Advice - Roofing Air Gap Issues - Cold Roof

[Jake Cole]

Looking for some advice on roofing if I may:

I’m building a cabin with cold roof in SW UK and concerned about the air gap between insulation and OSB deck.

The roof comprises (inside to outside):
12mm Q mark plywood
Thick vapour barrier (black DPM) that is fitted best I could but without butyl tape to seal screw holes and stapler holes.
170mm deep rafter filled with 150mm PIR insulation which only gives me a 20mm air gap (50mm is recommended building regs). I messed up the materials order.
That’s where I’ve paused building.

Roof is 10 degrees single pitch roof
Spans 4.2 meters
I’ve already ordered 32/1000mm box profile metal roofing sheets
High usage holiday cabin

I can see 3 options going forward:

A) No OSB on top of rafters - just membrane, counter battens then metal roofing
Q) is this weather proof enough? Warm enough? Too much airflow?

B) OSB fixed to rafters (20mm air gap), breather membrane, cross batten then metal box profile roofing. Gaps on front fascia and eaves for airflow.
Q) enough airflow? 50mm is most people's recommendation. 

C) option B but with battens on top of rafters to lift OSB and increase air flow (cross and through ventilation)
Q) is this too much airflow?)

Any advice or thoughts much appreciated. I know cold roofs are prone to moisture build up so I want to get this right.
Cheers,
Jake

[Redbeard]

Of the 3 I prefer C. As long as you keep the cold ventilation air on the 'right side' you cannot really have 'too much air' (flow). An empty loft void is not regarded as 'too much air', so your vent gap won't be. The flow is to some extent governed by the cross-sectional area of the 'ins' and 'outs'. What vent provision will you have? In terms of the gap after adding battens I don't think you want less than 50mm.

 

Is the roof already built up to the top of the rafters? If not I would take down the ply inside and add 25mm PIR, taped at all joints and perimeters, to cloak the thermal bridge. It's regarded by some as permissible to have the VCL a little into the sandwich, so you do not have to move your existing VCL. You'll just be doing belt and braces by taping the 25 PIR as a VCL too.

[Jake Cole]

Hi Redbeard, thanks for your thoughts on this.
 

RE venting, I plan to leave a 25mm gap on fascia and eaves with insect mesh all the way along.

 

I could not add more insulation for thermal bridging due to budget - not ideal I know. And I've built too much to take down the ply now. 

 

What are your thoughts on the need for OSB? I can't see it being much of an insulator. Most people I've asked say it's a structural anti-racking measure, but with the counter battens and 12mm ply ceiling ply, I can imagine it being pretty rigid without the OSB on top of rafters. If I batten at 400mm centres, with box profile roofing, it should be OK to walk on. Thoughts on this appreciated. 

 

Cheers, 

Jake

  

[Redbeard]

Who specified the OSB? What do they say?

 

Do you have no secondary waterproofing layer (breather membrane)?

[Jake Cole]

Hi RedBeard, this is my design, no one else involved, just a cacophony of things I've read and researched. I've also already bought the 9mm OSB that I was going to use for roof but that can be used for other things. 

This would be the build layers with no OSB:

12mm ply

Vapour control layer

170mm rafter filled with 150mm PIR board

25mm batten on top of rafter to increase air gap

Breather membrane 

2x2 counter battens (so I can use 32mm roofing screws and not puncture the membrane unnecessarily). 

Box profile roofing 

[Iceverge]

8 minutes ago, Jake Cole said:

Hi RedBeard, this is my design, no one else involved, just a cacophony of things I've read and researched. I've also already bought the 9mm OSB that I was going to use for roof but that can be used for other things. 

This would be the build layers with no OSB:

12mm ply

Vapour control layer

170mm rafter filled with 150mm PIR board

25mm batten on top of rafter to increase air gap

Breather membrane 

2x2 counter battens (so I can use 32mm roofing screws and not puncture the membrane unnecessarily). 

Box profile roofing 

 

That sounds fine.

 

It'll give you a U value of about 0.22. 

[Temp]

If the breather membrane is vapour permeable the (normally 50mm) ventilated gap isn't so critical...at least not on a tiled roof. Might be more so on a sheet metal roof.

Edited July 10, 2023 by Temp

[Jake Cole]

Thanks @Iceverge, 

 

Appreciate some confirmation that it will be OK. 

[Jake Cole]

Cheers Temp,

 

I was expecting to raise the rafters even without OSB as the instructions say to fit membrane with a 10-15mm sag that acts as a drainage channel. That would leave 5mm or less in some places. Might be fine and I'd much rather not have to raise the raters but I'd be concerned that there would be unwanted moisture build up. I wish I knew for sure but I guess one must prepare for the worst. 

[Steve247]

I've been researching a lot over the past 2 months re cold roof scenario for my build. I really didnt want a massively thick over sized looking roof. However, my BCO informs me that there are many failures of cold roofs ( week by week it seems ) even if the guide lines for ventilation and proper detailing are followed which is quite worrying!! This was also raised by a roofing carpenter I used.  Check out London Flat Roofing who will not do a cold roof and have lots of very good advice I think in particular 'through and cross ventilation'.  Needless to say I've now decided to go with a warm roof to avoid risks.  BS6229:2018 and reports from SPRA do not advice use of cold roofs. 

 

I think if you really have to do a cold roof get a full design done that will provide a warranty for the design. Then whoever does the work needs to be completely OCD on workmanship and you select the best performing materials money can buy, then hopefully you'll be ok. 

 

 

Part extracts for info........ 

3.2.5. THE COLD ROOF

The principal thermal insulation is placed at or above the ceiling (i.e. below the deck), resulting in the roof covering and structural deck being substantially colder in winter than the interior of the building. The structural support will typically form a ‘thermal bridge’ between the high and low temperature zones of the construction. It is very difficult to insulate a cold roof system to current mandatory levels without introducing thermal bridges and/or increasing the risk of condensation accumulation within the system. It can also be difficult to install an effective AVCL below the insulation layer and providing sufficient cross ventilation above the insulation. In addition, the mandatory requirement for uninterrupted external air circulation limits the application of the system where abutting elevations or changes in building geometry occur. 

Therefore, ‘cold roofs’ are very unlikely to be a feasible option and are not recommended in BS6229:2018

It is also worth noting that mushroom vents have proved to be ineffective.

 

An unavoidable cold roof construction should have the following;

• A waterproof breather membrane installed on the cold side of the thermal insulation. The breather membrane should be air-tight but vapour permeable e.g.<0.25MNs/g taped and sealed according to manufacturer’s instructions. 
• A continuous and fully sealed AVCL on the warm side of the thermal insulation, correctly installed. (Battens can be used on the underside of the deck from which the ceiling can be supported. This creates a void directly above the ceiling minimum 25mm deep allowing ay service cabling and pipes to pass through without penetrating the AVCL) 
• A minimum 50mm deep ventilated void above the breather membrane with maximum cross ventilated width between openings, not be greater than 5m allowing free movement of air to the underside of the deck.
  S1/2020 vS1 

 

[Jake Cole]

Hi Steve,

 

Yes, I would probably go for a warm roof next time height allowing. 

 

It's a self build, no one else involved.

 

I think I can get the cold roof to work. The screen shot below is what I'm going to do. Breather membrane will go between the rafter raisers and counter batten. Tin will go on as the top and final layer. The design below is what Steve Roofer recommends if working with a cold roof (cross and through ventilation). 

Cheers,

Jake