another sanity check, we're installing whole building insulation externally. I'm trying to get into the design of the heating elements, which will be 'underfloor heating' on the wall. Overall heat requirement target is ca. 85 kWh/m2. Big building in eventually 3 apartments. Separate zones/controls for each apartment. Also airtight as possible ( no idea what we will achieve yet ) with MVHR.

The 1st couple of things I'm confused on is that I've read a couple of recommendations that it should be installed on the external walls and that most manufacturers presume the installation of an internal insulation behind the pipes.

I had presumed that I would install on internal walls mainly opposite the external walls with windows, without insulation, so that any heat 'lost' behind will simply add to the whole building, and therefore I will design as far as possible to have the heating elements also 'back to back' on the same internal wall where possible. But this is just me making it up.. am I missing something with the external wall recommendation? I suppose in a way this is the same as putting radiators under windows because that is where it is the coldest, does that also make sense in a well insulated house, or better to keep heating elements away from external walls?

it's a renovation, I was aiming for around 50 kwh/m2 and hope it will be even better, but I have to work to what the energy consultant believes to be realistic and he wouldn't allow me to go for a target below 85.. so I will plan with that in mind.. plus this is Germany and we can get -20 degree night time temperatures and prolonged periods of -10 / -15 some years and we are in a flooding zone, so we might occasionally need to pump in extra heat to help dry out

Posted By: Silkyit's a renovation, I was aiming for around 50 kWh/m2 and hope it will be even better, but I have to work to what the energy consultant believes to be realistic and he wouldn't allow me to go for a target below 85.. so I will plan with that in mind..

That sounds like a good reason to find a different energy consultant. On the face of it, it's a ridiculous constraint, but then we don't have have the full context.

Is tick box what you want? Or do you want something built right? Given that extra information, I would definitely find a different consultant.

OK, it seems like you're keeping two sets of books. But you still need somebody to keep the private set of books, and if you want advice from us, then you need to tell us what parameters you're actually trying to build to rather than what you're telling the government.

hey thanks for the comments,

Yes this is Kfw, there are different levels, the consultant I speak of is someone who processes the applications, a bit like those people who do the energy certificates in the UK, they do a short training course and then they are allowed to do it. They are no geniuses and only able to work to rules of thumb.. and tick boxes. To get to the lower levels, i.e. sub 55 Kwh would need the thermal bridges to be calculated by an engineer, which as the Germans say.. "It doesn't pay..". Please don't be fooled into thinking that everything is done better or correctly in Germany.. they can build well but at a hell of a price, and the majority of people are still stuck with old fasioned ideas about the correct way to do things, the same as in every other country no doubt.

@djh I'm just at the start of the process of working this out in detail, and I've come to this position because of having to consider where pipes will be routed between rooms. So in my initial reading I came across some literature that says wall heating shoud be installed on external walls, and looking at some installation guides there seems to be a presumption that insulation should be placed behind the pipes. So at this stage I just wanted to understand the reasoning behind this, I'm very much taking the holistic approach, and willing to break with convention where it makes sense. The overall building design is based on having a technical room centrally located on the ground floor. I have a services shaft located in another adjacent central room ( 3 bathrooms stacked ). So my rough strategy was that I would make connections in and around this shaft and keep pipe runs to a minimum and mainly place the heating on internal walls. If I must, or if there isn't enough space on internal walls only there are options available to get onto the external walls too. But I don't understand why I should favour external walls over internal, and if I use internal walls I don't understand the need for insulation, if the any lost heat will be lost into another room that is being heated.