Insulating a 1930s semi room by room

Which walls in a 1930s semi are actually external?

Fewer than people expect, and that is good news for the budget. In a typical 1930s semi:

• The front wall, usually with a two storey bay to the lounge and the main bedroom.
• The back wall, to the dining room or kitchen downstairs and the back bedroom and bathroom upstairs.
• The side or gable wall, running past the hall, stairs and landing, and often one side of the box room and back bedroom.
• The party wall shared with next door is not external. There is a heated house on the other side, and I do not insulate it for warmth.

Many of these houses have solid 225mm brick walls with no cavity, although some 1930s houses were built with early cavity walls. Which you have changes the whole plan, so it is the first thing I check. If you have a cavity, cavity wall insulation may be the simpler first step and internal wall insulation may not be needed.

What complicates each room?

Every room has its own fittings that sit on the outside wall. Knowing them early avoids surprises on the day.

Electrics and plumbing are moved by trades I trust, and I coordinate them so you are not managing three firms. The front rooms with bays are covered in detail on my bay window insulation page.

In what order should I do the rooms?

Start with the problem, not the floor plan. The order I usually suggest:

1. Rooms with mould that keeps coming back, especially a child's bedroom. See black mould in a child's bedroom.
2. The coldest bedroom, often the front bedroom with the bay or the corner back bedroom.
3. The lounge, where you sit still for long evenings and feel cold walls most.
4. Rooms you are refurbishing anyway. If the kitchen or bathroom is being replaced, insulate the outside wall while it is stripped out. It saves doing the work twice.
5. The hall, stairs and landing last, because they are used briefly and are the most awkward to work in.

Bedrooms are also the easiest rooms to empty, so they make a good first job while you see how the process works.

Why plan the whole house if I only do one room?

Because the weak points are where rooms meet. If the front bedroom is insulated this year and the box room next to it in three years, the place where the two walls join needs to be detailed so the insulation runs continuously. If it is not, a cold strip is left at the junction, and that strip is where condensation finds a home.

One plan for the house means:

• the same system, or compatible systems, in every room, so a breathable wall is not boxed in by a sealed one next door;
• a consistent finished thickness, so skirting, window boards and door linings line up from room to room;
• the moisture check done once for the whole wall, not guessed room by room;
• the ventilation planned for the whole house, because a warmer, tighter home needs moisture taken out at source.

Whole house in one go is what I recommend when it is possible, because it is quicker overall and the details are simpler. Room by room is perfectly workable as long as the plan exists first.

What about the floor joists in the outside wall?

This is the detail that matters most in a solid wall house, and it is easy to miss. The ends of the first floor joists usually sit in pockets in the outside brick wall. Insulating the inside of that wall makes the brick colder, and if it is also damp, the joist ends can stay wetter for longer.

That is one of the reasons I carry out moisture modelling before choosing a system. It shows whether the wall can still dry out with the new build-up, and whether a breathable system such as wood fibre and lime is the safer choice for that wall. It is also why I check the outside first: pointing, gutters and render that let rain in need fixing before any insulation goes on. The comparison of thermal plasterboard, wood fibre and PIR explains the difference between breathable and sealed systems.

How much does each room cost and how long does it take?

I price by wall rather than by square metre, because each wall has its own windows, sockets and junctions. As a guide:

A corner bedroom has two outside walls, so it is two walls. A front bedroom is usually the main wall plus the bay. The finished wall comes out about 20mm to 80mm including plaster. Full detail is in my internal wall insulation cost guide, and every job gets a written quotation after a free home visit.

Can I live in the house while rooms are done?

Yes, and doing it room by room makes that easier. Only the room being worked on is out of use, it is sealed off from the rest of the house, and floors and furniture on the route in are protected. Kitchens and bathrooms need more planning, because you need somewhere to cook and wash for those days. I go through what to expect day by day in living at home during internal wall insulation.

How do I start?

With a free home visit. I look at every outside wall, check whether they are solid or cavity, look at the outside for anything letting water in, and talk through which rooms bother you most. As a Retrofit Assessor I then suggest an order and a system for the whole house, and quote the first room or rooms in writing.

You can see a finished 1930s room in my case study, read more about internal wall insulation, or book your visit.

Common questions