Standing Seam Roof Build Up

[CJO]

After some advice and experiences from those on this forum website regarding our potential Non Ventilated Warm Roof.

 

I have spent an evening/afternoon reading previous threads about this subject matter and that has raised a couple of questions.

 

Our internal roof pitches will all be open plan (vaulted) and our house will be an upside down layout.

 

Our architect and SE have specified the following build up from inside to out.

 

12.5mm plasterboard

Service void

Vapour control layer

9.5mm sheathing board

125mm Rafter

75 PIR insulation between

100mm on top

18mm sarking board

Standing seam metal work roof.

 

My partner and I have increased our expectation of our self build and want to make it achieve passive elements.

 

I’m thinking…

 

12.5mm plasterboard

Service void

Vapour control layer

9.5mm sheathing board

125mm Rafter

Cellulose infill for sound and insulation

100mm PIR on top

22mm OSB

Tyvek Metal Underlay

Standing seam metal work roof.

 

Potentially have an extra 50mm deep rafters to play with as the SE downsized the Architects spec…

 

Reading the previous threads regarding noise and condensation risks has got the grey matter questioning methods.

 

I have read a lot of Du Ponts website regarding roof build ups, watched Roofer Steve on YouTube explaining vapour control layers and also looked at VM Zincs information regarding build ups. Our doesn’t sound impossible.

 

Is there a concern infilling our rafters would result in needing deeper rafters?

 

Look forward to reading any responses.👍🏻

 

[Redbeard]

What you describe is a hybrid Warm roof - some on top of the rafters and some between. In some circumstances there is a risk of the dew-point occurring at the interface. A condensation risk analysis (CRA) can guide you, but a colleague had a good rule of thumb: If you have two-thirds of the R value above the rafters and one-third below, the interface is unlikely to get cold enough to reach the dew-point. If I remember correctly the lambda of cellulose is (or was when I used it) 0.036W/mK, and PIR is 0.022W/mK. So for 125 cellulose it's 3.47m2K/W (actually less, as that does not take into account the 'intrusion' of the rafters)  and for 100 PIR it's 4.54m2K/W, so not too far off but probably not two-thirds/one-third. A CRA (particularly if done in the WUFI software) will be more accurate.

 

Edit: A thought: If noise is an issue why not consider rigid wood-fibre on top? 140-180kg/m3 as against about 32kg/m3 (IIRC) for PIR)?

Edited Sunday at 15:31 by Redbeard
Noise and WF

[Nickfromwales]

5” rafters are quite small? At what frequency and what length? 

[SimonD]

1 hour ago, CJO said:

Vapour control layer

9.5mm sheathing board

125mm Rafter

Cellulose infill for sound and insulation

100mm PIR on top

22mm OSB

Tyvek Metal Underlay

Standing seam metal work roof.

 

If you do osb sheathing @ 11mm you can use this as your VCL, just tape and seal all the joints.

With respect to your buildup sub the metal roof, you need to be a little careful. There are only a small number of standing seam metal roofing manufacturers that explicitly have a buildup that doesn't require a ventilation gap so double check with them that the Tyvek metal underlay system on warm roof is accepted by them (I know it's been designed to allow condensation to drain from below the metal roof and there are approved systems) - and double check with building control. You are also much better of using sarking boards, which are the preferred substrate, second is plywood and 3rd is osb. Pull out resistance of your standing seam clips is greatly reduced in osb where you really need to use screws not nails, which most contractors will use because they won't want to spend the time screwing the clips into the board. So you have to look at this in the round and how the metal roofing contractor designs the clipping for the wind exposure of your house.

[Iceverge]

Any drawings of your roof and wall interface? 

 

If it's complex then I would recommend moving your airtight layer outboard of the rafters. 

[CJO]

Apologies for the delay and thank you for very quick responses to my questions.

 

@Redbeard thank you for explaining the rule of thumb, I had a small read about it was initially thought it was about the amount of rafter covered not relative to thevR values of insulation used (I'm not fibbing when I say I had a small read on it). I am now looking at woodfibre sarking boards following @SimonD suggestion, they seem to also provided a very good vapour permeable substrate.

 

@Iceverge I was on the understanding I need a nice strong vapour control layer internally and all materials leading up through the roof ideally need to be more permeable than that... until we reach the standing seem roof.

 

@Nickfromwales a quick bit of maths has the rafters working out to be around 3500mm. They will be butted up to an I beam ridge that will be packed out with timber. Roof pitch is 40 degrees.

 

Please have a good nose through the drawings, our architect is good but hasn't provided us with true detailing. Our SE drawings a more accurate and informative.

 

I look forward to any more insights you can provide.

24066-R01 - Structural Calculation Report 240925 (dragged) 5.pdf 01923-HBA-DR-1008_BR02_Proposed Section AA copy.pdf 01923-HBA-DR-1012_BR02_Window Junction Details copy.pdf

[Russell griffiths]

I wouldn’t consider osb under the sheets, pull out strength of an 18mm screw is not great. 
25mm sarking boards for me.