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Hi All, we want to get our house insulated - there is nothing in the wall cavity, so thought we were looking at 90mm foam covered by new rendering.  The first two local tradespeople who came to look were talking about doing just that, but the third said that if we were going to get external insulation, that we should also get cavity wall insulation or we are wasting our time - he doesn't do cavity wall himself, so would have nothing to gain from suggesting this.  Earlier that day, the second person to visit had said that people don't want cavity wall insulation nowadays and it is usually taken out.

Has anybody else had insulation installed in the last few years?  Was cavity wall mentioned as a good idea?  Is 90mm and rendering the standard?

My wife is the brains behind the job as she is the one who finds the house cold in winter - I live in fleeces and just put thicker ones on.

Thanks in advance for comments and advice received.

I'm interested to see what replies you get (considering insulation options in our old, cold, stone house!). There are still government grants in some areas to get cavity wall insulation, and the energy saving trust are still promoting it, despite more and more evidence that it can wreck a house and affect house insurance etc. I personally wouldn't fill a wall cavity with insulation, the idea of external insulation sounds sensible if you can't do it internally and still leave a gap to prevent bridging for damp. Our house has stone walls 24" deep and an internal cavity/framed wall (very typical on these old Scottish houses). I think I will take off the plasterboard and put insulation in the framed stud wall (leaving the cavity free), then re plasterboard. Seems the neatest option for me at least. 

That's interesting. I've only ever seen a solid wall externally insulated. Now you have mentioned it, it would make sense that the heat would travel through the internal brick/block skin and then rise in the cavity to escape at the eaves level. Meaning it wouldn't be 'trapped' by the external insulation. 

90mm of external plus a full cavity would be above new build regs I think. There have been problems with some cavity fills, as damp could track across the cavity. The external insulation should stop that happening if you were to have both. Do you already have enough in the loft ? 

I'm a builder , but no insulation expert. Interested to see what others think. 

Absolutely not an expert on this, but our last house had cavity wall insulation and caused us no problems at all. Don't know if it was designed and built like that, or if we were just lucky, but we had no dampness from it. 

Be very careful with cavity fill and condensation - it can destroy the house if done wrong.

Some areas of the country, especially on W coast are more prone to have driving rain on their walls. That's why cavities were originally used in buildings, to separate the (hopefully) dry inner wall from a damp outer wall. 

You haven't said where in the country you are and if you're near the coast and if sheltered or not. You also haven't said what age and construction your house, beyond it has a cavity. Without this even if a genuine insulation expert comes by, they may not be able to advise.

FWIW .. I spent time years ago (before every  cavity was given a grant to be blocked up with shredded cr*p) doing a very boring job (pun intended) of staring inside wall cavities with a borescope. We used to check that wall ties were present and not rusted away and that the cavity was empty and not blocked with rubble and mortar. A blocked cavity was seen as a damp problem back then. Fashions changed only a few years after that, I bet all the cavities I signed off as correctly empty (air flow) got blocked up with shredded paper and mineral wool only a few years later.

I resisted so many door knocking cold calls in the past for people trying to get me to sign up for free cavity insulation. They could never understand why I refused "but.. it's Free!" 

I was always suspicious of cavity wall insulation, but current house has it and no damp problems that I can see.

I'm going with adding insulation on the inside (to keep the costs down by DIY).

Interested to keep an eye on this thread though, as my parents place could do with better insulation.

Historically had issues with bridging in the cavity wall of their extension, so certainly wouldn't want to fill the cavity without knowing that the outer wall would stay dry, but I guess external insulation should guarantee that?

Is it be worth some research on internal insulation to improve the main living zone of your home - how open plan is it ? Would it be acceptable to leave upstairs / bedrooms as they are / cooler ? 

The insulation will work, but to do the job well you might want to move the windows out, inset windows will let in less light. Either way they'll need to new trim etc.. The same with doors. 

