Blocks for inner skin

[Jane W]

Plot purchased: tick!

Topographical survey: tick!

Beginning process of deciding architects/architectural technicians/project manager etc: In progress. 

 

I was just having a little discussion with a potential project manager about walls.

Our plot is tight for the build we ideally want both in terms of space, and available money( the usual thing!).

We're building brick and block with full fill dritherm or similar. Thinking about options. I do not want aerated lightweight blocks.

Anyone got suggestions for cost and space effective solutions to meeting insulation standards, looking at choice of blocks and insulation combinations?

I'd be over the moon if there was a medium density block/dritherm type insulation combination that would make a standard cavity wall width! but guessing there's going to be compromise. I'm also aware that wider cavities also have other cost implications...

 

[nod]

We’ve used solid concrete blocks for the last two 

[Jane W]

10 minutes ago, nod said:

We’ve used solid concrete blocks for the last two 

Ah ok. Could I ask some further questions....Were they medium density (which is what I'm hoping to go for) or dense? What was the cavity width and fill you used, and u-value you achieved? Did you get any resistance or higher quotes from the brickies for solid concrete?

[Jane W]

2 hours ago, Jane W said:

if there was a medium density block/dritherm type insulation combination that would make a standard cavity wall width

We want a wet plaster rather than plasterboard so wondering if insulating plaster would be a possibility?

[nod]

1 hour ago, Jane W said:

We want a wet plaster rather than plasterboard so wondering if insulating plaster would be a possibility?

we used medium density block and we went with full fill aAs there was a fire at the German factory where the insulation boards where made and all the major house builders bought the remaining stock 

 

I chose to use insulated plasterboard on the external was As I’ve done with the current build Magnetic plaster on the study and children’s bedrooms 

Wet plaster can have a cold feel to it 

I run a plastering business and we are still asked for sand and cement + skim on occasion Definitely better for fixing to and similar price to dot and dab 

 

[Ben1984]

On 16/10/2024 at 16:23, nod said:

we used medium density block and we went with full fill aAs there was a fire at the German factory where the insulation boards where made and all the major house builders bought the remaining stock 

 

I chose to use insulated plasterboard on the external was As I’ve done with the current build Magnetic plaster on the study and children’s bedrooms 

Wet plaster can have a cold feel to it 

I run a plastering business and we are still asked for sand and cement + skim on occasion Definitely better for fixing to and similar price to dot and dab 

 

Hi nod,

 

I hope you don't mind me dragging up your old post but it sounds like you might be a good person to seek advice from. I am trying to finalise the details for our self-build; we are doing a brick and block cavity wall construction and we want to avoid dot and dab which makes it a bit more difficult to achieve a decent u-value in the walls. Could we get away with a thermalite type block on the inner skin if we are planning to use sand and cement render with a skim top coat? Or is that asking for trouble trying to render onto that type of block? 

 

I hadn't considered insulated plasterboard, it alway seemed like a very expensive option. I'd be very interested to hear your rationale for choosing this if you would be kind enough to share it.

[nod]

I run a plastering business and I could have done either If dot and dab is done correctly You can achieve good airtightness Our builds both scored two Which is ok for high ceilings and three vaulted ceilings 

Don’t use thermaite The white blocks are fine with sand and cement 6-1 mix 

Parge coat is a waste of money Unless for sound Make sure your block work is pointed and bar jointed 

[Conor]

Just have a wider cavity.

[Ben1984]

28 minutes ago, nod said:

I run a plastering business and I could have done either If dot and dab is done correctly You can achieve good airtightness Our builds both scored two Which is ok for high ceilings and three vaulted ceilings 

Don’t use thermaite The white blocks are fine with sand and cement 6-1 mix 

Parge coat is a waste of money Unless for sound Make sure your block work is pointed and bar jointed 

Thanks for taking the time to reply - which white blocks are you reffering to? 

I would settle for a score of 2, but am aiming for something slightly better. Wet plaster would make this easier to achieve, and it also address some of the other things I dislike about dot and dab.

[Ben1984]

6 minutes ago, Conor said:

Just have a wider cavity.

Yeah, this is an option I am considering. But even if I go with a 200mm cavity it still makes sense to use a thermally efficient block for the inner skin if it can take the S&C render without cracking

[nod]

20 minutes ago, Ben1984 said:

Thanks for taking the time to reply - which white blocks are you reffering to? 

I would settle for a score of 2, but am aiming for something slightly better. Wet plaster would make this easier to achieve, and it also address some of the other things I dislike about dot and dab.

Wet plaster won’t make any difference 

Tarmack blocks 

[Iceverge]

24 minutes ago, Ben1984 said:

still makes sense to use a thermally efficient block for the inner

 

Have you done the calculations on it? 

