How do you stop a rooflight overheating?
In order of effectiveness: stop the energy before it reaches the glass, then reduce what the glass lets through, then give the heat that does get in a route out at high level. Everything else is a refinement of those three. Anything fitted on the room side of the glass is working on heat that has already arrived, which is why internal blinds disappoint people.
What follows applies both to a rooflight already in place and to one still being specified. If it is still on paper, the glass choice does most of the work for very little money. Whether your room is at risk in the first place is covered on will a rooflight make my room too hot, and the specification detail sits on energy efficient rooflights.
Shade it before the glass, not after
Solar energy that is stopped outside the building never becomes heat inside it. Energy stopped by an internal blind has already passed through the glass, warmed the blind fabric, and is now being released into the room air by convection.
The difference is not marginal. External shading typically excludes seventy to ninety per cent of the solar gain. An internal blind of the same fabric manages twenty to forty. If only one measure is going to be taken on an existing rooflight, it should be an external one.
External blinds, awnings and shutters
For a pitched roof window, every major manufacturer offers an external awning blind that runs in side channels on the outside of the frame. The mesh is dark, it is largely see-through from inside so the view survives, and it is the most effective single item available for a loft bedroom. Solar and electric versions avoid any need to open the window to deploy it.
For a flat rooflight or a lantern the options are architectural rather than accessory. A brise soleil, a pergola with slats sized to the summer sun angle, or a deep overhang at the edge of the roof all shade the glass without touching it. A deciduous tree does the same job and helpfully stops doing it in October.
Solar control glass, and when it can still be specified
The g-value is the fraction of solar energy the unit passes. Clear double glazing sits around 0.65. A neutral solar control unit sits around 0.30 to 0.40. Halving the g-value halves the incoming energy, and on three square metres of flat glass at midday in June that is roughly 800 watts of heat that never enters the room.
It is the cheapest fix by a wide margin, and it is only available at order because the coating is applied during manufacture. If the rooflight is being replaced anyway, or the units are still being made, this is the decision to get right. Which glass does what is set out on what glass stops solar gain.
Applied films on existing glass
Where the unit is staying, a solar control film applied to the inner face of the inside pane is the retrofit version. A good professional film will take a 0.65 unit down to around 0.45, which is a real improvement but not the same as factory glass.
Two cautions apply. Film increases the amount of solar energy absorbed within the glass, which raises pane temperature and, on a large toughened or partially shaded unit, can add thermal stress. And films on the internal surface of a sealed unit can affect the warranty, so it is worth checking with the manufacturer before committing. Films on the external surface weather faster but avoid both issues.
Internal blinds, and how to make them work harder
Internal blinds are the most commonly fitted and the least thermally effective, but they are not useless if they are chosen for the job. Three things improve them.
- A reflective backing. A metallised or white outward facing surface bounces energy back through the glass before it becomes heat. A dark fabric absorbs it and then radiates into the room.
- A sealed side channel. A blind running in channels traps a still air layer against the glass, which slows convection carrying heat down into the room.
- Deploying it before the sun arrives. A blind closed at eight in the morning on a south facing plane does far more than one closed at two when the room is already warm.
Blinds for rooflights need side channel provision built into the unit, so on a new installation the decision belongs at order rather than at handover.
Letting the hot air leave at the top
Warm air collects at the highest point in a room, which is precisely where a rooflight is. An opening unit at that point gives it an exit, and cooler air is drawn in through windows lower down with no fan and no power. That is the stack effect, and the taller the room the harder it works.
The useful detail is that the opening needs a matching inlet. A roof window opened alone in a sealed room does very little. The same window opened with a ground floor window or a door open at the far side of the space moves a large volume of air. In an extension with a lantern, opening a rooflight vent and a back door together is the difference between a stuffy room and a comfortable one.
Night purging, and the mass to store it in
Essex summer nights are usually six to ten degrees cooler than the afternoon peak. Opening the rooflight and low level windows through the evening and overnight flushes the accumulated heat out and pre-cools the structure for the following day.
It works properly only if the room has something to store coolness in. A screed floor, a tiled finish or exposed blockwork absorbs the night air and releases coolness through the following morning, cutting two or three degrees off the afternoon peak. A timber floor over joists with a plasterboard ceiling has nothing to hold it, so the benefit evaporates by ten in the morning. If a floor finish is still to be chosen in a room with a lot of overhead glass, this is a reason to lean towards tile or polished screed.
Mechanical help, and where it belongs
A rain sensor on an electric rooflight closes the unit automatically if a shower arrives, which makes overnight purging practical when nobody is going to be awake to manage it. Some units accept a thermostat or a timer as well, opening at a set internal temperature and closing at another.
Beyond that, a ceiling fan does not lower the air temperature but it moves air across skin and makes a room feel two or three degrees cooler for a very small electrical load. Air conditioning works and is a last resort, because it is being asked to remove heat that the glass specification could have excluded for a fraction of the running cost.
Which measure to reach for first
If the rooflight is not yet made: specify the g-value first, then decide whether the unit opens, then think about shading. That sequence costs least and delivers most.
If it is already in: start with external shading, because it is the only measure that stops energy before the glass. Add or improve purge ventilation next, since an opening unit can often be retrofitted more easily than the glass can be changed. Then look at film, then at internal blinds with reflective backing. Cooling equipment comes last, once the free measures have been used up. Worth noting too that if the glass itself is due for replacement, upgrading the specification at that moment costs very little extra, and that route is covered on can you retrofit better glass into a rooflight.
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