As there are a number of figures thrown around, relating to U-values in new homes or extensions it is worth a reminded of some of the figures used in current Building Regulations.
Building Regulations Overview
The Building Regulations cover all aspects of building a dwelling, including fabric heat loss and air permeability. They are periodically reviewed to upgrade standards, with a major overhaul every decade. Insulation standards were first introduced in the 1970s, with the standards of required energy performance dramatically increasing over the last 50 years. Building Regulations determine the energy performance of both new homes and any extensions to existing homes, which are covered in two documents:
- Part L1A, Conservation of fuel and power in new dwellings
- Part L1B, Conservation of fuel and power in existing dwellings
The current regulations are the 2010 Building Regulations, incorporating amendments in 2013 and 2016. The regulations allow some freedom for building designers to chose how to achieve the overall energy performance level.
Example: A design could chose to have better levels of insulation for the floors and less for the walls as long as that achieved the standard of performance required.
For new build homes, there are two levels for a designer to consider:
- Limiting Fabric Parameters: The minimum requirements for an individual building element. However if minimum standards were used throughout then the home would fail the overall TER (Target CO2 Emissions Rate).
- Concurrent Notional Dwelling Specification: A set of requirements for all building elements which together should meet the overall standard (the TER).
| Area | Limiting Fabric Requirement W/(m2K) unless stated | Concurrent Notional Dwelling Specification W/(m2K) unless stated |
| Roof | 0.2 | 0.13 |
| Wall | 0.3 | 0.18 |
| Floor | 0.25 | 0.13 |
| Windows | 2.0 | 1.4 |
| Doors | 2.0 | 1.0 – 1.4 depending on glazing |
| Air permeability | 10 m3/(h.m2) at 50 Pa | 10 m3/(h.m2) at 50 Pa |
In general, if a specification below that of the Notional Dwelling Specification is used, then this should set alarm bells ringing, as the shortfall would need to be addressed by a higher specification elsewhere.
Extensions
For extensions, then it is considered unreasonable to ask a dwelling owner to knock down and replace existing walls, although they can be asked to upgrade them. So the focus is on new thermal elements and the standards for these are:
| Area | New Thermal Element W/(m2K) unless stated |
| Wall | 0.28 |
| Roof | 0.16 – 0.18 depending on pitch |
| Floors | 0.22 |
| Windows & Doors | As part L1A |
| Air permeability | No specific requirement as this is a whole dwelling requirement. However approved construction details should be used. |
Existing thermal elements should be upgraded to an improved standard if they exceed a u-value of 0.7 for walls/floors or 0.35 for roofs. The improved u-value required is:
| Area | Improved U-value W/(m2K) unless stated |
| Wall-cavity insulation | 0.55 |
| Wall – external or internal insulation | 0.30 |
| Floor | 0.25 |
| Roof | 0.16- 0.18 depending on roof pitch |
Hi!
Our new build house has been built with a u-value of 0.19 for the roof. It was designed and built with rooms in the roof. Looking at your table, it seems like it should be different. The new build company are saying that it is within building regulations. Is it? What should it be?
Hi Katy,
When designing a house, two factors have to be taken into consideration. The first is the minimum standard of individual components (limiting fabric parameters). For the roof component this would be 0.2 and your roof u-value is within this. The second factor is the notional Concurrent Notional Dwelling Specification. This is an example mix of components that together ensure the house would meet the building regulations overall. It allows the designer to achieve the same overall energy performance result in different ways rather than specifying only one way of doing something. In your case the 0.19 u-value of the roof component would be compensated for by better values elsewhere (such as the walls, windows or doors).
Hope this clarifies
Kind regards
Jon
Interesting article on the complexities of thermal control in new housing identifying the considerable difference in performance required by the “Fabric limiting requirement” and the “Concurrent notional dwelling specification”- Central government needs to clarify these design parameters and take account of heat loss and gain from construction techniques that have inadequate natural ventilation provision for healthy living-
High levels of Covid19 transmission in confined poorly ventilated living conditions is highlighting this concern!!
I remember a quote from a traditional builder years ago saying “open the window and put another log on the fire” !