Vapour control barrier ?

[markiespud]

Hi everyone

I am at a bit of a stumbling block on our roof, we are building/ part renovating an old barn and putting in a warm roof.

Our roof makeup is basically tiles/ batten/ membrane/ batten/ 150mm pir then a sawn timber board (cosmetic) and finally open joists. My question is (despite numerous searches) do I need a vcl or vcb under the pir as it is already foil faced and a vcl will not be able to breath and the foil is already a vpb....

Can anyone advise me ?

Thanks

[Marvin]

Tricky one. I had discussions with Building inspector about flat roof and vpb under celotex. Inspector wouldn't agree not to install it, despite all edges/joins silver taped. 

 

I would ask Building inspector.

Edited July 22, 2022 by Marvin
minor change.

[DragsterDriver]

In my experience they won’t touch it without a vcl.

[Dan F]

We didn't use a VCL for our warm roof.  But this was only because i) we have high levels of airtighness ii) we were using natural slates (which provide more natural ventilation). 

 

I read through the BS at the time to ensure this was OK but can't remember all the details now.  But but as far as I remember if we didn't have either of those two criteria a VCL was required.

[Gus Potter]

1 hour ago, DragsterDriver said:

In my experience they won’t touch it without a vcl.

Yes it's essential. As we are introducing lots of insulation into buildings we need think carefully about trying to stop air with a high water gas content getting into places where it will later condense into a liquid and not be able to get back out easily. A VCL is essential.. Alutrix is one of the products you can research.

 

 

 

 

[markiespud]

Thanks everyone !

In terms of air tightness it should be pretty good as it's a Isotex icf which will be membraned on the inside (passive purple or parged). It's not our first rodeo but this is my first warm roof and for obvious reasons It needs to be right...

Thanks Marvin, I did email the BI and hopefully will hear back next week.

I will have a look at the Alutrix Gus, thanks !

[Dan F]

3 hours ago, markiespud said:

Thanks everyone !

In terms of air tightness it should be pretty good as it's a Isotex icf which will be membraned on the inside (passive purple or parged). It's not our first rodeo but this is my first warm roof and for obvious reasons It needs to be right...

Thanks Marvin, I did email the BI and hopefully will hear back next week.

I will have a look at the Alutrix Gus, thanks !

What tiles are you using? I can try to dig up the info if its useful.

[markiespud]

Koramic old hollow pantiles Dan, any help appreciated !

Mark

 

[DragsterDriver]

On 23/07/2022 at 00:28, Gus Potter said:

Yes it's essential. As we are introducing lots of insulation into buildings we need think carefully about trying to stop air with a high water gas content getting into places where it will later condense into a liquid and not be able to get back out easily. A VCL is essential.. Alutrix is one of the products you can research.

 

 

 

 

It took me a fair while to understand it all! 

[Dan F]

2 hours ago, markiespud said:

Koramic old hollow pantiles Dan, any help appreciated !

 

Sorry, what I said earlier was incorrect, it'd been a while and didn't remember the details.  In our case we have a single membrane for airtighenss and vapour control on the internal-side of the roof insulation and an external breather membrane. What it was that came up with building control in our case was ventilation, not the membranes we used. (sorry for confusion!)

 

Our roofer and building control wanted to vent the void under the (natural) tiles, but our architect had proposed mortar-bed ridge/hip tiles with no ventliation.  In the end, we want down the mortar-bed path route because, according to the BS this is fine when: i) you have an internal VCL, ii) the roof covering (natural slates in this case) is air permeable  and iii) the external membrane has low vapour resistance.  In all other cases you did need to vent.

 

You may be able to use foil-backed PIR as a VCL but I you'd need to tape and seal all joints, so it would probably be easier and more effective to use a membrane underneath the PIR I think.

Edited July 23, 2022 by Dan F

[JohnMo]

Basically the exact same route on our build, only difference is we have a dry ridge (mortar bed isn't allowed on new builds in Scotland) and we have sarking boards (another Scottish thing) below the breather membrane.

[DragsterDriver]

It should be a vented ridge. The heat behind the tiles will rise upwards not down to the eaves 

[markiespud]

Thanks for all the replies everyone and I have ordered a VCL in the form of Tyvek Airguard reflective, whilst I have your attention can you let me know what fixings you used through the insulation ?

I have tried Helifix but they will not go over 200mm fixings unless I specifically say that I don't want them to take into account for roof weight/ slippage and I need 225mm as I need to go through the batten/ pir and 16mm sarking into the rafter...

I cannot see that thermal bridging will be a problem with 150mm pir, can I use something like this :

https://www.efixings.com/6mm-structural-wood-screws-tx-star-flange-head-zinc/?variant=20160

Thanks again

[ADLIan]

BBA certs and MIs normally show use of AVCL. Is the 150mm pur all above the rafter? If so chat with Helifix as the fixing spec must resist wind uplift but also the dead load of the roof tiles and it’s tendency to slide down the roof slope. Needs specialist design to counter this especially as the tiles and battens are cantilevered away from the roof structure and the tendency to want to bend the fixings. Timber stop rails can help here. Also check with insulation manufacturer as they should be able to help