I am going to replace a single-glazed glass rooflight over the hall with a velux window. I want to keep the light coming in, but want much better heat insulation as the hall is a very large space. The glazing options I have seem to be double-glazed or triple-glazed with varying U-values – 0.89, 1.0, 1.2 and 1.3. I want to reduce my heating bills if possible, but the main reason is to make my flat feel warmer. Is it worth paying for a better U-value, or would a slightly worse U-value make only a negligible difference?
I currently have condensation problems with the current window. The velux window comes with a ventilation bar with dust and insect filter. The window will be in an inaccessible place, so is it worth leaving the ventilation bar permanently open or will this negate the heat insulation from the double/triple glazing. If I leave the ventilation bar closed, will this cause condensation?

Thanks in advance for any advice!

  • Adam

    The simple act of upgrading the single glazing with a double glazed unit will bring about the best gains in your case. A triple glazed unit will perform better but I am not convinced the extra cost involved will bring about much advantage in terms of heat retention.

    A U-Value is basically a measure of heat loss through a structural element. It is calculated on the rate at which heat transfers through 1 square metre of a structure.

    Unless the window is enormous then you will be losing vastly more heat through the roof structure and walls as these are the much greater surface areas. Velux, in their standard sizes aren’t huge mean you will be losing only a small amount of heat through it.

    As soon as you replace the single glazed window with a new one your condensation issues should all but disappear. Condensation forms when warm moist air comes into contact with a cold surface. The single glazed glass panel provides the perfect cold surface for condensation to form on. A double or triple glazed window will dramatically reduce the ability of the cold to penetrate to the inner pane of glass, thus reducing the ability of the warm air inside to deposit moisture.

    I would advise leaving the trickle vents open all year round. If you want to eliminate condensation you need to maximise ventilation. Your energy use should remain as it is at the moment. Any net loss you make by allowing a small amount of cool air in via the vents will be gained by having a double or triple glazed unit in place.

    You probably want to focus primarily on the style of window first and then decide what you want to spend on its eco credentials. Don’t forget to factor in the cost of the flashing kit and be sure to hunt down a suitably qualified installer. Ask your friends for a recommendation if you can. Here is a useful website to give you some idea of cost and other things to consider:


    Good luck with it.

  • Post a comment
    You must be logged in to comment. Log in