Hello, and welcome to this week’s ‘Back to Earth’ podcast with me, Chris Brookman. This the show for architects, builders and surveyors all about the use of sustainable building materials. In this episode specifically we’re going to be answering questions all about cavity wall insulation. 

Cavity wall insulation’s a topic that I get asked an awful lot about and come across a lot of situations where it’s used either inappropriately or it isn’t used when it should be used. 

So I’m going to answer all the questions that we’ve been asked over the last year or so and hopefully answer everyone’s questions about it.

First of all, what is a cavity?

There’s two different types of walls essentially. You can have solid walls which are literally a solid brick thick, or you can have a cavity wall which is essentially two walls – so you’ve got an inner skin of normally brick or block, which is normally about 10cm thick. You then have a cavity which can be anything between 50 and 100 ml wide, and then you have the external leaf which is another 10cm of brickwork or blockwork.

The idea of having that cavity is to prevent moisture from moving from the outer skin and migrating towards the inner skin. So that cavity allows any moisture that gets through the outer skin to drain down and go down into the ground before it manages to reach the inside skin.  It was a way of drying out buildings when there was no insulation in them basically.  It kept the weather out from the outside and it kept the interior relatively warm compared to having a solid wall.

What does cavity wall insulation actually do?

Well, it’s very, very, very basic. It essentially just fills that gap between the two walls. There are various different forms of it and actually there’s a lot of reasons why it should be carefully considered.

The next question is – should you actually fill a cavity wall?

It’s something that I get asked most often actually.  I would generally say that you should only ever fill a cavity wall if you’re going to externally insulate a building.  If you’re not adding external insulation, then I would generally recommend that you never, ever, ever fill a cavity wall. It’s there for a reason.  It’s keeping the building dry and it really should not be filled.  The reason for that is very simple – if you fill that cavity, moisture can then track from the outer skin all the way through and actually reach the inner skin, and also any moisture that escapes from the building during the winter months and gets into the cavity will condense in the cavity and that moisture wets the insulation and again that migrates back towards the interior.  So adding cavity wall insulation can potentially actually give you damp problems.

So, which cavity wall insulation is the best to use?

Well, there’s various different types. You can inject polyurethane foam into your cavity. You can blow different types of fibre – different types of fibreglass, normally, into the cavity. You can also blow polystyrene beads into the cavity and those are the main ones.

In terms of which is best it really depends on how well they’re installed because actually they’re all pretty good at what they do. But I have to say my money’s probably on the polystyrene beads, and that’s simply because polystyrene beads flow. It means that if you basically pour the polystyrene beads in at the top of the cavity, they will work their way all the way down to the bottom and they’ll also go under things like window sills so you actually get insulation properly worked in under the window sills.

You can also get slightly greener alternatives. There’s a product called Kenwood CW1000 which is essentially a mineralised woodchip and again it’s quite a fine material that can be blown into a cavity and because it flows, again it will get under the window sills, but it’s made from woodchips basically and so is a rather greener alternative.

The one important thing to bear in mind is that this product can’t be left in contact with the ground, so you generally have to put in a certain amount of synthetic insulation at the bottom of the w