Extension Cavity Insulation

[joe90]

So, reading the above perhaps I can offer a different point of view. Rather than slating the brickies we should concentrate on “the builder” , yes they get all the bad rep as well but if the builder, who employed a bricky, spelt out what is required,  the cost of the installation was incorporated in the build price and if a project manager were involved then there is no reason why a good job cannot be achieved as it was on my build. As stated above somewhere it’s not rocket science.

i do think it’s a shame we no longer hear of the term “clerk of works” , an overseer of standards of work..

[Gone West]

33 minutes ago, joe90 said:

i do think it’s a shame we no longer hear of the term “clerk of works” , an overseer of standards of work..

+1

[Roger440]

4 hours ago, joe90 said:

So, reading the above perhaps I can offer a different point of view. Rather than slating the brickies we should concentrate on “the builder” , yes they get all the bad rep as well but if the builder, who employed a bricky, spelt out what is required,  the cost of the installation was incorporated in the build price and if a project manager were involved then there is no reason why a good job cannot be achieved as it was on my build. As stated above somewhere it’s not rocket science.

i do think it’s a shame we no longer hear of the term “clerk of works” , an overseer of standards of work..

 

The whole system is broken.

 

Few people have pride in their work. Theres no supervision. The result is inevitable.

 

Summary, no one gives a shit.

 

If you did supervise or oversee, it would cost more, so theres zero incentive to do that either. People will buy the houses no matter how poor they are. 

 

Ok, a bit extreme as some clearly do care, but they are in a small minority, so the bulk of building will be poor. And is.

 

I dont think theres "a fix". Only inevitable decline in standards.

[Canski]

5 hours ago, Iceverge said:

A blocklayer is more than capable of installing cavity batts correctly. It's a technically much simpler thing than putting a cavity wall together. 

 

The industry never trained anyone to do it properly, manafacturers make unworkable products (rigid cavity boards), and there's usually zero oversight and reward for a job well done.

 

We still pay per block so effectively want the insulating job done for free. Brickies who make a good job of it loose money becuase they havn't been paid for their time.

 

 

Setting up the incentives so negatively for any human and getting a bad result, it's a bit rich to blame the individual. 

 

 

 

 

I’m sorry but installing insulation takes time and I always charge for it. It is not something that should be included in the rates the same as feature bands, cutting up gables and cutting the bottom course of blocks where others have lost the ability to use a laser level correctly. I won’t take on projects with tongue and groove insulation because it’s not fit for purpose. Cutting and installing full fill insulation requires the following skills. 
1) be able to use a tape measure

2) be able to use a long sharp knife and a cutting slot. 

3) be able to think ahead

4) checking each junction as you go. 
5) protecting work properly as brickwork progresses

Gaps in insulation is my pet hate and the lads who work with me know it and have known it for years. I can guarantee that there isn’t a single gap in any of the cavity insulation on the 2 houses that I have just built for myself. 
 

Like I said it’s not rocket science but it does take time to get it bang on. Time should be paid for. 
Some of the bricklayers that have worked for some of this forums members were most likely used to working on a ‘rate per 1000 all in’ to include all the sundries. I stopped giving my time away for free 30 odd years ago. You pay peanuts you get monkeys. 
 

I do agree that there are plenty of poor bricklayers about but to tar them all with the same brush is pure stupidity. 

[Roger440]

9 minutes ago, Canski said:

I’m sorry but installing insulation takes time and I always charge for it. It is not something that should be included in the rates the same as feature bands, cutting up gables and cutting the bottom course of blocks where others have lost the ability to use a laser level correctly. I won’t take on projects with tongue and groove insulation because it’s not fit for purpose. Cutting and installing full fill insulation requires the following skills. 
1) be able to use a tape measure

2) be able to use a long sharp knife and a cutting slot. 

3) be able to think ahead

4) checking each junction as you go. 
5) protecting work properly as brickwork progresses

Gaps in insulation is my pet hate and the lads who work with me know it and have known it for years. I can guarantee that there isn’t a single gap in any of the cavity insulation on the 2 houses that I have just built for myself. 
 

Like I said it’s not rocket science but it does take time to get it bang on. Time should be paid for. 
Some of the bricklayers that have worked for some of this forums members were most likely used to working on a ‘rate per 1000 all in’ to include all the sundries. I stopped giving my time away for free 30 odd years ago. You pay peanuts you get monkeys. 
 

I do agree that there are plenty of poor bricklayers about but to tar them all with the same brush is pure stupidity. 

 

This post alone, marks you out as an extemely rare.

 

For clarity im not of the view that most bricklayers are poor. Im of the view most cant install insulation. Thats not the same thing.

