What insulation for 38mm service void in MBC passive wall

[markharro]

We don't strictly need to insulate this as the main wall void is pumped with cellulose but if I were to what would be the best product to use? 

[Thorfun]

Seriously? Why bother? Save your time and money!! It’s already insulated and airtight to passive house standards. Any additional insulation will give very little gain I would’ve thought. Plus you’ll likely have to uprate your electrical cables as they’ll be running through insulation. 

[Russdl]

9 hours ago, markharro said:

 

We don't strictly need to insulate this

 

I’d say “don’t” for all the reasons @Thorfun highlighted plus I guess you’d be cutting something down to do it and it would take an age for minimal gain. 

[markharro]

9 hours ago, Thorfun said:

Plus you’ll likely have to uprate your electrical cables as they’ll be running through insulation. 

Thanks @Thorfun thats the sort of answer I like. It was actually our architect who suggested doing this but of course he's not the one doing the work. You point about cabling is interesting. Last week the electrician started on this and we now have a lot of cables in these void but also in the 90mm voids between rooms and in the ceiling voids. I will be putting insulation in these latter two areas for acoustic reasons. I assume that this should be obvious to the electrician and whatever cabling he has installed will cope with it?!

[Thorfun]

16 minutes ago, markharro said:

Thanks @Thorfun thats the sort of answer I like. It was actually our architect who suggested doing this but of course he's not the one doing the work. You point about cabling is interesting. Last week the electrician started on this and we now have a lot of cables in these void but also in the 90mm voids between rooms and in the ceiling voids. I will be putting insulation in these latter two areas for acoustic reasons. I assume that this should be obvious to the electrician and whatever cabling he has installed will cope with it?!

we filled our 89mm studs with 50mm Rockwool as well. no issues with the cabling that our sparky mentioned. I presume it's because the cables are running in the insulation but rather sat on top like this photo

 

 

but maybe @ProDave can give a definitive answer?

[markharro]

Ah thanks.....so on another topic why only 50mm of insulation? I have been reading about sound insulation and there appears to be a school of thought that says you don't completely fill the void? More good news if correct as it will save on rockwool costs! And another tricky issue is trying to work out how much soundproofing is needed between rooms/floors. I see you can get carried away with double/acoustic plasterboard; resilient bars etc and then even more specialist products. But how do you make a sensible decision on what to use where when you won't know if its enough until the whole interior is constructed and you are living in the house?

[Thorfun]

On 11/09/2023 at 09:38, markharro said:

Ah thanks.....so on another topic why only 50mm of insulation? I have been reading about sound insulation and there appears to be a school of thought that says you don't completely fill the void? More good news if correct as it will save on rockwool costs! And another tricky issue is trying to work out how much soundproofing is needed between rooms/floors. I see you can get carried away with double/acoustic plasterboard; resilient bars etc and then even more specialist products. But how do you make a sensible decision on what to use where when you won't know if its enough until the whole interior is constructed and you are living in the house?

this subject has been debated quite a bit on here and i don't think there is a perfect solution! you could absolutely go overboard with insulation, resilient bars, soundbloc etc but then you need a gap at the bottom of the door to let air in/out for the MVHR! so sound will escape that way.

 

in the end we decided to use resilient bars in certain places where we wanted to reduce the sound transfer, e.g between my study and the kid's bedroom above and we used 100mm Rockwool in the ceilings between the joists and 50mm on internal walls. we haven't used soundbloc anywhere and are using 12.5mm standard plasterboard.

 

was that a good choice? ask me again once we've actually finished building and have lived there for a while!

 

my conclusion was that we'd never eliminate the transfer of sound without spending a lot of money and detailing every wall to ceiling to floor junction with sound sealant etc so we just tried to take the edge of it all.

[Russdl]

Whilst the thread is drifting slightly from the OP’s original question I can say, regarding sound transfer, our experience is the air gap under the doors does not permit sound transfer as you may expect. On carpeted floors, if you close the door the noise pretty much goes away, and that is the case for carpet inside the ‘noise’ room and carpet/hard flooring outside the ‘noise’ room. 
 

However, if there is hard flooring from one side of the door to the other (downstairs toilet in our case, tiles run from the hallway to the toilet without a threshold) then that’s a different story, the gap’s not so good then, sound does find its way under that gap.
 

I turn the music up. 

[Gus Potter]

On 11/09/2023 at 09:07, markharro said:

I will be putting insulation in these latter two areas for acoustic reasons. I assume that this should be obvious to the electrician and whatever cabling he has installed will cope with it?!

No the spark won't and you are heading for big trouble if you start to insulate cables that are designed to be in a void for example.

 

Your spark will have carefully sized the cables based on if they are in a void or have insulation close. You are now adding insulation on a whim.

 

This is very serious and you need to talk to your spark. I have seen fires occuring in extensions to houses as a result of design changes like this... the house down the road from me went on fire for this kind of reason.

 

Your other designers (and that now seems to include you as you are making technical design decisions) should be alerting you to this.. or do they not have enough experience and that includes you too?

 

Now I may seem like a bit of a tosser but if you are making design descisions like this and you get it wrong then your insurance is invalid and the rest of the design team will probably leave you carrying the can!

 

In summary I can see you are enthusiastic about what you are doing .. which I very much appreciate.. but you probably don't realise that something that seems innocuous (you are tring to make the house warmer) can have a dangerous impact.

 

Now if you don't believe me about the wiring implications.. have you checked if what you are doing is going to shift the dew point in the wall and do you know what could happen if you do? .. you won't probably significantly but I say this just to make a point.

 

I laid it on thick above but for your own safety.. give the spark a call on the phone.. if you don't then there may be no electrical IEE completion certificate from the spark.. and that is often a problem.

Edited September 13, 2023 by Gus Potter

[Kelvin]

Here’s an article that discusses it. 
 

https://professional-electrician.com/technical/thermal-insulation-and-its-effect-on-flat-twin-earth-cables-niceic/

[markharro]

Thanks @Gus Potter I appreciate your advice. I have spoken to the spark today and all is ok. I have concluded that its better not to bother insulating these shallow voids - there is more than enough stuff to do to last a lifetime in any event.

 

Thanks @Kelvin for the article link- useful conclusion appears to be to route cabling above any thermal insulation. I suppose in walls that means just on the outside of the insulation layer rather than buried in the middle.

[mike2016]

My Architect specified Moy Metac or equivalent for the 50mm service cavity as part of the overall insulation package. I have 220mm Moy Metac within the timber frame itself and 50mm kooltherm inside of that. Was going to swap the kooltherm out with 100mm Gutex for breathability. Just need to decide if I should fill the service cavity or add another 50mm to the Gutex to be done with it!! I'll be flagging with the Electrician if I fill the service cavity or look at some trunking option to carry this cable inside the service void if this offsets the derating somewhat but I doubt that exists? Or surface mount everything and go full industrial!! The depth of the wall affects drain positions etc so I need to make this call up front! Good topic for me to follow!