Excessive cracks and shrinkage in plasterboard ceiling

[GaryM]

3 minutes ago, andy said:

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.

 

I've been looking on google searching zinc and standing seam suppliers sites and so far I can only see example like mine. I'm trying to find examples of double boarding and it's advantages.

 

I will talk to my architect, who despites comments made on this tread does seem very capable and know what he is talking about.

[andy]

10 minutes ago, GaryM said:

 

I've been looking on google searching zinc and standing seam suppliers sites and so far I can only see example like mine. I'm trying to find examples of double boarding and it's advantages.

 

I will talk to my architect, who despites comments made on this tread does seem very capable and know what he is talking about.

As I say I could be wrong here but your membrane has to go on something structural to contain your insulation layer but I don’t see that in your build up.

 

My build up is VMZinc standard construction as specified by both my architect and VMZinc installer.

[GaryM]

@andy looking at the install  document on VMZinc, the main difference is they have put the membrane at the bottom of the void where as in my design the membrane is on the ply under the standing seam.

 

Like you no expert, but from what I can tell, with a  membrane at top there is no need for one at the bottom of the void. I will talk to the architect soon.

[andy]

Yeah sorry the membrane I have is under the zinc but the crucial bit I think is the 50mm ventilation void open to the eaves but hopefully your architect can explain more.

[tonyshouse]

I am concerned about the dimensional stability of the wooden sheets , they will expand a little with heat, a bit more with moisture and should have been laid with gaps between them. They won’t necessarily shrink back when they dry out depending on lots of things. 
 

metal roof will collect condensation on the underside in some conditions insulated ot not and ventilated or not

 

i prefer insulated metal sheets 

[andy]

57 minutes ago, tonyshouse said:

 

metal roof will collect condensation on the underside in some conditions insulated ot not and ventilated or not

As I understand it, that’s why you put a breather membrane between metal roof and plywood or whatever your sarking is.

[MarkyP]

I have vm zinc standing seam on my loft dormers, each dormer is clad in exterior grade ply with a membrane and standing seam over. Under the ply is a ventilated batten space, then insulation between and under the rafters.

 

the fluffy stuff referred to earlier might have been OK if they'd used a 0.032 high density mineral wool like frametherm or rafter roll but if just standard loft insulation fluff at 0.044 then not ideal. Personally here I would prefer a combo of dense wool 200mm between and perhaps 50mm phenolic under rafter to get both a pretty decent u-value and  a good decrement delay and sound insulation of the dense wool layer. But agree with others that ventilation under the metal would have been a standard detail unless very conciously designed otherwise. Some consideration of wind tightness of the mineral wool would also be a factor if a ventilated void was introduced.

 

there are examples of fully filled build ups with no ventilation under either sarking or even a "non-breathable" metal roof but I think these are often with meticulous attention to airtightness detail and backed by warranted condensation risk analysis. Not sure the architect described here is in that bracket though.

 

I have a hybrid roof - after much deliberation I have the south and south west roof pitches insulated with fully filled 150mm dense flex wood fibre with 75mm PIR under rafter, this is under OSB sarking and fully filled, no ventilated void under the sarking but ventilated counter batten space above the sarking under the roof slates, so there is lots of outward drying potential for any moisture that does enter the insulation layer, boosted by the solar gain on the slates which heat up in the sun. The roof pitches which don't get full sun are different, 100mm phenolic between rafter with ventilated void (eaves and ridge) under sarking and 75mm PIR under rafter. My BC inspector did take a few mins to understand my logic but couldnt see any issue with it, but he did say he'd never come across a roof with two different insulation strategies in opposite pitches before but he got used to my strange ideas during the project! This was a large loft conversion so I was limited in insulation depth by existing rafters. My trade off on the south facing pitches was a small drop in u-value for a large improvement in decrement delay to reduce risk of summr overheating. Those roof pitches without full sun I went for the best u-value I could achieve within the depth available to me without compromising room space too much so used phenolic and PIR. 

[Russell griffiths]

I don’t think there is a way around this, you need to rip a 4foot square of ceiling down and have a look and draw exactly what you have ended up with. 

I personally think you have a cock up and a ticking time bomb. 

Inadequate insulation. 

Poor ventilation. 

No vapour barrier. 

Equals cold roof with condensation, equals roof timbers rotting and early failure. 

 

If this is not your fault, then why worry get the ceiling pulled out Monday and start the remedial work. 

Your builder cannot substitute pir for a glass batt without major re design. 

[Ferdinand]

In your reflections do not forget the cost difference between woolly insulation and PIR, which at 210mm would be £30+ per sqm.

 

Where did the extra money go, if any?

[joe90]

58 minutes ago, Russell griffiths said:

I personally think you have a cock up

+1

[joe90]

On 14/05/2021 at 10:50, MarkyP said:

the fluffy stuff referred to earlier might have been OK if they'd used a 0.032 high density mineral wool like frametherm or rafter roll but if just standard loft insulation fluff at 0.044 then not ideal.

can you not get the builder to confirm exactly what he used?

[Pete]

On 13/05/2021 at 20:38, GaryM said:

@andy looking at the install  document on VMZinc, the main difference is they have put the membrane at the bottom of the void where as in my design the membrane is on the ply under the standing seam.

 

Like you no expert, but from what I can tell, with a  membrane at top there is no need for one at the bottom of the void. I will talk to the architect soon.

I have a SSR roof and have experienced no problems. If my memory serves me well my make up is as follows (inside to out)

PLasterboard
Battening (service void)

VCL

18mm OSB

Wooden I Beam/380mm with full fill cavity insulation (not PIR)

18mm OSB

50mm battening (vent/void)

18mmOSB

Roofing membrane

SSR

[Russell griffiths]

1 hour ago, Pete said:

I have a SSR roof and have experienced no problems. If my memory serves me well my make up is as follows (inside to out)

PLasterboard
Battening (service void)

VCL

18mm OSB

Wooden I Beam/380mm with full fill cavity insulation (not PIR)

18mm OSB

50mm battening (vent/void)

18mmOSB

Roofing membrane

SSR

That looks a good make up. 

[GaryM]

Hi All, thanks for all the feedback.

 

I want to correct a couple of assumptions, the current build is up and above building standards.

 

It does have breathable membrane, it does have 50mm+ ventilation, it does have adequate insulation and it does have a vapour barrier.

 

The builder did drop the ball with using glass wool instead of PiR, but glass wool is permitted for that use. At time our was in very short supply. There build quality is up at a high standard and they know what they are doing. I am disappointed the specified PIR was not used.

 

One important item I didn't mention before is we do have over 20m2 of triple glazing in the space.

 

Having discuss with architect, builder and an independent expert. We feel the probably cause will be heat in the room due to solar gain, we will be doing some testing, looking at the heat in the room and within the ventilation void.

 

It's also worth pointing out the joints were redone using reinforced mesh tape and so it seems a lot better

 

We need to keep an close on it.