 

 

[joe90]

34 minutes ago, Canski said:

I’m sorry but installing insulation takes time and I always charge for it. It is not something that should be included in the rates the same as feature bands, cutting up gables and cutting the bottom course of blocks where others have lost the ability to use a laser level correctly. I won’t take on projects with tongue and groove insulation because it’s not fit for purpose. Cutting and installing full fill insulation requires the following skills. 
1) be able to use a tape measure

2) be able to use a long sharp knife and a cutting slot. 

3) be able to think ahead

4) checking each junction as you go. 
5) protecting work properly as brickwork progresses

Quite, 

36 minutes ago, Canski said:

I do agree that there are plenty of poor bricklayers about but to tar them all with the same brush is pure stupidity. 

Quite

25 minutes ago, Roger440 said:

For clarity im not of the view that most bricklayers are poor. Im of the view most cant install insulation. Thats not the same thing.

Quite 

[DevilDamo]

Unless it was to massively improve the u-value, I don’t know why internal wall insulation was even specified in conjunction with a 100mm cavity.

 

May also working out the cost and time difference between IWI and wider lintels.

[Iceverge]

10 minutes ago, DevilDamo said:

Unless it was to massively improve the u-value, I don’t know why internal wall insulation was even specified in conjunction with a 100mm cavity.

 

May also working out the cost and time difference between IWI and wider lintels.

 

Wider lintels won't be needed unless you're using those silly catnic things. 

 

Just use separate precast concrete one on each leaf.

 

 

[joe90]

7 minutes ago, Iceverge said:

Just use separate precast concrete one on each leaf.

+1 and no cold bridge.

[lookseehear]

As someone who is just about to trust his builder to install full-fill batts, can anyone provide practical advice? My builder is skeptical about the whole thing. We've got a roll of knauf loft roll knocking about which he's been using to insulate some pipes temporarily until they get moved, and he keeps saying to me that the loft roll and the cavity batts are basically the same thing and showing me a wet piece of loft roll which got wet and now resembles something the cat dragged in.

 

My specific questions are:

1. Do you fill right down to the bottom of the cavity or leave a small gap at the bottom to avoid moisture wicking up?
2. If using cavity trays, should the batts be packed below and above in contact with the trays or a gap left?
3. How well does it need to be protected while block-laying? It's guaranteed that we'll see bad weather while the walls go up. I assume just a layer of DPC to keep an area dry. What about actually installing it in the wet?
4. Anything to note with wall ties or junctions with existing walls?
5. I assume you air on the side of overfilling vs underfilling, ie slightly compress the insulation into place?

Reading through this thread, I'm wondering whether EPS beads are more foolproof. What about the fire performance though? U value in a 150mm cavity looks v similar to Knauf Dritherm 32.

[DevilDamo]

42 minutes ago, Iceverge said:

Just use separate precast concrete one on each leaf.

Suppose that does work with a rendered block wall as per the OP’s situation. 

[Gordo]

16 hours ago, Canski said:

I won’t take on projects with tongue and groove insulation because it’s not fit for purpose

100% agree. I always recommend pumped beads insulation as on balance believe it is the best option.
 

I have seen so many cavities with built in insulation with gaps, not uniform, mortar snots on top of insulation etc. most trades are paid on quantity not quality and with no or poor supervision we get what we get. Then when you are looking for quality the trades default to what they have done or seen done where quantity is king and think you are expecting too much of them.

 

Most speculative builders the foreman hardly leaves his hut to see what sub-contractors are doing as he knows what he will see and if he complains it isn’t worth it to him. He probably had subcontractors forced on him by the bean counters and would struggle to replace them if they left and the next crew wouldnt be any better. NO ONE REALLY CARES. They all say they aren’t going to be living in it and won’t be pay heating bills. There’s no incentive to do any better.
 

Everyone thinks Building Control or NHBC are their supervisors Hahahaha They are often turning a blind eye too now for similar reasons to foreman. The number of times I’ve left a site shaking my head, thinking “What a shit show” the industry has become, and that is with blinkers on.

 

The whole system is broken. Unless you do a self build and try your best to work around it or get a small hands on contractor who you can trust to do same. All more expensive and time consuming if it’s worth it to you. 

Edited Monday at 12:47 by Gordo

[lookseehear]

On 05/01/2025 at 21:56, lookseehear said:

As someone who is just about to trust his builder to install full-fill batts, can anyone provide practical advice? My builder is skeptical about the whole thing. We've got a roll of knauf loft roll knocking about which he's been using to insulate some pipes temporarily until they get moved, and he keeps saying to me that the loft roll and the cavity batts are basically the same thing and showing me a wet piece of loft roll which got wet and now resembles something the cat dragged in.

