Cavity Wall Insulation: The 16 Key Questions

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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 100mm thick. You then have a cavity which can be anything between 50 and 100 mm wide, and then you have the external leaf which is another 100mm 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 Cemwood 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 wall to prevent that material from going down into the ground.

How much does cavity wall insulation cost?

Well, that depends generally on who’s installing it. Most of the time you can get grants for that kind of insulation.  If you can’t, I guess you have to look around for competitive quotes, but generally the cost is pretty minimal, basically because it’s covered by the grant.

If you buy a house that already has cavity wall insulation, I would look at it very, very carefully, particularly for damp issues internally.  There is one simple way of rectifying any of those problems and that is quite simply to put external wall insulation on, and that will warm up the cavity inside. It will allow it to dry out and it will rectify the problems that are experienced inside.  That’s a slightly simplistic way to discuss it, but in essence that’s exactly what happens.

Another question I get asked is what is full fill cavity wall insulation?

Generally full fill cavity insulation is something that you build in rather than blow in afterwards.  Full fill simply means that when you’re building the wall, you’re fully filling the cavity between the inner and outer leaf rather than leaving an air space between the insulation and the external leaf, as was common.

One other question is – does cavity wall insulation require Building Regulation Approval? 

Actually it does.  Most of the time the installers notify Building Control but if they don’t, it is a notifiable procedure. So it’s important that Building Control are involved in it and have notification of it.

Another question I get asked is – does cavity wall insulation need renewing?

Generally it doesn’t.  There’s no reason that it should break down and degrade into anything else, but if alterations are carried out to the building and the cavity wall insulation is allowed to come out of the cavity, then it will need topping up and some of it will need replacing. So, generally not unless it’s disturbed in some way or lost.

Will cavity wall insulation stop damp?

Generally not. If anything, it’s more likely to cause damp unless you’ve got external wall insulation.  So that’s basically a no. 

Will it stop rats?

Mice and rats generally will move through anything. It doesn’t matter whether it’s synthetic or natural.  If there’s a source of food there, they will tend to move through it.  So, no, again it won’t stop rats.

Will it stop mould? 

Generally not because it will tend to allow moisture from the exterior skin to reach the interior skin, again, unless you’ve got external wall insulation.

Will cavity wall insulation reduce noise? 

Potentially it can do, if some of your noise is being transmitted through the actual wall.  But what you tend to find is that noise travels mostly through windows, through roofs, through air bricks, through doors, that kind of thing.  So it has the potential to reduce some noise transmission through a wall, but most of the time it will be pretty minimal.

Will cavity wall insulation dry out?

The answer is actually yes.  It does take a long time because the materials that you use for cavity wall insulation are extremely poor at moving moisture around and allowing themselves to dry out. But generally, yes, it will slowly dry out, so if you have a situation where you’ve got wet cavity wall insulation, the best thing to do really is to install some external wall insulation and preferably a vapour permeable wall insulation to warm up that cavity and allow it to dry out.

Will cavity wall insulation make my house warmer?

Well, one would hope so, but as per my previous comments – only if you have external wall insulation, because potentially the additional moisture that can migrate through it, will actually negate that additional benefit of warmth.

Can cavity wall insulation be removed? 

Yes, it can actually, and actually now that there have been so many buildings with it installed and so many of those buildings are having problems, there are actually companies now that go round and remove it.  As I said, unless there are serious structural issues, I wouldn’t remove it. I would simply add external wall insulation over it and allow it to dry out. But potentially there can be issues with the rusting of cavity wall ties, which are definitely exacerbated by wet insulation.  So if you need to, then absolutely you can remove it.

Does cavity wall cause condensation? 

Generally it doesn’t cause condensation but it allows the passage of moisture from the outside to the inside. So, I would have said, no, is the answer to that.

To summarise, basically what I’m saying is cavity wall insulation is only recommended, certainly by us, if you’re adding external wall insulation and that’s all about the moving of the due point from the cavity to out into the external insulation so that you don’t get condensation in the cavity in the winter.  It also is about preventing driving rain and moisture actually reaching that external leaf of the wall, which then can track through to the inside. 

So with that external insulation on there, you can be reasonably confident that you’re not going to be getting moisture tracking through from the outside and you’re not going to be getting condensation forming in the cavity in the first place – which would track back in as well.

So if you aren’t adding external wall insulation, the best thing you can do is use internal wall insulation but definitely do not fill the cavity.

If you have any further questions about sustainable building materials or cavity wall insulation, please feel free to email me at chris@backtoearth or alternatively give me a ring on 01392 861763.

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