Laying loft insulation over cables (Ed.)

[MK22]

Hi all, I’m wanting to lay down some loft insulation over the boards in the loft. The boards don’t go right up to the edges, and I can see wires running under the boards from the edge. Obviously once they’re under the board I have no idea where they go (I didn’t lay the boards). I’m aware insulation should not be laid over wires, but is it safe to lay insulation over boards which is over wires?
Thanks.

 

[Wayners]

Insulation needs to be against the ceilings to insulate.
Also I've seen many crushed cable under loft boards

 

[Dereekoo]

Insulation needs to be against the ceilings to insulate.
Also I've seen many crushed cable under loft boards

Why is that, surely the air gap between the ceiling/boards/insulation act as a barrier to heat transfer in the same way that the air gap in cavity walls do. I would add that if at the perimeter of the boards the insulation isn't dipped down to run along the ceiling then this would be detrimental to the heat retaining properties of the air gap.

 

[Wayners]

If you put your hand under loft insulation in winter its warm.
Plaster ceiling is warm so no condensation forms as the dew point is high.
With your air gap that's not going to be warm, so you're ceiling will be cold and your going to get mould growing within week's.

 

[Taylortwocities]

If the wires in the loft are lighting then putting insulation over them should not be a problem. Where issues can arise is if cables are high-current (eg electric shower). Then it may be necessary to install thicker cable for that circuit.

 

[Dereekoo]

If you put your hand under loft insulation in winter its warm.
Plaster ceiling is warm so no condensation forms as the dew point is high.
With your air gap that's not going to be warm, so you're ceiling will be cold and your going to get mould growing within week's.

Heat migrating through the plasterboard warms the air gap, boards/insulation above the air gaps maintains the heat? Assume that if the OP existing cold ceilings do not have mould growing across them adding insulation above will not suddenly cause such an effect otherwise everyone insulating their loft beware. As an aside U value of ceiling with just plasterboard = 2.7 with 100 insulation/air gap/boards = 0.4

 

[Wayners]

@Dereekoo

The air gap would need to be air tight though? Any air gap will just blow cold air in lowering ceiling temperature.
They op is not creating a warm roof or cold roof.

I'm not sure what it's called as don't exist as a known way to insulate correctly.
What would this way of creating a insulated ceiling called?

 

[Dereekoo]

Call it ceiling insulation, as stated its not a roof so cold or warm roof nuances don't come into it and any water vapour passing through ends up in the roof/loft space to be taken away ( or not) as it would have been prior to any boards or insulation being fitted. Would agree the air gap needs to be stable ( could never be air tight) and not acting as a wind tunnel hence need for insulation to be dipped down to ceiling level at edges of boards to alleviate any wind tunnelling effect

 

[FrodoOne]

surely the air gap between the ceiling/boards/insulation act as a barrier to heat transfer in the same way that the air gap in cavity walls do.

Why would you think that an air gap would act as a barrier to heat transfer, in "cavity walls", or anywhere else?

As we all (should) know
"Heat can travel from the hotter to the colder - by Conduction, Convection and Radiation
and that's a physical law"

(Shades of Flanders and Swan.

and
https://mathshistory.st-andrews.ac.uk/Extras/Thermodynamics/ )

In any "air gap", heat can travel by both Radiation and, more importantly, by Convection - with Conduction being minimal.

However, while the Radiation might not be very significant,
in a large "air gap" Convection certainly will be significant!
The (warmer) "Convection" air currents will (eventually) come in contact with a colder surface and transfer the heat in them to it.
They will then sink down, to be re-warmed and repeat the cycle!

That is why "insulation" is installed in "Cavity Walls".
The main function of this "insulation" is to reduce, as far as reasonably possible, heat-transfer - by means of "Convection".
It also reduces heat-transfer by means of Radiation.

 

[Dereekoo]

Why would you think that an air gap would act as a barrier to heat transfer, in "cavity walls", or anywhere else?

As we all (should) know
"Heat can travel from the hotter to the colder - by Conduction, Convection and Radiation
and that's a physical law"

(Shades of Flanders and Swan.

and
https://mathshistory.st-andrews.ac.uk/Extras/Thermodynamics/ )

In any "air gap", heat can travel by both Radiation and, more importantly, by Convection - with Conduction being minimal.

However, while the Radiation might not be very significant,
in a large "air gap" Convection certainly will be significant!
The (warmer) "Convection" air currents will (eventually) come in contact with a colder surface and transfer the heat in them to it.
They will then sink down, to be re-warmed and repeat the cycle!

That is why "insulation" is installed in "Cavity Walls".
The main function of this "insulation" is to reduce, as far as reasonably possible, heat-transfer - by means of "Convection".
It also reduces heat-transfer by means of Radiation.

I guess like a brick wall or plasterboard or window or insulation an air gap acts as a "barrier" to heat loss/transfer. The effectiveness of the "barrier" differs for each material. Working out the U value of a 215 solid brick wall gives a value of 3.58, if I put the standard air gap thermal resistance value of 0.18 into the calculation the U value becomes 2.18 which for many years was the accepted norm for housing until it was decided to improve heat loss by installing insulation in the gap.