Basement Construction

HardHouse is planned to incorporate a basement to house a media room/cinema, a gym with the potential to add a small “endless” exercise pool at some point in the future and a storage room. It will cover the full footprint of the house, and so be quite large, in the region of 140m2. This will allow us to build the large house we would like without compromising on the garden size or upsetting our neighbours by blocking their light. Ours will also have two exit wells, one at the front and one at the side of the house.

The construction techniques for basements need to fulfill two criteria that don’t apply in the same way to structures above ground and which effectively limit the choice of materials. As far as I can see there are no real Green options. The unique criteria as I see them are:

  • provide a retaining wall for the soil around the basement to prevent the whole structure collapsing
  • provide a watertight layer to ensure the basement is kept dry from groundwater

The basement will also effectively become the foundations for the build above ground but this is also true, to a certain extent, of a ground floor supporting a first floor, so I’ve not put this in the list as a specific basement requirement.

All the solutions I have seen use a poured concrete base slab cast in-situ and then one of a variety of different options to construct the walls, still using concrete products. In all cases the ceiling structure choice is fairly open. If you’ve had enough concrete and you don’t need internal loadbearing you can switch to timber, or you can continue the high mass trend.


I would prefer to have the waterproofing on the outside of the structure for several reasons:

  • Allowing water to penetrate through the insulation and the main structure of the walls and then relying on an electric pump to take it away has got to be bad thermally and environmentally.
  • Having the membrane on the inside means that you have to be very careful to ensure that no-one accidentally puts a hole in it, by drilling to hang up a shelf or similar.
  • The need to have an internal drainage cavity eats valuable space inside the house .

Consequently I’m in favour of external waterproofing solutions. This raises some concerns over the robustness of the membranes to normal builders during installation, and the fact that if it is ever penetrated it’s going to be a hell of a job fixing it.

The best option seems to be an inherently (mostly) waterproof main structure backed up by a tough external tanking solution.

This is then supported by the use of a geo-textile membrane outside of the tanking to encourage drainage of any water to the bottom of the wall, gravel outside of the insulation layer to do likewise and a French drain running around the walls to a deeper sump to allow the water to drain. All of this reduces the water pressure seen by the basement and consequently puts it under less stress therefore reducing the likelihood of any failure.


It’s clear to me that the best approach for insulation is to have it on the outside of your structure, and this is as much true below ground as above it. It really helps to minimise thermal bridging and to retain the thermal mass inside the building to help regulate temperatures.

The main difference is that below ground you need to use an insulation that is tolerant to water, and essentially this is limited to some form of polystyrene. Both expanded and extruded polystyrene are tolerant of water and do not lose any appreciable performance even if saturated.

However, the thermal performance of these is less than the more esoteric plastic based insulation that can be used in dry conditions, so more insulation is needed to achieve the same U-values.

Of course, the ground itself provides some insulating properties and huge thermal mass which is recharged by the sun on a regular basis. As such, the “ambient” temperature outside of the basement is more stable than that above ground, and never reaches the extreme low temperatures. Consequently less insulation is needed, which partially compensates for the lower performance. The below ground insulation will still need to be thicker than that above ground though, especially for a PassivHaus.

This approach means that the insulation has to be installed underneath the ground slab, and on the outside of the vertical walls, outside of any waterproofing layer. The insulation under the ground slab has to be strong enough to support the weight of the whole building and fortunately extra strong versions do exist (with minimal reductions in thermal performance) specifically for this purpose. Care also needs to be taken to ensure that there is a good overlap of the under-slab insulation and the wall insulation to make sure there aren’t any thermal bridges.

Exit Wells

Unless a basement is to be used exclusively for storage there needs to be at least two independent escape routes from each habitable room in case of fire. One way of doing this is to have two independent and isolated staircases rising into different rooms above ground. However, this option is highly dependent on the floorplan of the house and is practically very difficult to achieve for most designs.

The alternative is to have exit wells with doors to the outside world and an external stair case to ground level. This is also problematic, in that the exit wells take up space in the grounds of the house, look particularly unattractive and present particular challenges both for waterproofing and thermally.

