorset stays true to its shape.
Aluminum doorsets are similar to steel doorsets but the frame incorporates a thermal break.
A door is only as good as its weakest component and glass is usually the weakest component. Double glazed units incorporated in doorsets perform poorly if they are of a large vision panel. A doorset of this nature would be better glazed with triple units. A doorset that incorporates a small vision panel does not necessarily need to be upgraded to a higher specification.
It is worth checking if doorsets incorporate warm edge spacers in the glazing units as this would reduce the cold bridge.
Things people overlook
A few winters ago, we bought an insulated door to replace the old one and tried to reduce the chill in the hallway. The new door didn’t solve the problem because the fitter left a big gap on the old frame and closed it up with a bead of silicone.
The gap around the door frame should be sealed with expanding foam or other insulation. Silicone doesn’t solve the problem, and just covers it up.
Letter boxes should have an internal draught excluder brush. If the design of the door allows it, move the letter box to an external wall.
The threshold should have a seal that touches it along its entire length. Close the door to check. If a piece of paper can be slipped under the door, then there is a big gap along the bottom.
It is easy to miss older back doors and the keyholes that let a breeze through, after you have checked the locks on the front of the house.
Making things worse
The replacement door needs to be able to withstand attack, but that doesn’t mean it can’t be energy efficient as well. A door that has support and structure is better at retaining heat and is harder to break through.
Is it worth the expense?
Definitely, if your door is something like old, rotting timber and you can see daylight around the edges. Old doors can let in a lot of drafts. The thermal doors of today can replace your door and improve the look and feel of the entrance and give you a surprise reduction in your energy bills.
If your door is in reasonably good shape you could always think about improving the seals, thresholds and letter box to give you a more energy efficient door at a much lower cost.