Adding extra insulation to the wall service-void?

[Dreadnaught]

I have my SAP calculation for my forthcoming bungalow and all generally looks good. The roof and floor have U-values of 0.08 W/m².k. However, the walls are a bit low, about 0.18 W/m².k. I did this on-purpose to maximise floor-space but it has turned-out a bit lower than I expected so I am thinking of adding an additional layer of insulation inside the the airtightness layer of say 25mm or 50mm of PIR.

 

I was planning a 25mm service void anyhow. 

 

My questions:

1. Could I just fill that service void with sheet PIR everywhere except where services run? That would do the job without stealing more floor area.
2. I am aware that electrical cables may need de-rating if they are insulated. To counter this, could I just leave, say, 50mm air-gap each side of a cable run?
3. Are there any other problems with largely doing-away with a service void by filling it with insulation?

(For service runs, as a bungalow with a warm roof I will have 200mm of open space between the metal-web joists in the flat roof).

(The external walls will be 240mm I-beams at 600m centres full-filled with blown-cellulose/Warmcel.)

(The airtightness layer is Pro-Passiv Smartply.)

(I am aware of condensation-risk analysis and I am confident that adding such a small amount of extra insulation won't cause a problem).

Edited March 6, 2021 by Dreadnaught

[willbish]

My first thought is that sounds like a significant amount of work/hassle for a very minor improvement in performance.

 

Cutting in the PIR is a job you wouldn't be able to finish until all your services are in place. By which time you'll be itching to get the cavities closed and move on, not cutting tiny bits of insulation.

 

I'm not that familiar with timber frame constructions but if you want to increase the insulation wouldn't it be better to put a continuous layer before the service cavity.

[westcoast]

I'd like to know the answer to this too,

Im planning on doing away with the service cavity and use 50mm insulated plasterboard everywhere, except where services run.. 

[Dreadnaught]

Yes I came to the same solution too. 

52.5mm seems to be a standard depth for insulated plasterboard: 12.5mm plasterboard, 40mm of PIR insulation.

Looking at YouTube videos, its seems easy to chase-out channels for cables, back boxes, etc., so you can use it even in areas where you have services.

 

I am not yet at the stage of installing plasterboard but am thinking of using this when I do.

[Jenki]

On 30/05/2022 at 15:44, Dreadnaught said:

Yes I came to the same solution too. 

52.5mm seems to be a standard depth for insulated plasterboard: 12.5mm plasterboard, 40mm of PIR insulation.

Looking at YouTube videos, its seems easy to chase-out channels for cables, back boxes, etc., so you can use it even in areas where you have services.

 

I am not yet at the stage of installing plasterboard but am thinking of using this when I do.

It's easy to chase out the insulation when your looking at the back of the board. But to first fix, and then start chasing out the boards to suit before you fit them will end up a PITA.

[saveasteading]

Our groundworker (come general builder)  says that we can't use eps backed plasterboard, even with a service gap, unless the cables are in ducts. 

I doubt this but will check it out, unless anyone here knows already.

[Iceverge]

sounds familiar, something to do with the eps and the plastic coating of the cables. 

[Miek]

 

[AliG]

PIR backed plasterboard is fine, it does not have the same issues with cables that EPS does.

 

The issue to watch for is allowing for the cables not to overheat. This is probably a question best put to your electrician. As I understand it, they normally want the cables attached to something that is a better conductor than insulation to help keep them cool. If not the cable sizes have to allow for the fact that they may get warmer. TBh I think this is not normally an issue in a new house with proper cable sizes, but the electrician should be able to tell you.

 

I would look at your plans and see how many actual cables and pipes actually need to go in the outside layer, I found it was very few. If I put my mind to it I could have put almost every socket on an inside wall.