Mind the Gap/SAP – Go for Passivhaus

There is a gap between real energy consumption and consumption calculated by SAP (standard assessment procedure), known as the performance gap. In spite of this sometimes enormous gap it still used by the majority of engineers to size heating loads for homes, boiler sizes and everything else related. The savings made by adding Solar thermal and solar electric are then put forward to show what massive reduction in energy use is going to be delivered.

In the paper cited in the last blog (A Comparative Study of the Effects of Thermal Mass in New Dwellings in Scotland by Janice Foster) two identical dwellings, one of high mass and one of low mass had their energy consumption monitored and then compared to their SAP calculations. Both buildings had identical architectural designs (design layout, U-values, etc.) and were in the same location and orientation and so SAP predicted the boiler heating load for both would be 20 kWh/m2/annum. The actual consumption was 67 kWh/m2/annum for the high mass building and 89 kWh/m2/annum for the low mass.

Apart from showing how thermal mass can benefit modern, highly insulated buildings by reducing heating requirements by up to 20% it showed the monumental discrepancy between calculated and simulated energy consumption. This is very worrying given that the output of SAP is pivotal in a lot of domestic scale decisions regarding heating and lighting and also policy decisions from government.

I recently looked at the SAP (2009) calculation for the Silverton Passivhaus building and again found a very significant difference between the actual and calculated energy performance of the building. However, this time the building was using at least 30% less energy than calculated rather than the norm of using significantly more. I say at least 30% less as SAP doesn’t include cooking in it’s calculations whereas the total energy figures for the house do.

Maybe it’s time we switched over to using rather more complex modelling tools, such as PHPP (the spreadsheet used to model Passivhaus buildings) which will actually reflect the real performance of a building in most cases. The situation we are currently in appears to be that in most cases SAP does not represent the energy consumption likely.

Admittedly the reasons for this are largely due to the generally poor build quality of UK housing combined with massively variable occupant behaviour. The build quality of Passivhaus certified buildings is much higher than the norm as without this they will not achieve the levels of airtightness required. Also, the simplicity and attention to detail with which they are designed makes building them much easier from the outset.

If we are to really push our 21st century housing in to the 21st century we need to design it, model it and build it properly. It is time we stopped being apathetic about the way our housing is built and demand houses that perform as designed and don’t waste energy. Afterall, it is as important to make the volume house builders build homes that don’t waste energy as it is to demand that the energy used to heat and light them is not exorbitant.

If you’d like to read more about (including the SAP calculations) the Silverton Passivhaus then see the case study here.

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Thermal Mass

I was sent a very interesting thesis entitled ‘A Comparative Study of the Effects of Thermal Mass in New Dwellings in Scotland’ by Janice Foster. The paper researches how thermal mass affects overheating in buildings.

Most people would intuitively say that thermal mass would prevent buildings from overheating by absorbing the excess heat from either solar gains or internal gains and in some cases this is true. However, as with all things related to buildings and people, it is not that simple.

First and foremost there needs to be adequate night time cooling to dissipate the heat stored from the day. If this is not present then the thermal mass will exacerbate the overheating by radiating heat in addition to the other gains. Secondly, to actually work the mass need to be thermally coupled with the interior of the building. Simply, this means that the mass is in direct contact with the interior of the building and not hidden under carpets, behind service voids or in the case of ICF, not behind a layer of insulation.

The thesis also discusses how thermal mass, used correctly, is more useful in the South and East of the country where there is more sunshine and the average temperature is higher. High mass buildings typically have an internal temperature around 2 degrees lower than lightweight buildings and so as the climate changes developers and homebuyers should be looking for heavier-weight constructions.

There were two surprising points within the document, both around the cooler end of the year. One was that high mass buildings can reduce heating requirements by a staggering 20% by simply reducing the fluctuations in internal temperatures and storing heat gains from the day. The second was the gap between real energy consumption data and that calculated by modelling software such as IES and SAP. On one building SAP calculated the energy consumption to be 20kWh/m2, IES was 35kWh/m2 whereas the actual consumption of the building was 89 kWh/m2. Such staggeringly inaccurate assessments, especially by a tool which is a mandatory requirement of building control, beggars belief. How on earth can we pretend to be able to model buildings and make informed decisions on how to reduce heat loss when the output of such programs is so monumentally wrong?

The final point to make is about orientation and the difference it makes to heating and cooling requirements. South really is by far the best orientation from an overheating and a heating requirement point of view. To see a copy of the thesis contact Janice Foster at j.foster@gsa.ac.uk.