Thermally, the walls of the exit wells really need to be outside the thermal envelope of the house, and this means no, or minimal, structural connections between the two builiding elements that would act as a thermal bridge. For most houses this probably isn’t a significant issue, but for a PassivHaus it is a different story and this presents multiple problems, mainly making sure that the exit well is supported and doesn’t move away from the main basement causing cracks. I suspect that some structural connections between the two building elements might be necessary, but this needs to be minimised.

There is also a small difficulty with the insulation and rendering on the basement wall inside the exit well. Ideally, you would use the same insulation and wall covering as above ground leading to a smooth and continuous surface from the exit well to the main building. The logical conclusion of this would be to use polystyrene above ground as well as below it. However, the relatively poorer performance of EPS over phenolic means that the above ground thickness needs to be even greater, and on a constrained site this extra wall thickness will have an impact on the internal floorspace available.

If you use polystyrene for the exit well and phenolic for the main wall and want to achieve the same level of insulation then there is a step in the wall thickness, which is ugly.

The other solution would be to use phenolic for the exit well walls as well as those above ground, but this means that water needs to be prevented from getting behind the render, otherwise it will damage and impact the performance of the insulation and repeated freezing cycles will stress the render and run the risk of cracking. Doing this is not so simple.

Of course, having separate elements also introduces the potential for water seepage along the “joints” which would look unattractive and could tend to fill the exit well or at least cause some puddling that needs to be removed. (Of course, it will also potentially rain into the well, so some way of removing ground water will be necessary anyway.) The exit well could easily be included in the tanking, which would at least prevent ground water seeping in, but would also be vulnerable to differential movement stretching or damaging the membrane or the way it is sealed to the concrete.

I did think the waterproofing between the basement and the exit wells was going to be difficult, however in a recent discussion with the local representative of one of the main waterproofing material suppliers it seems that it isn’t a problem and can easily be dealt with. The “tanking” membrane can easily be bridged across the two structures and is both tough and flexible enough to cope with any differential movement (within reason).


Concrete Block and Tanking

Probably the current mainstream choice, constructing the walls out of concrete blocks, is something that will be familiar to most builders. As such it shouldn’t be too hard finding someone to do it and should be reasonably cost effective.

While still being based on concrete, many concrete blocks consist of significant proportions of Pulverised Fuel Ash (PFA) as an aggregate with the concrete binding the block together. This means that much of the material is a waste product and as such you are preventing it being disposed of in landfill or other nasty ways. It’s still concrete, but better than using entirely new materials.

The blocks are usually supported by steel rebar set into the floor slab to provide extra strength. While this has good retaining and loadbearing properties it’s not inherently waterproof and will need to be tanked thoroughly to prevent water ingress.

Service channels for wiring etc. can be cut into the blocks in the usual way.

To me, the tanking is the biggest risk with this type of construction. A blockwork wall will not be flat so tanking adherence will not be perfect and there are too many joints and places where leaks could develop over time. I don’t feel entirely comfortable with it and while I’m not ruling it out at this stage, but would need some convincing.

Insulated Concrete Formwork

Insulated Concrete Formwork (ICF) is a relative newcomer to building construction, and while there are some advantages, I don’t think they outweigh the disadvantages.

ICF basically consists of hollow polystyrene (or similar) blocks that are stacked to form the wall shape and then filled with poured concrete. Rebar is often needed to strengthen the structure. The polystyrene blocks are very light and easy to cut and it is possible to get custom shapes made (curves etc.) if you desire and, of course, the insulation is built in. This makes it very flexible and fairly easy to put up.

In principle the concrete is poured in one go providing a continuous boundary. In practice there are still potential leakpoints around the ties between inner and outer skins of the formwork and around the joint between the floor slab and the walls. Consequently, tanking would probably still be necessary, and might be complicated by the insulation on the outside of the concrete. Additionally pouring the concrete can actually be harder than it appears at first glance, particularly with unusual shapes or rebar.

Another disadvantage is that all the concrete actually ends up coated on the inside by polystyrene insulation which prevents it working adequately as thermal mass. (I have heard one story of a house built with ICF, and then the builder laboriously hacked off the internal polystyrene to get the thermal mass back….) In addition, since the concrete is partway through the insulation some have claimed that the concrete can end up being relatively cool and become a trap for interstitial condensation. If true, this could cause hidden mould issues which are probably not healthy.

The concrete is highly likely to use non-recycled aggregate and as such it’s probably not very environmentally friendly.

