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.