Water Vapour and heat constantly flow into and out of any construction and its materials this is commonly known as the building breathability, this includes the roof.


In most cases, warm moist air rises where it generally cools, as a result of this cooling, the air becomes denser thus reducing its capability of carrying water vapour, and this is where condensation is formed. The point at which condensation forms is also known as the ‘dewpoint’. The dewpoint can form within the fabric/materials of the roof construction; this is known as ‘Interstitial Condensation’.

‘U’ Value Calculations & Condensation Risk Analysis

Calculations are carried out in strict accordance with BS5250: 2011 Code of practice for control of condensation in buildings.

The calculations include;


  • The existing build-up and its current thermal performance, this will also show any condensation risk currently evident. If condensation is occurring within the existing build-up at a level which exceeds acceptable standards, a calculation will be supplied offering a thermal upgrade using the minimum thickness board size available to remove the risk.
  • Existing uninsulated surfaces e.g.: Felt over Concrete or Timber Decks/Asphalt over Concrete, etc., a calculation will be carried out to provide a ‘U’ Value to meet 0.18W/²mK or to meet the client’s requirements.
  • New or newly exposed roof decks, a calculation will be carried out to provide a ‘U’ Value to meet 0.18W/²mK or to meet the client’s requirements.

Note: A greater thermally efficient ‘U’ Value requirement will increase the thickness of the insulation; this could prove problematic in detail areas where upstand heights are limited. The inclusion of gutter channels in these situations will overcome such issues without affecting the thermal performance as this will be calculated into the design as required.

Refurbishment: The current roof build up may not comply with Part L Conservation of fuel and power of the Building Regulations, Approved Document L1B for existing dwellings which states that if renovation work is carried out then a thermal upgrade is required. This upgrade should improve the U-value to 0.18 W/m²K or better. This should be considered when refurbishing more than 50% of the roof area.