Removing ceiling and insulating between rafters...

[SanToki]

Having bought an old converted coachhouse from the turn of the century that had all it's character boarded up when it was turned into living accommodation, I am in the process of trying to bring back out of much of it as I can. The upstairs floor has a sloping roof joining partway down the side walls, with a ceiling that covered up about 1 metre of attic space above, which I've taken down, exposing all the rafters - which I now need to insulate, but I understand I need to leave a 50mm gap to promote airflow. The roof itself does have vents in the tiles in various places, but I'm struggling to understand how 'ventilation' will actually be achieved - as there is no air access at the bottom (i.e. soffit/fascia area). Once the ceiling is on, it's effectively sealed.

My original plan had been to wedge 75mm celotex inbetween the rafters right up to the felt, surely this would be a better solution, as - if there are no voids - no air can condense and cause problems?

 

[Burnerman]

You must leave that 50mm gap....if you don't there will be a disaster in a few years.
Consider soffit ventilators to allow air in - they have screens to keep bugs out, and require a hole about 60mm diameter. Vented ridge tiles are also available, and your building inspector will advise you on the correct requirements.
John

 

[freddiemercurystwin]

Failure to install soffit vents is not an option as is pushing the insulation up to the underside of the felt as well as short-sighted and non-compliant. By the way 75mm of Celotex will not get you through the regs. More like 110mm depending on rafter depth and centres. As mentioned consider ridge vents too.

 

[SanToki]

Consider soffit ventilators to allow air in - they have screens to keep bugs out, and require a hole about 60mm diameter.

Thanks for the response, I shall look into whether this is feasible. When you say soffit ventilation, does that mean that just a single ventilation hole at the bottom is needed – that there does not need to be another hole at the top as well? (i.e. in an entrance/exit fashion).

Another line of thought; if I were to fit something like 25mm cross battens horizontally across the rafters on the inside, and then insulate *on top* of that with K17 boards (i.e. so that the gaps between the cross battens were not filled), presumably this would allow the existing tile vents to perform adequately as air would be able to circulate into the roof space and between the rafters?...

 

[freddiemercurystwin]

I'm gonna bow out of the thread and leave it to Burny!

 

[Burnerman]

I can't quote any building regulations to you as I don't know them, but if you consider yesterdays requirements (no ventilation at all) with todays requirements (effectively a howling gale up there) then we see that dampness has to be avoided at all costs.
I think your cross battening sounds fine if you don't mind losing some of the loft space, but whatever happens you mustn't allow insulation material to contact the underside of the slates - condensation will definitely occur.
If the place was mine, I'd consider plastic soffit ventilators (the small round ones with the insect grille) at the bottom of each rafter, together with 3 or 4 ventilated ridge tiles. This way you would get good ventilation.
Its probably best for the roof pro's to step in now!
John

 

[SanToki]

I think your cross battening sounds fine if you don't mind losing some of the loft space
If the place was mine, I'd consider plastic soffit ventilators

As I say, I'll definitely get that looked into when the building chappy comes round next week, but I’m not sure yet how feasible that actually is due to the design/nature of the building – there is definitely no soffit/fascia arrangement on one side, and I'm not totally sure that the other has facility for them either. An alternative of vented tiles is obviously doable, but it would need a set for every rafter cavity…

I just thought the cross battening would be an easy solution as it’s effectively just building upon the system that is already in place and working (and done that way the insulation would actually be 100mm away from the tiles so is double the required distance). Yes it will mean some ceiling depth is lost, but it looks like that is unavoidable.

Cheers all, any more tips/advice more than welcome…

 

[SanToki]

I’m not sure yet how feasible that actually is due to the design/nature of the building – there is definitely no soffit/fascia arrangement on one side, and I'm not totally sure that the other has facility for them either.

Just got home and checked, and no there isn't any soffit/fascia on the other side either, so soffit vents are not an option

 

[Burnerman]

I guess vented roof slates is the way forward here, then.
John

 

[freddiemercurystwin]

It doesn't matter if there's a soffit or not you just stick a vent in above the gutter/below the tiles eg http://www.monier.co.uk/products/pr...ge/product/redvent-25-over-fascia-vent-2.html

 

[SanToki]

It doesn't matter if there's a soffit or not you just stick a vent in above the gutter/below the tiles eg http://www.monier.co.uk/products/pr...ct/redvent-25-over-fascia-vent-2.html[/QUOTE]
Now that looks interesting (I'm really not liking the idea of 16 vent tiles at £30 a shot!).
Would something like that be sufficient on it's own without higher up (i.e. ridge) ventilation?

 

[freddiemercurystwin]

No if you have an open roof and you are insulating/boarding the rafters there must be ventilation at top as well. Other vents are available eg http://cgi.ebay.co.uk/Roofing-6-Met...Ceilings_Walls_Roofing_ET&hash=item2c5b59a37d

This is ideally what you need to achieve but you can get similar results using tile vents near the ridge too.