PIR walls insulation loose...nightmare

[6hel]

Hello experts!

Average home owner here, knows little about building and insulation.

Large home extension being done (Half way through).

External walls with blocks inside, and outside (rendered), 100mil cavity, 90mil PIR insulation boards (Ecotherm), 10 mil air gap.
u-value = 0.17 (Ecotherm website), maximum allowed by new regs = 0.18. All good so far.

Inspector not happy because some of the PIR is against the outside blocks, others are against the inside blocks, others are in the middle.
Question 1: not having all the PIR boards against the inside block will it invalidate my u-value of 0.17. If yes, is there a way to calculate the new u-value given the existing condition?
Question 2: best way to solve this? Walls boarded and plastered inside. Outside all the rendering has been done.

 

[freddiemercurystwin]

The risk is from damp transferring from the outside to the inside, it's very hard to maintain a 10mm cavity. Surely your builder should be rectifying?

 

[6hel]

The risk is from damp transferring from the outside to the inside, it's very hard to maintain a 10mm cavity. Surely your builder should be rectifying?

Thanks for highlighting the damp transferring risk. Will it also affect the u-value calculations?
Yes about the builder rectifying it but I am trying to understand 1) if it can be rectified 2) the best way to rectify it (fastest).
There is a back extension, a two floor side extension and loft conversion. Rendering done, plastering and tiling done (except ground floor)... so I can't afford the time it will take to knock everything down and rebuild (I am renting another place while works are being done) and the builders will probably run away if that's the only solution.
I read in other places about injecting foam/beads but I don't know if this can be a solution in my case.

 

[23vc]

How does the inspector know where the PIR is if the walls are already built?
Aren’t these boards meant to be pinned to the inner skin with some kind of special fixing? Possibly start punching holes in outer skin in order to do this where it’s needed…..? Tricky one

 

[freddiemercurystwin]

It's quite an issue if Mr BCO won't pass it without it being rectified, presumably he's spotted it here and there and of course how does he (or you) know where it occurs and where it doesn't, the insulation should be held against the inner skin with wall tie insulation clips, have these not been used? If the external skin has been built well and rendered properly then the risks would be lower than if the external skin's been thrown up and render poorly applied but it's Mr BCO who will need to be satisfied. The U value may be affected but that's the least of your worries I would say.

 

[6hel]

How does the inspector know where the PIR is if the walls are already built?
Aren’t these boards meant to be pinned to the inner skin with some kind of special fixing? Possibly start punching holes in outer skin in order to do this where it’s needed…..? Tricky one

Ground floor still had walls cavity exposed around bi-folds.
Punching holes from outside is not ideal (rendering done all around) or even not possible (one side of the extension is less than 50mm from neighbor garage wall.)

 

[6hel]

It's quite an issue if Mr BCO won't pass it without it being rectified, presumably he's spotted it here and there and of course how does he (or you) know where it occurs and where it doesn't, the insulation should be held against the inner skin with wall tie insulation clips, have these not been used? If the external skin has been built well and rendered properly then the risks would be lower than if the external skin's been thrown up and render poorly applied but it's Mr BCO who will need to be satisfied. The U value may be affected but that's the least of your worries I would say.

Wall tie are there but not always keeping the PIR against the inside leaf. Poor job form bricklayers.
External render has been done by a specialised company with 10 years experience in the job. That's maybe why Mr BCO comment was more about U-value rather than damp. Initially he said the insulation would not meet 0.18, then I showed calculations from ecotherm with u-value = 0.17. That's when he said the calcs are irrelevant because the boards were not installed as per ecotherm instructions.
In order for Mr BCO to be satisfied I need to provide solution(s) to discuss with him. That's what I am desperately looking for.
He won't propose solutions.

 

[kp00110]

Sounds like you better ask him what's his favourite bottle or to name his price

 

[Mrs Ten Names]

Fix new insulation on the inside of the inside walls, prior to plastering, at your builder's expense.

 

[23vc]

That won’t stop the perceived moisture bridge issue if that’s what the BCO thinks

 

[freddiemercurystwin]

Mr BCO is apparently more concerned with U Values.

 

[23vc]

Insulated PB for the win then, surely…

 

[freddiemercurystwin]

Seems that way.

 

[noseall]

External walls with blocks inside, and outside (rendered), 100mil cavity, 90mil PIR insulation boards (Ecotherm), 10 mil air gap.
u-value = 0.17 (Ecotherm website), maximum allowed by new regs = 0.18. All good so far.

Are they the rebated ones? And has he placed DPC at jambs and corners?

7n.jpg

• noseall
• 3 Sep 2024

 

[noseall]

The risk is from damp transferring from the outside to the inside, it's very hard to maintain a 10mm cavity. Surely your builder should be rectifying?

Standard (10mm) with the new rebated PIR. Very expensive but equivalent to full fill. It matters not whether the blockwork or brickwork (internal/external) is built first. We've been fitting it for 18 months now.
You are required to fit DPC at corners and rebates.

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