Ventilation gap between plasterboard and insulation

[KeilBrae]

I live in an old cottage where upstairs accomodation is in the roof. I do not want to loose headroom by applying blanket foil insulation to the rafters followed by 2x1 batons and plasterboard.

Therefore I am going to us either Cellotex or Kingspan (as thick as the space will allow) wedged in between the rafters followed by either foil backed plasterboard or insulated plasterboard.

What should the gap be between the plasterboard and insulation to allow ventilation?

Can I place the Cellotex up against the sarking boards (the house is old so I don't have any felt).

Are there big thermal advantages using a 20 mm insulation backed plasterboard (like the Kingspan Kooltherm) compared to a foil backed 12.5mm plasterboard? The Kooltherm is quite pricey which is why I ask this. Are there any alternatives available?

Many thanks in advance for your advice

 

[leewill]

you should lave a 20mm gap between the kingspan in the rafters and the sarking boards and a 20mm gap between the kigspann and the plaster boards.

 

[^woody^]

You will not need any air gap if you use celotex which fits tight up against the sarking boards and is tight against the plasterboard - ie fully fills the void between the rafters

The probelm is, can you rely on the tiles keeping all the rain out, or will some get in to the rafters?

Foil backed plasterboard is not for insualtion, its for use as a vapour check.

To avoid any patern staining of the rafters on the face of the plasterboard (a cold bridge caused by the rafters) it may be wise to have a continuous layer of insulation (12mm celotex or 25mm polystyrene) across the underside of the rafters before plasterboarding

 

[WillB77]

To avoid any patern staining of the rafters on the face of the plasterboard (a cold bridge caused by the rafters) it may be wise to have a continuous layer of insulation (12mm celotex or 25mm polystyrene) across the underside of the rafters before plasterboarding

Can I attach Celotex and plasterboard to the rafters using a single fitting? ie screw long drywall screws through platerboard, then Celotex into the rafters? If so this seems alot cheaper than Kooltherm.

 

[^woody^]

Use some gripfill or similar to stick the celotex to the rafters and use a couple of screws, or screws and thin large washer to temporarily attach the celotex. Tape the joints with aluminum tape to form the vapour-check

Then, again gripfil the celotex and stick the plasterboard up, but cross bond it with the celotex joints and then screw it every 200mm as normal