(Solved) How can I ventilate a boarded roof

[thyristor44]

Hello
I have a slate roof that I am wanting to re slate. Its a lean to and was a hay loft that I re slated about 35 years ago but is beginning to leak.
Its formed from 9x3" timbers ( rafters? )
I live in an exposed site and would like to sark straight on to the 9x3s with 15mm osb, counter batten it, membrane, then laths and slates.
My problem is there is 2" of insulation with a ceiling below the top of the 9x3s. How could I ventilate this space as It is a cold roof. I can fit soffit vents to the bottom of the roof but the top where it butts up the the stone wall of the house is a problem.
Any ideas please
thyristor44

 

[catlad]

Have a look at manthorpe or marley web sites for abutment vents.

 

[freddiemercurystwin]

Assume there is a good gap between the insulation and the underside of your new boarding?

A few manufacturers out there eg http://www.glidevale.com/downloads/glidevale_roof_ventilation.pdf page 25

 

[thyristor44]

Assume there is a good gap between the insulation and the underside of your new boarding?

Probably not a lot, maybe an inch.

 

[freddiemercurystwin]

If boarded an inch should be OK.

 

[thyristor44]

If boarded an inch should be OK.

I have been thinking about that inch and it may not be there in places.
If I decided to add more insulation on top either before or after the new boarding is there a way I could make it a warm roof and not need to ventilate at all.

 

[freddiemercurystwin]

A warm roof has all of the structure inside your insulation bubble therefore the structure remains warm hence the term 'warm roof'. You could fill the void completely with insulation so you had no void at all, this would be a hybrid of both a cold and warm roof. But the problem with this is that any moisture that does get through your insulation will condense as it gets colder ie nearer the outside. Ordinarily this would occur in your ventilated air gap and be taken away out through the vents but as you would not have a gap the moisture will have no where to go so your timber and insulation could end up wet and eventually the timbers could rot.

Therefore to make a proper warm roof you should fit your existing insulation tight up against the new boards and fit an additional layer of insulation (say 40mm) on top of the boards, then the membrane then fix counter battens then your tile battens on top of those. eg http://www.icopal.co.uk/Products/Pi...esign_Guide/Warm_Pitched_Roof_Ventilated.aspx Though adding additional layers on top of your existing pitch line may bring with it problems with existing flashings where it meets the wall not being deep enough etc.

 

[thyristor44]

Nothing is easy. If I did board it what would be the best way of adding 2" of extra insulation. Would the insulation go between the counter battens on top of the boarding?

 

[freddiemercurystwin]

A rigid PIR insulation like Celotex or Kingspan, then the membrane then counter battens in line with the rafters then the slate battens on top.

 

[thyristor44]

Sorry to be ignorant but would I be fixing the counter battens directly on top of the celotex.
Would be much easier if I could.

 

[freddiemercurystwin]

http://www.kingspaninsulation.co.uk...8ccc61be/Kooltherm-K7-Pitched-Roof-Board.aspx Figure 3a

 

[thyristor44]

Thats a good bit of information, thanks alot. It pretty well gives me all the alternatives.
Thanks again
thyristor44