The biggest challenge will be internal moisture it has to go somewhere, i wouldn't fit until you have spoken to someone who had it done to a house of similar construction at least one winter ago. Plus how do they deal with air vents in brickwork, existing damp courses etc..

Unfortunately cavity walls (brick and block) are terrible designs for building modern energy efficient houses and should have been stopped decades ago, but that's another discussion.

We got cavity wall filling in our previous flat, it made quite a difference.  The space under the stairs, next to an external wall, stopped getting mouldy and the flat was generally warmer.  We had no problems.

We had cavity insulation free by the council. I didn't pay enough attention to by bills to comment on how much of a difference it made.

Considering that cavity insulation is a fraction of the cost of external insulation, I wouldn't even be considering external insulation when you have cavity insulation as an option.

They can both cause damp. The main cause is unless you do something to reduce humidity in your house, when it hits a cold surface it will condense. When this cold surface is your entire wall, it's spread so thin, it doesn't matter. But if your wall is now warmer apart from one spot that's still cold (no cavity in that spot for example, plinths etc) then it will condense a concentrated amount of water.

External insulation does this a lot since they have a habit of missing bits like where you have down pipes, cabinets etc.

I'd like to also echo the point about unless you deal with the cavity (some have capped tops), external only insulation could be a waste of money. 

As others have said, it all depends on your house construction (and the state of repair of the construction) and the aspect.

My house is a 1980s three storey, with render cladding that I carefully maintain, tucked in a bit of a hollow in Oxfordshire, so basically the perfect candidate.  I had no concern at all about moisture ingress, and monitored carefully for condensation.  The house remained bone dry inside after it was done (free, about eight years ago).  Made a really noticeable difference - from the inside, the external walls felt warmer to the touch and the bills certainly dropped.

I am in West Yorkshire, the biggest wall (we are on the side of the valley) is one that faces the weather coming off the moors.  The house was built in the mid-50s and the current design is brick and render.  We have had issues with damp - when we moved in a got damp work done, they said don't have cavity wall filling, it causes bridging of the damp, but the recent bloke said the external finish will prevent any moisture coming through

For those also considering getting some insulation work done, the last bloke also mentioned (but was the only one) that for house insulation improvement work like that, it can be done with a VAT exemption.  He said that he would send a link when he sends a quote - I haven't had any of the quotes through yet.

Thanks for all the replies so far, I hope it benefits others as well.

I've opted for internal insulation in mine since most of the plaster needed remedial work anyway and the decorative features, such as they were, were long gone when we bought it.

I considered cavity fill since it was available cheap but I know the pointing is flawed and the cavity gets wet, I also knew the wall below the DPC is really wet, I didn't want insulation in constant contact with that.

External foam was out of the question for me, it's a red brick terrace with decorative features, climbing plants and small roof overhangs, entombing it in a rendered box would have looked really odd.

If I run into new damp problems when it's all closed up I'll fit a powered ventilation system. It condensed in the cold corners anyway, the draught free insulated rooms are actually better now so I'm not overly worried.

In your shoes, before looking too hard at other options I'd do some more research into the various options for filling the cavity, the particular state of your house and how its peers have reacted to that treatment in your area. Or fix the draught first, see where that gets you to.

jk

We had ours done about 5 years ago. It made a big difference. We're not exposed to the worst weather or on the coast, so probably typical for many. I researched several types, from rock wool to foam. They dont do rockwool any more, or at least dont recommend it. Once it gets wet, it doesnt dry out. Expanding foam seemed quite good, but it can be patchy, and is impossible to get out. In the end, we went with polystyrene balls. They dont hold water and allow a certain amount of air movement. Its fairly quick as they are blown in. They can also be removed easily, if required.

Also worth lifting the floor boards and putting some rockwool there, especially against outside walls. If your a total geek, you can get a thermal imager add on for iphones. When its cold outside, it will allow you to see the bits you've missed.

