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Service cavity


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It's pretty straightforward to look up the effect of burying a cable in insulation. You may have go up a size to stay within safe limits.


I chose not to insulate the service void. Mine was only 25mm deep so it would have had a negligible benefit, and would have completed any future wiring runs.

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When you look at the benefits of anmdding insulation compared to an unventilated cavity, found it wasn't worth the effort or expense.


From build desk

The calculation method can sometimes produce the same result for a range of airspace thicknesses.

For example, the thermal resistance of air layers (i.e. undivided airspaces) with high-emissivity surfaces and a horizontal heat flow (the case for most unfilled cavities in external walls) is

0.18m2K/W for all thicknesses from 20mm to 300mm.


A mineral wool would give a 4 to 5 time better performance, a 25mm cavity with 0.032 mineral wool would add (excluding thermal bridging of wooden battens), 0.78 to your R value


The unvented cavity around 0.14.


Assuming your R value was already 8


Insulated U value ends up 0.114 (plus some for battens thermal bridges)

Empty U value ends up 0.123 (note the thermal resistance of wood and the air space is very similar, so little bridging to add.


So impact on U value is circa 0.01 or less 

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