Warm flat roof - builder installed 100mm insulation only

[Jit83]

Hi,

 

My builder has only installed 100mm of insulation on the warm flat roof of my extension. The top side is finished and watertight but the plaster needs to be completed on the underside. 
 

After I spotted this by shear chance, they said they’ll install a further 50mm underneath between the roof trusses, pushed right against the board.  They said the building inspector had suggested this. 
 

I’ve called the building inspector who’s now changed his story and said the builder needs to contact the insulation manufacturer. 

I’ve read reports that the above could cause risk of condensation and damp over the long term. 

 

Other than ripping it out and starting again, are there any other options? 
 

Can another 50mm be added on top of the waterproof membrane, and then another membrane installed on top?

 

Another thing the builder has said is they can install plasterboard with a membrane on the inside which would act as the vapour barrier. But I will be having spotlights so not sure if this is a valid solution. 
 

Any advice would be much appreciated. 

[JohnMo]

What do your drawings specify? - he should be building to the drawing not make it up as he goes along.

[nod]

18 minutes ago, JohnMo said:

What do your drawings specify? - he should be building to the drawing not make it up as he goes along.

Plus 1 

It amazes me when this happens 

Show him the drawing and tell him 

It’s says this 

Why have I got this 

[joe90]

It’s no wonder builders get a bad name when they can’t follow drawings 🤷‍♂️. I would make him rip it out and start again (I hope you have not paid yet? )

[Jit83]

The architects plans shows 150mm insulation which they have not followed. They’ve agreed the human error on their side. 
 

I’ve been paying weekly as they’ve progressed with the project. Won’t be making any more payments until this is resolved. 

Would a condensation risk analysis be the way forward (with the 50mm insulation against the boards)? 

 

[Alan Ambrose]

>>> Would a condensation risk analysis be the way forward

 

It might help - ubakhus.de is fairly self-explanatory. Post the condensation results up here and you'll probably get some more insight. 

 

What's the roof build-up? It might not be such a big deal to take off the waterproof layer and install more insulation. Post up the architect's drawing?

Edited May 13 by Alan Ambrose

[Jit83]

Architects drawings: 

 

[JohnMo]

Tell him to as per drawing nothing else, if he has to redo at his own expense, that's his issue - he should have read the drawing. 2 layer of 100 and 50mm on top is ok. But do vcl it as per drawing.

[nod]

2 hours ago, Jit83 said:

The architects plans shows 150mm insulation which they have not followed. They’ve agreed the human error on their side. 
 

I’ve been paying weekly as they’ve progressed with the project. Won’t be making any more payments until this is resolved. 

Would a condensation risk analysis be the way forward (with the 50mm insulation against the boards)? 

 

You got your answer 

Get them to re do it to plan 

 

[Jit83]

Unfortunately the builder isn’t willing to make any changes to the top side of the roof. They’ve carried out a condensation risk analysis which is attached below. 
 

I’ve never seen one of these before but I think it looks OK? 
 

1-MP-240513-101423-147.pdf

[Moonshine]

11 minutes ago, Jit83 said:

Unfortunately the builder isn’t willing to make any changes to the top side of the roof. They’ve carried out a condensation risk analysis which is attached below. 
 

I’ve never seen one of these before but I think it looks OK? 
 

1-MP-240513-101423-147.pdf 87.69 kB · 2 downloads

 

NO.....

 

it's not o.k, this risk analysis is for a low occupancy building, do not accept this

 

 

get them to put it right, it's what the drawing said. What else are they doing not to spec?

[Jit83]

I know, that’s what’s concerning me too. They’re meant to be a reputable company so it’s really frustrating. 
 

They have finally agreed to do the job properly after a lot of going back and forth. Thanks for the help on this. 

[goodbyegti]

Good to hear! Are the joists in the correct place? It's worth checking you have enough upstand on the parapet after the extra 50mm goes on top.

[Iceverge]

There's a lot of hocus pocus about flat roofs but when there's a continuous layer of external insulation like you have they're inherently safe. 

 

In fact I would be happy to install 100mm of mineral wool with a K Value of  0.035W/mK inside your roof provided there was some form of continuous mechanical ventilation like MEV or MVHR to keep internal moisture under control. 

 

I would not install a vapour barrier and would be super cautious of a vapour control layer. Done anything less than immaculately it would be a recipe for trouble.  There's a fair share of cognitive dissonance among insulation manufacturers who regretfully won't acknowledge that real life workmanship imperfections are a reality. 

 

To be pragmatic, if it was my house I would get the builder to install 100mm of mineral wool below the ply, omit any VCL or vapour barrier and fit some dMEV or dMVHR units in the house instead.

 

Mistakes can be made, punitive punishment just for the sake of it doesn't achieve much where steps can be take to rectify the problem without starting from scratch.