 

My specific questions are:

1. Do you fill right down to the bottom of the cavity or leave a small gap at the bottom to avoid moisture wicking up?
2. If using cavity trays, should the batts be packed below and above in contact with the trays or a gap left?
3. How well does it need to be protected while block-laying? It's guaranteed that we'll see bad weather while the walls go up. I assume just a layer of DPC to keep an area dry. What about actually installing it in the wet?
4. Anything to note with wall ties or junctions with existing walls?
5. I assume you air on the side of overfilling vs underfilling, ie slightly compress the insulation into place?

Reading through this thread, I'm wondering whether EPS beads are more foolproof. What about the fire performance though? U value in a 150mm cavity looks v similar to Knauf Dritherm 32.

Just wondered if anyone had any insight here, thanks in advance!

[Canski]

On 09/01/2025 at 01:10, lookseehear said:

Just wondered if anyone had any insight here, thanks in advance!

1) the insulation needs to go down to at least the bottom of your floor insulation. If you have room to go lower then do it. But there is no point of over insulating cavity walls to an area that is served by air bricks giving cold air flow beneath your floors. 

2)no gaps around the cavity trays or anywhere else. 

3) ideally protect with a roll of DPC on top of the wall, use a cavity tray roll to do this in one continual piece for each elevation. Try to plan the works so that the internal and external skins finish at the same height before covering. This is a pain in the butt if it rains during the day. You will need to weigh this down well. When it rains quite often the wind gets up. You will not always succeed in keeping the insulation dry but you will also not succeed in getting a 100% perfect build. At least you have made an effort. 

4) wall ties need to penetrate the insulation at reveals where the ties are spaced at 225 mm centres. I use a long sharp knife to achieve this cutting a slot the width of the knife ( about 25 - 30 mm)

5) over filling is better than under filling but not excessively. Any cut bats that I do on 150 mm insulation are typically 20 mm longer than the gap.

 

The most important thing is to cut the bats square. I have found the easiest way is to cut them them through a ‘slot’ If there’s room I do this in the scaffold and lie the bay on the scaffold and cut through the gap in the boards. When I work off the ground it’s easier to set up two trestles and a sheet of OSB or similar with a slot cut in it at least 500 mm long. I lay the insulation on this board then cut through the insulation and the slot. If it’s cut square you save on waste and have no gaps. Win Win !

 

Watch out for ….

1) Brickies attempting to cut it with their trowels. This makes me shout at them. It’s quicker and easier to do it properly. 

2) Brickies building higher than the insulation without covering it. I use lengths of 4 x 1 timber to catch any mortar droppings. You will use it elsewhere on your build anyway. When we reach window height we cut these to fit the panels and move them up or around the build as we go.  
3) at lintel height make sure that the gap below the lintel is full before placing the lintels. I try to cut this as an L shape around the lintel rather than cut a small piece below the lintel with a piece joining it. I then make an angle cut ( at the same pitch as the fall on the lintel) to pass the cavity tray through. 
 

I’m sure I’ll still get slated for this post but by using these methods you will achieve the best job possible. 

Alternatively find another better qualified trade to carry this work out while the bricklayers start the stopwatch to get compensated for downtime. 😜I’m just going to find a link for the knives I use. 
 

https://www.amazon.co.uk/Insulation-Scabbard-Cutting-Stainless-RockWool/dp/B09RW57LRL/ref=sr_1_1_sspa?crid=1P0E1NW2VAXET&dib=eyJ2IjoiMSJ9.684K2TWJ9ecZU2r20s7M2w4ShwmQYHp_4_4s3wJsF1sX3thu1LFat02fZ9ZjWiizdHfA-RauLUIFuTtgv-HKLUTAF9vifstTeWSkSiVZwNKenGr4DEYuc95QW7qfjew5y2SZkXgP5t89ZIGtHddUktN4AkHmOaZZJUg4SQs3Kxz6LcLEKslEcVaETycQkggid1bUrDN5k3cfN5-rtoUugMwGRUIqLR1NiFVUrvLbbuhZHrvRGdGyENqTIL6S8gNqLkmbQPOTrllelqmfxorPrOS4826YkNmy1qp8ieOZIWc.Q5ZptyHAlgRHUbhm1V-Df08zvXWWWLhHi5jDXsQYEY0&dib_tag=se&keywords=insulation+knife+rockwool&nsdOptOutParam=true&qid=1736500922&sprefix=Insulation+knife%2Caps%2C207&sr=8-1-spons&sp_csd=d2lkZ2V0TmFtZT1zcF9hdGY&psc=1&smid=A3CYBNKKYWTMTI

 

Edited 3 hours ago by Canski
Link added.

