Finishing off insulation - vapour barrier etc

cgray25

After several months (!) I'm now finishing off insulating our pitched roof with Kingspan - I've just got a couple of queries about the niggly parts I was hoping someone could help with.

Ideally I thought the kingspan should be running the entire lengths of the pitched roof, from the highest point of the ceiling right down to where it meets the soffits, but access to the lower points is proving tricky. (I've attached a diagram here: ). There are vertical support struts a couple of feet from the bottom - should I just board up to the struts, then perhaps board the vertical faces of the struts? Or would that minimise the effect of all the other insulation/be best to board down to the soffits if I can?

Also, there's a cavity wall just after the struts, there's quite a draft coming up the cavity - I gather it might be a bad idea to kingspan over this due to breathability (though I could be wrong), should I glass wool over the top of the cavity?

And finally, now that it's approaching plasterboard stage, is it advisable to put a polythene membrane over the kingspan before the plasterboard goes on?

TIA

latecomer

can you take a picture as its hard to imagine?

Is the draft not coming from the eaves? If so then you dont want to block this as its fundamental to preventing condensation/damp etc.

cgray25

Can do one better than that

Not got my camera handy but it's a chance to show off my paint 'skillz'

This image shows how the roof runs - the K shows where I've run the kingspan up to. The C shows the cavity and the vertical strut is marked in purple.

latecomer

Nice drawing

I'd certainly insulate down the vertical and out to the cavity at the back. I'm not sure whether you should go over the cavity or not though.

cgray25

Thanks latecomer, thinking about it boarding the vertical would create an internal 'pocket' which should be ok - that way I'm thinking the warm air will be kept in the room and if there is any draft coming from the cavity etc that should be kept out.

Ideally I'd board down to where the roof meets the floor but it's a nightmare getting in at it and I'm thinking the effect will be negligable...unless anyone can tell me otherwise.

mech_2

Definitely glass wool over the cavity. Reducing air circulation through the walls goes a long way towards gaining the full benefit of loft insulation.

macduf

What size are your rafters and what thickness kingspan are you proposing to put in? You need to allow a 2inch/50mm airgap between the tiles and the kingspan for air flow.
You need a polythene/visqueen over the entire "sloping" part of your ceiling and this means right down to your cavity if thats the case this is called a "vapour barrier" and prevents condensation.
A small "flat" ceiling at the top would be benifcial to you.
I would plasterboard the vertical struts kingspan in between the struts it would make your job easier and your suggestion is good to fill the top of the cavity with rockwool insulation.
Is it under building control?

cgray25

Thanks Mech.

Macduf - yes, I've left the 50mm air gap, using 75mm Kingspan. I had read about using vapour barrier sheeting - unfortunately I couldn't really sheet right down to the cavity because the vertical struts are before the cavity by about 1 foot and they get in the way. There is a horizontal bit of timber running across the vertical struts where they meet they roof. If you check the image linked above you'll see roughly what I mean/why it'd be tricky to insulate past the struts.

There is a small flat section at the top of the roof (it was partially converted by the last owner). I'd checked with the council - the conversion's got notes of comfort and, besides giving some info on insulation, they said I could basically do what I liked!

macduf

You dont have to insulate down to the cavity from the roof, if you kingspan the vertical struts and then there should be roof insulation on the small flat area leading up to the cavity. All is covered. Depending how keen you are there is a product out that comes in a roll and is around 25mm thick with insulation and a vapour barrier included you cover the whole area prior to putting your plasterboards on. Using this method gives your rafters some insulation, its not cheap but it would benifit your loft.

cgray25

Thanks for the info macduf - I did a bit of reading up last night though and it's left me a little confused, it said if your insulation is foiled on both sides (which mine is) then you don't need a vapour barrier. Having said that, I think I may just use some to 'finish' it anyway.

The other part that confused me was in relation to underfloor insulation - I've put some wool between the floor joists but that article said a vapour barrier would be required to stop rot etc. However, it said the barrier should be on the 'warm' side - I'm just wondering how that'd be done without lifting the floor? Kind of concertina it so that it goes between the joists then over/under them?