How to insulate cavity walls

If your house was constructed with cavity walls that are not insulated, then adding insulation could keep your home cosier and cut your heating bills. But it’s important to choose a method of insulation that’s right for your property – or you could end up with a damp problem.

In this section, you’ll find an overview of the options for insulating your walls, roof and floor.

Cavity wall insulation

Older cavity-wall properties were generally built without any additional insulation, relying instead on the air gap created by the cavity itself.

If this is the case for your property, and if you feel your home is losing too much heat, you may want to add insulation to your walls.

Options for cavity wall insulation:

  • inside the wall cavity
  • added to the wall, on the inside of the room
  • added to the wall, on the outside of your house

The right option for you will depend on a few different factors, like how much building work you want to do, the size of your rooms, your home’s location and how exposed it is, and your budget.

This is where an insulating substance (typically a foam) is pumped into the cavity though holes drilled in the wall. In general, CAfS does not recommend this for most buildings here in Cumbria, given that most parts of the county are designated as having ‘very severe’ or ‘severe’ exposure to wind-driven rain.*

You’re more likely to end up with damp. That’s because one of the main purposes of your wall cavity is to prevent condensation. By keeping an air gap between your external wall and your internal one, you prevent the warm air in your home from touching the colder external wall, where it would condense into droplets of water.

If you fill the gap, you create a bridge between your internal and external walls, making it more likely that rain water can work its way from your outside wall into the cavity.

* This is our general advice, based on expert opinion from construction professionals in Cumbria. Cavity-wall insulation can be a suitable option for some buildings. Before going ahead, we recommend having a proper survey and installation by appropriately qualified and insured installers.

This usually involves removing your internal plaster boards, fixing battens onto the wall and then attaching insulation to the battens.

For a cavity-wall home, typically you’d use closed-cell insulation (like Kingspan, Celotex or Isotherm) or insulated plaster boards. You could also opt for a semi-rigid batt-type insulation, like sheep’s wool or wood fibre board.

When fitting these, it’s crucial that the insulation doesn’t touch the wall. Keep an air gap to prevent condensation.

One disadvantage of this approach is that you’ll lose space in your room. If that’s a problem, an alternative is to apply a thermal plaster directly to the wall.

If your building is at risk of flooding, choose an insulation that does not absorb moisture, such as a closed-cell insulation. Put your plaster boards on horizontally, not vertically. That way, you may only need to replace the bottom board(s) if your home is flooded in future.

This is where you attach insulation and weatherproofing to the outside of your building. You might choose to do this on some walls, and then have internal insulation on others. There are quite a few different systems on the market for this, so we’d suggest doing your research to find the one that suits your building best.

Don’t forget to check with your local authority whether you need planning consent, as external wall insulation may change the look of your building.