Water storage tank

[Jons]

Good morning everyone

We have just obtained planning on a disused, earth covered, water storage tank and are in the process of deciding on the details of construction. We are not sure how to insulate the structure which has thick (about 900mm - 600mm) concrete walls. Do you insulate the outside and include the walls in the building envelope or insulate the inside and treat the walls as just a structural part of the building? We are planning to retain the earth cover on the roof and two sides with an extension on the east and south facing sides, so over all the building will be an "earth sheltered dwelling".

We would appreciate peoples opinion on the most appropriate option as I can see pros and cons in both approaches.

[Nickfromwales]

Hi @Jons, and welcome. 

Member @jamiehamy has done this exact same conversion which is documented well both here and on Ebuild ( our former hideout ) There is a lot of info from the start of the journey  and if you ask he may be able to link the EB content ?

 

[jack]

Welcome to Buildhub.

 

Can't help you with your question, but that sounds like an amazing project.

Edited to add: here's @jamiehamy's blog: www.theoldwatertank.blogspot.com 

[Solutions]

Hi Jons, 

 

I am not familiar with how your tank was build, some pics would have helped.

Normally i would treat the existing structure as a retention wall. Idealy looking to have watterproofing on the outside of the existing structure and an internal leaf with thermal insulation, secondary waterproof and drainage build on the inside.  

For the base, will probably be forced to add a watterproofing and drain system on top of the existing slab of the tank and get a new slab on top of it.

 

If what i say makes no sense pm me and i will send you some detailed drawings and pics of some basements done inside retention walls.

[Jeremy Harris]

I'd second looking at @jamiehamy's project, as they've tackled some very similar challenges with their water tank conversion.

 

The key thing is probable the one you've already identified, whether to insulate outside or inside the existing structure.  Outside is likely to mean a lot more work, but means you can gain the benefit of having the thick concrete at near room temperature, which will massively increase the thermal time constant, from the long decrement delay.  Inside is probably a fair bit cheaper and easier, but will reduce the decrement delay, but that may well not be important, depending on the orientation of the open side.

 

Having three sides and the roof earth sheltered will help, as it means that those surfaces will have a year around near-constant temperature differential of around 14 to 16 deg C. 

 

Another point about where to put the insulation relates to the internal space, and whether or not you can accept losing 200mm or so all around from adding internal insulation. 

 

I doubt that waterproofing will be a problem, as water tanks are always built way above the water table, so there should be little or no risk of ground water penetration.  Concrete that thick will also be very watertight, so the chances are you don't need much, if anything, in the way of further tanking.  The worst case may be that you have to fit a dimpled membrane against the concrete, draining to a small sump and drain outlet (which may well be just a gravity drain) and then internal insulation and lining inside that, pretty much like a basement.

[jamiehamy]

Hi Jon, 

 

Link to my blog contains most of what I've put online but feel free to ask any questions if you see any similarities. 

 

Pretty much as @JSHarris. said in terms of how to approach. 

 

We commissioned a structural survey on the tank first of all. This told us that the walls and floor/founds were sound however there one wall was bowed - (cast that way) and some remediation was required before they could be used structurally. There were some cracks too and other bits of spalling evident. 

 

We effectively built inside the existing walls which avoided complexities involved in tying into the original structure and made waterproofing much more simple. I did consider external insulation however the walls are chamfered and planning were keen they were retained visually. 

 

If you can post some pictures we collectively can probably provide you some more food for thought that is more targeted to your build. 

 

 

[Onoff]

@Jons, you'll have to go some to trump Blofeld's Lair aka @jamiehamy's garage!

[Jons]

Thanks for the advice so far - I've taken some pictures which may be of some help. (I'm not entirely sure this will work due to my technical incompetence!)

Outside of the tank

 

 

Inside of the tank

 

Section of the wall

[PeterW]

Looks nice ..!! Wonder how much they will charge you for a water connection ......

[jamiehamy]

Interesting - looks in a good condition inside - could be tempting to tank the lot,  insulate and scree the floor and line the 'external' walls with a timber framework. I'd keep those pillars exposed. Ceiling could be interesting - theoretically you could as Jeremy mentioned earlier insulate the outside and leave the internals exposed. Much depends on how you want it to look side. 

 

Btw - if cutting through deep walls,  a Husqvarna Cut n Break is a godsend! 

