Fitting insulation board.

I posted this in the Plastering forum in error, here it is again.

Hi, I was away for a few days and my son-in-law started fitting the insulation boards upstairs in the extension.
I had intended cutting the boards level with the top horizontal timber and infilling the vertical timber frame but have to admit his way looks more logical.
Is one way "better" than the other?. There is a sheet of 80mm in place with another sheet of 50mm still to be added.

Also is it easier/better to plaster the original outside roughcast wall or to strap and plasterboard it?.

 

Looks like your going to be heating that nice big void behind the stud wall.I personally would have gone with the cheaper and easier method of insulation
to the stud wall and good old mineral wool above the joist area which is as "cheap as" at the moment.
Unless you are going to use the space for cupboards etc then forget the above.
But your halfway there now so carry on
I think I would battern the wall out and plasterboard it, has it been painted ? or could you just dot and dab plasterboard to it, why not have your plasterer take a look

 

How much gap have you left above the insulation between the rafters? I think it should be at least 50mm to prevent the rafters rotting.

 

I think this depends on the kind of felting that is under the tiles. With a modern breathable membrane, you might not need an air gap. Likewise if no felting at all. With old-style impervious felting, you do need a 50mm air gap, with some means of ventilation, e.g. soffit vents at the bottom, vented ridge tiles in the void above.

This is based purely on research for DIY insulation of my bungalow when we had dormers put in. Because we have 3x2 rafters and needed 5" of Celotex, I battened them out to 6x2, and out 4" between and 1" over.

No doubt a proper builder or BCO will be along in a minute with better advice

Cheers
Richard

 

Being a stormy area of Scotland, the roof has sarking with tyvec under tiles.
The spec is for 130mm insulation board between pitched rafters with a minimum air gap between board and sarking of 25mm.
250mm of loft insulation between/over the ceiling joists.
I decided to cut off the lower parts of the boards and do it my way, (cue for a song!).
Cheers all.

 

Geraldthehamster, good practical use of insulation, as prescribed by Kingspan, and remembered the air gap. Info; Scotland mostly uses a ply wood covering onto rafters called sarking, then vertical lath up the rafter called counter battening followed by felt, then normal horizontal lath then tiles. Producing a very stable roof structure.

 

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