Dormer Window on an Eco House


When I bought the cottage, it already had an attic room with a dormer window. I loved this room when I first saw it, and it was partly what made me buy the house. However, it had turned out to be extremely cold in winter, so unusable as the studio I had dreamed of. It was also too cold to use as a guest room during the winter months.

Fortunately, since improving insulation is a main priority of the renovation, this room will be usable all year round when everything is completed.


What a Dormer Window is

In case you aren’t familiar with the term – a dormer is a window that projects out from a sloping roof (see photo above.) In an attic room, the walls slope, giving less space at full height than at floor level and having a dormer makes the room more spacious. It also means you have a view, which you often don’t with a skylight or other roof window, and gives the room a lighter, more airy feel.

For all of these reasons, I was keen to keep the dormer. As we went through the process of obtaining planning permission, we learned the cottage is in a conservation area and an area of natural beauty, so planning regulations are more stricter than in some areas. The new dormer would have to be the same as before.


Dormer windows – comparing new Eco with old and leaky

As we’ve already seen in previous posts, the quality of the roof is hugely important in an eco-renovation. Compared to an ordinary roof, an eco-roof is more complex in construction. To be properly insulated, the whole roof (Photo of before) had to come off. This wasn’t a big deal because it was old and needed to be replaced anyway.

As was standard at the time, the original dormer window and roof were constructed in a different way to the new one. They were made of single rafters that acted as the load bearing part of the construction. This layer was covered in a weatherproof outer layer, with plasterboard in the inside. Nowadays, in the UK, building regulations do require more insulation, so some sort of insulation would be used in the void. However, the walls and ceiling of the dormer wouldn’t be as thick because they would not use as much insulation as we used.


In contrast, we made our whole construction out of I beams leaving lots of space for the insulation. This is a more time consuming operation, but is much more effective at maintaining an even temperature in the house during winter and summer.

A complex construction

The construction of the dormer window needed a lot of care. It has to be strong, because it is part of the structure of the roof: how the Steico beams intersect is crucial, as we need to fill the gaps, with small bits of wood (as shown in the photograph). We also need to make sure it’s possible to get the blown-in insulation in to all the awkward corners, such as where the main roof meets the dormer construction at an angle. This is the main insulation layer for the roof, so it is important to get right.

Because it had to be integrated into the roof, the construction of the dormer happened in stages. As first the builders created the structure, and then covered it with plastic while they fitted the solar panels. When these were in place, they finished it off.


What to do about the sides of the Dormer

The sides, or cheeks as the builder keeps calling them, of the dormer are covered in lead sheeting. This is expensive, but while tiling with spare roof tiles would cost nothing on materials, labour would be a big cost, because would be fiddly to do. Over the lead is a rubberized roof, which isn’t completely flat so that the rain can run off either sides and down into the gutters.

On the underside of the roof, the builders will eventually fit an insect mesh. This will prevent insects getting into parts of the roof and causing a problem. Insects have loads of places to go in the garden, so it’s not as mean as it sounds.

Although a dormer might seem complicated, and is more expensive that just having a plain roof, it is definitely worth it. Because of the rubberized roof and lead cheeks it won’t leak, and its strong construction means it will last for many years.

Next time we will look at more work on the inside of the house, and the airtight layer. Do let me know what you think so far, I do love to hear what you think, so please make comments.

See you soon.





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