 

I suggest you do to get a feeling of how little a difference thermolites make.  I wouldn't use them. To cracky and brittle and expensive.

 

There's airtightness and then there's airtightness. Ok might be the best you can do with dot and dab if you're careful but wet plaster is the way forward if you want a really tight house. We got well below passive requirements on ours. 

 

 

 

[Ben1984]

10 hours ago, Iceverge said:

 

Have you done the calculations on it? 

 

I suggest you do to get a feeling of how little a difference thermolites make.  I wouldn't use them. To cracky and brittle and expensive.

 

There's airtightness and then there's airtightness. Ok might be the best you can do with dot and dab if you're careful but wet plaster is the way forward if you want a really tight house. We got well below passive requirements on ours. 

 

 

 

Yeah, I've run some calcuations (using some of the online calculators): based on a rendered block outter skin, 150mm cavity filled with mineral wool you would need to use a thermalite block on the inner skin to hit 0.18 u value. I suppose my question is really this: Can I hit 0.18 with a rendered 100mm block outter skin, 150mm mineral wool cavity and a wet plaster finish internally? Is there a block that I can use on the inner skin that is thermally efficient enough to hit 0.18 without causing me loads of issues with cracking?

 

My current thinking is that it can't be done. I would either have to use a thermolite type block to hit the 0.18 which would then be a nightmare to plaster onto or I could use a concrete block on the the inner skin but increase the cavity to 200mm to hit < 0.18. I would prefer to keep a 150mm cavity but not sure this can be achived given my desire for a wet plaster finish internally.

[Ben1984]

11 hours ago, nod said:

Wet plaster won’t make any difference 

Tarmack blocks 

Are you saying that wet plaster won't make any difference to air tightness? I was under the impression that it was much easier to achieve a good air tight score with wet plaster than it is with dot and dab. 

[nod]

54 minutes ago, Ben1984 said:

Are you saying that wet plaster won't make any difference to air tightness? I was under the impression that it was much easier to achieve a good air tight score with wet plaster than it is with dot and dab. 

I’m saying that if dot and dab is done correctly There is literally no difference 

If you go with wet plaster you won’t be able to use cavity closers Make sure the Brickies close all the reveals off old style 

 

Also wet plaster has a colder feel

My main reason for Using dot and dab 

 

As a business we do both and tapering and jointing 

Sand and cent and dot and dab work out roughly the same cost 

Due to the difference in cost of materials 

Edited Sunday at 09:13 by nod

[Ben1984]

14 minutes ago, nod said:

I’m saying that if dot and dab is done correctly There is literally no difference 

If you go with wet plaster you won’t be able to use cavity closers Make sure the Brickies close all the reveals off old style 

 

Also wet plaster has a colder feel

My main reason for Using dot and dab 

 

As a business we do both and tapering and jointing 

Sand and cent and dot and dab work out roughly the same cost 

Due to the difference in cost of materials 

Thanks, I appreciate your time and expertise. 

[nod]

10 minutes ago, Ben1984 said:

Thanks, I appreciate your time and expertise. 

👍

[torre]

2 hours ago, Ben1984 said:

hit 0.18 u value

I dislike Thermalites too. If it helps, for a new build the limiting value is actually only 0.26 (table 4.1 of Part L) which gives you flexibility to use a different block, as long as you make adjustments to improve efficiency elsewhere - it may be easy to add 10-20mm of PIR to your roof or floor, say, to achieve the same SAP score.

[Ben1984]

10 minutes ago, torre said:

I dislike Thermalites too. If it helps, for a new build the limiting value is actually only 0.26 (table 4.1 of Part L) which gives you flexibility to use a different block, as long as you make adjustments to improve efficiency elsewhere - it may be easy to add 10-20mm of PIR to your roof or floor, say, to achieve the same SAP score.

Thanks, this is excatly the kind of advice I was hoping for! I was already planning to add extra PIR to the floor so it sounds like I can have the wall makeup I want (150mm cavity and get rid of thermalites) without having to make any further adjustments. 

[JohnMo]

8 minutes ago, Ben1984 said:

Thanks, this is excatly the kind of advice I was hoping for! I was already planning to add extra PIR to the floor so it sounds like I can have the wall makeup I want (150mm cavity and get rid of thermalites) without having to make any further adjustments. 

But before running away think job done and I'll take the easy way out, note what table 4.1 actually says

 

NOTE: To meet the target fabric energy efficiency rate set out in Section 1, the energy efficiency

of some elements will need to be significantly better than the limiting standards in Table 4.1.

 

So it's an average, lots of walls at 0.26 means roof and floor have to loads better than min standard. So you may need to get to below 0.1 on floor and roof to compensate, based on what windows you have and area. There isn't really that much wiggle room.

