Why cavity wall insulation can cause damp and mould

We all hear about the benefits of having cavity wall insulation and the much promised saving on costly fuel bills but did you know 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.

However, 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 inside your home when you thought those days were over.

It’s a bit of a paradox really, I mean, surely insulation is a good thing?

Even the government get in on the act and offer 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 around £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 inside, 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 (see photo) and then pump millions of tiny polystyrene beads, the same type that is found in a bean bag.


Evidence of 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 coated with an exterior textured wall coating, you will not hide it with normal paint.

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

Quite simply, this can be due to many things such as 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.There’s also things like rubble in the cavity which can cause a good installation to go bad; properties not being ‘brushed off’ so insulation materials leak from your walls into neighbouring walls and leave gaps in your insulation.

When bad brickwork or pointing is evident, 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, which is what happened to the house below.


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.

Penetrating damp caused by cavity wall insulation

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”!

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 masonry paint won’t, so bear that in mind.

Look in the yellow pages to find your local cavity wall insulation company and be sure to ask them about the green deal or warmer homes scheme as you could get your home insulated for FREE!

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 may stop any water from transmitting inside and the mould MIGHT go away, but knowing what the actual cause is, and being able to look inside the cavity with a cavity inspection camera is usually only able to be done by qualified professionals. If you did have the specialist tools to perform these tasks, would you know what you are looking for?

If damp is an issue and has only been a problem after you have had cavity wall insulation fitted, then it is probably your first port of call to get this checked. Wall Cavity Claims offer a free and no obligation cavity inspection surveyor and if faults are highlighted, they have a panel of specialist solicitors who work No Win No Fee and can represent your complaint for compensation and damages.

It is extremely important you get it checked as mould and associated damp can cause severe health problems for you and your family.

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