Bay window insulation that finally warms the room.

Why does my bay window feel freezing, even with the heating on?

Because the bay is usually built very differently from the rest of the house. From inside a decorated bedroom it looks like any other wall, so most people never suspect it.

In many of the bays I open up around Sutton Coldfield, Great Barr and Walsall, the section below the window is a lightweight timber frame. On the outside there are hanging tiles; on the inside, a thin plaster lining, sometimes only about half an inch thick. Between the two there is often nothing at all, or a little old insulation that has slumped.

When that lining comes off, the problem is obvious: you can actually feel the wind coming through. That is why the bay stays cold while the radiator is on full, why curtains move on a windy night and why the window area is the first place condensation appears.

Which parts of the bay do you insulate?

The whole bay, not just the obvious bit. Cold gets in wherever the insulation stops, so I treat the bay as one piece.

• The panel under the window: the lining comes off, gaps are sealed, the timber frame is filled with insulation and a new insulated, airtight layer goes on the inside.
• The side walls and brick piers: insulated to match, so there is no cold strip between the bay and the main wall.
• The window reveals and head: a thinner board where space is tight, because these edges are where mould usually starts.
• The bay ceiling or soffit: where the bay has its own roof or a cold ceiling above it, I check it and insulate it if it is accessible and worthwhile.

The details are what make it work: every joint, corner and edge is sealed so the insulation is continuous and draughts cannot find a way round it.

Which insulation do you use in a bay window?

It depends on how your bay is built, and I decide after looking at it and modelling how moisture will move through the new build-up. I fit both breathable and PIR systems.

Timber frames and brick piers behave differently, and both need to stay dry. On many bays a breathable system, such as wood fibre with a lime plaster finish, suits the masonry and keeps its drying routes open. Where depth is very limited around the window, a slimmer PIR board can be the better choice, with careful vapour and air sealing.

This is the same thinking I use for internal wall insulation on whole walls. As a Retrofit Assessor, I will explain the choice in plain terms, so you understand why it is right for your bay, not just what it costs.

Why don't thicker curtains or a sheet of plasterboard fix it?

Because they cover the cold panel rather than dealing with it. The heat and the draught are still getting through the frame behind.

Heavy curtains help at night, but they also trap cold, moist air against the window, which often makes condensation and mould worse. Sticking new plasterboard over the old lining leaves the empty frame and the gaps untouched, and the cold air simply moves behind the board. Squirting expanding foam into gaps can stop one draught while leaving the rest of the panel uninsulated.

A proper job means opening the bay up, seeing what is actually there, sealing it, insulating it and finishing it so it stays dry. It takes a few days, but you only want to do it once.

Is insulating a bay window worth it?

If the bay room is one you avoid in winter, most people find it is. The change you feel is bigger than the size of the job suggests, because the bay is often the coldest surface in the whole room.

A cold panel and cold reveals pull warmth out of the room and out of you. Once they are insulated, the surfaces stay much closer to room temperature, so the room feels comfortable at a lower thermostat setting and holds its warmth for longer after the heating goes off. You also lose the cold draught that runs across the floor from the window.

I will not promise a figure off your energy bill, because that depends on the whole house and how you heat it. What I can say is that a bay room that finally feels usable in January is why most people call me.

Can you insulate the rest of the room at the same time?

Yes, and it is often the sensible moment to do it. If the bay sits in a solid brick front wall, insulating the wall on either side at the same time avoids a cold strip where the bay work stops.

Doing it together also means the room is only disturbed once, and the plastering, skirting and decorating are done in one go. At the survey I will show you where the heat is going and give you a price for the bay alone and for the bay plus the adjoining walls, so you can choose.

What will happen to my decorated room?

It will be looked after, and it will look like your room again when we leave, only warmer. This is the worry I hear most, and it is a fair one.

Before any work starts, you get a pre-start information guide so you know what happens each day. Then:

• Curtains, blinds and poles come down and are stored safely.
• Carpets and floors are covered, and furniture is moved back and sheeted.
• Dust is kept to the bay area as far as possible, and we tidy at the end of every day.
• Radiators, sockets and window boards are taken off and refitted on the new surface, with trusted trades organised if needed.

The bay walls come out slightly thicker, usually by a few centimetres, and the window board is extended to suit. Curtain poles go back on solid fixings. The new plaster is left ready for your decorator or for you to paint once it has dried.

Will insulating the bay stop the mould on my window?

In most bays it removes the main cause, which is a cold surface. Warm, moist air meets the cold reveal or panel and turns to water, and mould follows.

Once the bay is insulated, those surfaces stay much closer to room temperature, so condensation has far less chance to form. Ventilation still matters, especially in bedrooms where people sleep with the door shut. Where it would help, I recommend trickle vents or better extraction and organise them with trusted trades. There is more on this on my condensation and black mould page.

How much does bay window insulation cost?

From about £1,700 per bay, and most bays take 3–5 days. A slimmer PIR system typically costs from £1,700 and takes 3–4 days. A breathable system with lime plaster is around £3,000 and about 5 days.

The price depends on the size and shape of the bay, what is behind the lining once it is opened, whether the ceiling needs insulating, and what needs removing and refitting. After the free home visit you get a written quote that lists exactly what is included, along with my public liability insurance details and the guarantee for the work.

What does the bay look like when it's finished?

Like a normal, well-plastered bay, with crisp reveals and straight edges. The difference is in how it feels: no cold draught near the window, no chill when you sit in the bay, and the room warms up faster.

At handover we walk round the bay together and check the finish. You get an aftercare and maintenance plan explaining how to decorate, heat and ventilate the room, and I call about four weeks later to see how it is going. If you would like to see a full project from start to finish, read my 1930s bedroom case study or my guide on why bay windows get so cold.