wall ties and insulation inside a cavity

Why having cavity wall insulation can actually cause damp and mould

We often hear about the benefits of having cavity wall insulation and the much lauded saving on costly fuel bills, but did you know that in some properties it can do more harm than good?

Let’s first make it crystal clear though, this is not applicable to all homes with cavity wall insulation, in fact in some cases, having walls filled with those little white beads can drastically warm up a cold home and significantly reduce heating bills, which is good in anyone’s book.

There are some things you really need to be aware of if you are thinking about having it done, or have had insulation fitted and are now suffering from mould on the walls indoors when you thought those days were over.

It’s a bit of a paradox really, surely insulation in your walls is a good thing?

Even the government get in on the act and offered generous grants to all kinds of homeowners to get walls insulated and stop the waste of carbon and greenhouse gas, so what’s the deal here then?

The energy saving trust suggest that with this type of home improvement on your home the average savings for a detached house would be at least £250 a year and a semi, the savings would be £150 a year

(source: energysavingtrust.org.uk/Insulation/Cavity-wall-insulation) however when you think about it, it’s not a HUGE saving but it does look good on paper though eh?

How can something that, in theory, sounds great, actually be a bad thing? Let’s explain a couple of things first.

What is cavity wall insulation?

For those who don’t know, most houses built after 1930 have 2 walls in their construction, the inner walls that surround you indoors, and the outer walls with render, paint or pebbledash on them, for example.

There is a gap of only a few inches between the 2 walls or “leaves” which act as an air barrier, meaning that, in theory, all the cold and rain from outside won’t come inside because there is a gap.

Can you guess what happens when you fill that gap with insulation?

When insulation is fitted, a team arrives and drills lots of holes in the wall  and then pump millions of tiny polystyrene beads, the same type that is found in a bean bag.

finding out if your home has cavity wall insulation

It is worth noting that on many surfaces, especially on render, the workmen will leave nasty scars on the wall which, unless you subsequently have the walls painted or coated with an exterior textured wall coating, you will not hide it with normal masonry paint.

Appearances aside though, if cavity wall insulating is a good thing, why does it cause damp in some homes?

Quite simply, this is due to exterior walls being in a poor condition, due to either a lack of maintenance, severe weather conditions, or more usually, the fact that the external walls have lost their weatherproofing ability.

When this happens, the water that rains on to your walls outside, gets soaked up inside the bricks.

This normally would stop at the cavity and would not pass into the house, although a good way to spot this is if your interior walls feel cold to the touch, even when the heating is on.

So the filling inside the walls starts to transmit the moisture to the inner leaf or wall, causing damp.

This photo shows the effects of wind-driven rain on to a wall with cavity insulation.


mould around window caused by cavity filled insulation
mould around window caused by cavity filled insulation


The walls on the house we used as an example are from a house that has had very little in the way of maintenance and as such the external wall surface soaks up water.

If the cavities had not been filled, the black mould around the window frame and the ceiling would not have appeared.

It is worth mentioning that if mould appears on your internal walls, this is a very BAD sign and we discussed at great length some time ago, how detrimental mould can be to your health.

Can having cavities filled cause any more issues?

Yes. Another issue that can occur is what is known as interstitial condensation which is when the inner leaf of the wall after insulation becomes much warmer (of course) and this has a knock-on effect of pushing condensation away from the inner wall.

This can cause condensation to form WITHIN the cavity because the dew-point, with warmer air, gets pushed back and out towards the cold outer wall, condensing and forming in the brickwork itself.

This problem comes into it’s own during winter time when the water now trapped inside the bricks actually freezes, expanding as it does, and forming cracks to the bricks.

Over time this can cause seriously expensive brickwork repairs, or in the case of smooth render or pebbledash, it can push the wall surface away from the bricks, causing what’s known as “hollow render” or “blown pebbledash”!

If a house has pebbledash on the walls, when it starts cracking, it can be difficult and expensive to fix.

Is my home suitable for cavity filled insulation?

Probably yes, but it very much depends on the condition of your exterior walls.

If the walls are porous and let in water, you could do worse than to consider having the walls done too.

If your house walls are weatherproofed with a protective coating, they won’t let the water in, so you won’t get damp, plus the installation of the cavity fill will be successful and you will benefit from much lower heating bills.

Be aware though that you need the insulation fitted BEFORE you have any further works to the exterior walls.

A textured wall coating will cover up the scars left, but normal masonry paint won’t, so bear that in mind.

So if I already have cavity filled walls and mould appears, what can be done?

Quite simply, some attention and care to the outside walls will stop any water from transmitting inside and the mould will go away.

There are two main products that we would recommend for this, and both are guaranteed to stop all instances of penetrating damp, even if the mould existed before the cavity walls were filled.

The first product, a resin based exterior textured wall coating which is a service offered by ourselves and includes a full repair programme to the walls, fixing cracks, loose rendering and so on.

It comes with a 20 year warranty, against a life expectancy of over 25 years!

If you look at our gallery pages (in the menu bar at the top of this page) you will see what a difference it makes to a home.

The second product we recommend is a damp-proof wall coating which is suitable for listed buildings, houses made from unpainted stone, and brick houses too, as it does not change the appearance of the property, but affords outstanding weather protection into the bargain.

If your home HAS suffered from mould, then it also best to rent or buy a dehumidifier, although you may need to use it for many weeks before any problems start to clear up.

If in doubt about any of the above points raised, then your local cavity wall insulation company should be able to advise you on what is best, although of course bear in mind they may not be so impartial as we are!

So in answer to the original question posed: Cavity wall insulation IS a good thing but ONLY if the outside of your house is correctly weatherproofed.

To find out more, give us a call on 0800 970 4928 or click here to send us an email.

NOTE: We are not a cavity wall insulation company so don’t ask us for a quote for cavity wall insulation please! thank you

Scroll to top