Best cost effective cavity insulation

[Saints1]

Hello, 

So we have been provided planning consent and just going through the finer details. Waiting on building regs and the plans are with the structural engineer. I am aware that cavity walls are now to be 150mm thick, but with so many products out there, which is the best, yet cost effective cavity insulation to use. The house is a 1.5,(dormer style) 4 bedroom detached dwelling. 
Any help is greatly received.

 

[JohnMo]

PIR avoid. Easy to do badly, difficult to do well.

 

Would look at EPS beads, fully fills all voids, all gaps etc. make cavity as wide as practical. Minimum BR is pants 

 

[IGP]

100% this. 

  On 26/02/2025 at 14:05, JohnMo said:

PIR avoid. Easy to do badly, difficult to do well.

 

Would look at EPS beads, fully fills all voids, all gaps etc. make cavity as wide as practical. Minimum BR is pants 

 

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Example of PIR done badly is here 

 

 

[Crofter]

I've heard very good things about blown cellulose.

[ADLIan]

 

 

  On 26/02/2025 at 14:21, Crofter said:

I've heard very good things about blown cellulose.

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Not in masonry cavity walls I hope!

[SBMS]

  On 26/02/2025 at 14:21, Crofter said:

I've heard very good things about blown cellulose.

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Not in masonry walls!

[SBMS]

  On 26/02/2025 at 14:05, JohnMo said:

PIR avoid. Easy to do badly, difficult to do well.

 

Would look at EPS beads, fully fills all voids, all gaps etc. make cavity as wide as practical. Minimum BR is pants 

 

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I second this. 200mm walls, EPS beads, 0.15 u value. Gets everywhere. Leave brickies to do what they do well (lay bricks) not install insulation. 
 

I’ve designed out any manual labour/skill with insulation on ours.  EPS beads in wall and blown cellulose in roof. 

[Russell griffiths]

I would say it depends what this house is for. 
is it your forever home or are you going to flick it off in 3 years and build another one. 
 

forever home build it as best you can, selling it go for just passing building regs. 

[saveasteading]

  On 26/02/2025 at 18:36, SBMS said:

Leave brickies to do what they do well (lay bricks) not install insulation. 

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They don't build cavities well either, which is why I like full fill rock-wool. No mortar can get dropped down the cavity. If there is a way of pretending the insulation isn't right then I have missed it.

[Mike]

  On 26/02/2025 at 18:53, saveasteading said:

  On 26/02/2025 at 18:36, SBMS said:

Leave brickies to do what they do well (lay bricks) not install insulation. 

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They don't build cavities well either, which is why I like full fill rock-wool. No mortar can get dropped down the cavity. If there is a way of pretending the insulation isn't right then I have missed it.

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Better still, skip the cavities and choose solid walls with external wall insulation.

[SBMS]

  On 26/02/2025 at 18:53, saveasteading said:

They don't build cavities well either, which is why I like full fill rock-wool. No mortar can get dropped down the cavity. If there is a way of pretending the insulation isn't right then I have missed it.

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Our brickies leave out every fourth brick at the bottom of the cavity on external skin. They then get in regularly to clear out any snots and ensure all clear at end before beads are pumped in and those bricks replaced 👍

[Iceverge]

200mm full fill cavities with EPS beads are standard practice in Ireland with about a decade. 150mm is fine too but the cost difference is small. Maybe €10 per m2 of wall.

 

Get the brickies to turn a hose into the cavity wall for 5 mins before clocking off and knock off the mortar drop into the bottom of the cavity. Counter the dropped mortar by extending the cavity by one extra block below the floor level. This way the droppings will never get high enough to be an issue. 

 

 

[Crofter]

  On 26/02/2025 at 17:14, ADLIan said:

 

 

Not in masonry cavity walls I hope!

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That's what I get for not reading the question properly!