Internal insulation for block cavity walls

[Mike]

10 minutes ago, Roger440 said:

i dont want to apply to much depth internally so was thinking of combining [IWI] with cavity wall insulation (eps beads)

If you have a cavity that's certainly the best solution.

[-rick-]

17 minutes ago, Roger440 said:

All good stuff. Interested as as my 1980 extension is currently just a cavity block wall. 75mm cavity, no insulation.

 

75mm of beads will make a huge difference on their own. Do that and plug up any big air leaks and then see where you are.

[Julestools]

14 minutes ago, Mike said:

I briefly mentioned using cork as a spacer earlier; I'd use continuous strips, not just spacers every 600mm; without that kind of firm support 45x45 would be too flimsy. Personally I'd increase it to 45x75 if I was using timber.

I think I'd use 45x70 with (cork) spacers if using 100mm mineral wool; 45x45 with continuous cork  spacers if I opt for a (theoretically safer!) 75mm wool. Feels like I'm splitting hairs now.. It's hard to make a decision! 

[Gus Potter]

On 09/01/2026 at 14:22, Julestools said:

Hi all, I want to add internal wall insulation to my 1970's block cavity walls. The cavities were filled with a mineral wool type fluff in the 90's, which appears to be dry where I've been able to see it and there is no evidence of damp or mould in or on the walls. The external render is also in good shape. 

We are leaning toward a stud frame fixed to the inside of the external cavity walls with mineral wool bats like knauf RWA45 or RS45, with plasterboard over.

Our reasons being, it's cheaper than using celotex or insulated plasterboard, it's a breathable option which, from what I gather, would carry less risk of issues with condensation and mould and we don't mind losing a bit of internal space. 

I'd love to hear your thoughts and knowledge on this approach to  internallly insulating cavity walls. Is it a good idea to do it at all? Is a vapour-open breathable approach better, or should I use a vapour barrier, as some manufacturers suggest? Any and all thoughts welcomed! Cheers, Jules

Hiya @Julestools interesting thread and good questions. The following is a bit of back to basics, for the less initiated, maybe new folk to Build Hub. This is a very rough run down but intended to introduce folk to something that may be less familiar.

 

When we insulate walls we want to make sure that any water that condenses in the wall can be expelled outwards or inwards before it causes any damage. When you cook or breath out you expel "water gas". This does no harm. The harm arises when the water gas turns to a liquid.. water droplets. Mould, dry rot, wet rot, most bugs / fungie need liquid water. At the bottom of a wall we often see damage due to freezing water (spalling of bricks / render) when it becomes excessive. 

 

For the water gas to turn to a liquid it needs to hit what we call the dew point. This varies depending on how much water gas is in the air and the air temperature. For example if you live in the tropics the warmer air can hold much more water gas, lower the air temperature a little and you get heavy rain. In the UK it works the same way, but a bit less dramatic... but the UK weather is it's own animal, I'm not covering this just now.

 

If you have a 1970's brick cavity wall to start with with say 15mm of plaster on the inside and roughcast on the outside then the dew point is likely on the inside of the outer leaf of masonry, the obvious cold surface. The cavities are often drafty so lots of water gas will get vented. You can analyse this but in most UK weather conditions the water gas condenses on the inside of the outer leaf of masonry. 

 

Any condensation was designed to run down the inside of the outer leaf. In actual fact in the 1970's there was no condensation "analysis" as we now know it.. it just worked! These houses were designed based on the knowledge at the time and proven to work in most cases. If left alone they will continue to perform as originally designed, although not meet our modern insulation expectations.  Cavities at the time had residual air circulation. But now we want to make them warmer! To get a handle on this it's often a good idea to research what the designer at the time was thinking and what knowledge they had at their disposal. I love this stuff! Miss Marple! 

 

@Julestools "The cavities were filled with a mineral wool type fluff in the 90's."

 

That has two immediate knock on effects. The natural / residual air circulation in the cavity is blocked off. The cavity insulation moved the dew point inwards, so you no longer have the really cold surface of the inside of the outer leaf of wall acting as the dew point surface. The cavity wall insulation installers often sold this as they say the insulation will "wick" the water away safely downwards and not come inwards! 