[Canski]

On 06/01/2025 at 13:20, Gordo said:

On 05/01/2025 at 21:10, Canski said:

I won’t take on projects with tongue and groove insulation because it’s not fit for purpose

100% agree. I always recommend pumped beads insulation as on balance believe it is the best option.

That’s fine for a full house but for small extensions it would prove very expensive.
As a brickie who sees tight gaps around cavity trays I don’t trust the beads to get into some of these gaps. I know I that I can fill them 100% with cavity bats. I also know that I can install cavity bats neatly to the top of the wall plate and can see that it’s there before the roof covering goes on. 

[lookseehear]

1 hour ago, Canski said:

1) the insulation needs to go down to at least the bottom of your floor insulation. If you have room to go lower then do it. But there is no point of over insulating cavity walls to an area that is served by air bricks giving cold air flow beneath your floors. 

2)no gaps around the cavity trays or anywhere else. 

3) ideally protect with a roll of DPC on top of the wall, use a cavity tray roll to do this in one continual piece for each elevation. Try to plan the works so that the internal and external skins finish at the same height before covering. This is a pain in the butt if it rains during the day. You will need to weigh this down well. When it rains quite often the wind gets up. You will not always succeed in keeping the insulation dry but you will also not succeed in getting a 100% perfect build. At least you have made an effort. 

4) wall ties need to penetrate the insulation at reveals where the ties are spaced at 225 mm centres. I use a long sharp knife to achieve this cutting a slot the width of the knife ( about 25 - 30 mm)

5) over filling is better than under filling but not excessively. Any cut bats that I do on 150 mm insulation are typically 20 mm longer than the gap.

 

The most important thing is to cut the bats square. I have found the easiest way is to cut them them through a ‘slot’ If there’s room I do this in the scaffold and lie the bay on the scaffold and cut through the gap in the boards. When I work off the ground it’s easier to set up two trestles and a sheet of OSB or similar with a slot cut in it at least 500 mm long. I lay the insulation on this board then cut through the insulation and the slot. If it’s cut square you save on waste and have no gaps. Win Win !

 

Watch out for ….

1) Brickies attempting to cut it with their trowels. This makes me shout at them. It’s quicker and easier to do it properly. 

2) Brickies building higher than the insulation without covering it. I use lengths of 4 x 1 timber to catch any mortar droppings. You will use it elsewhere on your build anyway. When we reach window height we cut these to fit the panels and move them up or around the build as we go.  
3) at lintel height make sure that the gap below the lintel is full before placing the lintels. I try to cut this as an L shape around the lintel rather than cut a small piece below the lintel with a piece joining it. I then make an angle cut ( at the same pitch as the fall on the lintel) to pass the cavity tray through. 
 

I’m sure I’ll still get slated for this post but by using these methods you will achieve the best job possible. 

Alternatively find another better qualified trade to carry this work out while the bricklayers start the stopwatch to get compensated for downtime. 😜I’m just going to find a link for the knives I use. 
 

https://www.amazon.co.uk/Insulation-Scabbard-Cutting-Stainless-RockWool/dp/B09RW57LRL/ref=sr_1_1_sspa?crid=1P0E1NW2VAXET&dib=eyJ2IjoiMSJ9.684K2TWJ9ecZU2r20s7M2w4ShwmQYHp_4_4s3wJsF1sX3thu1LFat02fZ9ZjWiizdHfA-RauLUIFuTtgv-HKLUTAF9vifstTeWSkSiVZwNKenGr4DEYuc95QW7qfjew5y2SZkXgP5t89ZIGtHddUktN4AkHmOaZZJUg4SQs3Kxz6LcLEKslEcVaETycQkggid1bUrDN5k3cfN5-rtoUugMwGRUIqLR1NiFVUrvLbbuhZHrvRGdGyENqTIL6S8gNqLkmbQPOTrllelqmfxorPrOS4826YkNmy1qp8ieOZIWc.Q5ZptyHAlgRHUbhm1V-Df08zvXWWWLhHi5jDXsQYEY0&dib_tag=se&keywords=insulation+knife+rockwool&nsdOptOutParam=true&qid=1736500922&sprefix=Insulation+knife%2Caps%2C207&sr=8-1-spons&sp_csd=d2lkZ2V0TmFtZT1zcF9hdGY&psc=1&smid=A3CYBNKKYWTMTI

 

This is so so helpful - thank you!