Finally, any wiring will need to be kept away from the polystyrene and will probably need an internal service cavity covered in plasterboard.

As you can tell, this probably isn’t my first choice. In fact, out of the options identified, it’s probably last.

In-Situ Cast Concrete

In this case, concrete walls/panels are cast on site, complete with rebar for strengthening, either using timber shuttering, or laid down flat on the ground and then raised into position once set. This provides the benefit of an easily inspectable concrete wall, with good loadbearing and retaining properties. The concrete itself is inherently waterproof and it’s generally possible to pour the entire structure in one go making the only vulnerable point the one with the ground slab. It’s probably still a good idea to include some tanking on the outside to add a second line of defence.

It does leave the construction to the last minute, making it reasonably tolerant to change, but does introduce risks with bad weather getting in the way, and the need to wait for the concrete to set, adding to the installation time.

The concrete is highly likely to use non-recycled aggregate and as such it’s probably not very environmentally friendly.

Insulation needs to be added separately, again preferably externally and all of that concrete is available to help with the thermal mass of the building. Wiring and facilities can be channeled into the concrete in the same way as for concrete blocks.

It’s also more specialist than concrete blocks, so potentially less easy to find people to do it. Having said this, we have found two groundworkers that appear to have considerable experience in this area and initial cost estimates look reasonable, especially compared to some of the other options. One of them is a fully approved waterproof concrete and tanking specialist which could offer an extensive guarantee with the system.

Precast Concrete Panels

This is similar to in-situ cast concrete, except that the panels are made in a factory. This provides higher levels of accuracy, the ability to integrate insulation into the structure and to include wiring channels and boxes all in the factory.

The panels are then shipped to site and craned into position. Metal loops and rebar are generally used at the joints to connect the panels and then filled with a concrete pour, or mining grout. This results in a easily inspectable waterproof structure with excellent retaining and loadbearing structures. The process is virtually independent of the weather and is very quick to erect and seal.

Depending on the insulation options, a good amount of concrete is available internally for thermal mass and wiring channels are dealt with for you. It does mean that you need to finalise the design a few weeks ahead of time and that it is very hard to make last minute changes.

Depending on the supplier, the concrete used can be anywhere between totally new, or almost totally based on waste materials and different options are available for insulation, with some better than others.

There were a small number of dedicated basement suppliers in existence, but one of them has recently ceased trading, one of them doesn’t appear to have a real basement track record and the other is frighteningly expensive. All were/are clearly setup to achieve building regulations levels of thermal performance and would need additional measures to achieve PassivHaus levels, despite one of them being German.

Alternatively, there are plenty of companies offering pre-cast concrete panels for general structural purposes, and these could easily (and have) been used for basements or below-ground houses. The disadvantage here is that either the basement slabs have to be precisely constructed to allow pre-ordered panels to fit, or panels can’t be ordered until the ground slab is put in place and measured up. Either way introduces additional work and risk.

I’d probably still want to externally tank the walls and slab to be on the safe side and then install insulation outboard to meet the thermal performance targets, but this should be relatively straightforward.

One thing is for sure – based on the quotes I have received for a pre-cast solution so far, it looks like an expensive way of building a basement.

Party Wall

Of course, with a full footprint basement on our site we will end up excavating close to the site boundary and this means close to our neighbours. This means that we need to take adequate measures to prevent the walls of the hole collapsing and undermining our neighbours houses, with the disaster that would result, both timewise, financially and not least to the relationship with our neighbours.

The worst case scenario, both time and cost, would be a complicated and potentially expensive process using continuous auger concrete piling along both side boundaries, or at least those parts that fall within Party Wall.  However, we’ve had two groundworkers look at tour ground conditions, and both are firmly of the opinion that the cornbrash on which we are sat will be solid enough that we shouldn’t need to worry too much about special provisions, especially one one side.

We’re currently selecting structural engineers and will be embarking on Party Wall as soon as planning is through.


The main conclusion is that concrete is pretty much unavoidable for a basement construction, and that some form of cast concrete, preferably waterproof, is almost certainly more robust than a blockwork built system.

From a cost perspective it is starting to look clear that an in-situ cast system is going to be the most cost effective, and technically has some advantages. It’s probably the way we will go if we can find a suitable contractor.

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