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Sustainability in the Third World

I took part in a recent conversation about how to make more people aware of sustainability and renewable energy in Zambia. This is one of the more challenging and though provoking discussions I’ve had for a while as the culture is so different from the Northern European culture I’m used to.

The main thing that came across was that the systems, materials and processes that we use in the first world will not necessarily work in developing countries for several reasons. One of the most important is that 90-95% of the population live a very rural existence and could not hope to afford to use lots of energy anyway. This also means that they will never afford solar panels and quite frankly could teach us a thing or two about how to use energy and resources wisely. Those that can afford to will tend to use the more reliable sources such as generators as they are easy to fix and parts are available.

We also discussed how best to use earth in construction globally, to improve sustainability. Earth is a fantastic building materials for lots of reasons. It keeps buildings very cool in the summer, it has enormous thermal mass, it is very cheap and abundant and earth construction has very low embodied energy.

Some thought that hi-tech inclusion and mixing of soils was the best way to get more earth used in construction. Others thought that going back to using traditional methods would be best. However, I don’t think that there is one method that will suit all cultures and all markets.

In the developed world we have infrastructure, industry and industrialised processes, methods of assessment and also a housing market where people will pay lots of money for housing. This set of conditions could cope with producing a standardised product of blended soils from various locations. It can also cope with high tech methods of construction or prefabrication.

In less developed countries where the situation is different, educating people to use what materials are abundant may work much better as high tech methods are out of the reach of most people. The growing middle classes could use local materials and processes rather than trying to emulate what we in the developed world have created. This would suit local vernacular architecture much better and prevent these countries from becoming the all consuming behemoths that we have become.

What is your opinion? Add your comments below.

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25% discount on Baumit lime plasters and renders

We will be running a 25% discount on our remaining Baumit stock from the 16th of October until the 15th of November. If you want to come straight to the shop, click here or contact us at 7 Tuns Lane, Silverton, Devon. EX5 4HY – 01392 861763.

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Heatwave performance charts

From the data that we have collected from the Silverton Passivhaus since May, the most interesting period, in terms of the house’s performance, was during the heatwave in July. We saw temperatures in excess of 32 degrees which is a good test for the Passivhaus principle but also for the design input that we as a company have had on the building. If you want to download a drawing of the house, please click here for the PDF or click here for the data.

The house contains around 120 tonnes of concrete and steel that are fully within the insulation envelope. There are also around 20 tonnes of clay boards and around 6 tonnes of Fermacell boards which also add to the mass. The house uses a lot of wood fibre insulation wool and wood fibre boards for insulation. These have high thermal mass and should help keep the house cool in a heatwave.

The charts below, apart from looking like my 2 year old daughter has had a go with some colouring pens, show how the temperature varied with time during the heatwave in July this year.

This chart shows the South elevation. You can see from sensors 3 and 4 that the outer wood fibre boards are absorbing a lot of the heat from the direct sunlight with variations of around 15 degrees at sensor 4 (40mm below wall outer surface), 5 degrees at sensor 3 (140mm below wall outer surface) but buffering it so effectively that on the interior the variation is down to around 2 – 2.5 degrees. The exterior variation was up to 20 degrees between day and night and even during the day it is likely that there was at least a 20 degree difference between the inside and outside surfaces during the day.

The East and West face data set show relatively similar behaviour but that missing data from sensors 3 and 4 on the East face makes it difficult to see clearly how the wall behaved.

This data from the North wall shows the variations well but the temperatures experienced by sensors 1 and 2 is a couple of degrees higher that the interior temperature because there is a fridge nearby heating the area. Internal temperatures tended to peak when cooking happened in the evenings as the internal sensor was located near the kitchen area.

Overall the wood fibre insulation has performed well and done what is was supposed to have by keeping the building at a comfortable temperature inspite of high external temperatures. It would be great to be able to compare this to other Passivhaus buildings built with different materials to see how much is material dependent and how much is design dependent.

For more information on the Unger-Diffutherm wood fibre products see http://www.backtoearth.co.uk/products/unger-diffutherm-woodfibre-insulation-systems.

If you can shed any more light on these charts please feel free to contact me or comment.

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Our online shop

Now that some of you have started finding it, I thought I’d better mention that we’ve opened an online shop with all of our products on it.

We’ve opened an online shop with all of our products on it.

There, said it. Actually, not all of our products are on there yet but they will appear over the next few months so as you can buy whenever you like or just compare our prices to everyone else’s!!

Simply go to the product pages on the website and at the bottom of each page you can purchase the products individually or by the pallet.

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Passivhaus performance data

We have an ever growing database of information relating to the temperature and relative humidity of a passivhaus building. Externally we’re using a Davis weather station to produce a weather file and to monitor the internal conditions at the centre of the house.

