Excessive cracks and shrinkage in plasterboard ceiling

[GaryM]

Hi All

 

We have a standing seam roof, with a vaulted ceiling below and we are get excessive shrinkage in the plasterboards, gaps up to 5mm have opened up overnight.

 

Construction is  Aluminium standing seam roof > membrane > 18mmOSB > Posi beams with 250mm glass fibre insulation > foiled back plasterboard.

 

Plasterer is then spray coating plaster ( 2 layers x 1 mm thick), all joints have be meshed well.

 

Next day mesh had bulge  on joints and when remove revealed the 5mm gaps. They have redone joints and problem has reappeared, not quite as bad.

 

I have meeting with Builder and Architect to discuss the way forward, what is causing it and how to solve it.

 

Our get feeling is roof is heating up and cooling too fast, the plaster has dried in a day and it normally takes 2 to 3.

 

Any thoughts or advice?

 

Thanks

 

[Big Jimbo]

Have you got your Aluminium  roof on battens , and cross battens ? Is there a vertilation space under the Ali roof ? If you have no insulation above your posi beams, and only inbetween, you have a cold roof which should be vented.... I am no expert, but i don't like the sound of your roof make-up

[ToughButterCup]

1 hour ago, GaryM said:

...

Construction is  Aluminium standing seam roof > membrane >

...

 

Is the membrane being cooked by the metal roof ? Do you not need to ventilate?

[GaryM]

1 hour ago, Big Jimbo said:

Have you got your Aluminium  roof on battens , and cross battens ? Is there a vertilation space under the Ali roof ? If you have no insulation above your posi beams, and only inbetween, you have a cold roof which should be vented.... I am no expert, but i don't like the sound of your roof make-up

Aluminium roofs have to go on board. The should be a void over the insulation, the architect did specify 210mm PIR with 50mm void above, but buildings said they could not get PIR and use glass fibre instead. I am annoyed as Glass fibre is not as good as soild PIR, so I do have a worry that air is not ventilating as it should.

[GaryM]

7 minutes ago, ToughButterCup said:

 

Is the membrane being cooked by the metal roof ? Do you not need to ventilate?

That's my worry, I've just been going through drawings and as mentioned above, drawings specfiy 210mm of solid PIR with foil back with a 50 mm void above, builders said they could not get it and used 250mm of glass fibre instead.  So three concerns, has void been filed with glass fibre ,2 if used i guess foil on of PIR would have reflect heat and 3, the U value of glass fibre is only half of PIR.

 

So could this be the main problem or something else? i would image builder and architect will be pointing fingers, I just want to know what is causing it and get it fixed and move on

[Big Jimbo]

Gary M. Your roof is cold and should be ventilated. It sounds like it's not !!!! So you should have 250mm of pir between your ceiling joists, and instead you have 250mm of crappy fluffy stuff.

I'd be having that roof off and make them do it properly . P.S. While they are at it, make them put ventilation under the standing seam roof. That whole thing is going to cost a fair few quid to put right now.

[Russell griffiths]

I have 200mm of fluffy crap, but I also have 75mm of pir under the rafters all glued and taped. 

On a 30degree day the inside face of the pir is cold so no heat getting in. 

 

You cannot change pir for rockwool type stuff and expect expect the same result. 

 

I would expect that to be a very poor roof build up, it will be cold in winter and hot in summer. 

 

I would also think it is very very close to not passing building regs. 

 

Sorry, needs re doing. 

[joe90]

If your builder changed the spec without consultation before hand he is responsible and I would make him redo it at his expense. It’s just wrong!!! PIR and “fluffy stuff” have very different insulation values as you said.

Edited May 12, 2021 by joe90

[andy]

I have an anthracite zinc standing seam roof (so black, gets hot as hell) and the build up from outside is zinc, underlay stainless steel clout nailed to 18mm plywood, 50mm battens open to eaves and venting at apex, fibreboard nailed to i-beams with 350mm of warmcell followed by 12.5mm foil-backed plasterboard and then plastered (not sprayed).

 

This includes vaulted spaces and whilst we had a slight crack on the ends going towards ridge beam, I chipped it out and easifilled it to repair and painted and there's zero movement to date and no cracks.

 

As others have said, sounds to me like a lack of (required) ventilation void and glass fibre is no substitue for PIR alas... get it rebuilt to spec.

[nod]

3 hours ago, GaryM said:

Hi All

 

We have a standing seam roof, with a vaulted ceiling below and we are get excessive shrinkage in the plasterboards, gaps up to 5mm have opened up overnight.

 

Construction is  Aluminium standing seam roof > membrane > 18mmOSB > Posi beams with 250mm glass fibre insulation > foiled back plasterboard.

 

Plasterer is then spray coating plaster ( 2 layers x 1 mm thick), all joints have be meshed well.

 

Next day mesh had bulge  on joints and when remove revealed the 5mm gaps. They have redone joints and problem has reappeared, not quite as bad.

 

I have meeting with Builder and Architect to discuss the way forward, what is causing it and how to solve it.

 

Our get feeling is roof is heating up and cooling too fast, the plaster has dried in a day and it normally takes 2 to 3.

 

Any thoughts or advice?

 

Thanks

 

If you haven’t fixed insulated plasterboard to the underside of the joists You won’t be able to sort the cracking out

Ive seen this many times before when 15 mil boards are fixed directly to the rafters 

[GaryM]

5 minutes ago, Big Jimbo said:

That whole thing is going to cost a fair few quid to put right now.

