ICF'ers - Service void or no service void. That is the question....

[Mulberry View]

Our Architect drew a 50mm "Service Void". Now, I know it's not strictly needed with the ability to chase stuff in to the Polystyrene, but what have you done?

 

We have a Posi FF and roof, so plenty of service scope there.

 

I haven't got my mind off running an internal skin of PIR (25/50mm) instead in an attempt to lift the "weak" 0.24 u-value of the Nudura blocks, but I wouldn't do both the PIR and the void.

[Nickfromwales]

16 minutes ago, Mulberry View said:

Our Architect drew a 50mm "Service Void". Now, I know it's not strictly needed with the ability to chase stuff in to the Polystyrene, but what have you done?

 

We have a Posi FF and roof, so plenty of service scope there.

 

I haven't got my mind off running an internal skin of PIR (25/50mm) instead in an attempt to lift the "weak" 0.24 u-value of the Nudura blocks, but I wouldn't do both the PIR and the void.

Do you not have the XR35 block?

[Mulberry View]

5 minutes ago, Nickfromwales said:

Do you not have the XR35 block?

 

No, my build was spec'd in 2021 using their standard blocks. They advised us at the time not to bother with anything other than standard.

[Nickfromwales]

30 minutes ago, Mulberry View said:

 

No, my build was spec'd in 2021 using their standard blocks. They advised us at the time not to bother with anything other than standard.

Bum steer.....  

 

What's your AT score, or target AT score?

[Mulberry View]

27 minutes ago, Nickfromwales said:

Bum steer.....  

 

What's your AT score, or target AT score?

 

Not got it yet, but I really am hoping for 0.5.

 

The windows and most doors are Norrsken P31A which are capable of it apparently. The roof is a monopitch warm roof that has minimal opportunity for leakage. We'd said that if we don't hit our desired ACH that we'd consider Aerobarrier. I'm not planning to let that make me lazy though!

[Conor]

We fitted 100mm EPS on the outside to get our standard amvic blocks to 0.15. it's a LOT of work!!! But relatively easy. If you do PIR inside, you lose space and make fitting plasterboard more problematic - no guide lines for screws and screws will have to be something like 100mm+

 

We never did a service void, spark was happy to chase out with a hot wire cutter. Nothing else is really run in the external walls

 

[Nickfromwales]

On 18/09/2026 at 21:27, Mulberry View said:

 

Not got it yet, but I really am hoping for 0.5.

 

The windows and most doors are Norrsken P31A which are capable of it apparently. The roof is a monopitch warm roof that has minimal opportunity for leakage. We'd said that if we don't hit our desired ACH that we'd consider Aerobarrier. I'm not planning to let that make me lazy though!

Airtightness will trump insulation, and in winter, that, coupled with MVHR, will give you a plenty healthy-enough ROI on the house you've built, and then live in.

 

Adding IWI will be expensive, hard to execute perfectly (to achieve that 'goal'), and will impact on 1st & 2nd fix and GIA.

 

Leave it alone, you'll be spending a grand to save a hundred. Pointless (in actual, real life).

[Russell griffiths]

Forget fitting any additional insulation it is 100% not needed, forget the u value of nudura blocks for some reason it just looks poor on paper, they don’t add additional insulation in Canada you certainly won’t need it. 
 

you will wonder why you fitted a heating system at all once you move in, sort the airtightness and heating will be the least of your worries. 

[Russell griffiths]

We chased in and fitted conduit, loads of messy work, but I feel it makes the plasterboard feel more solid, by fitting the boards directly to the icf without a void. 
if you use sodal squirty foam behind the boards it bonds directly to the icf as well as screwing. 
 

 

[Susie]

Chase in the ICF with a hot knife, then plasterboard straight onto ICF, screw and adhesive when around windows if you have less fixing points at openings. 
If you have any sloped ceiling then think ahead if you need electrics or other in these areas. See our blog. We have no service voids but in the vaulted lounge/kitchen we used the roof rafters as they where deep we have insulation between rafters (rock wool) and underneath PIR so could chase into this. 

 

[Mulberry View]

1 hour ago, Susie said:

Chase in the ICF with a hot knife, then plasterboard straight onto ICF, screw and adhesive when around windows if you have less fixing points at openings. 
If you have any sloped ceiling then think ahead if you need electrics or other in these areas. See our blog. We have no service voids but in the vaulted lounge/kitchen we used the roof rafters as they where deep we have insulation between rafters (rock wool) and underneath PIR so could chase into this. 

 

 

Thanks @Susie, very helpful. I'll check your blog out.

[BotusBuild]

Chased in with a hot knife - I posted a video on here when I did it.

 

Most services are in our web joist ceilings.

 

We also have the standard 0.24 U blocks. We didn't go bonkers with AT and got a 1.8 result. I've since blocked up some major known leak points but can't be bothered with a retest. Our heating was not on much last winter.

[Nickfromwales]

On 19/09/2026 at 10:46, Mulberry View said:

 

Thanks @Susie, very helpful. I'll check your blog out.

Just remember you can’t run cables directly in EPS or other insulation, you MUST use containment such as conduit / trunking etc; that’s why “service voids” exist   

[Bancroft]

Doesn't really help you make a decision one way or another but I'm expecting to use a combination of chased runs and some battened areas.

 

My thoughts:

 

Chased runs should make the build quicker, and will reduce the materials/labour that would be needed if all walls were battened and then plaster boarded.  Granted, some of that cost might still be needed for conduits as @Nickfromwales pointed out but overall there should still be a saving.

I don't think the reduced insulation in areas where the chased cables are mounted will be sufficiently great as to impact the EPC/thermal value of the envelope.

Not battening all walls religiously gives more space internally.  100mm/4" isn't much but it's a lot to some...😳

 

Having said that, I'm keeping my mind open to whether some areas might benefit from having battens behind the plaster board.  For example if there are a lot of ethernet cables coming down a wall into a server/switch area; perhaps even where UFH pipes are emerging into a manifold.

 

Remain flexible.

[BotusBuild]

Here's that post I made reference to above

 

[Susie]

3 hours ago, Nickfromwales said:

Just remember you can’t run cables directly in EPS or other insulation, you MUST use containment such as conduit / trunking etc; that’s why “service voids” exist   

Depends on the size of room one cable in the far side of open plan room is better in conduit for our build than making a service void in all of vaulted ceiling. 

[Nickfromwales]

4 hours ago, Susie said:

Depends on the size of room one cable in the far side of open plan room is better in conduit for our build than making a service void in all of vaulted ceiling. 

Not the point I was making.

 

Cables run in insulation overheat, so in the regs it states you should avoid this; if you cannot then you MUST upsize the cable (considerably) or downsize the circuit protective device accordingly.

 

As a 16yr old apprentice for SWEB, we were asked to connect a 3kw 3bar electric fire up with 20m of 1.0mm twin & earth cable, and run a test on how hot it got. It actually coped OK sat on the concrete floor of the training centre.

 

We were then told to roll out 20m of loft insulation, lay the cable on it, and then to fold it over the cable, and repeat the same test. Then followed molten PVC and a very short but loud bang as the cable went nuclear.