[Jons]

15 minutes ago, PeterW said:

Looks nice ..!! Wonder how much they will charge you for a water connection ......

It's the £250 legal costs that Welsh Water want to charge me after I've undertaken a survey and produced a method statement on how I plan to remove the redundant equipment on site that I thought I owned - I find difficult to get my head round!!

[jamiehamy]

They are being very proper. Just get it all out and then submit a blank statement...! 

[PeterW]

10 hours ago, Jons said:

It's the £250 legal costs that Welsh Water want to charge me after I've undertaken a survey and produced a method statement on how I plan to remove the redundant equipment on site that I thought I owned - I find difficult to get my head round!!

 

So do you own it now ..?? If it was freehold with vacant possession then they don't own what is inside unless it's connected to the main at which point they need a wayleave anyway..! If they have signed and sealed the deal, you can remove the kit at their cost if you give them 28 days notice to either do it or you will. 

[Solutions]

I look at the pics and the caveman in me awakens ... 

Honestly, it looks amazing and from the pics it has huge potential. 

 

1. What is the foot print ?

2. How high is the ceiling at this moment ? 

3. Have you decided if you will unearth it or work only from inside ? 

4. How will you sort out the natural light? It does not seam to have enough hight to allow for light tunnels.

 

If you have no points of infiltration, it will be cheaper not to unearth it and just work from inside.

- sika on the concrete, or waterproof membrane, or both

- drain membrane, with a drain canal (and sump pump preferably)

- for the inner leaf, depending on budget, either stud with build in insulation, or thermal block with EPS.

 

Regardless if you chose stud or block, for the termal insulation I recommend you to look at polystyrene (specially at the graphite EPS). I know its common in UK to use mineral wool or Polyisocyanurate (kings PIR) but in my opinion they do not behave that well on the long run if exposed to humidity. 

 

And a suggestion about the pillars, clean them well and dress them up in stone. They will look nice and also act as a thermal mass. 

For the beams, you can find a good carpenter to dress them up in wood, ciseled, burned, stained and lacquered to look like an old single piece. 

 

[jack]

38 minutes ago, Solutions said:

And a suggestion about the pillars, clean them well and dress them up in stone. They will look nice and also act as a thermal mass. 

For the beams, you can find a good carpenter to dress them up in wood, ciseled, burned, stained and lacquered to look like an old single piece. 

 

I like the pillars and beams as they are!

 

Worth thinking about how to handle thermal bridging through the beams and pillars if using internal insulation.

[Jeremy Harris]

I agree, I like the pillars too, plus they look very definitely structural and there to support the roof loads, which will be pretty high, so removing them could be challenging.

 

Solving the thermal bridge problem, if going for internal insulation, is going to be pretty difficult, as insulating every one of those pillars would significantly reduce the space available, and would make them look out of proportion.

 

In terms of decrement delay and the heat capacity of the interior structure (thermal mass has no physical parameters and cannot be measured, so is a bit of a myth!)  then I doubt it's an issue, as being earth sheltered will massively reduce diurnal temperature variations anyway, even with one open side.  It looks quite deep to me, so the chances of getting significant solar gain through the one open side is probably pretty small, even if it faces due South.

 

I think it's well worth costing up both an internal and external insulation option, to be honest.  External insulation has the advantage of a larger internal space and greater freedom to utilise it in the design, but has the major problem of how to insulate the floor.  Adding internal floor insulation, with external wall and roof insulation, will still leave some massive thermal bridges, both through the pillar bases and around the edges (although deep perimeter insulation might mitigate the edge problem a bit).

 

Internal insulation solves the problem of the wall to floor thermal bridges, and means less work in removing soil, etc, but still has the big problem of the pillars and the thermal bridging they create.

 

All told I think the ideal solution (which may well be too costly) might be to remove the roof and the pillars, and internally insulate, adding a new green roof to get back the original external appearance.  This would be far more flexible in terms of internal layout, would allow the fitting of roof lights or light pipes to get daylight into the furthest reaches, and would also allow a great deal more design freedom.  You could opt for something like the design of Mole Manor, with a central atrium to allow light into the middle of the space, for example  The roof light over this could be made invisible from all the elevations easily enough, so from outside no one would know it was there (unless they flew overhead).