Make of that statement what you will as adults. 

 

@Julestools  " I'd love to hear your thoughts and knowledge on this approach to  internallly insulating cavity walls."

 

You are fortunate not to have any timber ( floor joists say) near the bottom of the wall. If so it becomes very tricky as the ends of the joists are more prone to getting wet. For all, BE VERY CAREFUL IF YOU HAVE TIMBER IN THE WALL!

 

You want to improve the U value of the wall. You insulate on the inside. You shift the dew point further inwards. Qualitively, if you improve it a lot you will shift the dew point further inwards, maybe even to the inside of the inner leaf or in it. Don't forget.. the calculations in the design codes are theoretical.. we need to apply common sense, this is most important!

 

Given the above lets take a pragmatic view. 

 

The first objective is to control the humidity in the house. Bathrooms and shower rooms need extra ventilation.. we know that. 

 

Next question is... we can select breathable insulation.. but the critical areas are bathrooms, kitchens. Folk paint them and tile walls. Say you sell the house or some really "keen" BC officer pulls you up as they say someone may use the wrong paint on the walls.. it's a fair question. Putting my SE hat on.. it is important! In some ways I think your are over thinking this!

 

The first objective is if you are going to create excessive water gas then you need to expel that as soon as you can, but you need to have a certain humidity in the house to stop your sinuces playing up. A house that is too dry is a nightmare!

 

In the round, if you are air tight, particularly in the wet areas, have good room ventilation then it may work. If you have ANY timber in the dew point zone then that is trouble. Yes to be CLEAR if you insulate on the inside you WILL shift the dew point inwards. Look to see what you could damage if you do this. 

 

To summarise. There is no point in trying to model the whole house, you'll drive yourself nuts! Just have a think about how a basic wall might work the way you are proposing, be conservative at this stage. Make sure you have no timber in places where water may condense. Make sure you can stop moist air by way of ventilation getting into the walls in the first place. 

 

I'm a desinger and SE in my day job.. there is a place for calculations and a time for pragmatism when dealing with older structures. 

 

 

@Julestools hope this helps, check you are spending your cash in the best way! It may be that you are not seeing the woods for the trees? To make a point.. if someone paints the walls with the wrong paint then all you efforts are moot. 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

[ADLIan]

Condensation occurring on the cavity face of the outer masonry leaf is irrelevant. There is a much greater amount of liquid water at this point following periods of wind driven rain. As part of the certification process (BBA for example) cavity wall insulation, built-in or retrofit, is tested to ensure it does not allow water from wind driven rain to cross the cavity, this water should still drain down the ‘cavity’ face of the outer masonry. Again not condensation related. 
 

Adding cavity insulation does not move the dew point ‘inwards’. Condensation, if it does occur, will still be on the inner face of the outer masonry leaf and may be considered as inconsequential.

[Roger440]

2 hours ago, Mike said:

If you have a cavity that's certainly the best solution.

 

Well it is. Apart from the fact im on the border of zone 1 and 2 for windriven rain.

 

The good news its rendered externally, base coat of render, but never recieved the final coat. 50 years ago. Its fine, but looks terrible. My current thinking is to use hardie plank or similar rather than try to do a render top coat, whick will give me, hardie plank, air gap, good render, inner leaf, before we get to insulation. So, done well, no water should ever get into the cavity.

[Mike]

1 hour ago, Gus Potter said:

To make a point.. if someone paints the walls with the wrong paint then all you efforts are moot.

A very good point. Since maintaining vapour-permiability will be essential following my current renovation, I'll be inscribing that, together with a few other key points, on a metal plate that will be screwed next to my MVHR unit for posterity - it may be worth considering.

 

In theory I shouldn't need to do that here in France because a mandatory 'house manual' is now required as part of the conveyancing process. It has to include, in detail (product names, technical characteristics, serial numbers, etc.), all the materials and equipment related to insulation, thermal performance, energy use & generation, etc., from initial build (if built after the law came in) through all subsequent modifications. It must also contain instruction manuals, servicing requirements & maintenance details - for example the need to use vapour-permiable paints.

