Insulation in service batten area

[colin7777]

The timber frame company have included 25mm service batten on the walls, has anyone put insulation sheets in the service batten area where no services have been run or even in where services run.

 

If is can be done without causing any problems with condensation etc, what type of insulation and does there need to be space left where electrical cables are run.

 

Colin

[Crofter]

Firstly, I would say speaking from experience a 25mm service void is absolutely the minimum size. If you have space to make it any bigger, say 37.5mm, then it will make life much easier when it comes to running cables and choosing back boxes etc.

IMHO insulating the service void is of marginal benefit- it is only 25mm, and it cannot obviously be continuous, so there will be significant gaps and bridging due to the battens and cable runs. It also puts insulation on the wrong side of the vapour barrier. It would be easier and more cost effective to simply add 25mm to your main insulation layer, but if you are very tight for space then yes it can be done.

 

For the cables, it's a fairly simple matter to spec them to be laid within insulation, it may put some of them up a size as they will run hotter.

[Redoctober]

Yes I agree with @Crofter - we had additional insulation to install once the TF had been erected. This went on prior to the vapour barrier being fitted and then the service batons went on thereafter - hopefully these images attached will assist - the first shows the TF having been erected and the second one will show the additional insulation having been fitted, then the vapour barrier fitted and sealed. The service batons are then fitted  followed by the plasterboard.

 

[CC45]

I put insulation in.  Its its own vapour barrier isn't it?

[Ed Davies]

For small gaps insulation makes less difference than you might think as the air gap is insulation itself, particularly if you've got a reflective surface on one or both sides to reduce radiative heat transfer. There might be better uses for the money spent on any insulation there, maybe more insulation in the roof or something.

[Jeremy Harris]

6 hours ago, Ed Davies said:

For small gaps insulation makes less difference than you might think as the air gap is insulation itself, particularly if you've got a reflective surface on one or both sides to reduce radiative heat transfer. There might be better uses for the money spent on any insulation there, maybe more insulation in the roof or something.

 

 

Yes, there's an approved method for approximating the λ for horizontal heat flow in a non-ventilated cavity in BS EN ISO 6946,  λ = 0.00858 + 5.518 x d W/m·K, where d is the gap thickness in metres. 

 

If anyone wants to determine the basic U value for any given thickness of something (ignoring surface thermal resistance) then just work out the thermal resistance, R, from thickness in m / λ, then U value = 1 / R

[Ed Davies]

3 hours ago, JSHarris said:

there's an approved method for approximating the λ for horizontal heat flow in a non-ventilated cavity in BS EN ISO 6946,  λ = 0.00858 + 5.518 x d W/m·K,

 

For gap widths of greater than about 10 mm this gives a resistance of about 0.17 m²·K/W (increasing only slowly as the gap width increases) which is equivalent to only about 3.7mm of PUR. If that's right, I take back my comment about putting insulation there not making much difference.

 

However, BS EN ISO 6946:1997 apparently gives a surface resistance for horizontal heat flows indoors of 0.13 m²·K/W and BRE Digest 108 says 0.12 m²·K/W. With two faces involved that'd be a resistance of at least 0.24 m²·K/W for the gap plus any contribution from the air itself. Additionally, a foil surface ought to have a higher surface resistance - I'd assume about 1.8 times but I've not seen any references discussing this. So I'm not entirely convinced.

[andyscotland]

On the u-value calcs ecotherm did for me, they rated a 25mm horizontal unventilated cavity with their foil face one side (plasterboard or blockwork the other) as 0.665 m²K/W. Fitting a separate polythene VCL against the insulation reduced that to 0.184 so the low-e facing definitely makes a big difference.

 

Not entirely sure where they get their figures as I'm conscious the 0.665 is quite a lot above the guides in BRE 108.

[Jeremy Harris]

42 minutes ago, Ed Davies said:

 

For gap widths of greater than about 10 mm this gives a resistance of about 0.17 m²·K/W (increasing only slowly as the gap width increases) which is equivalent to only about 3.7mm of PUR. If that's right, I take back my comment about putting insulation there not making much difference.

 

However, BS EN ISO 6946:1997 apparently gives a surface resistance for horizontal heat flows indoors of 0.13 m²·K/W and BRE Digest 108 says 0.12 m²·K/W. With two faces involved that'd be a resistance of at least 0.24 m²·K/W for the gap plus any contribution from the air itself. Additionally, a foil surface ought to have a higher surface resistance - I'd assume about 1.8 times but I've not seen any references discussing this. So I'm not entirely convinced.

 

 

I'm not entirely convinced, either!  It is the approved way of dealing with unventilated cavities in walls, though, I believe.

[ADLIan]

Jeremy - couple of numbers transposed in the BS EN number should be 6946. Also probably easier to remember that an airspace of between 25-300mm, horizontal heat flow and high emissivity surfaces each side (covers most airspaces you're likely to encounter) has a resistance of 0.18 m²K/W. Will be slightly lower for smaller airspaces.

 

Ed - the surface resistances only apply to inner and outer most surfaces of the wall (or roof, floor etc) and are not included in an airspace resistance.

 

Andy - a low emissivity surface does make a BIG difference to the airspace resistance - Ecotherm number looks correct. It will also affect the surface resistances but it is not often that you have low emissivity surfaces internally and/or externally in standard construction. Question to ask is does the Low E, bright, shiny surface stay bright and shiny in the long term!

[Ed Davies]

3 hours ago, ADLIan said:

Ed - the surface resistances only apply to inner and outer most surfaces of the wall (or roof, floor etc) and are not included in an airspace resistance.

 

What's the difference, from a physical point of view? I'd have thought they'd be much the same except that in a smallish closed gap there'd be less opportunity for convection so the surface resistances would effectively be a bit higher.

[ADLIan]

Ed.

 

Read the BS you refer to (there is a more recent version) for surface and airspace resistances and their definitions - values are tabulated but calculation methods also shown. As far as I am aware BRE Digest 108 was withdrawn many years ago (I remember using it at college), the basic principles still apply however but some of the values have changed.

[scottishjohn]

If it was me 

I would swop plaster board for foam backed type --then you have  insulation and keep you service void +extra sound proofing as well

use 37.5mm insulated plasterboard and you get 25mm of insulation 

sounds to me like the TF company are trying to keep to abosulte minimum frame thickness ,or it would just be deeper in the frame and could have thicker insulation behind the service void  to get spec you  need

140mm frame  ,rather than 170mm frame ?

 

Edited August 10, 2019 by scottishjohn