Condensation and roof windows

Condensation is usually more noticeable on roof windows and flat roof windows because warm, moist air rises to the ceiling. The problem, however, usually isn't the roof window. It’s the excess moisture in the room. Condensation is more prevalent in new homes. After a few years, new homes tend to dry out and have less moisture problems. Eliminating excess moisture by using a dehumidifier and providing adequate ventilation and air circulation will go a long way in preventing condensation.

Installing roofing underlay between the roof window frame and roofing material will help prevent condensation on the cladding.

When a leak isn't a leak!

Condensation can form on more than just glass. In some cases, the installer neglects to use roofing felt and insulation between the frame of the unit and the rough opening. This allows warm, moist air to escape from the room and come into contact with the underside of the flashing and/or cladding. The resulting condensation will drip back into the room between the frame and roof opening, giving the appearance of a leak. Having the roofer or installer add felt and insulation between the frame and roof opening should prevent this problem.

Understanding condensation

Condensation is the conversion of vapor to liquid. It occurs when the air becomes saturated with moisture and releases it in the form of water. It is recognised by the wet mist that sometimes clouds the interior glass on your homes windows.

This may also be the first warning sign that your home is retaining too much moisture elsewhere. And even if your house doesn’t actually feel damp, that excess moisture could be doing damage in unseen places.

What causes the excess moisture?

The air around us contains water vapor (humidity), and we add more water vapor to it by normal breathing, perspiration, cooking, cleaning and showering. When the air becomes saturated with excess humidity, it dispels the moisture by condensing it back into water. This is what shows up on your windows glass.

Why does condensation occur mainly in the winter?

Condensation occurs more often during cooler weather because a greater temperature difference exists between the warm interior of your home and the colder outdoors.

Warm air carries larger amounts of water than cold air. When warm, moisture-laden indoor air contacts a cool surface, such as a window pane, the moisture in the air forms condensation on that cooler surface. 

Fighting condensation - What to look for:

  • ​Mildew on walls, window seals and baseboards
  • Peeling or bubbling exterior paint
  • Deteriorating attic and wall insulation
  • Damaged floor tiles and deteriorating furniture
  • Rotting of structural wood between outside walls

Stopping excess moisture

Condensation is a moisture problem. And to fight it, you need to limit the amount of water vapor in your home. Start by eliminating sources that contribute to excess humidity.

  • Install double-paned insulated windows and doors with energy efficient Low-E glazing’s that help keep interior glass panes warmer
  • Install vents below windows to help warm the interior glass
  • Take shorter showers to reduce the indoor moisture
  • Cook with lids on your pots to reduce the moisture that is put into the air
  • Run ceiling fans to keep warm air from rising to the ceiling to help prevent condensation on roof windows and skylights
  • Run a dehumidifier during winter months and damp weather

Help your house breathe easier

Because you can’t stop all sources of moisture in your house, ventilation is also very important. Your home needs to breathe to fight condensation. Remember that cold air can hold less water vapor than warm air, so in the winter the air outside is often drier than the heated air in your home. Therefore, by allowing moist inside air to escape and dry outdoor air to enter, you can reduce your homes humidity level. 

You can take these steps to help ventilate your home:

  • Run kitchen and bathroom exhaust fans more often during the winter to expel hot moist air
  • Open a window in each room a few minutes daily to keep air circulated
  • Keep curtains, blinds and shades open, allowing warm air to circulate around the glass
  • Keep loft vents open, because hot air rises and can be released to the outside
  • Make sure your clothes dryer and gas appliances properly vent moisture-ridden air to the outside

How installing roof windows can help

Ventilation is a big step in the fight against condensation that’s why VELUX makes roof windows with several exclusive features that can help your house breathe easier. 

They open 
VELUX Roof Windows can be opened with manual or electric controls to release the warm, moist air within a home. 

Wood sash and frame help insulate 
All VELUX Roof Windows are constructed with quality wood, not metal or vinyl. Wood is an excellent natural insulator because it does not conduct heat like metal or vinyl. 

Energy efficient glass 
VELUX Roof Windows are available with Low-E insulated glass. Low-E keeps the interior glass pane warmer to lessen the chance of condensation forming.

Maintenance tips

Before leaving the factory, all VELUX Roof Windows are treated with a water-based fungicide and bactericide. After installation they should be given further coats of a good quality paint or varnish at regular intervals. 
If, however, condensation has been a problem, some discoloration or mold growth may have occurred and although the strength of the timber is unaffected, the result can be unsightly. 

Removing wood mildew

1. The mould must be killed using a sterilised agent. Scrape off any surface mould, then clean the area with a solution of 1 part household bleach diluted in 10 parts water. 

2. Rinse well with clean water. When thoroughly dry, rub down wood with medium sandpaper. Repeat steps 1 and 2 until mould is removed. 

3. Finally, coat with a proprietary clear wood preservative and then finish with paint or varnish as desired. 

Use of roof windows during the winter

During snowy weather, the ventilation flap should be kept closed. If the ventilation flat is open, the warm air inside the building will flow out and melt the snow above the roof window. The melted snow will freeze on the window covers and the make the window difficult to open. If you want to air the room, the window must only be opened briefly.

Even without snow on the roof, water may drip from an open ventilation flap in cold periods. This may take place when warm, moist air inside the building meets extremely cold fresh air. In these situations, the ventilation flap should now be kept closed. If you want to air the room, you are instead advised to open the window.

During changes of weather in the autumn and spring, the risk of condensation is increased.