If you live in or work on older buildings with solid walls—think stone cottages, Victorian terraces, or early 20th-century homes—improving insulation is often a top priority. But one big question tends to come up early: should you insulate the walls from the inside or the outside? Both methods can work well, but each comes with its own mix of pros, cons, and things to watch out for. In this guide, we break it all down so you can make an informed choice for your project.
If you’d rather hear Andy and Chris talk through the ideas in this article, you can watch Episode 2 of Can I Just Ask? below. 👇
What Is External Wall Insulation?
External wall insulation (EWI) involves fixing insulation boards or batts to the outside of the building and then covering them with a protective render or cladding. You’re effectively wrapping the house in a warm coat—and that has some big advantages.
Benefits:
- Great for moisture control: By insulating on the outside, the wall stays warmer and drier, reducing the chance of condensation building up inside the wall.
- Improved comfort: You can add thick layers of insulation without losing internal space.
- No disruption indoors: It’s perfect for homes that are occupied during the work—there’s no need to move furniture or strip plaster.
- Protects the building: It can shield the wall from rain and wind, especially if the existing masonry is porous.
Things to watch out for:
- Tricky details: EWI needs careful planning around windows, eaves, and pipes. It’s important to get the detailing right to avoid thermal bridges or water ingress.
- Not always possible: If your house is in a terrace or has shared walls, you might not have space to add insulation outside without stepping over boundary lines.
- Aesthetic changes: You’ll be changing the external appearance of the building, which may not be allowed in conservation areas.
What About Internal Wall Insulation?
Internal wall insulation (IWI) means adding insulation to the inside face of the wall—usually under new plasterboard or breathable lime plaster. It’s often the go-to method for heritage buildings or homes where the outside can’t be touched.
Benefits:
- Preserves the look outside: Ideal for listed buildings or street-facing façades.
- Can be done room by room: Handy if you’re renovating in stages.
- Quick comfort boost: Even a relatively thin layer of insulation can make rooms feel much warmer.
Key considerations:
- Condensation risks: As humidity from inside penetrates the insulation and hits a cold wall, condensation can form. That’s why the insulation and vapour control layers need to be carefully chosen and installed.
- Less room inside: You’ll lose a bit of floor space—though in most cases, not much.
- Disruption indoors: Furniture needs moving, walls need stripping, and you might need to upgrade wiring or heating as part of the work.
So, Which Is Better?
There’s no one-size-fits-all answer. It depends on your building, your budget, and what you’re trying to achieve. Here are a few questions to help you decide:
- Is the house occupied? If yes, external insulation might be less disruptive.
- Can the appearance of the outside change? If not, internal insulation may be your only option.
- Are you dealing with damp or porous walls? External insulation can help protect them.
- Do you have space inside or outside to work with? Think about boundaries, roof overhangs, and window reveals.
And remember: sometimes the best answer is a mix of both. For example, you might insulate the back of a house externally and the front internally to suit planning restrictions.
Final Thought
Insulating solid walls is one of the best ways to improve comfort and reduce heating bills in older homes. But it’s also a job that needs careful thought. Moisture, detailing, and materials all need to work together.
At Fibres Academy, we’re here to help you understand how to get it right—from choosing the right type of insulation to understanding how buildings breathe. The more you know, the better you can make your building perform.