Yep, at one stage I thought this was a complicated, specialist thing but chatting to someone recently the scales fell from my eyes - the ventilation kit is available from ebay and it's very DIYable. 

The short and rather unhelpful answer is...it depends. Sometimes it works, sometimes it can cause problems. A lot of it depends on the existing construction and the material used to fill the cavity. It's very unlikely to be 'foam', but more often some sort of inert, rigid insulation bead that is blown in through a series of small holes, which are later plugged.

Earlier this year I had some insulated plasterboard added to the internal face of a large, north-facing room in my house. It has 2 windows and an external door on the north wall and another large window on the east wall. The existing external walls were totally uninsulated and there's no underfloor insulation at present. I specified 60mm Celotex bonded to12.5mm plasterboard and it made a huge difference. (the Kingspan equivalent was a fair bit more expensive and only a very marginal performance improvement.) I also had enough space on the sight-line of the jambs, cill and head to have all those insulated too. I has a larger radiator installed too so that made a difference but even overnight having had the room at 20°, it would normally drop to about 10° by the morning over a cold night. Now in like-for-like conditions, it only drops to about 15-16°. I'll be trying to get into the solum space later this year and get some insulation under there to help wrap the envelope as best I can.

With this internal lining method, you do sacrifice some floor space but for my unusual wall construction, it was a better solution than external, rendered insulation or full-fill cavity insulation.

FWIW.. with a cavity, it should have sufficient vents but be closed across the top. Traditionally they cemented some slates (or whatever they had spare) across the top, so you shouldn't have free moving air rising up the cavity.

When left open at the top it has the tendency to make cold patches here and there on internal walls. So it *shouldn't* be the case it's open. This means that insulating external, such as thermal render, *should* still improve insulation even if the cavity is left empty because the air shouldn't convect it away.

Whether you are missing on extra insulation without benefit (because the extra render adds weatherproofing) is a matter of opinion. Personally (this is not advice!) I'd chance it for a dry sheltered place, but the description of weather driving off the moor onto the wall would make me want to keep the cavity and trade a little insulation for hopefully increased weather resistance. But that's just me! In your case you've had damp issues before and been advised by a damp specialist *not* to fill the cavity. If external renders and insulation is foolproof, how did the damp get past the original render? Personally I'd take the original damp specialists advice and leave it empty

I recently had cavity wall insulation installed after doing a lot of research, and also after a close friend also did. The key issue with cavity wall insulation, is what product you use to fill it. You need to pay more money for a closed-cell foam, such as BASF Walltite.

Almost all examples you hear of people having damp issues, they have used the cheaper open-cell, or loose-ball fill type to their cavities. All of the old government grant schemes only allowed for this cheaper insulation. This has given cavity wall insulation a bad name.

Open-cell insulation not only has inferior insulation properties, but it is porous and thus absorbs/transmits water. Driving rain penetrates your outer leaf of brickwork, and then is absorbed by the insulation. In addition to this, when the foam is installed it completely fills the cavity, making contact with the ground/substructure, and thus rising damp is also an issue with open-cell insulation. With closed-cell insulation this does not happen.

It is quite dear, I paid £28/m2 to fill my 50mm wide cavities, however I would not advise open-cell insulation regardless of how much cheaper it is.

Both myself and my friend had BASF Walltite installed and neither have had any issues whatsoever with damp. Mine has been in for 2 years now, his for 5 years.

1930s house with brick inner and outer skin. Had the cavity insulated about 12 years ago. Made the house warmer and easier to heat, cooler in the summer as well.

Only damp issue was cause by a leaking gutter, this was very localised and went away after rectifying the problem gutter.

We had very similar work done to our 1930s house a couple of years ago. It had pre-existing cavity wall insulation and render outside.

