Is my house suitable for cavity wall insulation?

Step one: check it's a cavity wall

Irish homes have three main kinds of wall: cavity walls, solid walls and timber frame (SEAI homeowner's guide to wall insulation). Cavity wall insulation only works on the first.

1. Measure the wall at a window or door.Measure the full thickness, inside to outside. As a rough guide, a wall of about 300 mm or more is often a cavity wall. A thinner one is more likely solid.
2. Look at the outside.Fair-faced brick laid in long stretches, all the same way, often means a cavity wall. Bricks laid with ends showing in a pattern can mean solid brick. Rendered walls give nothing away.
3. Think about the age.Many older homes, especially traditional homes built from stone or solid brick, have solid walls. Many later block-built homes have a cavity.
4. Ask, or get a survey.A building contractor or architect can tell you what wall you have. A cavity insulation survey answers it for certain.

Step two: check the cavity is empty

Some homes were built with insulation boards partly filling the cavity. Some had the cavity filled later. An installer drills a small hole and looks in with a borescope camera. They check how wide the cavity is, what insulation is already there, and any mortar or rubble lying in it.

SEAI's guide tells you to ask the installer directly: is the cavity suitable for fully filling with insulation, and how has that been assessed (SEAI guide)?

Step three: check the wall will stay dry

A filled cavity can let water across from the outer leaf if the wall has problems. Before filling, the installer should check for:

• Cracked or missing render, or failed pointing
• Damp patches already showing inside
• Gutters or downpipes leaking down the wall
• Rubble or mortar in the cavity that could bridge it
• A very exposed wall facing the prevailing wind and rain
• Air vents and flues that need sleeving so they keep working

Fix the wall first if any of these turn up. See cavity wall insulation and damp.

Homes that usually aren't suitable

• Solid walls. No cavity to fill. Internal or external insulation is the option. Compare the wall grants.
• Traditional homes. Older solid-wall homes are not covered by the cavity grant under Better Energy Homes. SEAI runs a separate Traditional Homes pilot.
• Timber frame. The cavity in a timber frame wall is there to keep the frame dry. Don't fill it.
• Cavities that are already full. A camera check shows it. If the cavity was filled with an SEAI grant, the second wall measure is internal or external insulation.
• Walls with damp problems not yet fixed. Fix the cause first.

Apartments and terraced houses

Apartments and terraced houses can be filled too. The grant is smaller because there's less outside wall: €850 for a mid-terrace and €700 for an apartment (seai.ie). In an apartment block the walls are usually shared, so the management company may need to agree.

The quickest way to find out

Use the grant check and leave your number. One SEAI-registered contractor who covers your area rings you, tells you what to look for, and confirms it on a survey.

Questions people ask