[Julestools]

10 hours ago, Roger440 said:

All good stuff. Interested as as my 1980 extension is currently just a cavity block wall. 75mm cavity, no insulation.

 

As the house is "compact", i dont want to apply to much depth internally so was thinking of combining it with cavity wall insulation (eps beads) 

 

As per OP this is a bit "hydrid".

Yes, I think I am a bit "hybrid" (not hydrid)! 😁

[Julestools]

9 hours ago, Gus Potter said:

Hiya @Julestools interesting thread and good questions. The following is a bit of back to basics, for the less initiated, maybe new folk to Build Hub. This is a very rough run down but intended to introduce folk to something that may be less familiar.

 

When we insulate walls we want to make sure that any water that condenses in the wall can be expelled outwards or inwards before it causes any damage. When you cook or breath out you expel "water gas". This does no harm. The harm arises when the water gas turns to a liquid.. water droplets. Mould, dry rot, wet rot, most bugs / fungie need liquid water. At the bottom of a wall we often see damage due to freezing water (spalling of bricks / render) when it becomes excessive. 

 

For the water gas to turn to a liquid it needs to hit what we call the dew point. This varies depending on how much water gas is in the air and the air temperature. For example if you live in the tropics the warmer air can hold much more water gas, lower the air temperature a little and you get heavy rain. In the UK it works the same way, but a bit less dramatic... but the UK weather is it's own animal, I'm not covering this just now.

 

If you have a 1970's brick cavity wall to start with with say 15mm of plaster on the inside and roughcast on the outside then the dew point is likely on the inside of the outer leaf of masonry, the obvious cold surface. The cavities are often drafty so lots of water gas will get vented. You can analyse this but in most UK weather conditions the water gas condenses on the inside of the outer leaf of masonry. 

 

Any condensation was designed to run down the inside of the outer leaf. In actual fact in the 1970's there was no condensation "analysis" as we now know it.. it just worked! These houses were designed based on the knowledge at the time and proven to work in most cases. If left alone they will continue to perform as originally designed, although not meet our modern insulation expectations.  Cavities at the time had residual air circulation. But now we want to make them warmer! To get a handle on this it's often a good idea to research what the designer at the time was thinking and what knowledge they had at their disposal. I love this stuff! Miss Marple! 

 

@Julestools "The cavities were filled with a mineral wool type fluff in the 90's."

 

That has two immediate knock on effects. The natural / residual air circulation in the cavity is blocked off. The cavity insulation moved the dew point inwards, so you no longer have the really cold surface of the inside of the outer leaf of wall acting as the dew point surface. The cavity wall insulation installers often sold this as they say the insulation will "wick" the water away safely downwards and not come inwards! 

Make of that statement what you will as adults. 

 

@Julestools  " I'd love to hear your thoughts and knowledge on this approach to  internallly insulating cavity walls."

 

You are fortunate not to have any timber ( floor joists say) near the bottom of the wall. If so it becomes very tricky as the ends of the joists are more prone to getting wet. For all, BE VERY CAREFUL IF YOU HAVE TIMBER IN THE WALL!

 

You want to improve the U value of the wall. You insulate on the inside. You shift the dew point further inwards. Qualitively, if you improve it a lot you will shift the dew point further inwards, maybe even to the inside of the inner leaf or in it. Don't forget.. the calculations in the design codes are theoretical.. we need to apply common sense, this is most important!

 

Given the above lets take a pragmatic view. 

 

The first objective is to control the humidity in the house. Bathrooms and shower rooms need extra ventilation.. we know that. 

 

Next question is... we can select breathable insulation.. but the critical areas are bathrooms, kitchens. Folk paint them and tile walls. Say you sell the house or some really "keen" BC officer pulls you up as they say someone may use the wrong paint on the walls.. it's a fair question. Putting my SE hat on.. it is important! In some ways I think your are over thinking this!

 

The first objective is if you are going to create excessive water gas then you need to expel that as soon as you can, but you need to have a certain humidity in the house to stop your sinuces playing up. A house that is too dry is a nightmare!