We wanted to add 150mm of external wall insulation to improve the thermal performance of the house. The key advice we got from architects was:

You have to have properly filled cavities or you get thermal bypass (as described above) - basically convection of warm internal air in the cavity. So we had the old cavity wall insulation removed and replaced with expanding foam type insulation (icynene in our case).

Adding external wall insulation may not have the desired effect if you don't also sort out airtightness - draughts cause a feeling of cold out of proportion to the actual temperature effect. So when we replaced the original 1930s steel windows (!) we had the new windows moved outwards in the wall and taped with airtightness tape - as were any other joints (water pipes etc.) 

We then had 150mm of EPS with silicone render over the top. You are highly unlikely to get an water ingress from the outside because of the water repellent outer layer and thick layer of polystyrene wrapping your house - however, you can get issues with condensation in a much more airtight house. We added a ventilation system to help with this - but still have some minor issues between the roof joists where we have cold bridges (the colder temperature of the interior wall where there's no insulation at roof level causes condensation to form in these areas). We're going to sort this by adding insulation to to the top of the wall between the joists.

In terms of the effect on comfort - the house in winter rarely drops below 18 degrees and that relies on having the heating on for around 2 hours per day at a low flow temperature (c. 40 degrees) - we're very pleased with it. 

If you have bare brick outer leaf, foam is a big no-no. The vertical joints in bricks laid with cement mortar are guaranteed to split away from one or both bricks due to mortar shrinkage. This means that rain gets driven through the resulting gaps by the wind and any insulation which bridges the cavity will transmit that damp to the inner leaf. Rendered external surfaces don't generally suffer from penetrating damp thus can have foam-filled cavities.

My Dad's house has 'megafoam' filled cavities, it's concrete block construction with 'grani-set' render and the foam was put in when it was built in '74. Dry as a bone, but has suffered black condensation mould in the east-facing bedrooms in the past due to them being the coldest parts of the house and warm, moisture-laden air rising into them and condensing out.  We've now got rid of the recirculating extractor fan in the kitchen and things are much better. The foam is a very weak, white substance, almost like meringue.  Central heating output temp is 50 degrees so I'd suggest it is working well.

External insulation has the advantage that it is keeping the walls warmer and thus the effective thermal mass of the house is much increased.  This is beneficial in terms of being able to take advantage of cheap rate electrical heating. As mentioned upthread, you ideally also need to keep the cavity from changing its air, but it's easy enough to cap the top of the cavity with cavity closures.  Warmer walls mean the rising damp level in houses that suffer from it will be lower too.  Outer cavity leaves will also dry out due to being protected from the rain, thus insulating better - think how much rain soaks into a brick wall and how much energy is used evaporating it again. Interstitial condensation will also be reduced, whereas internal 'dry lining'-type insulation will increase it. Apart from the need to re-fit windows and rainwater goods there are virtually no downsides - the only real one would be for proper old houses that need to 'breathe', but as houses that old don't generally have cavities that's unlikely.

Not what you want to here but other people's experience when it comes it insulation will be largely meaningless. Every house performs differently depending on construction method, condition, situation*, and usage. What works for others might not work for you and vice versa. Wall insulation is expensive, high risk, and in most cases represents a relatively poor return on investment. More meaningful gains are often achieved with additional roof insulation, draft-proofing, and better building maintenance. 

Unfortunately retrofit is a bit more complicated than whacking in some insulation. The people who know what they're talking about would recommend against single measures and instead promote the 'whole house' approach. This can be boiled down to 1. Understanding how your building performs now and 2. Designing a comprehensive scheme of retrofit based prioritising the health of the occupants and the building.

More guidance here: https://www.leti.uk/retrofit

*Situation: You've said upthread that you're house is exposed to driving rain off the moors. This is a texbook case of when not to get cavity wall insulation.

This has been really helpful for me, so thanks to all who have replied.  Loads to think through further, plus extra research as not all of it makes sense to me - not my area of expertise.

