Can you add insulation around an existing rooflight?
Yes, and it is usually worth doing, because the heat escaping around a rooflight frequently exceeds the heat escaping through the glass it surrounds. The perimeter is a ring of timber, plasterboard, small awkward voids and interrupted insulation, and on an installation done in a hurry there is often nothing at all in the 20 to 100mm immediately behind the frame. Improving it is a contained piece of work with three possible routes in: from the room by opening up the reveal, from the loft or roof void where there is one, and from outside while the covering is off for another reason. Which route applies depends on the build-up rather than on preference. The physics of the perimeter is set out on thermal bridging at the rooflight perimeter, and the whole-unit thermal picture on the energy efficient rooflights hub.
Most of the gain sits in the first 300mm around the frame
A junction between a rooflight and a roof is described in calculations by a psi value, a heat loss per metre of junction rather than per square metre of surface. A well detailed kerb junction lands around 0.05 W/mK. A bare timber kerb with insulation stopping at the deck can be four or five times that.
Run the arithmetic on a modest flat rooflight measuring one metre by one and a half. The glass area is 1.5 square metres, so at a whole-unit U-value of 1.2 the unit loses about 1.8 watts per degree. The perimeter is five metres, so at a poor psi of 0.25 the junction loses another 1.25 watts per degree. Bring the junction down to 0.05 and two thirds of that disappears. Nothing about the unit changed and the loss dropped by around 40 per cent. That is why the perimeter is the first place to look.
Three ways in, and what each one reaches
| Route | What it reaches | Disruption |
|---|---|---|
| From the room, reveal opened | The inner face of the kerb, the void behind the plasterboard, the vapour control layer, the collar around a roof window frame | One room, plaster and decoration, one to two days |
| From the loft or roof void | The sides and head of a roof window frame from above, the joist bays around a cold deck opening | Access only, no plaster, dependent on crawl space |
| From outside, covering back | The outer face of the kerb, the deck insulation returning onto it, the whole thermal envelope | Scaffold and weather planning, the complete answer |
| Over the reveal, insulated board applied | Nothing behind the existing lining, but adds resistance across it | Half a day, costs a little headroom and daylight |
From inside the room: opening the reveal and filling it properly
This is the route that suits an existing unit in a finished room. The plasterboard is taken off the reveal, which exposes the inner face of the kerb on a flat roof or the collar between the frame and the rafters on a roof window. What is usually found there is a void, sometimes with a scrap of mineral wool pushed in loosely and sometimes with nothing.
The work is to fill it completely with no gaps and no compression: rigid board cut tight and foamed at the edges where the geometry allows, or a firmly packed mineral fibre where the space is irregular. Compressed insulation loses a large part of its value, and a 10mm air gap behind a board short-circuits it, so the fit matters more than the material. The vapour control layer is then lapped from the ceiling onto the frame and taped, which is the detail on vapour control layers sealed to the frame. Splaying the reveal while it is open costs little extra and throws more daylight into the room, which is on rooflight reveals and why splaying them matters.
Insulated plasterboard over the reveal, and the headroom it costs
Where opening the reveal is unwelcome, a thermal laminate board can go over the existing lining: plasterboard with 25 to 50mm of rigid insulation bonded to it, with the joints and the frame junction taped. It adds real resistance across the whole reveal and it is a short job.
The trade is honest. Fifty millimetres of board on all four faces takes 100mm off the width and the length of the daylight opening, cuts the amount of light reaching the floor, and can foul the operating handle or the blind track on a roof window. It also does nothing about a void that already exists behind the original lining, so a cold bridge deeper in the construction stays where it is. It is a good answer where the original detail was reasonable and a poor answer where it was not.
From the loft or the void, on a pitched roof
Where a roof window lights a room in the roof and there is still accessible void alongside it, insulation can be brought tight to the sides and head of the frame from above without touching a single sheet of plasterboard. Manufacturers make an insulation collar sized to their own frames for exactly this, and it beats loose material pushed into a gap by hand.
The rule that governs the work is the ventilation path. A cold pitched roof needs a clear air gap, conventionally 50mm at the eaves and 25mm above the insulation between the rafters, running from eaves to ridge. Pushing insulation up tight against the sarking to fill a gap near a rooflight blocks that path and creates a cold damp zone directly above the new work. Rigid ventilation trays or counter battens keep the gap open while the insulation goes right up to the frame, and the two requirements are met together rather than one at the expense of the other.
From outside, where the kerb face is finally reachable
The complete answer is only available from above, because the outer face of a timber kerb is where the cold ring actually is. On a warm deck the correct build-up wraps the kerb in insulation on its outside face, through its full height, with the deck insulation lapping onto that wrap rather than butting loosely against the timber. Then the waterproofing dresses up and over.
Where a kerb was built bare, the timber runs from the warm inside of the building to the cold outside air with nothing interrupting it, and no amount of work from inside changes that. Correcting it means the covering coming back locally, the wrap going on, the deck insulation lapped and the waterproofing reinstated over the top. It is the right moment to raise the kerb height as well if the upstand was ever marginal. Build-ups are on upstands, kerbs and warm roof build-ups.
Doing it while a new unit is going in anyway
The cheapest version of this work is the one that happens while the frame is out for another reason. Fitting a new sealed unit or a new complete rooflight opens every one of the three routes at once, the scaffold is already up, and the labour for the perimeter is a fraction of what the same work costs as a standalone visit later.
So where a unit is at the end of its life and the perimeter is also suspect, the two should be planned as one job rather than two. On a replacement, the insulation is reinstated tight and continuous, the control layer is taped to the frame and the kerb is upgraded or rebuilt as part of the work rather than as an extra, which is described on replacing a failed or misted rooflight.
Checking the result on a cold morning
The perimeter is invisible once it is finished, so it is worth recording the before and after. An infrared thermometer or a thermal camera used on a cold clear morning, with the heating on and the outside temperature well below the inside, shows a cold bridge as a bright line following the frame. Photographs of the opening at each stage, taken before the plasterboard goes back, are the more useful record, because they show what is actually behind the finish.
The measurable outcome is the surface temperature at the coldest point of the reveal. Where that point sits comfortably above dew point for the humidity the room actually runs at, the black spotting that appears in February stops appearing. That is the practical test, and it beats a calculation.
Gloucestershire build-ups, and which respond best
Warm deck rear extensions across Gloucester, Tewkesbury and Quedgeley are the most common candidates, and the usual finding is a softwood kerb with nothing on its outside face. Interwar semis at Longlevens, Hucclecote and Charlton Kings converted in the 1990s often have a roof window with 100mm of mineral wool between rafters and a hollow collar around the frame, which responds very well to work from inside. Cotswold cottages with thick stone slate roofs have deep rafters and generous room to insulate, though the covering is expensive to disturb, so the internal route is nearly always the right one there.
Cold deck flat roofs, common on 1960s and 1970s rear additions across the vale, are the case that needs most care, because insulating harder without keeping the void ventilated makes the construction wetter rather than warmer. That build-up is on flat rooflights on a cold roof build-up, and it is checked before anything is added.
Next on this subject.
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