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Basement Insulation. Thermal Mass.
Improving efficiency. Reducing waste.
This page is just to get you thinking about what you want to tell your architect is your preference, which they are likely to include unless they have strong views to the contrary.
You need a small amount of knowledge about insulation, thermal mass, heat recovery ventilation and better ways to heat your home before you can come to your decision. This page starts off brief then I have put more detail, in case you want to learn more.
I provide plenty of evidence, further down this page that
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Soil is insulation.
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Concrete is insulation.
Whoever does your Standard Assessment Procedure (SAP) calculation, probably bought a spreadsheet with no mention of these wonderful insulating materials.
I invite you to download and supply that person with my evidence. Make them include soil and concrete.
Otherwise you will be spending lots of money on unnecessary insulation, as well as losing living space to insulation, for no benefit. Except that you get your now extremely expensive certificate more easily.
There is government subsidy available for heat pumps.
But heat pumps are only at their greatest efficiency when it is warm and you don't need any heat. You want most heat when it is freezing outside when heat pumps become much less efficient. They can even become the opposite. Requiring more energy than the heat they produce. Air source when it is freezing outside, and ground source after 3 years because you have frozen the ground and there is no heat left.
Another policy government has got entirely wrong is electricity generated from gas. It takes two KW of gas to produce 1KW of electricity. That's one reason why electricity costs you over twice as much for the same energy from electricity as gas.
What I would like you to focus on, is designing your new home to make maximum use of free heat from the sun, very cheap heat when solar or wind are producing electricity no one wants, and keeping the heat you have - also very cheap indeed.
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Conservative government policy was to reward you for using more electricity produced by gas, rather than rewarding you for using less energy.
At the Futurebuild Show 2024, there was huge concern.
Heat pumps are working more in cold weather than warm weather, obviously. They use such a lot of electricity when the source is at or about freezing that one customer's new electricity supply was increased from single phase to 3 phase when he mentioned an air source heat pump. When the demand for electricity is high, that extra electricity is all generated by gas. Emitting twice the CO2 as a gas boiler for more than 3 times the cost.
Currently, government policy is that you get a good SAP score for a heat pump that will destroy our infrastructure, and a bad SAP score for a ZEB (see 3) that will save it.
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I go into a lot more detail, with proof heat pumps don't work on my page about heating and cooling that did not seem to work for my clients.
The problem with free energy from solar or wind, is they generate intermittently. Not enough when we need it. Too much when we don't.
The biggest missing link is batteries. Three are available (that I know of).
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Store electricity.
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Store heat to warm your home.
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Store heat to heat your water.
1. Store electricity.
Batteries and the other kit are not cheap, I know. Their value will be in how much money you save, or how much you make selling stored electricity on.
You can buy mains electricity extremely cheaply at certain times - even free from Octopus energy if you have a smart meter and their Intelligent Octopus Go. They say when the grid is greenest. Presumably when wind and solar are generating too much to use. You could buy cheap to store it to use in your home when tariffs are much higher, saving the difference.
Cars cost a lot to run and an electric car already has batteries. It seems to me that solar panels that replace petrol costs or domestic tariff electricity costs have the greatest potential. You could have a solar panel system not connected to the grid. just lots of solar panels and lots of batteries. You might never buy petrol and still sell electricity to other EV owners, perhaps via Zapmap.
On October 10th 2024, The Morrisons nearby with thousands of solar panels on its roof, was selling EV charging for up to 76p for very fast charging, compared to my domestic rate of only 25.21p, per KWhour.
2. Store free heat to help warm your home.
Thermal mass means storing solar gain in your floors and walls. When the sun warms your home too much, some heat goes into your concrete. When the home cools at night, that heat comes out again. You reduce the need for air conditioning and you reduce the need for heat before you wake up.
Thermal mass is essentially free of investment, as well as free energy. You just need to make the right choices.
3. Store cheap or even free heat to heat your water, in a ZEB.
A ZEB is a zero emissions boiler. They store heat at a very high temperature to heat water on demand. Ideally, you will buy off peak electricity, going to waste from wind farms and solar panels, for very nearly free and use it heat up your ZEB 'boiler'.
For instance, Thermino heat batteries. and Tepeo ZEB. Others may be available.
Insulation.
Insulation should either be inside or outside your structure. Not both.
Either only heat and cool the air,
or soak up and store heat from the sun, washing, cooking, and your body heat.
Years ago, a client mentioned that a good friend of his is a Professor of Environmental Engineering.
