self build waterproof basement formwork membrane concrete icf

Basement (and house) Wall Formwork.

polarwall house
quadlock house    polarwall basement
These are ICF. There are a 100 brands or so. If any were really any good you would be able to buy them through builders merchants.
This is timber, built my way rather like ICF, therefore very simple. Much stronger at corners, keeps its dimensions, much, much easier to adapt.

The choice for basement formwork is between ICF that remains and formwork that is taken off and insulation added afterwards.

For years, few people questioned sandwiching concrete between 2 layers of ICF insulation. It seemed brilliant.

But it wasn't brilliant. It was a mistake.

Thermal Mass recently got added into SAP calculations.

Thermal mass is the ability for the heavy construction material to even out temperature changes by storing or releasing heat.

ICF salesmen will tell you that the concrete core in ICF is great thermal mass. But that is a lie. Part of your SAP calculation includes the Admittance Value (AD): "the resistance to heat flow at the surface of the material". By insulating between your thermal mass and your room you will probably reduce your 'Thermal Mass Parameter' from 500 or so (better than good) to around 70 (worse than poor). More on thermal mass here.

The real choice is between heating/cooling just the air or making use of thermal mass.
self build formwork basement

My method uses a lot of timber. Most of which gets re-used in the house or sold on, so it is usually very cheap.

These walls have already been filled to half height. Now the formwork has been completed to the top and the wall is ready to fill to full height. Filling in two pours avoids the honeycombing in concrete that leaks.

‌During 2019 I had a long, friendly chat with someone I know still selling ICF. He told me he supplied the first of 3 apartment blocks and only had one burst, for which he was proud. The customer, he said, chose another ICF for the second block and is now in dispute trying to claim £50,000 from that supplier. For the third block the customer has chosen steel frame and not ICF.

This basement was built by the selfbuilder with a few of his mates and some help and product from me.

self build basement formwork self built basement

I sold him a waterproof guarantee that his concrete and concrete joints would have no visible ingress of water for as long as I was alive and able to fix any leaks. That was in 2015.

Today that would cost (no VAT)
  self build basement
  1. Additive to make his concrete waterproof. £29 per m³. About £1,000 for this basement.
  2. Training and Supervision of concrete pours £300 a visit. 5 visits usually £1500.
  3. Poker hire £40 a day, 1 poker to use and a spare just in case.
  4. Fibreglass rods to get the slab level. About £42 for this basement.
  5. Fibreglass rods through the walls cast in leaving no holes. About £260 for this basement.
  6. Nuts for rods. About £335 for this basement.
  7. Steel waler plate hire. £1 each for up to 4 weeks. About £21 for this basement.
  8. I also have excellent, strong, external drainage membrane costing less than £4.25 a m²
The guarantee comes free if you buy all the above as appropriate and you do the work as I show you.

You cannot get a waterproof concrete guarantee with ICF because the concrete additive supplier cannot inspect the concrete and get voids filled before he issues the guarantee.


Heating/cooling just the air. For instance: ICF, timber frame, SIPs.

Please take a look at this other page about insulation. It opens in a new window and starts with a case study about a couple whose basement was built with 50mm/75mm ICF and their house walls above ground with 50mm/100mm ICF.

Soon after they moved in they fitted sensors on everything and found that during the first cold month they spent £26 on energy. They expected that success to continue and to sell excess electricity to the grid over the Summers and get that back.

But 3 years later I returned and instead of near zero energy cost they were spending £1,000 a year.

They aren't storing any heat in their walls, they had to install air conditioning in their bedroom and their Ground Source Heat Pump got more and more expensive as it froze their ground.

ICF failed to save any energy.
monitoring energy use


ICF out of plumb ICF is great to put together and still looks straight and plumb before you put concrete in.

But the reality is that that it stretches, leans and bursts under the weight of wet concrete.

Walls stretch and lose their dimension as the weight of wet concrete inside forces the wall to get longer and pushes panels apart. One wall getting longer makes the wall round the corner curved.
  this polystyrene spread even after the concrete was setting These polystyrene panels spread after the concrete was setting.