Yes that's my worry. Roofer, who was a skilled zinc roofer, said the metal was well ventilated, but he just fitted the membrane and metal, that was before all the fibre was fitted.

 

 

[GaryM]

1 hour ago, joe90 said:

If your builder changed the spec without consultation before hand he is responsible and I would make him redo it at his expense. It’s just wrong!!! PIR and “fluffy stuff” have very different insulation values as you said.

He change to "fluffy" and was putting up when I saw it, he said "We can't get PIR so we are doubling up on insulation, from 120mm to 250mm". I'm not a builder so just went with what he said. It was only when this problem occurred and I went back to the drawings and reading through the architects spec that I noticed he had specified 210mm PIR. 

 

Generally the builders have been good (but slow) and pride themselves on the quality of build, so from what you and others are saying, I am a bit shocked.

[Jilly]

On the plus side, at least you notice now before you have a future condensation debacle. 

[GaryM]

Just an update:

 

I've spoken to Architect: he is not that concern about the use for glass wool,  he says it should still leave a good 100mm get for ventilation.  The Glass Wool used had a better insulation properties 0.044w/mk which is the same as the solid PIR I believe.

 

[TonyT]

Sounds like the architect fu@@ed up.

 

[TonyT]

[joe90]

23 hours ago, GaryM said:

Posi beams with 250mm glass fibre insulation

 

 

18 hours ago, GaryM said:

architects spec that I noticed he had specified 210mm PIR

frankly that does not add up in my mind. PIR is  roughly twice as good as glass insulation but glass insulation fitted is only 40mm more than PIR!!!

1 hour ago, GaryM said:

The Glass Wool used had a better insulation properties 0.044w/mk which is the same as the solid PIR

would be good to see some figures on that assumption?

 

edit.... even if it was correct it does not work, excess heat is transferring to the plasterboard. That’s why you paid a professional, to get it right!!,! ?‍♂️

Edited May 13, 2021 by joe90

[PeterW]

2 hours ago, GaryM said:

The Glass Wool used had a better insulation properties 0.044w/mk which is the same as the solid PIR I believe.

That isn’t correct

 

PIR is 0.022-0.025w/mK, glass wool is 0.035-0.045w/mK

 

If you remember that uValues are calculated using the inverse (i.e. 1/uValue) then glass wool is half as efficient as PIR. 
 

19 hours ago, GaryM said:

but he just fitted the membrane and metal, that was before all the fibre was fitted.

So I expect that the builder has stuffed the insulation up under the OSB which will have removed any ventilation and you have condensation issues. The moisture from the drying plaster will have also gone upward and condensed on the OSB - do you know what membrane was used ..?
 

 

 

[GaryM]

4 hours ago, PeterW said:

That isn’t correct

 

PIR is 0.022-0.025w/mK, glass wool is 0.035-0.045w/mK

 

If you remember that uValues are calculated using the inverse (i.e. 1/uValue) then glass wool is half as efficient as PIR. 
 

So I expect that the builder has stuffed the insulation up under the OSB which will have removed any ventilation and you have condensation issues. The moisture from the drying plaster will have also gone upward and condensed on the OSB - do you know what membrane was used ..?
 

 

 

Ok I will double check the insulation values, I'm sure the glass wool I looked at was the same as the solid, but I will check.

 

I don't know the membrane of the top of my head.

 

  I am pretty confident the builder has not stuffed it up willy nillly, I saw them do a lot of it and they did take care and afterwards it was laying very even

[GaryM]

5 hours ago, joe90 said:

 

 

frankly that does not add up in my mind. PIR is  roughly twice as good as glass insulation but glass insulation fitted is only 40mm more than PIR!!!

would be good to see some figures on that assumption?

 

edit.... even if it was correct it does not work, excess heat is transferring to the plasterboard. That’s why you paid a professional, to get it right!!,! ?‍♂️

Thanks for the reply.

 

I'm rechecking the numbers and the architect is as well, we also have SAP calcs to refer to.

 

Note: at the moment we are assuming it is drying out to quickly, it seems the most logical reason

 

[GaryM]

@TonyTI'm re looking at the numbers.

 

Thanks for replying

[andy]

Do you have a cross sectional drawing of your build up like this?  Where the arrows drop down is the cold roof 50mm ventilation space with battens running vertically to allow air flow.

 

 

[GaryM]

 

7 hours ago, GaryM said:

Just an update:

 

I've spoken to Architect: he is not that concern about the use for glass wool,  he says it should still leave a good 100mm get for ventilation.  The Glass Wool used had a better insulation properties 0.044w/mk which is the same as the solid PIR I believe.

 

 

I now realise the above is rubbish and we have half the specified insulation in there.

[GaryM]

9 minutes ago, andy said:

Do you have a cross sectional drawing of your build up like this?  Where the arrows drop down is the cold roof 50mm ventilation space with battens running vertically to allow air flow.

 

 

 

Thanks for the reply Andy,

 

Yes we do, i think the difference is yours looks like doubled boarded and battened, is that right

 

 

Edited May 13, 2021 by GaryM
correction of information

[andy]

10 minutes ago, GaryM said:

 

Thanks for the reply Andy,

 

Yes we do, i think the difference is yours looks like doubled boarded and battened, is that right

 

 

 

It has to be double boarded as far as I am aware.  If you don't do that to allow the cold outside air to circulate between the board under you roof (18mm ply in my case) and the roof that containing your insulation (15mm DHF for me) it won't work.

 

I could be wrong but am I seeing in your drawing that the airflow is moving in the same space as the insulation?  If so, I cannot see that being correct.