self build waterproof basement formwork membrane concrete insulation

Insulating a home with a basement.

I'm not selling anything here. This page is my conclusion and recommendation after years of exhibitions and going back to customers and asking them how what they had chosen worked out.

ICF did not work out well. Nor timber frames or SIPs. Heat pumps generally failed.

What worked best of all, and for less cost, was thermal mass and heat recovery ventilation with air tightness.

  self build basement

ICF basement house   In 2012 I built a basement and all the house walls with ICF.

These clients initially thought they had achieved zero energy bills for their new home with U Values around 0.20 rather than 0.10.

Note: The official term is Net Zero Energy Building. EU Directive2010/31/EC defines NZEB.

You can see a video I made in 2014, two years later, explaining this basement and its heating here.

These clients thought that their fuel bills would be zero because
  1. Photo Voltaics on the roof,
  2. Ground Source Heat Pump bringing heat in,
  3. Heat Recovery Ventilation saving some heat before it is lost outside,
  4. Air Source Heat Pump in the basement heating their water cheaply by extracting even more heat from the still fairly warm air on its way out,
  5. 150mm of ICF insulation above ground,
  6. Roof trusses stuffed with insulation and
  7. Excess electricity sold to the grid.
When I made the video in December 2014 the weather had been very cold already. They calculated that in that first month they spent just £26 on buying energy. The rest came from their GSHP. On an annual basis, after selling excess electricity in Summer, they thought their bills would be about zero.

But when I returned again in April 2017 to catch up on old times, their bills were £1,000 a year for energy.

Something had gone wrong. Actually, two things in particular seemed to have gone wrong.
  1. The GSHP had got expensive. It no longer cost less than £26 a month to heat their house. It was costing 10 times that because the ground had become too cold.
  2. With no thermal mass because of the ICF insulation inside, they got too hot in Summer and installed air conditioning.

The guys selling ICF, timber frames and SIPs are all trying to sell you a building method that wastes the opportunity to store energy in thermal mass - in concrete.

True, it takes less energy to warm the air in a cold house of lightweight construction. But why should the house have got cold?

True, it takes less energy to cool the air in a warm house of lightweight construction. But why should the house have got too warm?

True, all these choices are more air tight than brick and block cavity walls used to be. But other methods of construction have upped their games since.

BUT NOTE: the ICF description only says "making it possible". Not that it will.

And NOTE: the timber frame description puts made from renewable woodland first. Not saving energy.

AND: the SIPS description only compares SIPs to "older technologies".

My conclusion is that ICF, timber frames and SIPs all perform poorly compared to newer technologies and masonry building fabric.

My money is on the future being concrete.

I Googled "benefit of ICF / timber frame / SIPs. These came up first:

ICF basement

timber frame basement

SIPs basement

Further down I explain how to generate most of your carbon-emission-free energy on site, this means selling excess electricity to the grid and paying back the carbon investment in cement.

Can a concrete house be zero carbon ...... ?

Can a concrete house also be free to heat ...... ?

The most common question I get asked is, should you insulate inside or outside a basement? I answer that lower down as well.

Heat Pumps.

The ground source heat pump and the air source heat pump guys have been trying to let you believe that the efficiency in Summer is so good that even in Winter you save money. But put on the spot many can't tell you that you would save anything on the coldest days when you need the most energy.

But if, at exhibitions, you ask how much does it all cost, it seems to me that those talking in the realms of £7,000 are the ones who struggle the most to convince you you will always save energy; while those talking about £20,000 are very confident you will save energy all the time.

However, saving money and saving energy aren't quite the same when it comes to renewables.

My customers who were, I think, disappointed with GSHPs seem to have saved money burying overlapping coils. Lots of pipe, not much soil to get heat from.

The expensive suppliers might tell you to have a straight pipe in a very deep borehole, say 150m deep, perhaps two boreholes.

GSHP basement GSHP basement

The expensive suppliers might be tapping into a far greater heat source than the cheaper guys.

I tend to think that the cheap guys use all the available heat in the ground quickly before the end of Winter; while the expensive guys have such a huge reservoir of energy deep enough to be warmed up by the centre of the earth that their installations succeed more often.

So why do people buy cheaper?

Maybe because cheaper could be free over 7 years.

The Which? magazine has a guide.

Perhaps the RHI, Renewable Heat Incentive, will cover the whole cost of a small installation more easily than it will cover the whole cost of an expensive scheme; except that with a cheap scheme you probably still need some expensive energy from the grid and another heating system as well for the days a small heat pump cannot cope.

£20,000, I'm not saying that is what yours needs to cost, might only be partially offset by the RHI, Renewable Heat Incentive, yet more likely to provide all your energy cheaply.

