Are flat rooflights and skylights energy efficient?
A current flat roof rooflight performs far better than the plastic dome most of them replace, and on a new opening it loses more heat than the insulated deck it went into. What decides the real performance is not the unit but the kerb beneath it, which on a great many installations is an uninsulated cold bridge running the whole perimeter.
Why the kerb dominates
Because it is a continuous path around the entire opening.
An uninsulated timber upstand connects warm room air to the cold outside around all four sides, and the frame sitting on it stays cold whatever the glass is doing.
On a modest unit that is several metres of cold bridge, and it appears in no U-value.
The horizontal position
The second reason a flat roof unit is harder work than a roof window.
Glass facing straight up radiates to the open sky, which on a clear night is effectively far colder than the air, and it sits above the warmest air in the room.
Both work against it, and the mechanism is on does a rooflight lose more heat than a window.
What a modern unit achieves
Comfortably inside the limiting standard, on a whole unit basis.
A double glazed unit with a low emissivity coating, a gas fill and a warm edge spacer sits well under the limiting value for a replacement.
What the numbers mean is on what U-value should a rooflight have.
What a plastic dome achieves
Very little, and it is why replacements are such an improvement.
A single skin dome is barely insulating at all, and even a twin wall one performs far below a modern glazed unit.
So on the majority of our flat roof work, the replacement is a genuine energy gain rather than a cost.
Air leakage around an old dome
The loss that is invisible and frequently larger than the glazing difference.
An old dome sits on a kerb with a perished gasket and a gap somebody filled with foam years ago, leaking air continuously.
Sealing that properly is worth more than the difference between two good units.
The vapour control layer
The detail that matters on a warm roof and is frequently cut and left.
Where an opening passes through a warm roof build up, the vapour control layer below the insulation is interrupted, and it has to be continued and sealed to the frame.
Left cut, humid room air enters the roof build up itself, which is slower and more serious than water on a frame.
Solar gain, which cuts both ways
A horizontal unit takes summer sun at close to the worst possible angle.
Which makes solar control glass more useful here than on a pitched slope, and it also means useful winter gain is available on the same unit.
The trade is on what is solar control glass.
Overheating in a kitchen extension
The commonest comfort complaint on this product.
A generous area of horizontal glass over a lightweight extension with no ventilation is exactly the arrangement that overheats.
Glass specification and an opening unit together are the answer, and neither substitutes for the other.
Whether the area limit applies
On a new extension rather than a replacement.
An extension has a notional limit on total glazed area, and a large lantern or several units can exceed it, which means demonstrating compliance by calculation.
The detail is on can a rooflight be too big for building regulations.
What it does to a heating bill
On a replacement, a small improvement. On a new opening, a small cost.
Either way it is a small number against what the walls and the roof around it are doing, and we would not build a case on it.
What it does for lighting
The one genuine saving, and it is modest.
A dark rear extension lit artificially through the day uses electricity that daylight replaces, which is a real if small reduction.
It does not amount to an energy efficiency measure and it is worth having.
Insulating an existing kerb
Possible on a replacement, and worth doing while the unit is off.
It generally means working at the upstand with the covering lifted or from inside, and it is the single change that most improves how the installation performs.
Doing it during a replacement is a fraction of returning to do it.
Doing it during a roof renewal
Where the arithmetic works best.
Where a flat roof is being recovered and insulated, forming the opening and the kerb as part of that work means the insulation and the vapour control layer are continuous by design.
That produces a materially better result than the same unit retrofitted later.
What we will not claim
That a new opening improves a house’s energy performance, because it does not.
What it can be is close to neutral, well specified and well installed, in exchange for a room that is transformed.
What to ask for
Three things rather than a datasheet.
The whole unit U-value, confirmation that the kerb will be insulated, and confirmation that the vapour control layer will be continued and sealed at the frame.
What gets insulated on a flat roof
We insulate the kerb and seal the vapour control layer as standard, because on a flat roof those two details decide the performance far more than the choice between two good units.
The whole picture is on the energy efficient rooflights page.
Thinking about this job? We survey across Worthing, West Sussex and the South Downs and give you a fixed price before any work starts, with a 10-year workmanship guarantee.