Shortly after, I received this paragraph, presumably aimed at the client but with my web page in mind.
"Putting the concrete on the room side of the insulation (ie ground-insulation-concrete-room) it will retain heat (like a storage heater) and reduce the rate (speed) at which the overall room cools down and heats up. It will make no difference (over a season) how much heat you use to heat the basement (if its kept at a reasonably constant temperature). It will take longer to heat if you allow the basement to cool down so if you wanted to use the basement infrequently for short periods (eg as a spare bedroom) and then leave it empty (and cool) then it may not be the best solution (although that could be overcome by adding more insulation internally)."
It seems to be saying, in particular, that all other things being equal, it will cost the same to keep a basement warm or to warm it every few weeks.
Basements are insulated by the ground anyway. A typical 100m² basement has a U Value of about 0.16 according to a BRE leaflet you can find lower down. This meets minimum requirements without any insulation either inside or outside the basement retaining wall.
Thermal Mass.
If you go for thermal mass, the storage heater analogy, your concrete will soak up some heat when the room is very warm. You won't pay to make your room very warm. It is very warm only when the room is flooded with sunshine. Therefore, thermal mass is concentrated where direct sunlight falls upon it.
When the room cools, probably overnight, the thermal mass will give you some heat so the room cools down less. Control your heating to give thermal mass a chance before you pay for energy.
A buried basement probably should be insulated inside. Whereas a basement with south facing bi-fold doors should be able to soak up and store solar gain.
Whether you add insulation inside or outside depends on whether you have any solar gain to store in thermal mass.
I discuss basement insulation and thermal mass in more detail below this line. I also touch on air tightness and heat recovery ventilation.
This page tries to turn you well away from ICF.
The best thermal mass is dense concrete.
Cement in the UK is responsible for only 1.47% of our carbon emissions (compared to an average 6% worldwide). Yet concrete can massively reduce the 28% of our emissions that are due to domestic heating - and likely to rise as we install more air conditioning unless we do something about it.
If you build your new home with concrete, solar gain and thermal mass, you will save more carbon dioxide in your first year than the carbon dioxide cost to make all the cement.
Every year after that you will save a huge amount of carbon dioxide. I prove this lower down.
The cheapest building to build, fill with the required equipment, warm, keep warm and ventilate will primarily concentrate on insulation, air tightness, thermal mass and heat recovery ventilation. These choices involve less equipment, less maintenance and the greatest efficiencies.
The home that has underfloor heating powered by ground or air source heat pump will only be comfortable for a short while. Your fuel bill will be more than you expect. Even more if you are too warm and you open windows. This is explained fully on the failure page. Page about heating and cooling that did not seem to work for my clients.
The family who feels comfortable spending the least money will put on a jumper when it gets cold outside, rather than turn up the boiler a notch. They will not have a heat pump to bring heat in from outside or air conditioning to send heat outside because both these types of kit are too expensive to buy and too expensive to run.
A quote from a speaker at a Futurebuild exhibition in London. "The most sustainable building will be a building that is never knocked down, and the most sustainable embedded energy will be equipment never thrown away."
The speaker asked that we use materials and equipment that can be upcycled at the end of the life of our building.
He or another speaker made the very serious point that our national networks and our equipment in our homes are capable of keeping us comfortable only 95% of the time.
If everyone tried to heat and cool to keep the same comfortable temperature at all times we are awake, we would have to double the capacity of the national networks and double the amount of equipment.
We only need wear a jumper or change into shorts 5% of the time to avoid doubling the equipment in our country.
His point is we need to reduce the amount of material we mine, process and move around the world to manufacture equipment just as much as we need to reduce our heating.
This image is a slide from a futurebuild 2020 presentation by the lead architect of the team that won the Sterling Prize in 2019 and many other prizes besides. This project, Goldsmith Street in Norwich, is a Passivhaus Superstar. it was interesting that this architect said it is not necessary to use Passivhaus Approved products. However, he said it was necessary that the builder was required to prove all his work was carried out well. Air tight had to mean air tight, no temporary sticky tape over the cracks.
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air tight
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400mm of insulation and triple glazed windows
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winter sun maximised
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summer sun reduced
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heat recovery ventilation (note that the unit is accessible where the filter can be vacuumed to remove lint weekly and filters changed easily 4 times a year)
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no cold bridges or energy leaks
A new council estate in Norwich won the RIBA Stirling Award (2019) because of, in part, its thick insulation and Mechanical Heat Recovery Ventilation.
