In this article we cover:
- Difference between a roof light or skylight and a roof window
- How roof windows help with overheating
- Where to put them
- Types of roof window
Roof windows bring in daylight, regulate heat, and improve ventilation. But they need careful planning to avoid making your house uncomfortably warm.
Poorly placed or oversized roof windows can contribute significantly to overheating. Good design is about balance, ensuring that glazing areas are proportionate, shading is included, and ventilation is properly planned.
Dealing with overheating
Building a certified passive house in Ireland sounds like a dream. Fresh, filtered air, no need for radiators, and a home so well insulated that a toaster could practically heat it. But there’s a catch.
Designers, caught up in the romance of floor to-ceiling windows and solar gain, sometimes go overboard. The result? Overheating. Yes, you read that right. In Ireland.
A common misconception is that overheating in modern low energy homes is caused by too much insulation or airtightness. In reality, there is no such thing — only poor design. Passive house principles do not just focus on keeping heat in during winter; they also ensure homes do not overheat in summer.
The real challenge is preventing excess solar heat gain and not introducing unnecessary internal heat from heating systems. This is where careful design, backed by science, comes in through the passive house methodology.
Thermal modelling early in the design process allows overheating risks to be predicted based on orientation and Ireland’s latitude, ensuring a home stays comfortable all year round.
It’s true, however, that with good insulation and airtightness comes the need for ventilation – for fresh air (not for cooling). And the passive house methodology requires that you install Mechanical Ventilation with Heat Recovery (MVHR).
Roof windows bring in daylight, regulate heat, and improve ventilation. But they need careful planning to avoid making your house uncomfortably warm.
Poorly placed or oversized roof windows can contribute significantly to overheating. Good design is about balance, ensuring that glazing areas are proportionate, shading is included, and ventilation is properly planned.
Dealing with overheating
Building a certified passive house in Ireland sounds like a dream. Fresh, filtered air, no need for radiators, and a home so well insulated that a toaster could practically heat it. But there’s a catch.
Designers, caught up in the romance of floor to-ceiling windows and solar gain, sometimes go overboard. The result? Overheating. Yes, you read that right. In Ireland.
A common misconception is that overheating in modern low energy homes is caused by too much insulation or airtightness. In reality, there is no such thing — only poor design. Passive house principles do not just focus on keeping heat in during winter; they also ensure homes do not overheat in summer.
The real challenge is preventing excess solar heat gain and not introducing unnecessary internal heat from heating systems. This is where careful design, backed by science, comes in through the passive house methodology.
Thermal modelling early in the design process allows overheating risks to be predicted based on orientation and Ireland’s latitude, ensuring a home stays comfortable all year round.
It’s true, however, that with good insulation and airtightness comes the need for ventilation – for fresh air (not for cooling). And the passive house methodology requires that you install Mechanical Ventilation with Heat Recovery (MVHR).
If you;re thinking of relying on open windows and trickle vents, remember that it is the equivalent of cutting a hole in your house and hoping for the best. You may get somewhat consistent fresh air if you’re lucky, but you will also lose heat in winter and let in excessive
warmth in summer. An MVHR system filters, distributes, and recovers heat, providing constant fresh air without the energy losses of traditional ventilation.
Picking the right spot
Not all roof windows are created equal, and where you put them matters. South-facing roof windows can be useful in winter, bringing in passive solar heat, but without external shading or smart glazing, they will crank up indoor temperatures in summer. It is like leaving a magnifying glass on your roof and wondering why your sofa is melting.
North facing roof windows, on the other hand, do not have this problem. They provide consistent glare-free daylight without overheating, making them perfect for bedroom and workspaces.
Even better, when placed high in the building, they act as thermal chimneys, allowing rising hot air to escape on those rare (but increasingly common) sweltering Irish days. A well-placed north-facing roof window with remote opening -ideally solar-powered so you do not have to climb onto the worktops every time you want fresh air – can naturally ventilate a home and make summer heatwaves far more bearable.
The science of staying warm
Triple glazing is non-negotiable in a passive house, and what is between those panes is just as important as the glass itself.
A standard window has air in it. The problem is, air allows heat to escape through convection – warm air rises, cool air sinks, and heat is lost in the process. Enter noble gases: argon krypton, and xenon.
- Argon is cost-effective and great for wider gaps
- Krypton is higher performing, perfect for thinner glazing where space is tight
- Xenon is the Tolls-Royce option; excellent but wildly expensive.
Why do denser gases insulate better? Simple. They are less prone to movement, which slows down heat transfer. If heat loss were a relay race, dense gas molecules would be slow to pass the baton.
The bottom line
It’s a good idea to add glazing to your roof, but make sure it’s a roof window, not a rooflight, that meets the passive house criteria. And preferably choose a north facing orientation, for steady light and summer cooling, because a south-facing roof window without proper shading will lead to overheating.
First Irish Rooflights
The Viking, inventive as they were, did not have the luxury of triple glazing. Instead, they used thin slices of animal horn to let in light while keeping out the wind. A functional solution, but we’ve come a long way since.
Modern Passive House roof windows now come with insulation collars and vapour barriers to prevent dreaded condensation buildup. Without these, your sleek new home could develop a hidden water problem, slowly rotting away like an old longboat left in a bog.
Roof windows vs rooflights
The terms roof window, skylight and rooflight are often used interchangeably, but they are not the same thing.
The main difference is that roof windows must be installed in plane with the surrounding roof, meaning they sit flush with the roof structure. Rooflights, or skylights, on the other hand, can be installed out of plane, meaning they can project above the roofline.
This distinction is not just a matter of terminology; it is covered under International Standards and CE marking. If a product is sold as a roof window, it must meet strict structural and performance standards, ensuring airtightness, weather resistance and durability in an inclined roof installation.
Skylights and rooflights, while still useful, do not always meet these same standards and are often more suitable for flat roof applications or aesthetic purposes rather than high performance passive house builds.
For a passive house, where airtightness and thermal performance are critical, roof windows that comply with passive house standards are generally the best choice.
For more about the passive house methodology, see passiv.de
