Skylight and rooflight fitting in Lydney

In June the sun is above the horizon long before seven, and a roof window looks straight into that. A loft bedroom with an unblinded window can be bright by five, and a blind built to black the glass out is the cure. Blackout blinds can be had for roof windows, and an electric or solar blind is the one to ask about when the window is too high to reach.

That is the first thing a loft conversion has to settle, because a bedroom is the room where light is least welcome. The second is height. A window that sits high in the slope lets in sky and not much else, and one set low enough to look through while standing gives you a view of the street or the garden. Which suits you depends on whether you want a view or just daylight, and it is simpler to decide before the opening is cut.

Escape matters in a loft bedroom too: the escape rules sit in Approved Document B, and the England section of our planning guide points to them. A roof window can be the escape window if it meets the escape measurements. We look at the arrangement on the survey and say what the room needs.

On a warm night the glass is the other consideration. A closed roof window under a summer roof traps heat, and opening it a little lets that heat go. A blind used by day cuts the sun on the glass, and the two together can keep a loft more comfortable than glass alone. The comparison of centre-pivot and top-hung windows shows how each type sits over a bed.

The room below also has its evenings. When the lamp is on and the blind is open, the glass turns into a dark mirror that gives the room back to itself, and from a neighbour's window the lit room shows through. Closing the blind at dusk is the simple answer, and a loft bedroom with one is a calmer place to read in bed.

Start with what the cloudiness really is. If it sits between the two panes and wipes off from neither side, the sealed unit has failed and moisture has got in. If it sits on the inside of the room-side pane, it is condensation from the room, and that is a ventilation matter. The two look alike on a dull afternoon, and only the first calls for a replacement.

A failed unit can often be replaced alone. The frame is kept and new glass is fitted into it, provided the frame is in good order. Where the frame has decayed, or the window is old enough that its fittings are worn, the answer is a whole new window. How that call gets made is set out in the guide to sealed units against whole rooflights.

There is a regulations point worth knowing. Fitting new glass alone, or a window into its old frame, falls outside the energy efficiency requirements that govern a controlled fitting. A whole new roof window is different. Its U-value should be no worse than 1.4 W/m²K, or it should reach Window Energy Rating band B, which is the figure in Approved Document L for a roof window in an existing home. A rooflight on a kerb has its own figure of 2.2 W/m²K.

A milky patch in the glass is the usual giveaway, and light comes through unevenly. The room below is dimmer than it ought to be, and the view of the sky has gone. A replacement brings that back, and the evening lamps no longer sit behind a smeared pane.

If the rest of the roof is sound, a swap involves less work than cutting a new opening, because the structure round the window is already there.

For the afternoon itself, the check is simple. Look at the window from the room on a bright day and from the garden if you can reach it, and note where the cloud sits. Take that note to the visit.