Pitched roof insulation concerns - help please...

11 Dec 2013
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United Kingdom
Hi all,
I have a problem with my Loft Conversion (have just bought the 1920's terraced house and the LC was already in)
Its freezing up there, due to the fact there is ZERO insulation, just plasterboard.
I am now in the process of insulating the pitched roof, however I cannot find a solution to the problem:

*The construction is made up of (from outside in)
1-Roof tiles
2-Roof tile battens (unsure of size at this time)
3-NON breathable membrane between rafters and tile battens
4-Roof rafters (2"x2") every 400mm
5-Roof A frame sitting central to the house (doesn't penetrate into void currently)

I understand the plasterboard has to go and new insulation has to be fitted.
However I am unsure how or what insulation I should be fitting and where? Do I need to push up 50mm solid Celotex/Kingspan flush up against the non breathable membrane sitting between rafters? Or should I try and maintain an air gap/ventilation of 50mm and just sit insulation board against the whole underside of the rafters? (there is no venting where the two roof faces meet on the apex) so there will be no X ventilation.

Space is an issue here as there is not much head height.

Is it worth leaving a vent space (50mm) between the non breathable membrane and the insulation to avoid condensation build up?

Or should I just go ahead and install 50mm Celotex between rafters flush against membrane and cover with an additional 20/30mm Celotex sheet over the whole space to prevent thermal bridging and finally provide a 100% sealed vapour barrier before plasterboard?

I would also like to mention, I have a rather expensive white 100% waterproof paint which I plan on using for the final coat to provide an additional backup vapour barrier.

Any and all comments welcome!!
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You must retain an air gap, ideally of 50mm. This air gap, to be effective must be open to the atmosphere at the eaves and the ridge, so as to allow any moist air to be sucked out.

The construction for a loft conversion carried out nowadays would call for 100mm Celotex between the rafters and a continuous layer of 40mm beneath, tape all the joints with a foil tape, then plasterboard.

However as space is at a premium then I would be looking at fitting something like a continuous layer of 75mm beneath the rafters at least this will go some way to keeping the place warmer than present. Then add your plasterboard after taping the joints. If your rafters really are 2x2 I would consider consulting a structural engineer to check they will cope with any additional loadings.

The ventilation at ridge and eaves is essential to ensure the timbers remains rot free.
Thanks for the reply.

I understand the 50mm vent gap, and that the regs call for it. However, is there a requirement for the gap if the possibility of moisture entering into the void is minimal to zero?

As imentioned, there is a NON brethable membrane currently in place, are you suggesting I remove this and somehow repla with a breahable mambrane? Obviously doing this would require the tiles to be removed. Sort of negating the point due to costs.

Being sensible, there could be room for 70mm total insulation board without sacrificing too much head room.. but 50mm of this would need to be sat between tne current rafters.

As mentioned the house is old, and sat half way up the A frame is a 4x3 90 degrees to the a frame sat into the wall either end, this supports the 2x2 rafters. Sorry forgot to mention that bit.

If I have to maintain a 50mm gap then I will have no choice but to scrap the idea of rennovating my loft conversion..
Frankly your post only underlines your misunderstanding as to why the vented gap is required, it is to remove vapour that gets through the insulation not through the felt. Being sensible and all!
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Ok, thanks.
If all joints/penetrations on the insulation board are sealed with silver aluminium tape, and then a waterproof acrylic paint over the plasterboard, wouldnt this prevent all moisture from rising up into the cavity removing the need for a vent gap?
Logically there are barriers that ought to prevent anything entering the insulation from below, inevitably though the air gap will be (slightly) warmer than the outside (that’s the point of the insulation right?) and as that (slightly) warmer air touches the felt, any vapour cannot get through, so condenses against the felt. And indeed if really cynical one could argue that if some vapours can get through the VB under the insulation then can it get through the felt too? However history tells us otherwise, that lack of ventilation encourages timber rot. At the end of the day you must make your own decision but my advice is as above, a ‘cold roof’ system, is ventilated above the insulation and will be a healthy roof because of it.
Why not strip off the old plasterboard and then fix some gen x muti foil insulation, fix with a tacks or stapler on the rafters then counter batten with a 2x1 batten and plasterboard,or use a insulation plaster board if you want,it's an old building and you can only do so much on the regs stuff,if head height is at a premium that's what I would do,oh and maybe add afew slate vents in roof or even better dry ridge system
ok, I didn't realise this stuff existed!

Do you mean pin it accross/under the rafters then plasterboard on top?
ok, I didn't realise this stuff existed!

Do you mean pin it accross/under the rafters then plasterboard on top?

Yea tack it to under side of rafter and then put a 25x50mm batten on top that then plasterboard,leaves a 50 mm air gap between felt and foil and a 25mm gap between foil and plasterboard
Cool, that makes sense.
What thickness would I need for the foil insulation?
Yes, after closer inspection, it seems this insulation is to be used in conjunction with another insulation.

Back to square 1 :'(

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