 

In the round, if you are air tight, particularly in the wet areas, have good room ventilation then it may work. If you have ANY timber in the dew point zone then that is trouble. Yes to be CLEAR if you insulate on the inside you WILL shift the dew point inwards. Look to see what you could damage if you do this. 

 

To summarise. There is no point in trying to model the whole house, you'll drive yourself nuts! Just have a think about how a basic wall might work the way you are proposing, be conservative at this stage. Make sure you have no timber in places where water may condense. Make sure you can stop moist air by way of ventilation getting into the walls in the first place. 

 

I'm a desinger and SE in my day job.. there is a place for calculations and a time for pragmatism when dealing with older structures. 

 

 

@Julestools hope this helps, check you are spending your cash in the best way! It may be that you are not seeing the woods for the trees? To make a point.. if someone paints the walls with the wrong paint then all you efforts are moot. 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Thank you Gus, for a great explanation of the whole insulation, ventilation condensation thing! (I think I'm going to have had enough of these words by the time I finish!) 

Perhaps I am over thinking.. and here is a good place to do it. 

 

Trying to be pragmatic, I'd say that if I moderately insulate with mineral wool (75mm?), ensure good airtightness and ventilation, then it is unlikely that the dewpoint will move to the inside or cause problems given that the inner leaf is already warmer due to the cavity insulation, would you agree? 

Thanks again. 

[Roger440]

8 hours ago, Mike said:

A very good point. Since maintaining vapour-permiability will be essential following my current renovation, I'll be inscribing that, together with a few other key points, on a metal plate that will be screwed next to my MVHR unit for posterity - it may be worth considering.

 

In theory I shouldn't need to do that here in France because a mandatory 'house manual' is now required as part of the conveyancing process. It has to include, in detail (product names, technical characteristics, serial numbers, etc.), all the materials and equipment related to insulation, thermal performance, energy use & generation, etc., from initial build (if built after the law came in) through all subsequent modifications. It must also contain instruction manuals, servicing requirements & maintenance details - for example the need to use vapour-permiable paints.

I wrote a house book when I sold my last house, the key feature of which was not to paint over the lime with modern paint!

 

 

[Gus Potter]

On 21/01/2026 at 22:31, ADLIan said:

Adding cavity insulation does not move the dew point ‘inwards’. Condensation, if it does occur, will still be on the inner face of the outer masonry leaf and may be considered as inconsequential.

I disagree. You say inconsequencial.. but it's not! The way we assess condensation risk is not to say "it never happens" it does under certain conditions.

 

What we do is to recognise this but say to ourselves say.. if it only happens say for a week or two in the winter things will dry out quickly before mould or fungie can establish.. In the long term timbers and other parts of the building fabric can live with that if the occurence is low.

 

Cavity wall insulation increases the risk of occurence of condensation as you move the dew point inwards and water gas condensing. Also if you fill the cavity then there is less free air circulating. 

On 21/01/2026 at 22:31, ADLIan said:

Condensation occurring on the cavity face of the outer masonry leaf is irrelevant. There is a much greater amount of liquid water at this point following periods of wind driven rain.

You are getting confused between water ingress and condensation. Your arguement is flawed. You either have a cavity or you don't.

 

You say that water penetration is the most risk.. if you have an open cavity then the water drains down the inside of the outer leaf, that is traditional design. A fully filled cavity works in a different way and does shift the dew point inwards and it's how often that happens is what matters. Yes the cavity wall insulation providers claim their insulation will wick the water down to the founds safely.. but any way you want to cut the mustard a shift in the dew point inwards at times when considering water gas moving from the inside to the outside of the wall does occur. 

 

Now in some ways lets say you do hit the dew point in a "bad spot". The first thing to ask is does this matter.. are we argueing a moot point?  Well it does matter if you are going to make timber /steel wetter more often. The other consideration is that this water needs to evapourate. This requires heat or lots of air draft or a bit of both. The heat can come from the inside of the building or from the outside free air. 

 

Remember that much of what we do when we are designing stuff is to seek the best compromise! 

 

Just to make things more complicated. Your house is cool inside, the outside air temperature increases and the humidity rises. So now every thing works in reverse! This happens quite a lot in Scotland and Wales. The humid air from the outside penetrates the walls. An easy way to get your head round this is when you have well insulated glazing. The temperature and humidity suddenly rises outside and you get condensation on the outside of the glass. 

