How Much Energy will you save using Warmshell Internal, the leading breathable Internal Wall Insulation system?

Internal wall insulation (IWI), like our Warmshell Internal system, is fixed to the inside of your walls. The amount of energy saved through the entire section of wall is measured using a "U Value" - a figure which tells you how many watts of heat are lost per m2 of wall. The lower the U value, the warmer your house will be. Here's some more information on what U values mean. 

Of course, different types of walls are more or less efficient to start with, and you can put different thicknesses of Warmshell Woodfibre on the wall. Our handy calculator below will help you assess how much energy your wood fibre IWI insulation board will save. If you need bespoke calculations please contact us.


solid stone wall u values


What are U values

Solid brick walls were common across Britain and Ireland until the end of the 19th century. 

There were 2 main types, a "brick and a half" wall (or 13.5" thick wall) and a "brick-thick" wall (9" deep). Both can be identified by "headers" in the bond pattern in the bricks


Solid brick walls can be made 60% warmer with as little as 40mm of Warmshell wood fibre board..

    uninsulated wall +40mm Warmshell wood fibre +60mm Warmshell wood fibre +80mm Warmshell wood fibre +100mm Warmshell wood fibre
9" solid brick wall
U value 1.67 0.68 0.52 0.39 0.32
improvement 0% 59% 69% 77% 81%
13.5" solid brick and a half wall
 U value
0.61 0.48 0.37 0.31
 0%  54%  64% 72%


Sometimes using insulation boards isn't possible, due to the shape of the wall, or the limited depth. You can also look at our alternative solutions: insulating using aerogel adhesive and blanket and Solo lime plaster; insulating using our lightweight, thermally efficient lime plaster and render, Lime Green Ultra; or insulating by casting lightweight hemp and lime using Lime Green Hemp Binder.

And of course, sometimes these products are used in combination to match the building; for instance thin aerogel insulation in the window reveals, but woodfibre for cost and moisture control on the walls. Remember to assess the moisture risk as well as your U value; sometimes too much insulation badly installed will cost you energy long term!

Solid stone walls are found almost everywhere and are much more variable than brick walls. But the basic method of insulating with Warmshell Woodfibre from the inside also works for stone as well as brick.

Solid Stone walls can vary in how warm they are depending on how thick they are, what type of stone they were built with, and how much mortar and air is present in the core of the wall.

     uninsulated wall  +40mm Warmshell wood fibre  +60mm Warmshell wood fibre  +80mm Warmshell wood fibre  +100mm Warmshell wood fibre
  300mm limestone wall  U value 1.80
 0.55 0.42
 improvement  0%  60%  69% 77%  80%
  500mm limestone wall  U value  1.45 0.63
 improvement 0%
 66%  74% 79%
  500mm sandstone wall  U value 1.50
 0.52  0.40  0.34
improvement 0% 56%
 65% 73%
500mm granite wall U value  1.45 0.63 0.49 0.37 0.31
improvement 0%  57% 66% 74% 79%



Other types of walls

There are a huge variety of wall types, which work in different ways. Warmshell woodfibre is designed for direct application to solid walls, for other types you will need expert advice.

Cob, daub and earth structures

Walls built of earth have a signficiantly different hygroscopic behaviour to brick or stone (clay grabs water and holds it). Insulating internally cools the wall down, changing the relative humidity of the wall permanently. No long term research exists on IWI on cob walls in the UK climate, and insulating them should be treated as highly experimental. Lime Green does not warranty or recommend installing IWI on earth walls.

Cavity walls

From the late 19th century onwards cavities between 2 leaves of brick became common. IWI can work well on these walls, but there are significant variations in the width of the cavity and how well it is ventilated, and how the leaves are tied together. Some walls may already have cavity insulation. Careful investigation of the wall to establish the exact design should be undertaken before IWI is considered and bespoke advice sought.

Timber frame walls

Historic timber frame wall, such as 'black and white' oak framed houses, can in many cases be succesfully insulated using breathable insulation like Warmshell wood fibre. Please contact us for further advice as these walls need special consideration.


What are U values and thermal bridges?

U-values measure the heat lost by conduction through an element (e.g. a wall or a piece of glass or a roof) in watts per m2 for every 1°C difference in temperature between the inside and out. So if your room has a U value of U=1 and is 20ºC inside and the outside air is 19°C, each m2 of wall will constantly lose 1 watt. If the outside air drops to 0°C, each m2 of that same walll will lose 20 watts. Change your U value from 1.0 to 0.5 and that 20°C temprature difference now only costs you 10 watts of energy per m2.

But walls don't consist of simple elements; thermal bridges "puncture" the insulation zone when you fit IWI (Internal Wall Insulation). For instance, a metal screw through the insulated board will leak heat quickly; joists and internal walls between rooms are very significant thermal bridges in IWI. These thermal bridges are a "point loss" and U values are too crude to measure them, instead y-values are used.

This gets complex, and to model it properly requires sophisticated software and time.

What many people, and even the Building Regulations, don't consider is that it is better to have a continuous layer of insulation, rather than just putting lots of insulated plaster board where it's easy to fit. Current regulations and green grant schemes reward very thick insulation installed badly, but this can lead to mould and long term rot in the uninsulated areas. Think of it like walking in the snow barefoot and hoping an extra hat will keep you warm. In fact 40mm of insulation board over all the wall surface in a room, including window reveals, will save as much energy as a 100mm board installed without insulating the reveals and thermal bridges. Your house won't get frostbite, but the condensation caused by rapid temperature changes from missing insulation in adjoing fabric can cause damp.

Warmshell Internal has detailed information on avoiding these issues; you can consult our design guide and our detail drawings.

For a comprehensive design with U values lower than about U=0.4, you can use SAP and WUFI software to work out how much insulation you need and where. These tools are very important if you are targetting low U values; for instance achieving Passivhaus standards (U=0.15 for a wall) with internal insulation is difficult and technically risky and will require extensive hygrothermal analysis of the building, the local weather and its design to see if it is even possible without causing long term damp problems.

Finally, it is important to realise that not all heat is lost though conduction; air leakage and drafts are a massive source of heat loss. Making rooms air tight is important, but you will need to consider ventilation. Fortunately, Warmshell Internal has low VOCs and naturally buffers moisture, so it is a healthier option for rooms with improved air tightness.


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