Comparing Flat Roof Insulation Thicknesses Q+A

GBE > Encyclopaedia > Code > Q+A > G#8976

Q+A Questions & Answers

About:

Question:

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.

Assumption:

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

Statement: 

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

GreenSpec Jargon Buster 

• POLYISOCYANURATE (PIR)
• 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 some of this, it needs development and clarification, see below)
• Non-renewable, high k value performance for thickness of thermal insulation, global warming potential

Assumption:

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 and middle of the road 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?

© AJ Architects Journal (GBE additions)

© GBE GBC GBL NGS ASWS Brian Murphy aka BrianSpecMan **
7^th January 2016 – 3rd January 2023

See Also:

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Green Building Calculator GBC

• GBC V2 has over 150 insulation materials, formats and densities tabulated
• By choosing between regulations, design standards or campaigns users set their U value targets
• Users can see the required thicknesses of each insulation material in each building element

• By switching targets users instantaneously see different thicknesses required

• 

  GBC UValue Etc V2 A16 BRM 210121

   

• Summary tables display: in use energy demand, carbon and costs; embodied energy, carbon and sequestered carbon

GBC Elements Summary V2 A15 BRM 240920

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GBE Links

Vacuum Insulated Panels (VIP) – Green Building Store

www.greenbuildingstore.co.uk/page–kevothermal-vacuum–insulated–pa…

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 …

www.kevothermal.eu/

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

High Performance Vacuum Insulation Panels

www.vacuum–panels.co.uk/

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

https://en.wikipedia.org/wiki/Vacuum_insulated_panel

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

www.kingspaninsulation.co.uk/Products/Optim-R/Optim…/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

https://www.marleyeternit.co.uk/Slimvac/SlimVac–Panel.aspx

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 GBC GBL NGS ASWS Brian Murphy aka BrianSpecMan **
^8th January 2016 – 3rd January 2023

Comparing Flat Roof Insulation Thicknesses QA G#8976 End.