Guide
Cavity wall insulation and damp
Filling a sound cavity wall shouldn't cause damp. Filling a wall that already lets rain in can. Here's how to tell the difference.
Cavity wall insulation doesn't cause damp in a sound wall that has been checked properly. It can let water cross the cavity where the outer wall already lets rain in, where there's rubble in the cavity, or where the wall faces heavy wind-driven rain.
So the survey before filling matters more than the product. Fix the wall first, then fill it.
Why a cavity is there in the first place
The gap between the two leaves of a cavity wall stops rain that soaks into the outer leaf from reaching the inner one. Filling the cavity with insulation that sheds water keeps most of that benefit. The problems start when there is a path for water across: a bridge of rubble or mortar, a gap in the fill, or so much water getting into the outer leaf that it can't drain away.
One Irish cost guide puts it plainly: "If the wall already has water penetration, bead can bridge moisture inwards and create a problem instead of fixing one" (ShamFix). SEAI's own guide tells you to ask the installer if the system will result in any dampness or condensation on the walls, or interfere with ventilation (SEAI guide).
What the installer should check first
- The outside of every wall: cracked render, open joints, failed pointing, damaged window sills
- Gutters, downpipes and overflows running down the wall
- The cavity itself, through a test hole with a camera: rubble, mortar or old insulation
- How exposed each wall is to wind-driven rain
- Damp already showing inside, and where it comes from
- Vents and flues, so the house keeps the air it needs
Damp after cavity wall insulation: what to do
- Write down where and when.Note which wall, which room, and if it's worse after heavy rain from one direction. Take photos.
- Look outside that wall.Check the gutters, the downpipes, the render and the sills. A leaking gutter is the most common and the easiest fix.
- Call the installer back.Give them your notes. They can check the fill in that area with a camera.
- Rule out condensation.Black mould in corners and on cold walls, worse in winter and in bedrooms, is often condensation, not rain. Ventilation helps.
- Fix the cause, then the patch.Treating the patch without fixing the cause brings it back.
Damp or condensation: how to tell
| Rain getting in | Condensation | |
|---|---|---|
| Where | Usually an outside wall, often the most exposed side | Cold spots, corners, behind furniture, window reveals |
| When | After heavy or wind-driven rain | Worse in cold weather, mornings, bedrooms and bathrooms |
| Looks like | A patch or tide mark that grows and shrinks with the weather | Black spotty mould, water on windows |
| Usual fix | Repair gutters, render, pointing or sills | Ventilation, steady heating, less moisture indoors |
Good insulation tends to help with condensation, because the inside of the wall is warmer. Ventilation still matters.
Questions to ask about damp before you agree
- Have you looked into the cavity on every wall, not just one?
- Is there any rubble or mortar in the cavity, and what will you do about it?
- Do any of my walls face a lot of wind-driven rain, and does that change the product you'd use?
- What repairs should I do on the outside before you fill?
- If damp shows up after, will you come back and check the fill?
Before you book
When the contractor rings, tell them about any damp you already have, and which way the house faces. A good survey answers the question before the work starts, not after. See is my house suitable?
Questions people ask
Does cavity wall insulation cause damp?
Not in a sound, well-checked wall. It can when the outer wall already lets rain in, the cavity has rubble in it, or the wall is very exposed. That's why the survey matters.
I have damp since my walls were filled. What now?
Note where and when it happens, check gutters and render on that wall, and call the installer back to check the fill. Rule out condensation too.
Can cavity insulation be removed?
It can be extracted, but it's a specialist job and costly. Check the wall properly before filling instead.
Is black mould a sign of bad insulation?
Often it's condensation, not rain getting in. It shows on cold spots and in rooms with a lot of moisture. Ventilation and steady heating help.
Sources (1)
Prices and facts on this page were checked against these published pages. We are not connected to these businesses.
- ShamFix: shamfix.ie
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