ICF blow burst   ICFs often burst at corners.

Bursts and concrete pouring out is common.

ICFs rely upon something built in crossing the gap and supporting both sides at once. At corners that strength isn't there because double sided formwork becomes single sided opposite the inside of the wall round the corner.

The ICF only holds where the panels are whole and double sided.

Perhaps you won't use ICF and you don't want to use my timber method either.

That's fine. You can still use my rods so you leave no holes with traditional plywood shutters or most systems you hire in. The rods are 18mm diameter. You can still enjoy a completely waterproof basement structure if you use a different formwork.

A brief description of my formwork method. It is very simple, it does not require a kicker and there are no holes left through it. If you have piles or underpinning they can be one side of your formwork.

resin rod formwork not insulated concrete formwork   We construct it all the way round in one go but only to half height.

First the props are put together and fixed in place. You can see that the wall will be upright and straight.

Next the scaffold boards or 6x2 one side are screwed to them putting in the rods as you go.

Then the timber the other side, the strongback timbers, plates and nuts.
  waterproof concrete basement resin rod formwork stripped

This is how we cast completely waterproof basement walls and completely air tight house walls.

From here onward I go into far more detail trying to convince you not to use ICF.
If you have already firmly made up your mind you might want to stop reading here.


Note the corner marked A.
Note also the enlarged detail showing the top of a prop.

problems with ICF
Resin Rod and Nut Method

Single sided resin rod against underpinning and double sided resin rod under construction by the customer.

resin rod nut wall formwork against underpinning and double sided

After 12 years on construction sites I began building domestic basements with ICF. I tried different brands, reduced the height of the panels, added ply at corners, poured in 2 pours, made much stronger braces, developed safer scaffold, and so on.

Having tried and tried has led to a number of very good innovations.
  1. No kickers. Kickers sink when properly compacted or they get made very badly. Kickers are notorious for requiring repair and being expensive to waterproof.
    Kickers are bad.

  2. The width of ICF formwork is fixed and so it takes much less to hold it upright. Much less than traditional shuttering.
    Fixed formwork width is good.

  3. Using 4x2 and hex head washered coach screws to make props that double up as scaffold. All the screws come out. All the timber is good for another use.
    Re-using your props in the house as stud partitioning is good.

  4. Steel mesh and spacer bars to hold it all perfectly upright putting no pressure on the formwork.
    Steel easily fixed by amateurs, because it comes welded together, is good.

  5. Using completely waterproof concrete instead of membranes, tapes, strips or pumps with no additional labour whatsoever.
    Right First Time for less cost is good.

  6. Traditional formwork relies upon steel threaded rods throughout that are withdrawn (through plastic sleeves) leaving hundreds of holes to fill.
    Our resin rods are cast in. Cutting them off flush is far easier than filling holes and therefore good.

  7. Supervising the work.

    Note from the interim report of the Grenfell Tower disaster: "Compliance, enforcement and sanctions processes are too weak".

    I supervise all the concreting with my admixture. I provide pokers I know will do the job properly. You will do as I say because you want my guarantee.

    Having someone stood doing nothing is considered bad only by those who don't pay for mistakes to be put right.

    And the true secret of avoiding expensive repairs by those who do.

  8. Removing the formwork to see that the concrete is faultless, and doing something about it if it isn't.
    You, your architect, engineer and building inspector will all be happier having seen the concrete is solid throughout. This is good.

  9. Cutting the cost of exactly the same insulation by buying it as large sheets and sticking it on with spray adhesive later, after the messy work is complete.
    Spending less and not having the insulation in place far longer so it gets damaged is good.

My timber formwork is much stronger than any ICF. It is much cheaper yet better than any traditional formwork (in the hands of anyone but professional formwork carpenters). The concrete can be properly compacted so you will get a better result - and if it is Waterproof you want then it is Waterproof you will get.

ICF is just polystyrene. If you fill it carefully, gently and without compaction you will leave voids inside. If you vibrate it to get rid of the voids, absolutely necessary to make it waterproof, no system of polystyrene is strong enough.

