Right - back to the theses.
Normally I'd agree,with you Superman, but adding 400-500 mm of quilted roof insulation may not be the answer.
Quite apart from anything else, it will tend to totally block the eaves insulation minimum 50mm air gap and cover services.
Any quilted insulation at first floor ceiling will be unworkable in terms of vapour checks if there are downlighters or badly fitted vents.
Deep insulation if this order really requires the services to be routed at high level to avoid damaging them and creates hazard underfoot.
Its one thing to have 150mm quilt between the 150mm joists, but with deep insulation you can't see where to place your feet in the attic.
Worse, if the services are above the level for safety they run a higher risk of freezing in the winter, which means this needs to be re-thought.
More than once I've seen 300mm of insulation stuffed into a space 150mm deep, the installer thinking they'll get the benefit of a 300mm insulation layer.
Quilted insulation 300mm deep must be installed to its uncompressed depth to work as designed, which is to trap a 300mm deep layer of air.
It works by preventing this air circulating, creating a composite material of air and glass fibres that doesn't transmit heat well.
I have seen insulation stuffed in to tightly that it had no chance of working as intended - nearly "solid" in fact..
If you don't provide 300-400mm depth to install it, don't install it and crush it - that's just money wasted.
In my opinion its far better to install HD boards following the line of the roof - and don't crush them either!
I would strongly suggest doing the hard work and installing these between the sloping roof rafters, leaving the 50mm gap behind venting eaves and ridge.
This is more difficult to do, and in particular to do well, but it means that the entire attic is kept warm, significantly reducing concerns about freezing pipes in the winter.
You do have to think more carefully about service routes and bathroom ventilation, but that will probably need to be routed via the Mechanical Ventilation and Heat Recover system these days.
You can come along later and install a plaster finish and vapour check to more HD insulation boards to the face of the joists, building up a really good level of insulation and air tightness.
With sealed breather paper to the outside of the joists, the house will be wind tight, but not vapour tight - it can breath but you won't be blown out if it.
There are several firms offering building systems which will do all of this, and you should take advice from your architect in relation to this work and the benefits of it.
You should install upgraded highest floor joists at this time - the increased material costs will be minimal and will pave the way for a future attic conversion.
This is yet another reason to install insulation following the line of the roof, it means the work of insulating the attic level is done once.
Here are the regulations you must build to by law
http://www.environ.ie/en/TGD/
Here is the new Part F
http://www.environ.ie/en/Publications/DevelopmentandHousing/BuildingStandards/FileDownLoad,1647,en.pdf
See Diagram 11 "Ventilating Roof Voids" , Section D "Ceiling Following Pitch of Roof" P. 28
Can I also suggest you look around AAM for helpful articles, both here and in the Homes and Gardens forum and also the askaboutlaw forum.
Can I also respectfully suggest you take a scan over the Self-Build FAQ in case there is information there which might help your endeavour.
http://www.askaboutmoney.com/showthread.php?t=126261
ONQ.
[broken link removed]
All advice on AAM is remote from the situation and cannot be relied upon as a defence or support - in and of itself - should legal action be taken.
Competent legal and building professionals should be asked to advise in Real Life with rights to inspect and issue reports on the matters at hand.