Insulating loft with mix of between rafters and existing between ceiling joists

Hi Victorian slate roof with gables. Existing 100mm glassfibre/rockwool between 5" ceiling joists then chipboard floor.
Reroofing and need to increase insulation for Building Control.
Easiest way to get near required insulation figures is to add between rafters 75mm solid PIR insulation with 25mm airgap before breathable roof felt and slates at time of re-roofing.
Was considering adding Vapour barrier between ceiling and existing 100mm glassfibre.
Still will not meet 0.16 U value "required" but hope to negotiate with BC on what is practical, economic and feasible in Victorian rood space. What issues are there in mixing warm and cold loft systems ? Any other economic suggestions etc ? Suggestions on how to negotiate with BC !
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Robert wrote:

I think you will have a problem. In order to get the combined effect of the two insulation methods there would have to be no air flow in the loft and you will then get a condensation problem. I feel you have to pick one method or the other and not combine them. As for negotiating with the BCO, You just need to produce credible calculations to show how your proposal meets the particular regulation.
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Being a little simplistic, can one not simply mount extra thickness of wood on the ceiling to accommodate the extra lagging there and then put the chipboard over it again?
I'm assuming there is not going to actually be an everyday used room up there, you just need a new roof and a floor to allow access from time to time. Brian
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On 29/07/2017 09:29, Brian Gaff wrote:

I was trying to avoid a major project in raising the existing attic floor ( which is already boarded) by 200 mm to accommodate 150mm of lagging and a airgap before the chipboard.
My fallback would be to use 90mm sheet PIR insulation across the ceiling joists and then put the chipboard backdown on top.
As the roof is coming off getting 75mm PIR put in for the cost of the materials between the rafters by the roofers is an "easy"option.
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Robert wrote:

Lay PIR on top of the boards, and maybe hardboard on top of that?
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On 28/07/2017 20:33, Robert wrote:

Put new layer of insulation across the loft floor at right angles to the joists.
If you need storage then build a false floor across (part of) the loft, raised above the new insulation.
Andy
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On 28/07/2017 21:56, Vir Campestris wrote:

+1
I've watched a couple of reroofings near me, where they put PIR between the rafters. It seems to be utterly pointless. The void will need ventilating somehow, and that will result in loss of heat.
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I don't think 75mm PIR foam between rafters is going to meet the current requirements.
Builders seem to think that breathable felt avoids the need for an airgap but this was not confirmed by my BC who actually asked for 30mm! My rafters are 225mm (chalet building) so not such a big problem.
I have a mix of warm and cold attic space. The warm space is 100mm PIR foam between the rafters with 37.5mm liner board (25mm PIR foam backing 12.5mm plasterboard) screwed to the timber undersides. Linerboard includes a foil vapour barrier which saves faffing about with plastic sheet and staples.
Cold space is 100mm PIR foam between floor joists and 100mm Rockwool over. Vapour barrier stapled to underside of ceiling joists before plasterboard.
As yours is not new build, I would have thought BC would be happy if you do the best you reasonably can?
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Tim Lamb

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On 29/07/2017 10:00, Tim Lamb wrote:

Agree but getting close. I need a R of 6.25 and the 100mm existing provides 2.27 . 75mm PIR is about 3.4. Anyone know what a lathe and plaster ceiling and 18mm chipboard is worth in insulation terms ? Its the issue of splitting the insulation between ceiling and rafters which I am concerned about. Do like to have a good understanding of all the issues before talking to Building Control !
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On 28/07/2017 20:33, Robert wrote:

UPDATE Spoke to actual BC officer (after submitting Notice and paying £108) - quite happy with fitting whatever solid insulationwill fit between rafters as long as breathable membrane and 25mm airgap. As long as the level of insulation is increased to whatever can be readily fitted using existing size rafters/joists not at all concerned that a U value of 0.16 will not be reached. Happy !
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On 02/08/2017 14:11, Robert wrote:

OOI, will you be sealing the gap between the eaves and the external walls?
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