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Sagging cavity wall insulation: why it happens, how do I know and what to do?

Posted by Helena Ripley on 13 July 2015 at 12:30 pm

Installing cavity wall insulation (CWI) can help to stop heat leaking through your walls and therefore decrease your spending on heating by up to £270 (based on an average gas heated detached home). However over time the insulation can sag or degrade.

Gaps in insulation can cause nearly as much heat loss as having no insulation at all. According to research by NASA 50% of the heat lost from a building can be attributed to gaps in insulation amounting to 5% of the surface area. A large amount of heat can be drawn out through a small uninsulated patch due to the temperature difference between the inside of your house and the outside. This is similar to a lot of air being released from a small hole in a balloon. So, it is important to make sure that your insulation is well maintained.

What types of cavity wall insulation are there?

There are a number of types of material that can be used for CWI – but all can compact or degrade over time:

  1. Mineral wool (also called mineral fibre or blown fibre) is by far the most commoly used.
  2. Polystyrene beads will help where access to the external wall is difficult as they can be dropped down the cavity from the eaves.
  3. Polyurethane foam might be recommended where the brickwork is crumbling since it easily fills gaps and cracks.

These are all blown into the cavity through holes drilled through the external wall.

Mineral wool is the same type of material as loft insulation but it is torn into small pieces. There are various types of beads available – expanded polystyrene beads, expanded polystyrene granules, polyurethane granules and perlite beads. Often the beads are bonded together to prevent them from escaping. Expanding foam is the most waterproof and less affected by damp but the expansive force of the foam can be a concern.

Why would there be gaps?

Mineral wool gradually compacts and causes gaps to form in the insulation, particularly in the top of the building. This is made worse if there is any damp within the cavity as water will weigh the insulation down; however this is a slightly different issue. Insulating beads can be lost into loft cavities and through air vents; and foam gradually degrades and loses its insulating properties; in some cases it can also release gases.

How can you check for gaps?

The quickest way to check whether you have any gaps in your cavity wall insulation is to hire a thermal imaging camera. Any gaps will be shown on the images as cold patches. Alternatively a visual inspection can also be made; this can be done by drilling into the wall and looking inside with a camera. This shows the extent of the sagging or compaction of the insulation. This isn’t something you should do yourself (see ‘A note on your guarantee’ below). A CWI installer would need to check this, so if you do think you have a problem you may need to contact your installer or another professional.

What to do if you have gaps in your CWI?

In most cases it is a simple matter of topping up the insulation, gaps can be refilled in the same way your house was insulated before – by injecting the insulating material through the wall. However if your insulation has become damp or water logged it will need to be removed at it will no longer do its job of insulating. This is a difficult, costly and time consuming process. Sometimes it is not possible to fill in the gaps, if they are isolated or in hard to reach places. If this happens it may be necessary to install internal or external insulation in the problem area.

What are the costs involved?

Cheap infrared cameras are available to rent. Prices start from around £85, but you get the camera for a few days, so perhaps might be worth clubbing together with others to make the most of it. Some community groups buy cameras as part of projects and some libraries or local authorities will lend them too, so these are good places to start.

Most surveys of cavity walls (using a camera) by CWI installers are free. However if you are covered by a Cavity Insulation Guarantee Agency (CIGA) guarantee any post insulation inspection cost might be covered. Your CIGA (Cavity Insulation Guarantee Agency) guarantee should cover any top ups necessary. Guidance on what to do if you think you have problem with your insulation can be found on their website.

The price of insulation removal varies with each property. A very rough guide to the cost of removing cavity wall insulation is about £1,800 to £2,500 for a three bed semi-detached house. A three bed bungalow would be cheaper, about £1,000 to £1,500. Tall or difficult to reach buildings would have to be quoted for on an individual basis. Again, this should be covered by your CIGA guarantee.

A note on your guarantee.

When you had your CWI installed you should have received a 25 year guarantee from CIGA. One of the terms and conditions of this guarantee is that the insulation must not have been altered or damaged. So if you think you have an issue with your insulation and want a visual inspection of the cavity you should contact your installer. If you don’t know who installed the insulation have a look for another professional. Trying to do a visual inspection of the insulation by yourself could invalidate your guarantee.

Photo: Heat Insulation

More information about Insulation on YouGen

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Need help with any Jargon?

If you have a question about anything in the above blog, please ask it in the comments section below.

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2 comments - read them below or add one


solarcrestComment left on: 19 April 2019 at 2:25 pm

cavity wall insulations like mineral wool and blown fibres soak up water in its liquid form but they do allow for vapour to diffuse through. you may have a two fold problem one being the capiliary effect of driving rain but more than likely the main problem is interstitial condensation.  as you increase the air temperature inside the building the moisture content and vapour pressure also increases and this vapour diffuses through the wall and condenses on the inside face of the external brickwork which will always be at dew point, this misery compounds over time. if this insulation isnt removed it will be the catalist for outer brickwork degradation and structural failure (freeze thaw cycle).

once this insulation is wet its basicaly the end of its thermal resistance it clumps together sags and doesnt dry. 

the best cavity walll insulation is polyuraythane foam which is resistant to water in its liquid form yet allows water vapour to diffuse through the wall assembly without a problem. it doesnt degrade or slump and will help fix any damage.



regards Richard

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Andy in Hawick

Andy in HawickComment left on: 31 July 2015 at 10:23 pm

When we replaced some windows we discovered vast areas of the wall cavity void. Some places had never been filled others had insulation in the lower part. We contacted the installer and they came and did a refill of all the walls. The installers said that the amount of material going in wasn't much different to what they'd expect with a virgin cavity.

Looking back at thermal images we'd taken only about three years after the walls had been filled initially, you can see the tell-tale signs of sagged insulation although that was not clear to us at the time.

The blown fibre used was described to us as celulose treated with silicone.

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