Cavity wall insulation- compromise? bricks and blocks don't match

[ab12]

Hi all

Can you please advise on the following scenario:  Outer leaf of extension will be imperial brick dimension 73mm height * 105mm width * about 225mm length (although the stated length is 228mm). Brick is called classic weathered orange brick, main distributor is Classic Brick Company and retailers such as Jewsons and EH Smith supply it. Inner leaf of extension will be blocks standard height of 215mm.

 

5 courses of brick with 13 mm mortar joint gives 417mm and 

2 course of block with 10mm mortar joint gives 440mm 

which leaves a difference of 23mm

See diagram

The reason for doing 13mm joint is to match with old Victorian bricks of 76mm height.

Now that means wall ties will be angled down from block to brick and Dritherm 32 cavity blocks (150mm width) will not sit flush with each other when piled up vertically leaving a small gap between adjacent cavity slabs

 

So I'm thinking will this not compromise the insulation effect in parts where there will be small gaps between the 2 slabs piled vertically and cause cold bridging?

 

How do we overcome this scenario?

 

thank you in advance

[Tony L]

Assuming the ties are coming into the bricks at height intervals that are compatible with the Drytherm, can't you just cut slices in the Drytherm so the last 100mm or so goes around either side of each tie?

[torre]

We have experience doing something similar. Build up the blockwork first (think that's recommended for full fill inflation anyway) so you can be sure you can keep the ties running downhill. An insulation saw makes the vertical cuts @Tony L mentioned easier. The insulation should match the blockwork coursing.

 

If you do the insulation yourself you'll be fine, if you leave it to brickies make sure they don't just let the tie drag the insulation down.

 

You're right the coursing will run out, think in terms of  5*86=430 Vs 2*225 = 450 though

 

You may want to use slightly longer ties to make up for them sloping down. You can also get two part ties but they're expensive.

[Mr Punter]

Why not use a row of 40mm coursing blocks / slips every 2 courses of blocks and fill the cavity with eps beads? 6 course of bricks outer, 2 course of blocks and 1 brick slip inner.  Brickie will make it work.

[ab12]

On 30/03/2025 at 20:11, Mr Punter said:

Why not use a row of 40mm coursing blocks / slips every 2 courses of blocks and fill the cavity with eps beads? 6 course of bricks outer, 2 course of blocks and 1 brick slip inner.  Brickie will make it work.

This idea is interesting. Can I just that this will satisfy this setup? Mansfield 215 x 100 x 40mm OPC Brick Slip 20N

Calcs:  6 course of 73= 438,  5 layers of  13 mm mortar thickness gives 65mm  Total outer skin 503mm

             2 course of 215 blocks ann 1 course of 40mm brick slip = 470mm, needs 2 layers of mortar which is usually 10mm each so total 490mm leaves shortfall of 13mm

 

Other option is cut the blocks into 53mm heights and use these instead of 40mm brick slips which means outer and inner leafs will equal 503mm 

 

1 other consideration recommendations for wall ties is every 450mm centered vertically but in the above setup they'll be every 503mm centered vertically. Is that going to cause an issue?

 

Thank your guidance

 

            

 

[ab12]

On 30/03/2025 at 18:30, torre said:

We have experience doing something similar. Build up the blockwork first (think that's recommended for full fill inflation anyway) so you can be sure you can keep the ties running downhill. An insulation saw makes the vertical cuts @Tony L mentioned easier. The insulation should match the blockwork coursing.

 

If you do the insulation yourself you'll be fine, if you leave it to brickies make sure they don't just let the tie drag the insulation down.

 

You're right the coursing will run out, think in terms of  5*86=430 Vs 2*225 = 450 though

 

You may want to use slightly longer ties to make up for them sloping down. You can also get two part ties but they're expensive.

Thank you for the input. So essentially with this set  up  the insulation will be cut into a triangular/slanted shape at the top end after the first installation but then every additional insulation batt piled vertically on the base batt will be cut into a triangular/slanted  shape at both top and bottom end to achieve a continuous layer of insulation? Bit like in the diagram shown

 

 

Edited yesterday at 12:17 by ab12

[ab12]

Also which insulation saw would you recommend for cutting the insulation batts- Dri therm 32.

 

Brickie will be installing the insulation. Are there any special instructions I need to issue?

 

Thanks

[Mr Punter]

When you are working out coursing you add a layer of mortar for every course.

 

For your 6 course of 73mm bricks with 13mm you have 516mm.

 

For your 2 x 215 block and 40mm block slip to coordinate, the mortar beds on the inner leaf would need to be 15mm which is as wide as you should sensibly go.

[ab12]

Thank you for this,  Mr Punter. Much appreciated

 

Will it compromise the overall build up of the wall or the structural integrity of the wall  if the wall ties are not at 450mm centre's vertically?

 

So for context its a rear single storey extension measuring 5.9m by 4.1m outer leaf with 150mm cavity

 

I've already purchased Dri-Therm 32 batts , the spec is 455mm height, 1200 mm length and 150mm thickness.

 

 

[ab12]

1 other option crossed my mind 

 

what if we do 5 layers of 73mm brick with 13 mm mortar giving

86*5= 430mm for outer leaf

 

so for inner leaf 1 lock at 215mm and other block at other 195mm with 2 layers of 10mm mortar

giving 215 + 195+ 10 + 10= 430mm

 

Can this be made to work aswell?

 

Thanks

[Canski]

Ask your bricky if he can sort it ?

[ETC]

Vary the bed depth.

[Brickie]

This product solves every problem you mentioned. Not cheap but probably the best solution,other than doing EPS beads. 
Just to add,you’ll need to impress upon the Brickies the need to actually bed these in,rather than poking them in afterwards. 

Edited 7 hours ago by Brickie