Traditional formwork is ply backed by 4x2s and upright soldiers, both sides, slammed against the kicker and squeezed against the plastic tube that the threaded rods go through. It takes many years of experience to gain the skills required. A dedicated team follows on to hide all the blemishes - not to carry out thorough repairs.

There are some very distinct advantages of timber formwork over ICF:
  1. Removing the timber will let you see that the concrete is good and repair it if necessary.

  2. Timber will be stronger and less likely to burst; especially at corners where many ICFs are weaker, even with bespoke corner blocks.

  3. Timber will be stronger and less likely to stretch in length under the load of concrete.

  4. Timber can be fabricated to be a single sided formwork against underpinning.
We have tried and tried, with experience of over 50 ICF basements, to fill and compact the concrete properly and make the ICF strong enough not to stretch, expand, lean over or burst. None of them. None of them are strong enough. Any ICF basement wall that kept its shape is not properly full of concrete.
  first photo of scaffold board basement formwork

If you are contemplating a swimming pool always use this timber formwork. ICF is a waste of money because the concrete is a much better substrate for tiles and you can easily place insulation the other side before you backfill.


icf corners spread or burst when being filled with concrete

icf corners spread or burst when being filled with concrete

this polystyrene spread even after the concrete was setting

icf corners spread or burst when being filled with concrete

icf corners spread or burst when being filled with concrete

icf corners spread or burst when being filled with concrete

And here you
see emergency
bracing where
is bulging
under the
weight of

I sold/used a white Canadian ICF for 15 basements, then tried a green Canadian ICF once, used plain ICF boards 300mm high 19 times, the same, plain ICF boards 200mm high 15 times and I once attended another brand's pour as the concrete additive supplier.

I have heard about most of the rest from the concrete pump operators, many of whom have pumped my concrete many times, got to know me and chat freely.

All ICFs either fail or they are left full of voids (unless poured one course at a time).

The Beefed Up plain board system, with boards 200mm high instead of 300mm, used half as many plastic rails again, and more than 50% more ties because I reduced the spacing between them. So it cost about £8 a square metre more, with the same insulation, to make it strong enough not to burst with my concrete inside.

I used this beefed up version 15 times by the end of 2012. Every one of those 15 customers complained about the mess at corners and dimensions changing once the concrete was inside.

The panels no longer bulged out into a curve, neither did they burst, but the walls still stretched in length.

Both these polystyrene systems spread 6mm or so, even after the concrete was setting.

Spending more just for strength, the insulation being exactly the same, still did not provide the quality customers want because dimensions changed.

No brand of ICF, beefed up or not, keeps its shape if the concrete is vibrated thoroughly enough to be waterproof.

Do not use ICF with waterproof concrete now that a better system is available.

     This is a close up of the corner marked A in the photo above. The panels opened enough for concrete to squeeze out making the dimension wrong and creating a right mess.

icf corners spread or burst  A

DIY Timber Formwork.
Insulation added later.

Easy to erect timber formwork easily strong enough for waterproof concrete and easy enough for any competent self builder. Easier than ICF because you can screw two bits of timber together. You cannot screw ICF to ICF.

I used the resin rod method single sided against underpinning and for double sided retaining walls many times.

The secret is to keep using the timber. Formwork should only cost £12.50 a m² if used again and sold on or used permanently elsewhere. 100mm of waterproof insulation added later will be about £12.50 a m² as well.
  • First fix props made from reusable 4x2,

  • fix one side of your scaffold boards to the props 7 courses high,

  • on top of the first board put 1m lengths of roof batten with a 20mm gap. These gaps are where the threaded resin rod, thin spacer nuts and big strong nuts go, so that you don't have to cut, notch or drill the reusable boards,

  • and 2no. 6x3 timber strongbacks either side.

  • remove the timber after the concrete has set and re-use all of it for more formwork and eventually use your 4x2 in your stud partitioning.
All the timber is reused or sold on so it is very cheap.

