Comparing Flat Roof Insulation Thicknesses G#8976

By January 7, 2016 February 27th, 2019 Q&A

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Comparing Flat Roof Insulation Thicknesses

Comparing Flat Roof Insulation Thicknesses


I was wondering if you knew of a solution which is thinner in depth than the required warm roof composition of 150mm wood, board and 140mm Celotex to meet building regulations on a very small single storey extension.


Thinness is essential and dictated by the surrounding construction and design objective.


Thinness for thinness sake is usually expensive and uses higher impact materials.

Assumptions based on question:

‘Wood, board’ is assumed to be dense wood-fibre board (whose k value is relatively poor compared with Celotex).

Interpretation of Question:

Celotex is PIR = Polyisocyanurate

GBE Jargon Buster 

  • A development on polyurethane where there is a slight difference in the constituents and where the reaction is conducted at higher temperatures.
  • PIR is more fire-resistant and has a slightly higher R value.
  • Applications include wall, floor and roof insulation. (GBE disagree with this, it needs development and clarification)
  • Non-renewable, high k value performance for thickness thermal insulation, global warming potential


Timber joisted flat roof construction?

What do we know?

Both materials can be:

  • fixed to top side of flat roof timber joists
  • fixed to bottom of flat roof timber joists
  • cut and fitted between flat roof timber joists with a reduction in overall thickness and reduction in performance due to bad fit

The easy answer:

  • Yes: Vacuum Insulated Panel (VIP) or Aerogel will always be thinner than other insulation materials
  • Both these alternatives are expensive, but the roof area is small so it may be worth spending money on the thinner materials.
  • VIP cannot be cut on or off site nor fixed with nails or screws.
  • One VIP manufacturer produces standards sizes.
  • Another flat roof VIP manufacturer makes bespoke sizes to suit.

The contradiction to the easy answer:

  • The building regulations Part L is only concerned with winter heat loss (k, R & U values)
  • GBE are also concerned about overheating caused by summer solar heat gain through the flat roof surface.
  • PIR will be good in winter and poor in summer
  • Dense wood fibre board can be good in summer and less good in winter
  • A combination of both may be good in summer and winter
  • VIP will be useless in summer and good in winter
  • Aerogel will be good in winter but GBE do not know how well it performs in summer

The complication:

The method of construction and workmanship may cause the application of these materials to not perform to the best of their abilities. 

Interpretation of Question:

  • Does the building regulations require the combination of the two materials to meet requirements?
  • Or are either capable of meeting the requirements separately?

Difficulty with the Question:

  • The two materials are found almost at the two extreme ends of the scale of k values.
  • See the following schedule from the Architects Journal (modified by GBE)
  • Calcium Silicate, Cellular Glass and Dense wood fibre are missing
  • Dense wood fibre will be at the top of this schedule.
  • The two thicknesses in the question cannot both be compliant with the Building Regulations Approved Document L
  • The two thicknesses of two materials in the question are not equal.
  • So it seems to be that both are needed?
Insulation thickness required to achieve a U-Value of 0.2 W/m2K
Dense wood fibre board
Cellulose fibre 235 mm
Glass wool fibre 220 mm
Stone wool fibre 210 mm
Extruded polystyrene foam 190 mm
Polyurethane foam with CO2 185 mm
Expanded polystyrene 180 mm
Polyurethane foam with pentane 150 mm
Phenolic foam 135 mm
Polyisocyanurate foam 130 mm
Phenolic foam with foil face 120 mm + Airspace
Polyisocyanurate foam with foil face 95 mm + Airspace
Aerogel 75 mm
Vacuum Insulated Panel (VIP) 25 mm

© AJ Architects Journal (GBE additions)

GBE wonder if the roof is shaded from the sun if so:
  • increase the thickness of PIR reduce the thickness of wood fibre board.
  • Replace some of the PIR with aerogel
  • Replace some of the PIR with VIP on top of the deck and ballast the roof membrane
If the roof is hit by the sun:
  • increase the thickness and use dense wood fibre
  • If you go thicker then other materials are suitable
    • Cork
    • Cellulose fibre
    • Cellular glass

Follow Up:

  • Having spoken to a Building Control Officer today they are happy to receive some technical information on a VIP or aerogel product which meets the same thermal properties of 140mm of Celotex and overall U value of 0.18 for the flat roof.


0.18 is more onerous than 0.2 in the table above so thicknesses will increase

Question 2

  • Any particular manufacturers you know of I could research options on their websites?


See GBE Links in right column

© GBE NGS ASWS BrianMurphy
aka BrianSpecMan
7th January 2016 – 9th January 2016

Comparing Flat Roof Insulation Thickness
See Also:

Having read through the GBE Q&A I must compliment you on how user friendly, informative and logical it was, well done.

Paul Bourgeois of Zero Carbon Britain, AECB & Greater Cambridgeshire Greater Peterborough Enterprise Partnership (LEP)

I am glad you liked it, thank you.
I respond in the context that an architect can be sued over comments made over the garden fence to a self builder digging out foundations.
So I know I have to be thorough: ask and answer all the questions around the first question.

BrianSpecMan of GBE

GBE Links

Vacuum Insulated Panels (VIP) – Green Building Store–kevothermal-vacuuminsulatedpa

Kevothermal vips insulation used in window reveals Green Building Store has just added bespoke Kevothermal Vacuum Insulation Panels to its range of …

Kevothermal Limited manufactures super Vacuum Insulation …

Vacuum Insulation Panels are manufactured by Kevothermal Ltd who have been producing VIPs in Shropshire, UK since 2006.

High Performance Vacuum Insulation Panels

EnviroHomes Vacuum Insulation Panels (VIP’s) out perform all conventional insulants to give an exceptionally thin insulation solution.

Vacuum insulated panel – Wikipedia, the free encyclopedia

A vacuum insulated panel (VIP) is a form of thermal insulation consisting of a nearly gas-tight enclosure surrounding a rigid core, from which the air has been …

Kingspan Insulation – Optim-R…/Overview.asp…

Kingspan OPTIM-R is a vacuum insulation panel (often called a VIP). It is an optimum performance insulation product which provides a high level of thermal …

SlimVac – High performance vacuum insulation panel

SlimVac – Vacuum Insulation Panels … Ultra thin insulation ranging from 10mm to 40mm; Achieves U-values upto 0.13 W/m2K*; Certified thermal conductivity of …

© GBE NGS ASWS BrianMurphy
aka BrianSpecMan
8th January 2016

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