 

 

 

 

 

 

 

 

 

Edited 5 hours ago by Gus Potter

[ADLIan]

@Gus Potter I’ve spent a long time testing and designing cavity wall systems and know the difference between the issues with moisture from condensation and/or wind driven rain. When I have referred to the ‘cavity’ face of the outer leaf I assumed this would be understood to be the inner surface irrespective of a cavity or cavity insulation being present. Condensation may be predicted in a standard insulated cavity wall however it be at the internal face of the outer leaf - it may be considered as inconsequential as there is only masonry in the general wall area, the amount of condensate will be minimal, the outer leaf will be saturated at times following periods of wind driven rain. Adding further internal insulation to an already insulated cavity wall may move the dew point inwards depending upon material used and thickness. If steel/timber studs are used and condensation is predicted adjacent to them then a review of the design is needed.

 

I stand by my comments. See BS 5250, insulation manufacturer details & BBA certificates. The BRE did work many years on ‘reverse condensation’ and it was considered not to be an issue.

[Mike]

premature posting - hang on...

Edited 2 hours ago by Mike

[Gus Potter]

1 hour ago, ADLIan said:

I’ve spent a long time testing and designing cavity wall systems and know the difference between the issues with moisture from condensation and/or wind driven rain.

Good.

 

1 hour ago, ADLIan said:

When I have referred to the ‘cavity’ face of the outer leaf I assumed this would be understood to be the inner surface irrespective of a cavity or cavity insulation being present.

Ah, us Engineers take this as do the rest of the construction industry take this as being the inner surface of the outer leaf.. I suspect you are making a straw man arguement. Seriously how do you get from the "cavity face" to the inside of the walls. I don't know what convention you are using? Maybe you can point to BRE 443 if I'm maybe barking up the wrong tree? 

 

1 hour ago, ADLIan said:

Condensation may be predicted in a standard insulated cavity wall however it be at the internal face of the outer leaf - it may be considered as inconsequential as there is only masonry in the general wall area, the amount of condensate will be minimal, the outer leaf will be saturated at times following periods of wind driven rain.

You are confused and mixing external penetration of moisture with the internal transfer of water gas outwards and where it condenses. 

 

1 hour ago, ADLIan said:

it may be considered as inconsequential as there is only masonry in the general wall area, the amount of condensate will be minimal,

Who deems that inconsequantial? Have a think about the basic thoery and work your argument through. Think about buildabilty, the elements of structure that connect into that. 

 

1 hour ago, ADLIan said:

the outer leaf will be saturated at times following periods of wind driven rain.

Agree, at times this can happen, depends if you have a porous facing brick or the wall is rendered for example. 

 

1 hour ago, ADLIan said:

Adding further internal insulation to an already insulated cavity wall may move the dew point inwards depending upon material used and thickness. If steel/timber studs are used and condensation is predicted adjacent to them then a review of the design is needed.

It does and that is what I've been saying, I agree with you!

 

1 hour ago, ADLIan said:

I stand by my comments. See BS 5250, insulation manufacturer details & BBA certificates. The BRE did work many years on ‘reverse condensation’ and it was considered no be an issue.

I have a copy of this. This reverse condensation issue was recognised. It probalby in my view is still valid. I mentioned this in my last post just to highlight how much this is very much an inexact science!

 

BBA certificates! Well they said that about the Grenfell Cladding! Funnily, years ago, I was involved in getting CE certification for a cold formed steel company.. so kind of know how that world works. My advice is.. if you are a self builder then learn a bit and trust your common sense. 

 

@ADLIan I half agree with you.. we can debate the language, and different apporaches. One key test is would you personally sign off a design when I'm saying watch out? Remember I'm an SE.. if you do stuff that is going to compromise my structure when I have fore warned you then if it fails you are in big trouble! It's big boy pants time!

 

I'm not sure  but the best thing to do would be to crystalize our thoughts and come up with something that would be helpfull to folk on BH? 

