Bay window insulation in Birmingham.

Which kinds of bay window do Birmingham homes have?

Mostly two, and they are built in quite different ways. That is why I never price a bay over the phone.

Many streets have a mix, because bays were rebuilt, rendered or reglazed over the decades. The house type is a starting point, not the answer.

How can I tell what my bay is made of?

A few simple checks give a good clue before anyone opens anything up.

• Look outside under the window. Brick all the way to the sill usually means a brick bay. Hanging tiles, timber boarding or a rendered panel under the glass usually means a lightweight panel behind it.
• Knock gently inside. A hollow sound below the window board points to a timber panel with a thin lining. A dull, solid sound points to brick.
• Feel it on a cold, windy evening. If you can feel moving air near the skirting or round the window board, there are gaps in the construction, not just a cold surface.
• Look up. A ground floor bay often has its own small roof, flat or tiled. A bay that runs up two storeys may share the main roof.

At the home visit I check this properly, along with the wall thickness at the reveals and a moisture reading. My guide on why bay windows get so cold explains what sits under a typical window board.

How do you insulate a Victorian brick bay?

Much like a solid wall, but with more corners. A brick bay on a Birmingham terrace is usually a single thickness of solid brick, so it loses heat in the same way as the rest of the front wall, only through three faces instead of one.

On old solid brick I usually prefer a breathable build-up, such as wood fibre boards with a lime plaster finish, because it lets the wall keep drying as it always has. Before choosing, I check the brickwork for damp and model how moisture will move through the new layers. The angled joints between the faces are where most of the care goes, because a gap there leaves a cold stripe that shows up as mould.

Period details matter too. Deep window boards, shutter boxes, picture rails and the plaster cornice inside the bay can usually be kept or reinstated. The Victorian home insulation guide explains why these houses need a gentler approach than a modern one.

How do you insulate a 1930s bay with a panel under the window?

By opening it up. The panel is usually the weakest part of the whole house: a timber frame, tiles or render outside, and only a thin lining inside, often with nothing in between.

1. The window board and lining come off so I can see the frame and check the timber is dry and sound.
2. Gaps to the outside are sealed, and the frame is filled with insulation.
3. A new insulated board goes on the inside, fixed and sealed so air cannot get round it.
4. The brick piers and side walls are insulated to match, so there is no cold edge where the panel ends.
5. The reveals get a thinner board, the window board is extended, and everything is plastered.

Space is usually tight around a 1930s window, so a slim PIR board with PU adhesive is often the right choice here, with careful vapour and air sealing. Where there is room and the frame suits it, a breathable system can work too. My comparison of thermal plasterboard, wood fibre and PIR sets out the trade-offs.

Does the ceiling or roof of the bay need insulating?

Often, yes, and it is the part most people never think about. A ground floor bay usually has its own small roof, and the space between that roof and the bay ceiling is commonly empty. On a cold morning you can sometimes feel the chill coming down from the ceiling as much as from the window.

If I can reach that space without damaging the ceiling or the roof covering, I insulate it with the rest of the bay. If the bay roof is leaking or the covering is near the end of its life, that has to be repaired first, and I will tell you so rather than insulate under a wet roof. On a two-storey bay, the upper bay ceiling often joins the main loft, so it can be done from above with the loft insulation.

What difference does insulating the walls of a cold room make?

In my case study of a cold 1930s bedroom, I insulated solid brick walls with wood fibre and lime. On a 0°C day with the room at 20°C, the calculated surface temperature of the main wall rose from about 14.5°C to about 18.4°C, and the estimated heat loss through that wall fell by about 71%.

Those figures describe the wall, not your heating bill, which depends on the whole house. What matters day to day is that a warmer surface stops pulling heat out of the room and out of you, and it is far less likely to collect condensation. In a bay, where you have three cold faces around you, that change is easy to feel.

It also helps with the black mould that so often starts on bay reveals and behind curtains. Ventilation still matters, especially in bedrooms. My guide to condensation on bedroom walls covers that side of it.

How much does bay window insulation cost in Birmingham?

From about £2,300 per bay. A slim PIR system is usually £2,300 to £3,000 and takes three to four days. A breathable system with wood fibre and lime is around £3,000 and takes about five days. The insulated walls come in by roughly 20mm to 80mm including plaster, depending on the system. You get a written quote after the free home visit, listing exactly what is included.

Grant schemes are means-tested and often go through large installers working to a standard specification, and the detail a bay needs is easy to miss in that kind of work. If you do not qualify, or would rather choose who does the work, you can still insulate privately, one bay or one room at a time. My guide for people who did not qualify for an insulation grant explains the options.

If the bay sits in a solid front wall, it is usually worth pricing the walls either side at the same time. The internal wall insulation in Birmingham page covers whole rooms and whole houses.

Which parts of Birmingham do you cover?

The whole city. North Birmingham, including Kingstanding, Erdington, Perry Barr and Great Barr, is around 20 to 30 minutes from my base in Bloxwich, Walsall. South Birmingham, such as Hall Green, Kings Heath and Moseley, is more like 40 to 45 minutes. Visits are planned in advance, and the work is the same wherever you live.

If you are in Sutton Coldfield, my bay window insulation service page goes into the day-by-day detail of a bay job, from protecting the room to handover.