Which Cavity Wall Insulation?

Cavity Wall insulation for typical existing homes comes in a number of forms:

  1. Beads
  2. Beads with adhesive
  3. Mineral fibre
  4. Foam

I recently looked at a couple of the more popular options in the UK and found to my surprise that there is very little technical information available on them. No technical details on installers or manufacturers websites and when I called one of the biggest installers I was told that there was no-one there who could answer my questions (despite being quite basic) and that I would either have to speak to a surveyor or have an on-site visit. So they are keeping some basic data hidden away. Not sure why as there is nothing really to hide. Also all the sites give out basic info on the process of installing, benefits etc, but there is no mention of any particular issues to bear in mind.

All of cavity wall products are injected and all will do the required insulation job on paper, however there are some major differences between them. I shall also highlight some concerns / factors that you might wish to bear in mind when choosing.

One major factor before you even start is to ascertain whether your property is actually suitable for cavity wall insulation. Many cavity walled properties that were built before WW2 did not have wide cavities and many are also full of builders waste. So make sure that your wall is capable of safely having insulation installed.

Second major factor is: How exposed is the site? Is it prone to wind driven rain? The way that cavities are designed to work is that the outer ‘skin’ is allowed to get wet and then the cavity keeps the inner wall dry. This means that the walls are actually quite poor at insulating, but that they are at least dry. By filling the cavity with a material, this can allow water to pass from the outer skin into the inner one. Not good! So your outer wall needs to be in good condition with intact render / mortar, not affected by leaking gutters or down pipes etc.

Third major factor is: Are you planning on any replacement doors / windows / extension works etc in the future? Some insulations are loose fill and so if you disturb the wall then they will just fall out. Thinking ahead will allow you to make the right choice in the longer term, otherwise you are just making extra work for you or for future occupiers.

Fourth major factor is: Do you have good seals around anything that goes through the wall? Insulation companies are not good at making good a wall before work. So if you have penetrations through the wall for waste pipes, drains, water connections, gas connections, air bricks etc, or even poorly sealed doors and windows the companies often just fail to inject insulation in these areas. Thermographic images taken on insulated properties often show that cavities are not insulated around ‘weak points’ like doors, kitchens (where there are lots of services), above window lintels etc. This causes cold spots in these areas and can lead to damp and mould forming.

So back to the choices.

  1. Beads – Beads on their own form some of the most insulating choices. The beads that are injected though are loose fill. They can also form a bridge between the two skins. They can rely more on gravity to fill the cavity rather than pressure when installing.
  2. Beads with adhesive – These are generally less insulating, but are pressure injected in order to make the glue work. They are regarded as being water proof and so should keep any water away from the inner leaf (assuming that they is installed correctly). The pressurised installation should mean that it is more likely to fill all of the cavity (and blow out of any weak points!)
  3. Mineral fibre – These are sold quite often as being fire proof. However, fires in cavities are not very likely and so the toxic fumes from beads are not too much of real hazard. Mineral fibre is also a loose fill, although it does bind together better than beads. It can also form a bridge between the skins if the outer skin fails (just like the beads).
  4. Foam – This is not fitted these days as there have been numerous cases where the foam has failed over time and left a right old mess in the cavity. If you did have foam installed years ago then you may wish to revisit the condition of your walls. There are companies who will remove failed foam and re-install a more appropriate insulation. A thermographic image should identify if your walls do have a problem.

Overall then, there is of course a need for cavity wall insulation in many houses, but you need to think about the suitability of the walls, the need for continual maintenance of the outer skin to ensure that it is not compromised and also the choice of the material for the fill. Good luck!