[Mike]

  

On 22/01/2026 at 08:34, Julestools said:

I'd say that if I moderately insulate with mineral wool (75mm?), ensure good airtightness and ventilation, then it is unlikely that the dewpoint will move to the inside or cause problems given that the inner leaf is already warmer due to the cavity insulation, would you agree?

As you may have gathered, what seems like a simple question is actually quite complex. Dew point is important, but dew point calculations don't take into account several key factors, such as the abilities of the particular building materials to move moisture through the structure by capillary action, nor the adsoption properties of some materials (the ability to remove vapour molecules from the air and bind them within the natural fibres such as hemp and wood - not the same as absoption - before it condenses to liquid), then desorb it again. External factors are important too - hence my point about the condition and exposure of the external wall and the state of the DPC - because, as mentioned by @Gus Potter, insulating the wall changes how and when it retains and releases moisture. Then you have factors such as the presence or absence of vapour control layers, vapour permeable paints and MVHR...

 

Anyway, to answer your question, I can only think of 4 ways of deciding on an insulation thickness:

1. Major manufacturers of vapour control layers will run it through their model and come up with a recommendation based, of course, on using their VCL, which must be installed to very exacting standards. You'd be wise to include a service void with a VCL to cut the risk of damaging it (now or in the future).

2. Find a consultant to carry out a WUFI analysis. However WUFI is highly dependent on the data used for the building's internal environment (variations in temperature and humidity throughout the year); the default data options provided are very finger-in-the-air and don't take into account factors such as the presence of MVHR, which can significantly cut moisture levels. That situation can be improved by using their companion software to model the environment of the specific building as a whole, but that's going to rack up the cost significantly. I'm not entirely confident in the standard external environment files used either, in view of the expected climatic changes, though at least those are more easily assessed and changed and have less impact on the output.

3. Take a cue from research such as that carried out by Historic Scotland. If only there were more of it available to cover a greater range of buildings and locations.

4. Go no lower than the more-or-less accepted 'safe' U-value limit of 0.5W/m².K

 

Edited 1 hour ago by Mike

[Gus Potter]

On 09/01/2026 at 14:22, Julestools said:

Hi all, I want to add internal wall insulation to my 1970's block cavity walls.

 

56 minutes ago, Mike said:

As you may have gathered, what seems like a simple question is actually quite complex.

Mike that is a well though out answer. Service voids, great. Your command of the theory is great as is many other posters. 

 

But what is @Julestools going to do and the options?

 

For me the thing is to see if there are timbers that may suffer say, if not then step forward, what is buildable with least risk of condensation? You'll always find some calculation that shows moisture!

 

My starting point when working on older structures is to say.. let's look at what each room is used for. Is it a bedroom, a kitchen or a utility room where you might be drying washing. Here we look for the simple solution and that makes it easier to have confidence that the insulation will work. 

 

Now at some point Building Control may come back with a query, but that is a good thing! As a designer we think the problem though and come up with a reasoned case. This is the art of design. it does not need to cost the earth to build!

 

3 hours ago, ADLIan said:

I’ve spent a long time testing and designing cavity wall systems and know the difference between the issues with moisture from condensation and/or wind driven rain.

 

I have done lots of testing also! I have also done my fair share of finite element analysis.. As an SE I can tell you that the premise that SE's take is that all models are wrong, it's just a giude to the behavoir!.. your's will be too @ADLIan! Models are an aide, don't hang your reputation and hat on these! You claim you have tested.. you should know better and you need to understand the limitations of an FE model and if you have tested stuff then lets see your evidence! 

 

56 minutes ago, Mike said:

Take a cue from research such as that carried out by Historic Scotland.

They have some good stuff if you have time to read. 

 

But in summary. If you stop the moisture levels building up in the rooms in the first place then you head off most of the potential problems! It's that simple! Simple is a good starting point for old houses, then you tweak stuuf as you go. It's actually fun once you have grasped the basics. 

 

I'll say again much of this stuff is basic common sence once you have got your head around the rough idea. The big challenge is actually building it and putting it all together. Here we want something simple and that will not cuase a builder to rip you off!

 

 

 

 

Edited 59 minutes ago by Gus Potter