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Warmshell Internal helps reduce your heat loss through solid walls, saving you energy and money.

Retrofitting as little as 40mm of our Warmshell Woodfibre insulation board will reduce the heat loss through your a typical solid wall by over 60%! But saving energy isn't as simple as just about making your house hotter; Warmshell Woodfibre passively manages the moisture in your walls, unlike insulated plasterboard or foil faced plastic insulation, to create a comfortable, healthy indoor climate and protect the fabric of your house. We've designed Warmshell to work with the physics of solid wall buildings, not against them - a method recommend by the UK Government as best practice.


Read on to find out about the U values and energy savings that our IWI system can offer, or return to the main IWI system page

"The lowest risk designs for internal wall insulation are consistent with the existing moisture strategy of the building. For solid walled (traditional) buildings...capillary active insulations (such as woodfibre) and vapour open adhesives and finishes shall be chosen."


Department for Business, Energy & Industrial Strategy, Retrofit Internal Wall Insulation Guide to Best Practice, 2021


U values are a number showing how much heat "leaks" though an element, like the floor, the roof or a wall.

The lower the number the better, but you can never reach zero. And remember that any internal insulation offers diminishing returns, each extra mm of insulation saves less than the previous one.

U values have other problems, so while you might be asked to reach a certain figure to comply with regulations, you must carefully consider other factors below...


    Warmshell Woodfibre Board Thickness
     No insulation + 40mm  + 60mm +80mm +100mm
9" brick wall U value 1.65 0.68 0.52 0.39 0.32
% improvement 0% 59% 68% 76% 80%
13.5" brick and a half wall  U value 1.34 0.61 0.48 0.37 0.31
 % improvement 0%  54%  64% 0.72% 77%
500mm limestone wall  U value 1.40 0.63 0.49 0.37 0.31
 % improvement 0%  55%  65% 75%  78%

What U value do you need?

When you insulate a house you will probably have to meet a certain U Value to comply with Building Regulations. In England and Wales these are given in Approved Document L, in Scotland guidance is in the Building Technical Handbook and in Northern Ireland in the Building Regulations Technical Booklet.

Different figures are given for new build and retrofit and in all cases the documents recognise that Internal Wall Insulation can have draw-backs and risks for old buildings, so they allow a degree of flexibility and compromise.

As a very general rule of thumb, if you upgrade the walls you should aim for a figure of U=0.70 or lower, The figure of U=0.30 is often quoted but should only be used where the payback and other risks like condensation are acceptable.

You can read a more in-depth explanation here

The problem with U values is that they don't consider thermal bridges or air leakage

When you include these in the design, it becomes apparent that with any internal insulation on a solid wall there is often little point in aiming for a U value lower than about U=0.30; lower figures look better on paper, but there is no real-world energy saving.

You're better off spending more effort on air-tightness and detailing, using our design guide and example detail drawings.