 

Base U value rates are for building regs section 1 in England are...

 

External walls U = 0.18 W/(m²·K)

Party walls U = 0

Floors U = 0.13 W/(m²·K)

Roofs U = 0.11 W/(m²·K)

etc...

 

I really wouldn't compromise on U values or airtightness, it will cost you every heating day there after, plus you will have way more heating days as well.

[Iceverge]

 

 

There's an option with the lightweight blocks on the inner leaf. I'm not sure of prices in the UK but in ireland they're three times the price about, €20 more per m2. 

 

 

If you were to use the below 200mm config you could easily drop from batts of 0.032W/m2K to 0.037 which are much much cheaper. 

 

 

 

 

 

Knauf cavity 032 Batts at 150mm are £22.58/m2 and 036 batts at 200mm are £16.38 . About £6/m2 less. 

 

https://www.insulationshop.co/150mm_knauf_dritherm_32_ultimate_cavity_slab.html?srsltid=AfmBOooJlH4JJCo9Jukz208_v9VOXP0dZLtipM00uCQZqYL56HiU5rk3

https://www.insulationshop.co/100mm_dritherm_cavity_slab_37_standard_knauf.html

 

 

Add in a slight difference for longer stainless steel cavity ties and you're still £20 per m2 better off going for the wider cavity. That could be easily £5k for a house. 

 

 

 

 

[Ben1984]

9 minutes ago, JohnMo said:

But before running away think job done and I'll take the easy way out, note what table 4.1 actually says

 

NOTE: To meet the target fabric energy efficiency rate set out in Section 1, the energy efficiency

of some elements will need to be significantly better than the limiting standards in Table 4.1.

 

So it's an average, lots of walls at 0.26 means roof and floor have to loads better than min standard. So you may need to get to below 0.1 on floor and roof to compensate, based on what windows you have and area. There isn't really that much wiggle room.

 

Base U value rates are for building regs section 1 in England are...

 

External walls U = 0.18 W/(m²·K)

Party walls U = 0

Floors U = 0.13 W/(m²·K)

Roofs U = 0.11 W/(m²·K)

etc...

 

I really wouldn't compromise on U values or airtightness, it will cost you every heating day there after, plus you will have way more heating days as well.

Thanks John. 

 

I am not looking to comprimise much on the u-value of the walls. I could achieve 0.18 if I used thermalite type blocks on the inner skin, but I don't like thermalites generally and certainly not keen on the idea of plastering directly onto them. Swapping thermalites to medium density concrete blocks would increase the u-value of the wall by 0.01, I can't imagine this will make a significant difference to my heating bills. However this was a problem when I was, incorrectly, under the assumption I had to achieve 0.18 on the walls.

 

I still might opt for a 200mm cavity for the increased u-value, but it's helpful to know that it's possible to achieve the wall make up I want with a 150mm cavity.

[JohnMo]

11 minutes ago, Ben1984 said:

200mm cavity

Then bonded poly beads fully filled?

[Ben1984]

8 minutes ago, Iceverge said:

 

 

There's an option with the lightweight blocks on the inner leaf. I'm not sure of prices in the UK but in ireland they're three times the price about, €20 more per m2. 

 

 

If you were to use the below 200mm config you could easily drop from batts of 0.032W/m2K to 0.037 which are much much cheaper. 

 

 

 

 

 

Knauf cavity 032 Batts at 150mm are £22.58/m2 and 036 batts at 200mm are £16.38 . About £6/m2 less. 

 

https://www.insulationshop.co/150mm_knauf_dritherm_32_ultimate_cavity_slab.html?srsltid=AfmBOooJlH4JJCo9Jukz208_v9VOXP0dZLtipM00uCQZqYL56HiU5rk3

https://www.insulationshop.co/100mm_dritherm_cavity_slab_37_standard_knauf.html

 

 

Add in a slight difference for longer stainless steel cavity ties and you're still £20 per m2 better off going for the wider cavity. That could be easily £5k for a house. 

 

 

 

 

Thanks, another really helpful response. What software/calculator have you used to produce this? It appears to be vastly superior to what I have been trying to use.

I have the space to accomodate a 200mm cavity, I was reluctant to go down this route initally because of the increased costs. Your post suggests that a 200mm cavity could, infact, be more cost efficient that a 150mm. I will need to look into this in more detail.

 

 

[Ben1984]

2 minutes ago, JohnMo said:

Then bonded poly beads fully filled?

Possibly! I had, until this morning, been looking at ways of making a 150mm mineral wool cavity work.

 

Based on the responses i've recevied so far I am now inclined to favor a 200mm cavity. I don't know enough about the bonded poly beads to compare them with mineral wool in a 200mm cavity. I will need to look into this.