Time will tell if they still work in 5 years or if no one changed the filters.
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This is from a Futurebuild speaker as well.
The speaker spoke about his team's design starting with a plot of the solar exposure before deciding how best to design a Passivhaus.
The utility room and other rooms not requiring much heat were on the North side while the main roof faced South. They included a Snug at the Western end that gets the evening sun.
Big windows faced South and stone tiles on 150mm thick concrete on 200mm of insulation provided a thermal mass store to soak up daytime sunshine.
Overhanging roofs and balconies provided summer shading.
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Mechanical Heat Recovery Ventilation.
Mechanical Heat Recovery Ventilation, when it works, is brilliant saving a lot of money for a small investment. But MHRV needs maintaining. One customer who is very pleased with his MHRV says he pays £25 a month by Direct Debit for all his heating, cooking and hot water for a 5 bedroom house with a flat incorporated for his son.
He questioned my doubts about MHRV. I said my concern was I had been persuaded that shutting a MHRV unit away in a corner of the loft where no one ever changes the filter can stop it working, meaning no energy saving and possibly toxic air if no fresh air gets in.
He told me that his MHRV unit is very accessible and as well as changing the filter 4 times a year, quite expensive but very simple, they take the filter out every fortnight and vacuum up the lint that collects in it, rather like cleaning a tumble dryer filter.
He said it is amazing how much lint collects in only two weeks. His MHRV saves him a lot of energy and makes his air lint and dust free throughout the house. It is as if the MHRV does half his vacuuming for him, so it makes sense to vacuum the filter frequently and change it before it clogs up with pollen and so on that vacuuming cannot remove.
Thermal Mass is Insulation.
In October 2024, a building control body published a spreadsheet, as a web page you can open and save here, with all the insulating products available and their insulation values.
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One of the best I found was Celotex GA4000 with a lambda value of 0.22, meaning 70mm hits a target U Value of 0.18.
I would like to try to compare that to 200mm thick reinforced, waterproof (particularly dense) concrete.
The thermal mass information in the thermal mass leaflet available below, explains that heat only penetrates dense concrete 100mm or so during a day with sun on it. At night the heat returns to the room. It does not keep penetrating further.
And it makes clear that heavyweight, dense materials have better thermal mass potential than lightweight and less dense materials.
First, the pages from the U Value web page for various concrete blocks. I have highlighted the best.
It cannot be said that concrete does not insulate. It does.
And soil? Soil is described by scientists as 'thermally inert'. Of course, that means about 2m of soil, not 100mm. But it explains why ground source freezes the ground over a winter and the temperature does not recover over a summer.
I would like to make the same claim about particularly dense, waterproof concrete. Especially where it has soil backfilled against it.
What is the U Value of 100mm of particularly dense waterproof reinforced concrete with a compressive strength in excess of 50N/mm2 ?
The best performer is the Stowell Concrete dense Concrete Block with a compressive strength of 22.5N/mm2. I checked their web site here. Their 100mm dense 22.5N/mm2 block has a lambda value of 1.52.
That means 100mm has a U Value of 1.78.
I confidently claim that 100mm of waterproof concrete has a U Value of 1.00 or better. Buried in the soil as a basement floor or retaining wall much, much better. The BRE leaflet lower down says U Value of 0.16.
The EU Directive that bought us energy performance certificates, I think we were 8 years late introducing them, also 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.
An expert at a workshop (Whole Life Carbon at the Futurebuild exhibition on March 7th 2019) described that in order to get data his team analysed a new home just completed by a mass national housebuilder. The SAP calculation approved by building control was a U Value of 0.16. But the actual U Value, mainly due to poor air tightness and cold bridging, was worse than 0.30.
Waterproof, reinforced concrete, even above ground where you might not need waterproof, is undoubtedly air tight and no cold bridge, if the work is all supervised so it is done well.
"From an energy perspective, it would be difficult to have too much, and generally the more thermal mass the better"
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This guide was given free at the Futurebuild Exhibition and is available for free from www.concretecentre.com. Click on this image to download it in full.
It will help the person doing your SAP calculation include the benefit of thermal mass.
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"Revisions to the Standard Assessment Procedure (SAP) for Part L1 of the Building Regulations and more challenging requirements for addressing overheating in new homes."
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"Ongoing improvements in glazing and window technology, which makes passive solar design more effective (lower heat loss and improved solar gain)."
Part of your SAP calculation will include a figure for TMP. 100 is low, 250 is medium and 450 is high.
