Thinner and better alternative to 150mm PIR in cold roof insulation

[Adsibob]

2 hours ago, Cpd said:

You had a plan - lowering the floor and building a green roof. 
that plan is now shot as you don’t want to lower the floor due to cost. 

you need to go back to the drawing board and start again as your now trying to get a square peg into a round hole. 
I think by starting to replan with the new restraints you could come up with a better option. 
Also take on board what Russell has said about the existing design not being best practice. 
I had a similar situation and had to go down the underpinning route..... 

Hi @Cpd and @Russell griffiths thanks for your feedback. The green roof was not my idea and was more or less imposed on us by a rather ridiculous local authority planning office. We live in a conservation area. This was effectively our fourth attempt to get planning having applied twice and appealed once before hand. We won't be applying again or starting again as this is quite a small aspect of the overall project and i'm not going to tell my builder to stop working and incur an additional 12-16 weeks rental costs at the flat we are living at to take a punt that the planning department might be more reasonable on this ocassion when they have demonstrated that they are complete morons over the last two and a half years.

 

The green rough is a very basic one that shouldn't weigh much (we are talking about an area of about 10 square metres, with the most minimalist greenery i can find), but I note your concerns and will raise this with my structural engineer. 

 

As for the leaves, all the nearby trees are actually evergreens so leaves won't really be a problem. I will have to schedule a yearly or twice yearly inspection of the roof, which I can probably do myself as that wall is only 2.4m high. 

[Temp]

Would they notice if the whole roof was accidentally one course of bricks higher?  I guess the neighbours might?

Edited February 4, 2021 by Temp

[Pocster]

17 hours ago, PeterW said:

@Adsibob These are VIPs - Vacuum Insulated Panels. 
 

https://www.kingspan.com/gb/en-gb/products/insulation-boards/insulation-boards/optim-r

 

They are bespoke and made to exact size

They cannot be cut, drilled or altered on site 

They have to be protected from damage by another product (Kooltherm In this case)

They are expensive - about 3 times the price of the same uValue per square metre of PIR. 
 

They are an insulation of last resort. Why do you need to reduce ceiling height ..? You are gaining possibly 15mm as the overlay panels go below the joists and the Optim-R goes between. If you have 150mm joists, you can fill to 125mm with PIR and then use 30mm of Kooltherm over the top to get the same uValue for about 35-40% of the cost of VIPs. 

I used vips in my ground level flat roof due to the minimal depth I had . The problem is they come in fixed sizes and obviously you can’t cut them ?

[PeterW]

So... make this a cold roof and feature the joists below the roof ..?? What is the span you need to cover..? Wall plate to wall plate ..??

 

Build up becomes joist,

12mm OSB

150mm bonded PIR

12mm OSB

Rubber membrane

geotex

geo mesh (30mm) with soil/ vermiculite 

sedum mat

 

Plasterboard goes on the joists direct, or even recessed slightly to show the joists.