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How much internal insulation do I need to apply to my solid walls?

Chris Brookman
Member Admin

Probably less than you’d think. Internal insulation boosts comfort much more than external insulation, for the same thickness, by isolating the huge mass of masonry from the interior. This allows the plastered surface of the insulation to warm up much faster and radiate heat back into the room, improving the sense of comfort experienced. As a rough guide, in terms of comfort, you need twice as much external insulation as internal insulation to provide the same level of comfort.

When using wood fibre insulation we usually suggest that a 60mm board provides the optimal amount of insulation from a cost-benefit perspective, although 40mm would be the thinnest and 120mm the practical thickest layer that we’d suggest. 60mm of insulation would typically give you a U-value of around 0.46 W/m2K on a brick wall.

Building Control often request that you target a U-value of less than 0.3 W/m2K but this is often without basis as the actual Building Regulations only ask for a material improvement and for the building fabric to be less than 0.7 W/m2K. This is on the basis that the cost of installing internal wall insulation is unlikely to achieve a simple payback of less then 15 years and that the original wall has a U-value of considerably more than 0.7 W/m2K. Therefore, 60mm of insulation would be a perfectly acceptable target.

To see some example specifications please click on the following links:-

40mm internal wall insulation

60mm internal wall insulation

100mm internal wall insulation

To see other specifications for external insulation, timber frame insulation, roof or floor insulation, please feel free to use our Specification Generator to create a specification that fits your situation.

These specifications will help you build spaces that are both warm in winter, cool in summer and quiet, even in urban locations.

Topic starter Posted : 09/03/2021 5:46 pm

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