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.

Most had a very warm basement, even without heating, but often the rest of the house did not work out as well as expected. Most would fail the new rules that should be coming in the year after next.

The EU Directive that bought us energy performance certificates requires new homes from 2020 to be Nearly Zero Energy Buildings with most of their energy from renewable sources. Definition of NZEB near the bottom of the page.

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.

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. These days, the mass housebuilders cover over all the holes in the masonry and insulation with airtight plasterboard and skirting board. The cold still gets in.

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 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 ASHPs too small for the job 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 of her blog 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.

About 20 years ago, the Building Research Establishment, BRE, produced a paper about the U value of a basement without insulation.

The U Value of an average domestic basement, just because it is buried, is about 0.16 before you add any insulation.

The point, therefore, is that a basement neither needs much insulation nor much heating.
ICF basement U Values ICF basement U Values ICF basement U Values

Finally, my recommendation.

I would
  • Maximise thermal mass: all the floors, all the walls and the flat roof all built with waterproof, always completely crack free, waterproof, reinforced concrete.

  • Heat Recovery Ventilation.

  • Excellent air tightness. Windows, doors and service entries as well as the concrete structure.

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

                        - Just the above, I am led to understand, is likely to exceed Passivhaus Standard, depending on your windows.

  • 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. Less under the basement.

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 solar panels might not be completely weatherproof so my waterproof roof is the flat roof above the bedrooms.

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. Your heating source should be set to put heat in 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.
You might only need a couple of electric radiators. 1 KW might be all you need to slowly bring the temperature back up to 20oC after it dropped to 19oC after the weather outside turned to freezing.

You could vault your ceiling / loft; in which case you have to find a way to fix your solar panels and / or roof tiles without causing a thermal bridge.

Can a concrete house be zero carbon over its lifetime?

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

The benefit of the concrete house is its airtightness and thermal mass. Once up to temperature, you hardly need anything to keep it there. The savings are:
  1. Hardly requiring any energy.

  2. No need to invest in any expensive GSHP, ASHP or biomass equipment for space heating.

  3. More energy than required produced on the roof and the excess sold to the grid.
You still have to choose your domestic hot water provision. The cheapest might be electric hot water heated partially by solar energy and brought up to temperature by electricity.

The environmental benefit of concrete is not in how much renewable energy you get from an investment of tens of thousands of pounds, but how much less energy you use during its incredibly long life time than building any other way.

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.

I have said you might not need GSHP, ASHP or biomass. These things are sold because salesmen explain how the RHI makes them virtually free. My proposed dwelling would probably not use enough energy to get the full RHI.

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.

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

Phil Sacre

From DIRECTIVE 2010/31/EU

Article 9
Nearly zero-energy buildings

1. Member States shall ensure that:
(a) by 31 December 2020, all new buildings are nearly zero-energy buildings;

2. 'nearly zero-energy building' means a building that has a very high energy performance, as determined in accordance with Annex I. The nearly zero or very low amount of energy required should be covered to a very significant extent by energy from renewable sources, including energy from renewable sources produced on-site or nearby;

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