It depends mostly on the nature of the cavity. A traditional brick cavity wall is a bairly ventilated cavity. In this instance there is very limited air movement between the cavity and external. There will be weep holes to allow any moisture or condensation to drain. In walls of this type the cavity and layers to the exterior of the cavity provide some insulation and adding external insulation is the least technical difficult option as it moves the dew point outwards and has a low risk of causing interstitial condensation.

Some wall types have fully ventilated cavities. See rainscreen cladding for example. In this case there is no point in insulating outside the cavity as air movement will bypass the insulation.

Cavity insulation will improve the overall performance of the wall in addition to external insulation. 

Internally insulating is the most technologically challenging as it brings the dew point closer in to the house. Brickwork is comparatively moisture impermeable and if your internal insulation drops the wall temperature below the dew point at this interface you get condensation form within the wall.

Effective use of vapour barriers can mitigate this and this can be checked by following the method detailed in BS EN ISO 13378 and if you have any maths skills you can do it in excel.

I got some good data on lime plaster, render and cement from Glasgow university recently.

This allows one to work up good vapour open designs for old single skin walls using lime or hemp plasters, wood fiber or wool insulation and then the rubble filled lime mortar walls that still conform to all the BS standards for the avoidance of condensation in buildings.

Bare in mind that thermal mass is not your friend if you want to intermittently heat your house.

It can be very low energy to have a very fast thermal response and only heat the house when you want to be in it. All that thermal mass moves thermal energy from periods of occupancy when you want it heated to periods it's empty and you don't.

"I got some good data on lime plaster, render and cement from Glasgow university recently." 

Have you got a link for this?  I've got an old brick house, some 18th C and some 19th C brick (mix of double and single skin) with some thatch and some tile.  Be interested to see this.

David

https://link.springer.com/article/10.1617/s11527-021-01759-x

My mistake, it was Edinburgh university. Unfortunately there was apparently just a small pot of funding and Prof. Banfill has now retired.

Its specific to Scotland but possibly useful elsewhere. I was quite surprised by how vapour porous the lime mortars were. I have been meaning to build up a 3D finite element model to look at the moisture diffusion properties of a lime mortar rubble fill wall built out of whinstone but I haven't had either the time or access to the software for the last six months.

The idea of his project was to provide good data for inclusion in standard 2D interstitial condensation calculations or within WUFi or other numerical methods.

I recently bought some lime mortar and lime putty and found some information here:

https://www.lime.org.uk/applications/illustrated-guides-and-how-to-videos.h...

There was an article in yesterday's Sunday Times (16 July 2023) about eco improvements to your house which has a section on insulation which might have some helpful info

Heat pumps, solar panels, insulation: will they save you money?
Eco-friendly improvements often come with hidden costs
https://www.thetimes.co.uk/article/heat-pumps-solar-panels-insulation-will-... (paywalled)
http://archive.today/2023.07.16-002237/https://www.thetimes.co.uk/article/h... (not paywalled)

We had a couple of windows installed last year and some of the insulation fell out when they cut the holes in the wall - they basically replaced the lost part by shoving a large block of Celotex into the cavity at the sides of the window and a bit above the new lintel (the bit below it obviously didn't fall out.

They were saying that the flock-style stuff in our house isn't too bad, as some falls out but then it stops. Polystyrene balls, however, continue to fall out and it's a nightmare to deal with - especially if it's a ground floor window!

I was going to get cavity wall insulation in my old house, as there were a few grants at the time.

The chap that came to do the survey drilled a hole in the wall and poked his scope in and said they wouldn't be able to do it, as it already had some insulation in there (rock wool style, I think) and mixing types was a bad idea.

Was a bit of a surprise that there was any at all - I'd thought the house was too old to have had it during construction and there weren't any of the tell-tale drill holes in the sides of the house from adding it later.

Closed-cell foam products are impermeable, so the issue you state of water being transmitted across the cavity, does not occur.

