Insulating a former Concrete Water Balancing Tank

[Dave and Helen]

Hi, 

 

I'm hoping someone will have some bright ideas on how we can insulate a former Severn Trent Concrete Water Balancing Tank, which we intend to integrate into a new residential dwelling. 

As photos speak a thousand words, I've attached some images to show our project and the tank. 

 

In addition to tanking, we need to insulate the concrete tank, which will be partially used for garaging and habitable space. The structure is mass-poured concrete that is stepped, with a thickness of 600mm at the top and 1.2m at the base. 

The surface is irregular and not straight as it's rough poured concrete, with no rebar. We're thinking of using exterior wall insulation (EWI), which is fixed back to the concrete, and then forming an outer structure to form skin and protect it. Planning won't allow us to use render, which I'm okay with, but as most EWI installations, like those attached to old council houses, are rendered, this poses a problem.

We ideally want to form a new brick face for the tank, but because it's stepped out, if we choose to create a skin straight up from the base and include a 150mm cavity (100mm EWI and 50mm air gap), the distance between the back of the brick face and the existing concrete would be 760mm, far too wide to be safe. If we stepped the new brickwork in to follow the existing steps (as shown in the attached Sketchup drawing) how could we support the weight of the brickwork whilst retaining an air gap between the new brickwork and EWI, or without flattening the EWI if there was no air gap? 

 

We've seen quite a few water tank conversions, but all of them appear to have straight walls.

An alternative would be a timber frame and cladding, but I'd be concerned about how this would withstand the test of time. 

 

I'd be interested to hear any suggestions anyone may wish to offer. 

 

Kind regards

 

Dave C

[Mr Punter]

Is it the case that the planners will not let you demolish this?  It is certainly not pretty.  It does have a pyramid vibe though so you could decorate with hieroglyphics.

[Tom]

Might it be simpler/easier/cheaper to make an insulated "tent" inside this to use as habitable space?

Other than that I'm afraid I'm fresh out of ideas. Would your architect not have a plan in this respect?

[joe90]

8 minutes ago, Tom said:

to make an insulated "tent" inside this to use as habitable space?

That is what I thought, timber framed internally and insulated just for the habited bit (not necessary for the garage I guess) , brick skin outside just to hide the ugly concrete. 

[Conor]

Trying to do anything on the outside of that structure will be a nightmare. Do all damp management and insulation on the inside. 

[Redbeard]

Not the same thing at all in terms of aesthetics, but you might have a look at Green Building Company's Stirley Barn project.https://www.greenbuildingstore.co.uk/technical-resource/cre8-barn-stirley-farm-enerphit/

[Mike]

In interesting structure that may well outlast the house!

 

Clad it as a roof, not a wall - for example by building a half-barrel vault / parabaloid over the face of it. Could be clad in black sheet metal or zinc (to match your cladding?) or planted as a green 'bank'.

 

 

Otherwise, for brickwork, a company like Ancon could no doubt come up with a custom supporting structure.

 

[Dave and Helen]

Many thanks to all for the comments so far. The planners wouldn't allow the existing structures, the original 1930s pumphouse and the tank to be demolished, not that we really wanted to, we love the challenge of working with what we have. Also, the arches in the tank are quite amazing... at least to us. 

Creating an internal structure is an option, but we didn't ideally want to lose any internal space. As large as it is, once the walls come in it could feel a bit tight.

Also, the three rows of arches cross the internal structure. If we create an internal timer frame on the outside walls, wouldn't there be a cold bridging issue where the arches meet the walls, as we wouldn't be able to insulate around them very easily? 

 

I guess if we did create a timber frame, tank and insulate on the inside, we could then simply face the outer walls. 

I understand for a structural warranty, there's a requirement for two forms of tanking. The honeycomb structure with a drain at the bottom on the inside, behind the timber frame or plasterboard, but I guess we'll also need to tank the outside where the outer wall is below ground? 

 

I've attached some additional photos of the inside. 

 

Really appreciate your feedback.

 

Dave C

[saveasteading]

How splendid.

Building regulations document Z  for converting a ziggurat.

If it was me, I think I'd do it all inside and keep the look.

It would be the same principles as converting a stone building. 

In Scotland the regulations allow pragmatism: "as far as reasonably practicable" is often stated. In England it isn't, but the building control officer may be pragmatic.

We worried about losing internal space but it was the right decision.

Yes, there will be a cold bridge through the arches but they are a stunning feature.

 

Are you keeping the pump?

 

[joe90]

Those internal arches are cool and a lovely feature. Being a devils advocate, if your worried about cold bridges then create a warm roof above and clad externally with EWI, there are some “fake” (composite) timbers out there which look good and won’t rot like wood. As there is not the pressure internally from a million gallons of water could you not cut off the two most lower steps? it looks like you have successfully cut into the structure in places. Just saying.

[Russell griffiths]

I’m going to go against everyone and say insulate it on the outside. 
use eps, buy it direct from the manufacturer and they will cut it to the thickness you need, put it on the external steps to bring them all out to one level plane, then build an exterior skin in brick using the same brick bond as the pumping station. 
long stainless ties back to the concrete, or build it 9inch and have an independent skin. 
 

 

[saveasteading]

4 minutes ago, Russell griffiths said:

insulate it on the outside. 

That's practical of course, but the interest is gone. And the arches decorated in moorish style?

[Roger440]

If you insulate  externally, the floor will still be connected to the ground, and the arches connected to that. So the arches will still be a cold bridge regardless. Unless you clad them.

[joe90]

8 minutes ago, Roger440 said:

If you insulate  externally, the floor will still be connected to the ground, and the arches connected to that. So the arches will still be a cold bridge regardless. Unless you clad them.