Some figures for you to come back and refer to:
Entirely timber frame (and presumably SIPs and ICF) about 70.
Traditional (brick cavity, beam and block, some load bearing and some stud partitions, timber upper floor and roof) about 200.
Entirely heavyweight construction 500 to 650 depending on size and other variables.
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Do Building Regulations require insulation throughout?
Building Regulations might seem to, but actually we have to pass the SAP or SBEM calc test and there could be an exception.
Researching this during a conversation with a client, who paid me for my £125 service, I found the following: and this seems to tie up with a couple of clients who built their basements without insulation everywhere.
The guide above, Thermal Mass Explained, states on page 15 that thermal mass capacity is limited to the first 100mm. This seemed unfair to me, especially in light of the fact that the failure of ground source heat pumps prove heat passes through soil very slowly - so why shouldn't all the concrete and the soil beyond count toward basement thermal mass?
This publication,
A_brief_guide_and_free_tool_for_the_calculation_of_the_thermal_mass_of_building_components
has a link to a page with a link at the bottom to a
free spreadsheet produced by the same people.
Please investigate this for yourself if SAP calcs are a mystery to you.
The little revelation I found here is that from an insulation perspective the soil beyond the concrete structure can count toward the insulation.
The soil beyond the basement cannot count toward thermal mass but it can count toward insulation.
The simple conclusion seems to be that basement floors don't necessarily have to be insulated where they have at least 2m of soil beyond them before open air. The exception would be around a window or patio door.
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This first sketch is supposed to show how it should be cheap and easy to build a passivhaus using only reinforced concrete and external insulation.
If the insulation is a complete envelope there need to be some footings for face brickwork.
But if the soil beneath the basement is included as sufficient insulation the footings are no longer needed.
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Basement Floor Insulation.
In one of the sketched sections through a house, above, I deliberately did not include basement floor insulation.
There are 3 choices.
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Insulation over the structural floor slab beneath the screed. The usual.
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Insulation under the structural floor slab. A screed is still necessary to get a flat floor because it is not possible to get a perfectly flat floor with waterproof structural concrete that is stiff and sets quickly. (Although doubling the dose of my powder in waterproof concrete for £69 per cubic metre certainly helps).
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No insulation.
It is proven, elsewhere on this page, that 2m of soil between the basement structure and the open air is sufficient insulation.
It is also proven that if a completely buried basement receives no direct sunlight, solar gain, that whether that basement is allowed to cool and warmed up every few weeks, or if it is kept warm, the energy required is the same.
Our variables are
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Solar gain that can be stored in thermal mass.
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Whether all parts of the basement are more than 2m from open air.
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Whether some parts of the basement, for instance beside a patio door to a sunken patio, are within 2m of the open air.
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Whether the basement has external drainage, which would interrupt the 2m of soil rule, or groundwater that could wash heat away.
Trying to minimise insulation while maximising solar gain and thermal mass might mean having insulation between the structural floor slab and the soil for 2m from the open air but no floor insulation otherwise. Wall insulation might stop more than 2m down from outside ground level, except within 2m of the patio door.
This is way beyond my expertise and experience but I hope raising the issue is helpful.
We should be asking ourselves what actually works?
First and foremost, good workmanship - which might require continuous, effective inspection to refuse to accept anything sub standard - works a lot better than rushed work and sub standard materials.
Note the quotation above about the mass built house designed to 0.16 but built to worse than 0.30 by a national house builder.
And the other note above the six-point sketch of the Sterling Prize-winning homes "necessary that the builder was required to prove all his work was carried out well"
If you are a self-builder you have the chance to demonstrate to the country that carbon, energy use and emissions can genuinely be greatly reduced. And that it need not be expensive to do so.
Cavity wall insulation.
Another gem of a simple idea from my happy MHRV customer.
He put his cavity wall insulation in himself. He has blockwork inside and outside that is 50mm of rockwool then 150mm of insulation board, such as Celotex, QuinnTherm or Recticel, then his cavity then his brick.
He explained that the rockwool follows the contours of his blockwork, ensures no air gap between the blockwork and his rigid board and ensures that joints in his rigid board are closed by rockwool.
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About 25 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.
Unfortunately, you cannot include a figure for 'cave' in your SAP calculation, but you can include a figure for thermal mass.
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Profit from Photovoltaic panels yourself.
When I looked into photovoltaic panels and a bank of batteries purely and solely for charging fully electric cars, and not even connected to any house wiring, it looked very profitable, especially if you sold energy to other car owners during sunny weather when you could generate more than the capacity of your battery bank.
