Insulating The Door
An insulated door is worth it in a London winter for three reasons, and only one of them is the heating bill.
Short answer
An insulated door makes the garage several degrees warmer, which matters if there is a room above, if the garage is heated, or if anything in there can freeze. It is also substantially quieter and considerably more rigid, which means it dents less and lasts longer. The energy saving alone is a weaker argument than the other three.
The real reasons
A room above the garage. An uninsulated door makes that floor cold whatever is in the ceiling, because the garage air is at outdoor temperature.
A heated or partly heated garage, where an uninsulated door is the largest heat loss in the building.
Things that freeze: paint, adhesives, a second refrigerator, stored liquids, a water line.
Noise. An insulated door is markedly quieter in operation and also cuts street noise.
Rigidity. A foam-filled sandwich panel is far stiffer than a single skin, which means it dents less and holds its shape.
And the house itself, because garage air gets drawn into the house through the service door and the shared wall.
What the ratings mean
Doors are quoted with an R-value, and the number quoted is frequently the centre-of-panel figure rather than the whole-door performance. The construction matters more: a polyurethane foam-filled sandwich panel with steel both sides outperforms a polystyrene insert in a single-skin door at a similar quoted number. Ask what the construction is, not just the number.
Construction types
Single skin: one layer of steel, no insulation. The cheapest and the least rigid.
Single skin with a polystyrene insert: an improvement, and the panel is still a single skin so the rigidity gain is modest.
Sandwich panel with polystyrene: steel both sides with the foam between, which is a real rigidity gain.
Sandwich panel with injected polyurethane: the foam is injected and bonds to both skins, which is the best performance and the most rigid.
The seals matter as much as the panel, because a well-insulated door with a failed bottom seal is leaking at the gap.
And a thermal break at the panel joints and the perimeter, which the better doors have.
Retrofitting versus replacing
Insulation kits for an existing single-skin door exist and give a modest improvement.
They add weight, which means the springs may need rebalancing or replacing — this is the part people miss, and an unbalanced door with the original springs is a short-lived arrangement.
They do not add rigidity, because the panel is still a single skin.
They do not improve the seals, which should be dealt with separately and are cheaper.
Replacing with an insulated sandwich-panel door gives the full benefit and is the better answer where the door is near the end of its life anyway.
And whichever route, do the seals, because that is the cheapest part of the performance.
Common questions
Is an insulated garage door worth it in London?
Yes, mainly for a room above, a heated garage, noise and rigidity.
Can I insulate my existing door?
A kit gives a modest gain and adds weight, so the springs may need rebalancing.
What should I ask about?
The construction, not just the R-value. An injected polyurethane sandwich panel is the best.