Windows and Doors
Windows and doors are typically weak points in the building's insulation. Even good quality modern windows and doors will be thinner (and therefore less well insulated) than the surrounding walls. Condensation forms on windows because they are usually the coldest surface in the room. Poorly-sealed windows and doors are also a significant source of draughts which can significantly increase the heat loss.
In this thermal image of a sash window it's clear that the window is much colder than the surrounding walls. This is a combination of the window's poor insulating qualities (despite it being double-glazed) and draughts flowing through the joints.
Thermal image of a front door showing cold areas around the edges and letterbox indicating draughts. Thankfully draughtproofing a door is a simple job that you can do yourself.
Reducing heat loss at windows and doors
Unfortunately replacing doors and windows is an expensive job and may not be advisable if you have attractive, well-built period features. Victorian windows were routinely built with good quality hardwood frames that may still have plenty of life left if well looked after.
Double-Glazing
Modern glazing usually comprises at least two panes of glass with a gap between and efficient airtight edge seals. High-performance triple-glazed windows are now available with all manner of clever coatings and inert-gas-filled voids. These offer far better insulating properties than traditional single-pane glazing. It is easy to check whether you have single or double glazing by inspecting the junction between the glass and the frame.
If your existing windows are in poor condition then upgrading to double- or triple-glazing can be a good upgrade, but it’s often very expensive.
Secondary glazing
Rather than replacing the existing windows, this is a separate window installed inside the existing one. This offers the benefits of double glazing (the performance is usually better!) without spoiling the appearance of the original window. If you’re installing internal wall insulation this will increase the depth of the window reveals and provides a golden opportunity to add unobtrusive secondary glazing. This is usually cheaper than double-glazing as well.
Secondary glazing such as this can provide more effective insulation than double glazing, while retaining the period character of any original windows.
Image credit - www.sheerwaterglass.co.uk
Insulating doors
Most people only have a couple of external doors but these can still be a major source of heat loss. Older timber doors are usually poorly-sealed around the edges, and you also have keyholes and letterboxes for draughts to get through. Draughtproofing is usually the most effective method for improving the thermal performance of your doors.
If you have an unheated basement, remember to draught-proof the basement door as well as the external ones.