Victorian slate roof with gables.
Existing 100mm glassfibre/rockwool between 5" ceiling joists then
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
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 !
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.
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
This newsgroup posting comes to you directly from...
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.
I don't think 75mm PIR foam between rafters is going to meet the current
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
As yours is not new build, I would have thought BC would be happy if you
do the best you reasonably can?
Agree but getting close.
I need a R of 6.25 and the 100mm existing provides 2.27 . 75mm PIR is
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 !
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.
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