Preparing Your Walls for New Insulation After Removal

If your cavity wall insulation has just been taken out, or is about to be, you may be wondering what has to happen before the walls can be insulated again. The short answer is that the preparation happens inside the wall and around it, not on its surface. The cavity must be completely clear, whatever let water in must be fixed, the wall ties must be sound and the walls must dry out. Only then can anyone judge whether the wall should be filled again at all.

This post explains each of those steps, what you can safely check yourself and when you need a professional. It also covers why, in exposed parts of Wales and the South West, refilling the same cavity isn't always the right answer.

Why there's no wall surface to prepare

Cavity wall insulation isn't fixed to the face of a wall. Many homes built since the 1930s have two walls: an outer leaf of brick and an inner leaf of brick or block, with a gap between them called the cavity. Retrofit insulation is blown into that gap through small holes drilled in the outer leaf.

So there is no need to smooth, fill or level the brickwork for new cavity insulation. What matters is the state of the cavity itself and of the outer wall that keeps the rain out. Advice about clean, flat surfaces and fillers belongs to internal or external wall insulation, which really are fixed to a wall face.

Make sure the cavity is completely clear

Old fill left behind is the main thing that undoes a refill. Pockets of wet mineral wool, clumps of polystyrene beads or old mortar resting on the wall ties can bridge the cavity, which means they give water a path from the outer wall to the inner one. New insulation blown in on top would hide them, and the damp would come back.

A proper extraction finishes with a check using a borescope, a small camera on a flexible tube pushed through the drill holes, so you can see the cavity is clear along its length and down to the base. Ask for that check, and for photos, before the access holes are made good. Our guide to the cavity wall insulation extraction process explains how the removal itself is done.

Fix the reasons water got in

If the old insulation was wet, something let water reach it. Refilling without fixing that will simply soak the new material. Common causes include:

• worn or cracked mortar pointing, or porous, flaking bricks

• cracked render, or render that holds water against the wall

• leaking gutters, downpipes or overflows running down the brickwork

• blocked weep holes, the small gaps above windows and doors that let water drain out of the cavity

• soil, paths or patios built up against the wall above the damp proof course

You can check most of these safely from the ground. Look along the gutters during heavy rain, use binoculars for the upper brickwork, and note any staining below windows or downpipes. Leave anything at height, and any repairs to the masonry, to a professional.

Check the wall ties while the cavity is open

Wall ties are the metal ties that hold the two leaves of a cavity wall together. Older steel ties can rust, and rust swells, which can crack the mortar joints and, in bad cases, let the outer wall bulge.

With the old fill out, the ties are easier to see than they will ever be again. If the borescope shows heavy corrosion, deal with it before new insulation covers them up. Our wall tie surveys and replacement service covers this, and the two jobs can be planned together.

Give the walls time to dry

Wet masonry dries slowly, often over several months, and longer through a Welsh winter. Insulating a wall that is still wet traps the moisture inside it. While you wait:

• keep the heating on steadily rather than in short bursts

• use extractor fans and trickle vents, and open a window when drying washing indoors

• photograph old damp patches every few weeks to see whether they are fading

• hold off redecorating until the marks have stopped changing

If damp patches stay the same or grow once the cavity is clear and the outside repairs are done, the cause may be something else, such as rising damp at low level. Our damp proofing and treatment service can track down the source.

Ask whether the wall should be refilled at all

This is the step most often skipped. How well a wall copes with cavity fill depends a great deal on how much wind-driven rain hits it. Building guidance divides the UK into four exposure zones, from sheltered to very severe, and the two highest zones take in almost the whole western coastline and much of Wales. At least one major building warranty provider won't accept blown-in insulation behind plain, unrendered brick in the very severe zone.

That doesn't mean your home can't be insulated. Depending on the wall, the options may be refilling with a material approved for its exposure, leaving the cavity empty, or choosing internal or external wall insulation instead. A wall facing the prevailing weather may need a different answer from a sheltered one.

If you do go ahead with new cavity insulation

Putting insulation into a cavity wall counts as building work under the Building Regulations in England and Wales. Use an installer registered with a scheme that lets them self-certify the work, otherwise it has to go through building control. Before you agree, ask:

• what material they will use, and whether it is approved for your home's exposure

• how they have checked that the cavity is clear and the walls are dry

• what guarantee comes with the work, and who stands behind it

Keep all the paperwork with your house documents. You will need it if you sell, or if anything goes wrong later.

Cavi Clear Ltd surveys and removes faulty cavity wall insulation, cleans out the cavity and makes good the access holes, and can check your wall ties while the cavity is open. If you're thinking about re-insulating, book a survey through our cavity wall insulation removal service first, so the wall is clear, sound and dry before anything new goes in.