 

As a source of ideas, it might be worth looking back at some of Arthur Warmby's designs, and through the archive of the British Earth Sheltering Association.  They have long since disappeared from the web, but I was a member years ago and downloaded quite a lot of their articles, but I must have archived them somewhere, as right now I can't find them.  The chances are they are saved on an old portable drive somewhere, though, so if you're interest I could try and find them.

 

 

Edited July 5, 2017 by JSHarris

[Solutions]

... If the money are there, YES, ideally you would want to unearth it and do everything from outside. (See the first suggestion made, before the pics were posted)  The problem is the costs, JSHarris, can tell us from his own experience how fast ground works cost can escalade. 

 

That is why i suggested treating the existing structure as a "retention chamber" and work from inside as on a traditional basement.

The floor is the easiest part to sort, because by default he is forced to introduce a new layer (ideally full slab) on top of the existing one to connect with the extension areas (south and east) and to accommodate the water proofing and drains. The thickness (raise) that will be imposed by the drain depth can be filled with high density polystyrene (like you use in a resting slab).  

 

In regards to braking the thermal bridges, you can do a reverse brake from inside like you do in retrofits. Yes, it will eat up space by thickening the columns by an extra 10cm , but it will be way cheaper then unearthing. 

 

Plus, if the ceiling is under the freezing depth (60-80cm for UK if i remember correctly), the earth layer should provide a natural insulation with a relatively constant +4C during winter time. 

 

Personally I would always go for external insulation and proofing, but sometimes the deciding factor is the money

 

 

[Jeremy Harris]

The earth layer will be around 8 deg C below about 0.6 to 1m, not 4 deg C, and may well be warmer than this.  The earth under our passive slab stays at around 9 deg C, and all the caves in South Wales I've been in were around 8 deg C, perhaps a bit warmer in the really deep and dry ones, colder in the ones with active streamways.

 

A rough and quick calc suggests that you need more than 100mm of PIR around those columns to mitigate the thermal bridge to an acceptable level, there seem to be so many of them that the surface area is large, and the thermal conductivity of concrete, plus the relatively thick section suggests a fairly high heat loss without very good mitigation, plus a substantial condensation risk.  I think 150mm of PIR might be the minimum, plus a very well sealed VCL, then cosmetic cladding.

 

I think that taking the roof off and removing the columns, then building inside the tank may well be the better solution.  This has so many advantages, that it seems almost a no-brainer, especially as the new roof structure could be made lighter, allowing greater spans, and yet still retain an earth covering to maintain the external appearance.

 

The construction of this tank will have been driven very much by cost, so the cast in-situ floor, walls, support columns and roof won't have been engineered with domestic use in mind, and the roof is almost certainly massively over-engineered, to allow for the deep earth covering, and possible machinery use surcharges on top. 

Edited July 5, 2017 by JSHarris

[Jons]

I am finding the discussion really helpful in considering the options.

Just to clarify - the photos I took may have been misleading as we only have two columns.  I've attached the plan we have which shows the existing rectangular tank with the extension on the east and south sides. The roof is about 150m thick concrete with a similar depth of soil over. We are planning to put roof lights over the bedrooms and snug with the kitchen, dining and sitting area in the extension. We are not looking at a Grand Designs approach with much of the work being done by ourselves and local builders. Any clarification of details please ask.

[jamiehamy]

Rooflight wise any reason they are not bigger? We're likely getting one for our garage which will have something like 25mm glass on the outer pane meaning the roof space - if flat - is fully usable? Can be tinted with a mirror tint for privacy. Just a thought!  

[Jons]

No particular reason for the size - it's more for natural light as we are planning to spend most of our time in the extension, moving into the 'snug' in winter evenings. We will not be using the roof as we have plenty of garden/field space around the building.

[Construction Channel]

Afraid I can't add anything to help as you are already in very experienced hands, But I will say it looks like a fantastic project. 

 

We like pictures around here. Lots and lots of pictures  

 

Ed

[Solutions]

Hi Jones, 

 

If the concrete and earth layer are only 15cm thick ... JSHarris is right, it would worth more to unearth it and work everything from outside.

(Apologies, I thought you had some serious ground on top of your roof)

 

In regards, to the temperature debate, the 4C i uses for reference was from my exp with London clay (see the image attached).

I have to admit that I am not too familiar with Wales soil (besides the nice sandy beaches) 

A lot of info and interesting reports can be found on the BGS website, including info about the thermal conductivity of various types of soil.

 

 

C012 (1).pdf