It is important to properly insulate round windows. Traditionally a lot of thermal bridges have routinely been built into our window details. For example by using masonry to close the cavity at the reveal, metal lintels that extend from inside to outside (how insane is that!). Additionally insulation is often not present under windows and even in the reveals due to poor workmanship or incorrect installation.
Draughts are frequently present under window boards, either emanating from the cavity or sometimes directly from outside. This coupled with this frames that are not always fully sealed to the masonry allowing draughts to enter; these draughts can most easily detected by noticing blackening of the cracks or dark shadowy streaks on the frames. We found a couple of extreme examples of draughts where it was possible to see straight over the window head from indoors or straight out under the window board and across the top of the cavity, and on out under the outside window sill and straight down the garden! Always look for cracks and gaps first then check with the back of your hand feel for draughts, the ultimate search weapons are feathers, lit candles, infra red thermometers and thermal imaging cameras.
The following set of pictures illustrates how to rectify window reveal and window board problem in homes that have been dry lined.
Further discussion on problems with a WC and understair void: Retrospective cloakroom airtight fix (discussion on Green Building Forum – external link)