There is greater interest at this time of the year in reducing heat loss from our homes and reducing fuel bills. Most home-owners correctly concentrate on the areas of biggest heat loss by upgrading the insulation to their roof, walls or windows. However a quick check will reveal whether your home is losing unnecessary heat from unlined wall vents.
My own house, and my neighbours, all have this problem. As a result the cold air is able to spread out behind the walls, and insulation, and cool the house. The heat loss is worse during windy conditions. I cannot understand why the builder did not line the vents properly, but the houses are about 16 years old and attitudes to insulation and heat loss were looser then.
It is part of the building regulations that vent ducts should be composed of rigid continuous insulated pipework.
The first step to check whether this problem affects your own home is by simply removing the vent cover to investigate. If the vents are unlined the easiest solution is to install a continuous insulated duct between the internal and external vents. This is straight forward enough unless the external and internal vents are not in line, in which case more work is required to line them up, usually this means adjusting the inner wall to suit. See pictures below of this work being carried out in my neighbours house. Thanks for the photographs Ronan.