However, ground temps tend to be above air temps ( @SteamyTea will load a spreadsheet shortly) and I would prefer a cold bridge to floor than roof.🤷‍♂️).

[SteamyTea]

1 minute ago, joe90 said:

However, ground temps tend to be above air temps

Not that often. Maybe on a few really cold winter days.

At the moment the air temperature is 13⁰C and the ground temperature is 6⁰C.

The sea temperature is 10⁰C.

So most of the time the ground is colder than the air, and especially the air in a building.

And people wonder why I say that just adding mass to a building makes it colder.

[saveasteading]

43 minutes ago, saveasteading said:

but the interest is gone.

You could say to visitors...this used to be interesting but we hid it.

 

Other relevant matters.

How much of it will be garage or otherwise unheated space?

Will it be subdivided by (insulated) walls?

Are you building over the top?

Or what I'm getting at is: does it need much insulation to external walls and floor?

[saveasteading]

10 minutes ago, SteamyTea said:

the ground temperature is 6⁰C.

But what is the ground temperature under the slab of an unheated building? 11° perhaps? 

[joe90]

On 11/03/2024 at 17:23, Dave and Helen said:

Planning won't allow us to use render,

Why 🤷‍♂️ it’s not listed (is it?) so why not, planners tried to tell me to render my build (common around me then) but I refused and proved a local large farmhouse near me was built in brick like I wanted (I won 🖕).

[SteamyTea]

12 minutes ago, saveasteading said:

But what is the ground temperature under the slab of an unheated building? 11° perhaps? 

Not much higher than the surrounding uncovered ground.

We don't thermally insulate our houses by sticking on another layer of bricks.

Clay has a k-value ~3 W.m^-1.K^-1. So not very good.

By the very nature of building, the ground needs to be well compacted, so often a worse thermal conductivity.

It is also a nature of physics that the greater the temperature difference, the faster it looses energy. So what does get through the slab, is then into a semi infinite heat sink. 

 

A lot of people think that the ground under a house makes for a good thermal store, so good that every house has one.

 

There is a reason that insulation has a low k-value and a low density, it stops the energy leaking out from where you want it.

[joe90]

However, as demolition is not allowed any works done will be a compromise so I guess the OP has to decide how to proceed and decide between insulation vs aesthetics.

[Mr Punter]

Cutting the openings looks like it was quite a task.  Neatly executed.  Were they on a price?

[Mike]

8 hours ago, Dave and Helen said:

Creating an internal structure is an option... the three rows of arches cross the internal structure

Don't create an internal structure - I'm with @Russell griffiths - insulate externally - those arches should definitely be on show! Plenty of opportunity for an interesting lighting scheme too.

 

8 hours ago, Dave and Helen said:

I understand for a structural warranty, there's a requirement for two forms of tanking

Hmm - never tired, but not sure how easy it would be to get a structural warranty on a 1930s structure (as opposed to on the new portion of the build).

 

If it's been storing water I'd guess that the concrete may already be reasonably water-resistant? If so, then controlling any humidity is controlled through MVHR + breathable construction may be adequate.

 

 

[Dave and Helen]

Wow, so many replies and opinions. I'll see if I can answer some of your various comments here, but I think I need to take advantage of the ability to start a blog in here.

 

So with regard to the structure. As you can see from the various pictures of the tank, you'll have noticed a brick pumphouse in the background. Planners wanted us to build one structure, so we worked with our architect on a proposal to build one house, linking the pumphouse with the tank, with a new section in between that would also enable a new structure to be built on top of the tank, with a good half of the roof area being used for a roof terrace.

 

The tank space will be a 3rd garage, with the removal of one of the existing arches to form a larger garage space, supported by steelwork. The remaining space will be habitable, but mainly a cinema and games room, after all, there aren't any windows. The floor in the tank slopes down to the overflow at the lowest point, which exits the tank to form a natural drain, so we're planning to take advantage of that. We'll then level the floor and insulate above. We can't insulate under the arches, but I've been told by another architect we met that 'cold bridging' will only penetrate so far up the concrete pillars. If this is the case, insulating on the outside would make better sense, as the concrete tank would then be within the envelope of the structure so the heat would be retained in the concrete... does that make sense? 

 

We cut the various openings last summer when it wasn't so wet. It took the contractors three weeks to complete and cost just over £14k, so to me, it was a real bargain, and they did an amazing job. We stockpiled the rubble, and together with the foot or so of concrete we excavated from the pumphouse floor, worked with a local farmer who owned a portable crusher driver off a PTO on his tractor and crushed the lot. We ended up with about 200t of great hardcore. 

 

The planner wouldn't let us use render because the house next to us has been pebble-dashed all over with what looks like porridge, so they weren't keen to see any more. However, I'm sure we could persuade them if we really wanted it. 

 

We want to retain the stepped exterior, but make it more presentable.

A Ziggurat, I love that... and I've learnt something new. We've had a friend call it Mini-Pitchu, but that's much much better!! 

 

I'm off to explore how to create a Blog so I can keep you all up to date on the progress.

 

Many thanks for all your comments.

 

Dave C 

[joe90]

36 minutes ago, Dave and Helen said:

The planner wouldn't let us use render because the house next to us has been pebble-dashed all over with what looks like porridge, so they weren't keen to see any more.

Just because your neighbours did a shit job should not preclude you from doing a good job.

38 minutes ago, Dave and Helen said:

However, I'm sure we could persuade them if we really wanted it. 

Give examples of clean crisp render you want to use. (If you still want render).