Velux vs Somfy Motorised Blinds for a South-Facing Conservatory

A Velux roof window carries its own blind track moulded into the sash frame, which means the fabric runs in a fixed channel a few millimetres from the pane. Somfy takes the opposite path: it sells the Sonesse and Roll Up motor tubes as components, and a fabricator wraps a chosen fabric around the tube and mounts it in a separate cassette. That single structural difference is why the two systems behave differently in a conservatory where the glazing runs at a slope and the sun tracks across the roof for six to eight hours.

Where the fabric sits relative to the glass

On a Velux GGL or GGU roof window, the factory blind, the DKL blackout or the MHL awning, rides in the aluminium side channels of the sash. The gap between fabric and glass is sealed at the edges, so the air pocket trapped behind the blind does not circulate freely. For solar gain that matters: an external MHL awning blind intercepts the radiation before it passes the pane, and Velux quotes heat reduction figures for the awning far above any internal blackout, because once infrared energy is inside the glass it becomes trapped long-wave heat.

Somfy fabric sits wherever the fabricator mounts the cassette, which in a conservatory is usually below the rafters on a Pinch pleat or roller system spanning the whole roof plane. The fabric is 60 to 150mm off the glass, so warm air convects up behind it and vents at the ridge. This handles glare well and evens out the light, but it does far less against the raw solar load than an external Velux awning does. A conservatory that overheats needs the interception to happen outside the glass line, and only one of these two systems offers that as a catalogue product.

The short version on protocols

Velux uses its own io-homecontrol radio inside the KLR200 and App Control, and Somfy uses io-homecontrol too on its higher tier plus RTS on the older range. Because both sit on io-homecontrol, a single Somfy TaHoma hub can often drive Velux roof-window blinds alongside Somfy roller blinds, which is the one genuine interoperability win between them.

Measuring the reveal before anything is ordered

A roller blind in a UPVC casement fails most often because the reveal was measured at one point instead of three. The width should be taken at the top, middle, and bottom of the opening, and the smallest of the three governs the order, because UPVC frames are rarely square after a decade of thermal movement. A 4mm discrepancy across a 1200mm span is common, and a blind cut to the widest measurement will bind on the narrow side.

For a face-fix inside the reveal, the depth from the glass to the front edge of the plaster line decides whether the cassette clears the handle throw. Somfy roller cassettes on a Sonesse 30 tube need roughly 90mm of depth for the bracket and end caps, so a shallow reveal forces a face-fix onto the plaster instead. Velux roof-window blinds sidestep this entirely because the track is already part of the sash, and the only measurement needed is the window pane code stamped on the top data plate, a value like MK04 or UK08 that maps directly to the blind part number. Getting that code wrong is the single most common Velux ordering error, because the code is not the physical size in millimetres, it is a Velux size index.

When the mounting lands on UPVC rather than timber, the fixing pulls into a hollow chamber. UPVC casement brackets for Somfy blinds should locate into the reinforced steel section inside the frame where present, or use a chamber-spanning fixing, since a self-tapper into unreinforced UPVC will strip under the repeated torque of a motor tube starting and stopping several times a day.

What the blinds cannot fix on their own

A south-facing conservatory built before the 2010 Part L revisions often has single-skin polycarbonate or early double glazing with a poor centre-pane U-value, and no blind changes the U-value of the glass. The night-time heat loss through that roof runs in the opposite direction to the summer gain problem, and a motorised blind does nothing about it once the sun is down.

Secondary glazing addresses the heat-loss side by adding a second sealed pane inboard of the original, creating a still air gap of 20mm or more that cuts conductive and convective loss through the vertical elevations. Ventrolla and similar systems fit a draught seal into the existing frame rebate, which closes the air leakage path around the opening lights, the route that carries more heat out of an old conservatory than the glass itself on a windy night. None of this competes with the blind decision; it sits underneath it, because a motorised blind on a leaky single-glazed elevation is managing glare on a structure that is haemorrhaging heat through gaps the blind never touches.

The practical sequence is to seal and, if needed, secondary-glaze the fixed elevations first, then choose the roof blind for solar control. Doing it the other way round means the blind is specified against a thermal problem it was never able to solve.

Ventilation that the blind must not block

Part F of the Building Regulations sets background ventilation rates, and a trickle vent in the head of a conservatory frame is often the compliant route to that airflow. A full-plane Somfy roller blind mounted tight to the rafters can seal off the ridge ventilation path if the fabric is drawn fully closed, which turns the space into a sealed box on the hottest afternoons and defeats the convective venting that the off-glass gap was supposed to provide.

The design fix is to leave a deliberate gap at the ridge or to stop the blind travel short of full closure, a limit that both io-homecontrol motors can be programmed to hold. On a Velux roof window, the ventilation flap in the sash top operates independently of the internal blind, so the trickle path stays open regardless of blind position, which is one reason the roof-window approach suits a room that must meet a background ventilation figure without manual intervention.

A worked cost and coverage comparison

Take a lean-to conservatory roof of 4m by 3m, four glazing bars, south elevation. Fitting it out with Velux means the roof itself must be Velux roof windows, so this only applies where the roof already carries GGL or GGU units; three MK08 windows with factory MHL external awning blinds and io-homecontrol motors runs to a four-figure sum per opening once the awning motor and controller are included, and the external awnings are the item doing the real solar work.

A Somfy install on a conventional glass or polycarbonate conservatory roof covers the whole 12 square metres with a pleated or roller system on Sonesse tubes, priced by the square metre of fabric plus the motor count, and a single TaHoma hub drives the lot. The Somfy route is usually the only option when the roof is not a set of Velux windows, and it wins decisively on even light distribution across a large plane. Where the roof is genuinely a bank of Velux windows and summer overheating is the complaint, the external MHL awning intercepts radiation that any internal Somfy fabric, sitting inboard of the glass, can only re-radiate back into the room.

The decision therefore turns less on the motor brand and more on whether the shading sits outside or inside the glass line, and on whether the roof is a Velux window system at all. A large south conservatory with an internal-only blind will still climb in temperature on the clearest afternoons, because the energy is already past the pane before the fabric ever sees it. That leaves an open question worth resolving before ordering: on this specific roof, at the hour the sun is highest, is there any external shading path available, or is every option on the table an internal one?