The Loft Conversion UK
Guide To The Thermal Envelope

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When it comes to a loft conversion the UK building regulations are very specific about what is acceptable in regards to the thermal envelope of a property.

With a loft conversion the insulation of the new room is hugely important as it effects the energy performance of the property. And this, in turn, of course effects how much energy you are using at home and how much you are paying for it.

By converting the loft you are removing the existing insulation and adding new thermal elements, such as windows, that have to meet rigorous new standards which are all covered in the building regulations.

The Loft Conversion UK Guide To The Thermal Envelope

According to the building regulations a property is classified by its thermal envelope.

This means that anything that can leak heat, such as windows or the roof, are classed as thermal elements and are allocated an allowance of heat loss that they must not exceed.

Unfortunately there are complications in that there are different targets depending on whether or not the new project is renovating an existing part of the house or building something completely new.

With most loft conversions the building regs will recognise the project as a renovation if the existing roof remains unaltered.

If, however, as is usual, skylight windows are added than the they will be classed as new and will have to meet more exacting standards.

If your project includes a dormer than the structure and windows will be classed as new and if more than 25% of the roof has been altered, then that too will be classed as new.

All these points are covered in Document L of the building regulations and are really all about U-values.


The easiest way to explain u-values is that they designate how much heat loss is acceptable from the different parts, or elements, of the house. The higher the u-value the more heat is allowed to escape.

Some typical examples of required U-values are:

New dormer walls 0.30
New dormer roof 0.20
Renovated side wall 0.35

The reason u-values are of such importance, aside from having to comply with building regulations in the case of new loft conversions, is that every home now has to have an energy performance certificate(EPC).

The EPC needs to be produced when selling the house in the future so ensuring now that energy conservation and performance is at its optimum will be a good investment for the future.

For more info see the related pages below

Choosing A Builder For Your Project

The Foil Alternative

The Loft Conversion UK Guide To Converting Your Loft

Using Loose Fill Insulation In Your Loft

The New Polyisocyanurate Material

Using Foil Insulating Material

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