That's based on an assumption that the foam is continuous and unbroken. Any break in the foam, for instance at a wall tie or caused by poor workmanship, will create a capillary through which moisture can travel. In real life retrofitting an unbroken material is almost impossible. This is significant because the purpose of a cavity is to form a capillary break. This may or may not matter depending on the situation of the building. 

Cavity walls were originally developed for exposed buildings that suffered from wind-driven rain, where previously the solution was to build thicker walls. They became ubiquitous because cavity walls are cheaper, but in many locations they're not strictly necessary and cavity fill is low risk. In exposed locations (or for poorly maintained buildings) cavity fill remains risky. 

Because every building is different and sits within its own micro-climate, anecdotal evidence (eg. "I did it and it's fine" or "I did it and now I have to knock down my house") is largely meaningless.

There's lots of good advice here so I don't have much to add. It really does depend on the type of construction, the nature of the cavity, the local climate and ground conditions.

Without knowing what your particular details are, the only other thing I would advise is not to install cavity wall insulation if you don't have a good damp proof course. If water can rise from the ground up into the walls, anything inside the cavity can get damp and the reduction of air circulation in the cavity means that walls will take longer to dry out after rain, possibly causing internal finishes to become damp and / or mouldy. I've no doubt there are plenty of properties with no DPC that have had retrofitted cavity wall insulation, some will be fine, and some will be suffering from problems - again it all depends on a number of factors.

What makes a 'good' damp proof course? If you can see a line of black polythene or bitumen felt, or even slates in the horizontal mortar joint near the ground, then it should be good enough to stop rising damp. If its an older building, a plinth of low permeability stone or engineering bricks is a reasonable DPC. I am dubious about how effective an injected DPC is because it is often only the outer leaf that has been  injected, and injection DPCs have been installed in walls that aren't really compatible (the worst case being stone faced, rubble filled walls, where they don't do a thing, but damp injection companies will still take money for doing this work). If you can ever see a horizontal line near ground level between damp bricks / stones / blocks, and dry ones on a wet day then the DPC is working.

Agree about internal insulation if it is appropriate to your situation. We had an offshot room with walls of a single skin of breeze block and it was cold. We had the plasterboard that was nailed to 2x1 battens on the walls replaced with 50mm kingspan and then had new plasterboard and the difference is very noticeable.

Dave

Anybody know whether pebbledash is vapour permeable? Genuine question. Large chunks of our house are covered with it. Ugly, but low maintenance, extremely weatherproof and zero damp, but if I were looking external but breathable (as you suggest) insulation would it all have to come off?

Depends if it's made with cement or not... Stick a bit in some vinegar and see if it fizzes. 

Many traditional buildings were historically rendered with roughcast, which looks a bit like pebbledash but is made with lime. It was there for added protection from the weather but there was (and still is) an unfortunate trend for people to scrape this off to expose the stone underneath. 

I've got a copy of BS EN 12524:2000 at home so will see what it has to say. However I'm sure that pebbledash products range from massively and you will never know without either finding the original paperwork or doing testing.

I read something about putting vinegar on it and watching for fizz - thus lime. I convinced myself that there was fuzzing but so hard to be sure.

The pebbledash is I'm sure pretty old.

Thanks to all the replies, they have been very helpful for me and it would seem to many others as well.  The quotes are beginning to come in now, so decision time is soon.  Not yet heard from the person who mentioned getting the job VAT free, so still waiting to get the details on that.

Historic England also have some good technical CPDs on the use of limecrete.  Although limited, their testing shows  NHL 5 is not much better than concrete in breathability terms.  We now spec NHL 3.5 for floors and 2 for laying and pointing. 

Not sure if this has been mentioned elsewhere, but when i had the cavities filled in my brick ex council house in hebden bridge, the contractors insisted on banging a big hole in the wall to ensure the gas fire had sufficient oxygen! No probs with the insulation btw.