£4,000 and the RHI might cover the heat pump cost completely, but you might not get much free heat that way and you could still be buying expensive energy from the grid as well.

It seems to me, that small ASHPs and GSHPs without access to enough soil struggle in cold weather - especially if your home got cold when the temperature dropped suddenly outside and you want a lot of heat fast.

Whereas a larger heat pump heating a house that is well insulated and protected from a sudden drop in temperature outside will cope adequately at all times.

Another technology that caught my attention is Seasonal Thermal Energy Storage. STES.

Icax says on its website: "It is a characteristic of earth that heat only moves very slowly through it - as slowly as one metre a month."

Icax tells us what we hear all the time, that it is very expensive to store electricity. But, they say, it can be virtually free to store heat. What they seem to promote are two systems both with coils alongside each other buried in the ground.
  1. By Summer, solar energy is used to warm up the ground.

  2. By Winter, that same ground has the excess heat taken from it much more efficiently by GSHP because the soil is warm to begin with.
When I put this to heat pump suppliers at an exhibition, they doubted you could have enough surplus energy in summer, from solar panels, to put enough heat into the ground to provide for a whole Winter. Are Icax right? I don't know. There is an academic exploring a similar idea. A chartered engineer and chartered geologist, she is a research fellow whose blog describes her excitement at the idea. Two pages here: testing and monitoring.

Should we be storing heat in the soil beneath our basements?

We would need the heat to be very deep beneath our basements so that it didn't come out into our habitable space too early when we didn't want it to.

If you have already dug 3.5m down the only safe way to go further might be with piles or a borehole. If you hit water then any energy you tried to store would be washed away instead.

I'm not sure storing energy underground could be viable.

Finally, my recommendation.

I would
  • Maximise thermal mass: all the floors, all the walls and the flat roof.

  • I would have an Air Source Heat Pump, probably in the loft.

  • Heat Recovery Ventilation.

  • Excellent air tightness.

  • Continuous insulation under the basement, all over the walls and over the roof outside the thermal mass.

  • I would cover the roof in solar panels, perhaps a mix of photo voltaic and water heating.

seasonal heating cooling envelope Continuous insulation outside the thermal mass walls and floors. Shown here in yellow.

A Chinese manufacturer, Himin, can make a roof entirely of solar panels. You needn't have a roof with panels fixed to it. Just one roof of panels.

I am making the assumption that it might not be completely weatherproof so my waterproof roof is the flat roof above the bedrooms. The ASHP and hot water tank can go in this loft area. Perhaps the HRV and rain harvesting filtration as well. So you might house all this equipment in a weathertight wooden shed beneath the sloping roof.

I have shown extra footings to provide a foundation for brickwork, shown in brown. A careful choice of wall tie will avoid thermal bridging.

I have shown this sketch to heat pump suppliers at exhibitions and the major positives were:

  1. The thermal mass would help hold the temperature inside constant. The heat source pump should be set to put heat into underfloor heating at the actual desired temperature, and allowed to be thermostatically controlled 24/7/52. Always on.

  2. The thermal mass would not be shocked by a sudden temperature drop outside. Therefore, there should never be a sudden demand for heat.

Can a concrete house be zero carbon over its lifeteime?

Can a concrete house be free to heat including the cost of all your heating equipment, energy saving equipment and insulation?

Asking for both is a pretty tall order, whereas, just zero carbon that cost you a lot of money might be fairly simple over the lifetime of the building. Or zero energy bills as a result of a large investment that might not be recouped before expensive equipment needed replacing might be easy as well.

I am convinced that once built and lived in, my proposed home would require the least energy to keep it warm and cool compared to any other scheme, unless that scheme cost several times as much.

But my best-possible energy saving might create another issue. You might not need to generate enough carbon-free energy to get the full RHI over 7 years.

You could find a heat pump just the right size that the RHI might give you back 80%. While you are using it your energy will be almost free so that saves £1,000 to £2,000 a year. Hopefully, though, the heat pump that gets you the best RHI return also gives you enough heat all the time, or else you will need some expensive energy from the grid as well.

Even after the RHI stops you will still have cheap heat. The less heat you need the more you save compared to an average home built years ago.

If you sell excess electricity from solar panels to the grid every summer you will be paying back your carbon investment having used a lot of cement.

If you sell the grid more than 3 times what you buy back in Winter your fuel bills will give you a refund. But you might need more panels to do that than the RHI will reward you for.

The blend you choose will depend firstly on your priorities, secondly on your pocket and lastly on whether you pay for exceptionally thorough design.

You might get zero carbon completely free of cost - after your children's children pass on the benefits in their will. If everyone cuts carbon emissions now our great grand children might still have a habitable planet. It isn't just about money.

You might not like any of my thinking. Fair enough. I'm not selling it, I'm only sharing it.

Phil Sacre

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