It is a fact that our electricity suppliers can pay 3 times as much for wholesale electricity at 5.30pm as they paid 45 minutes earlier before we got home and switched everything on.
This is because the wholesale price is the agreed price for the most expensive electricity being generated, which is usually gas. When other generators cannot meet demand, gas fired power stations are brought on stream and everyone gets paid more. House of Commons Library explanation.
It is likely that we will soon have to pay 3 times more for early evening electricity compared to mid-afternoon electricity. So it must make sense to face our photovoltaic panels at where the sun is when we want electricity.
We can already choose to be charged a different rate throughout the day for electricity (E.g. Intelligent Octopus Go), and choose not to take any electricity when it is at its most expensive. Some will charge up batteries from the grid when the price is near zero (this might be because solar panels or wind turbines are generating more than we can use mid-afternoon). Perhaps their bank of batteries will be in their electric car. They just plug it in.
Then, during early evening peak demand and peak price, they will sell excess electricity from their car to the grid when they get paid, let's say, 30p per kWh. No panels involved. Just a phone app.
STOP PRESS January 23rd 2023
I wrote this text box in about 2019.
Today, in the news, people can pay less for electricity if they signed up and avoid using a lot of electricity between 5pm and 6pm.
STOP PRESS March 25th 2024
Today, on the BBC News website, "The watchdog said it has launched early discussions about a "dynamic" price cap that changes as consumers became more flexible."
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Taking a shower or rinsing washing up under a hot tap is pouring expensive energy down the drain. Consider showering in the bath with the plug in. Don't drain the bath before the water cooled and you kept the heat.
Fabric First Approach. With almost maximum concrete.
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Continuous insulation outside the thermal mass walls and floors. Shown here in yellow. Minimum 2 metres of soil between the basement floor slab and the outside air.
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. Other manufacturers make photo voltaic roof tiles.
I am making the assumption that solar panels might not be completely weatherproof so my waterproof roof is the flat roof above the bedrooms.
At most times, a rise in temperature from solar gain, cooking, heating, washing and body heat is stored in the concrete until released during the night resulting in less heating required first thing in the morning.
You might only need a 1 KW plug-in electric heater on a timer and timed to use the cheapest electricity to make a difference to the whole house before you wake up.
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The benefit of the concrete house is its air-tightness and thermal mass without any cold bridging. Once up to temperature, you hardly need anything to keep it there because of cooking, washing and your own body heat.
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.
Can a concrete house be zero carbon over its lifetime?
What have we learned so far?
The simple ideas work best.
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First and foremost, good workmanship - which might require continuous, effective inspection to refuse to accept anything sub standard - works a lot better than rushed work and sub standard materials.
Note the quotation at the top of the page about the mass built house designed to 0.16 but built to worse than 0.30 by a national house builder.
And the other note above the six-point sketch of the Sterling Prize-winning homes "necessary that the builder was required to prove all his work was carried out well"
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The academics concluded that
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insulation, air tightness, solar gain and thermal mass;
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as well as equipment sufficient to keep us completely comfortable only 95% of the time;
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that would all last many, many years;
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without dragging materials from half way around the world;
are all the right direction.
I cannot see that we can be zero carbon without generating something that we give to the grid as well as using very little energy ourselves.
Ofgem tells us that the average home uses 14,900kWh of energy from the grids each year.
Our first task, building a new home, must be to use a lot less. Supervision to improve air tightness and wise choices to maximise thermal mass.
Our second task is to use less equipment and make the equipment last longer. More wise choices.
Our third task is to contribute our excess solar energy we generated to others.
How much energy might be needed to produce the cement to build a concrete house with a basement that requires virtually zero heat from the grids to keep comfortable?
6,000kWh
Compared to 14,900kWh used annually by the average home.
A concrete house would break even with an average house in under a year,
Every year after that a concrete house would save, probably, 10,000kWh without any further investment in cement.
According to a number of web sites, including The International Energy Agency, the UK's cement required 116kWh to produce each tonne.
This simple spreadsheet cement needed.xlsx concludes that cement needed to build the structure of a standard new self build home, 12m x 8m internally over 3 floors including a basement, would require cement with a cost of about 6,000kWh.
The spreadsheet is not protected and you can change it, so I obviously cannot be responsible for something you may have changed.
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We might find it better to repay society with our excess solar energy if we generate and store electricity for Electric Vehicles, rather than sell solar power to the grid when it doesn't need it.
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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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