For single sided formwork against underpinning and concrete piles resin the threaded bar into the concrete behind and build in the same manner filling between the single sided formwork and your underpinning.

use 4x2 6x3 resin rod thin nuts and big nuts with scaffold boards

Remove the props and uprights the day after the pour and the nuts and scaffold boards after the concrete has hardened.

Cut the resin rods off flush after they are finished with.

Inspect the concrete.

Add probably 100mm of waterproof insulation outside before you backfill.

Traditional Formwork

Traditional formwork needs a kicker to clamp against either side. Kickers always leak and they are very difficult to waterproof later.

Traditional formwork leaves walls full of holes after threaded steel rods are removed.

To be cost-efficient they cast walls in sections and move panels with a crane or big digger. More joints create more leaks.

Traditional and hired formworks are far from a DIY system, you need years of experience.

  traditional formwork with kicker   This photo is a link to a site with
what they call a classic example
of a failed kicker joint.
traditional formwork kicker failure

Concrete against underpinning and concrete piles.

Traditional Single Sided Shutter.

The bottom is pushed against a kicker.

The big pans are assembled and lifted by crane.

To be efficient with the hire cost they cast walls in short sections and move panels along daily, not giving the concrete any chance to cure properly.

This is not a solution for a domestic basement, which is why ICF is cast close to piling or underpinning, not against it.

  traditional formwork
This is the sort of set up normally required to pour against piles with a single sided shutter. I would question whether anyone could do a good job pouring such height, but apart from that this stuff is clearly not going to be cheap nor moved around by a couple of men.

ICF against concrete.

traditional formwork   All Insulated Concrete Formworks rely upon being double sided. Therefore they need to be built independently of underpinning.

That means leaving a space in between that needs to be filled up somehow.

It also means leaving insulation in between which is probably a complete waste as well as a waste of space.

The side of a basement against underpinning is against very thick concrete the other side of which is underneath the building being extended. There is a strong scientific argument that no actual insulation would be needed against this wall because any heat lost would take a very long time to escape to open air, which is the definition of Insulation.

DIY Timber Formwork against concrete.

Actually cheaper than double sided timber formwork because there need only be timber one side if the other side is the underpinning or concrete piles.

resin rod resined into underpinning to create single sided formwork

Using the right resin to anchor threaded rod into the concrete is simple. You don't even need to be accurate because the threaded resin rod is flexible and still just as strong.

Another regular user of brands of ICF that I haven't used myself sent me this after I sent him a friendly email saying that a pump operator described, during one of our pours, a torrid day they had experienced the week before. I am keeping it anonymous, obviously.

"the burst occurred at a T junction where an internal ICF wall joined the outside and although they had braced it with a bit of 12mm ply it was not enough, but the final straw was that the 10mm aggregate from **** had at least two stones that were 40mm at least. When they cleared up the concrete they found these two stones and one plastic web that had been broken off!! The client was on site and was happy with the end result and concluded the causes were not with my men on site - in fact he found the stones and web! We have experienced at least one burst on nearly all of our ICF jobs and there is not an identifiable pattern to the causes. We have had bursts on full blocks in reasonable sized walls about 3 courses up which has to be the block. We have used self compacting and stiff mixes with pokers, we have tried delaying concrete trucks between pours, we always work around the walls so as not to fill up from one point and none of it is a guarantee against bursts!!

I have to say that I have never been comfortable with ICF for basements and upset **** the other day when I told them so - for the reasons you state of not being sure the concrete is correctly compacted but also the fact that concrete does not chemically bond to the webs leaving a potential microscopic passage for water at each rib."

Like us, this company is also experimenting with non-ICF ways and whilst they have not publicly stated they will not use ICF again they do not want to.

At long last a very experienced practitioner (me) has gone public with the problems common to every brand.

Design Professionals do not like ICF because they cannot see that the concrete is complete throughout.

Contractors do not like ICF because it is fragile, bursts and carrying concrete back up in buckets is heart as well as back breaking and overtime is expensive.

Customers do not like ICF afterwards because walls bulge and lose their shape so rooms will be smaller after the plasterboard hides the problem. And rendering the outside means having to shave foam off first to get a flat plane but losing insulation they paid for.

I think ICF is finished.

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