How should this be constructed?
The first layer of Celotex fills the gapes between the rafters, and a second layer leaves a flat surface. But how do you fix the plasterboard on that? Do you try to remember where the rafters were, and use six inch screws, or is it recommended to fit horizontal battens onto the rafters, insulate the second layer between those, then fix to the battens?
Surely it's easy enough to mark the position of the rafters on the bottom and top edges of each slab, when you fit the second layer? And then transfer the marks to the plasterboard when you fit that? A chalk-line might be handy, too...
David
Roland Perry wrote on 17/05/2006 :
A nail at the top and bottom placed such that a piece of string run between them would be roughly in the centre of the rafter - will take care of that problem.
In message , at 19:20:23 on Wed, 17 May 2006, Lobster remarked:
Yes, although that leaves me with the six inch screws. But we have two votes for that, so thanks for the advice.
Why haven't you fixed PB to the joists, put thick insulation between, and thin insulation over?
Do you have soffit and ridge ventilation? Or is the roof contructed with a breathable sarking material?
In which case the usual technique is to leave a 2" air gap, then place
2" of insulation between the rafters (assuming they are 4" obviously), followed by say 25 - 35mm over the top.If you screw the celotex overcoat to the rafters then you can see the screw heads as a guide.
No need for battens - you will need longer screws - but *not* 6" unless you want to push the tiles off the roof! I used 60mm drywall screws for this.
See the second photo down:
In message , at 23:45:02 on Wed, 17 May
2006, Chris Bacon remarked:I'm not sure I understand the sandwich there. Are you saying it should go rafters-with-thick-insulation-between, then plasterboard, then thin insulation, then what?
Don't geddit - why 6"? Unless I'm missing something, you've only got 2" celotex plus plasterboard actually over the rafters, so you'd probably get away with 3"?
David
The message from John Rumm contains these words:
Seems to me the OP is intent on putting 4" of insulation between the plasterboard and the rafters in addition to some between between the rafters. Seems a bit like overkill to me unless he is living North of the Arctic Circle.
I mis-read the question. Cider played a part, as did general stupidity. Apologies. Ignore that post.
Not really. In most cases, it is required to meet building regulation u-Values. It also prevents cold bridging through the rafters which can lead to condensation, staining and mould on the rafter lines. However, I suspect the intended bottom layer will be less than 4". Normally, 50mm is quite enough.
Christian.
In message , at 06:35:04 on Thu, 18 May 2006, Lobster remarked:
OK, perhaps I'm exaggerating a bit. Five inches.
The rafters are 3", as is the celotex (two layers of). So if I used a four inch screw, then about half an inch would be in the rafter.
Why six inches of celotex? That's what I've been told is required by da building regs.
In message , at 09:06:53 on Thu, 18 May 2006, Roger remarked:
Three inches, but otherwise correct :)
Be careful on layer sizes. The bottom layer must not be larger than the top layer. i.e. if you have 75mm below the rafters, you need 75mm minimum between the rafters to avoid interstitial condensation. However, you may still need to maintain a 50mm air gap, which would require 125mm rafters (i.e. 5").
If you happen to have breathable membrane sarking, then Kingspan allow you to use no air gap at all according to their installation guide, allowing
75mm between and below the rafters only 75mm deep. I don't know if Celotex claim the same.Christian.
Where did you read that? I understood, and continue to hope, that interstitial condensation is prevented by a vapour barrier which in this case is the foil covering of the insulation.
Christian.
Thats for insulation between and *over* the rafters. The OP is insulating between and underneath the rafters. The Kingspan pdf isnt that clear but the comment about deep barge boards points to over the rafters. With the over rafter method some water vapour would be able to penetrate via the rafters, with under rafter method the water vapour path is completely blocked.
The previous version of the guidance suggested otherwise, but I'm actually inclined to agree that this might have been a mistake with the previous version and that your interpretation is correct and the newer guidance has been updated to reflect this. Unfortunately, at no point does the new guidance mention using more than 25mm below rafters, so it never actually covers the situation and it remains ambiguous.
Christian.
Have something to add? Share your thoughts — no account required.
Ask the community — no account required