We’ve also installed temp/rh probes in to the timber framed walls. These are in the service void, in between the OSB and the UdiFLEX wood fibre wool, at the interface between the UdiFLEX and the UdiTHERM  wood fibre boards (300mm from OSB) and also at the junction between the UdiTHERM and the UdiSPEED wood fibre boards (400mm from OSB). There are a set of these sensors on each elevation (e.g. N,S,E,W). If you’d like to see the house and how it is constructed see our Flickr page.

The construction of the walls are (from the interior) 12mm Fermacell, 38mm service void, 15mm taped OSB3, 300mm UdiFLEX wood fibre wool, 100mm UdiTHERM wood fibre board, 40mm UdiSPEED wood fibre board, 7mm UdiPERL self coloured render system. The U-value is around 0.1 W/m2K.

We’ve had a few issues with stability of the connection between the sensors and the computer which is logging data but we seem to have this under control now. Apologies for the holes in the data, hopefully this will disappear from here on in.

If anyone would like the data please contact us. I’ll host it in a public dropbox and send you a link.

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What is the best insulation for a house?

What material makes the best insulation is really down to what is required of it. If you’re at one of the poles in a research station and it is 50 degrees below zero, the relative humidity outside is nearly zero and you don’t see the sun for months at a time then something synthetic is going to be your best bet.

However, we in the UK live in a temperate and wet climate. The sun might shine strongly one minute and then rain pour the next, the relative humidity varies between 50 and 100% and the average temperature varies between zero and 25 degrees Celsius. In this situation insulation needs to do more than just slow the passage of heat. It needs to be able to deal with moisture and for occupant comfort, it would help if it could absorb heat too.

Insulation is often regarded as a very one dimensional product. Lower conductivity = better performance = better product to use. However, because of the amount of moisture inside and outside our buildings and the ways in which this moves through the building fabric, insulation needs to be able to cope with moisture. It should be able to absorb and release it without changing it’s thermal performance. In doing so it prevents accumulations of moisture in sensitive components such as timber studs or rafters.

It is also beneficial for insulation to not only slow the flow of heat but actually absorb it too. Our buildings are becoming ever more thermally efficient and are more at risk of over heating, whether from occupant behaviour or from solar gains. This means that to stabilise the internal environment heat needs to be stored in the materials in the walls, floors and ceilings of our houses and allowed to slowly release.

Whilst we are technically advancing all the time we still have a very long way to go to be able to produce materials as complex as plant fibres. These materials not only form excellent insulation materials but they also have a much higher specific heat capacity and so store many times more heat than synthetic materials. In addition to this there is research to show that these fibres use water as a ‘phase change’ material and are able to store and release heat energy by allowing water to change from liquid to vapour and back again in their pores.

Wood fibre is one of the best insulation materials as it combines high heat capacity and high density and is able to transport and store moisture very effectively. It is also a very effective insulant but able to store large amounts of heat energy, ensuring stable internal environments within buildings. Hemp, flax, straw and cellulose all do this too but normally to a lesser extent as they tend to be made in to lower density products.

For another perspective on the importance of thermal mass in buildings read this article http://www.molearchitects.co.uk/pdf/GREEN%20BUILDING%20MOLE%20cavendish.pdf

This post is based on my experience of living in a certified Passivhaus building using, of course, Unger wood fibre for insulation and thermal mass and EBB clay boards for thermal mass and acoustic insulation. What is your experience? Do you have a house insulated in a different way? How does it behave? Let me know by adding a comment.

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What are the best insulation materials for a Passive House?

This is a question we find a lot of our customers asking, especially after having walked around a trade show. There are, of course, many different ways to build to the Passive House standard and many types of insulation but from my experience some are better than others.

There has been a rush from manufacturers of insulation materials to design buildings to achieve the thermal and airtightness requirements of Passive House. However, many of them are very light weight and ignore the fundamentals of what makes a comfortable building with a pleasant internal environment.

The thermal mass contained in a building makes a significant difference to how the building behaves when heat is either captured (from sunlight coming through windows) or produced internally by cooking, tumble drying, vacuuming and from electrical devices. Some people will dispute this as the assessment tool for Passivhaus, known as PHPP, takes account of all of this. However, it appears rather conservative on this area and occupants of the certified passivhaus buildings that are in existence in the UK mention over heating as their only concern. Most of these buildings are made from relatively high thermal mass materials so it remains to be seen how lighter weight buildings will perform.

The same principles apply to passivhaus buildings as any other in that a more thermally massive building will be more comfortable in our variable climate. Materials such as wood fibre insulation or Porotherm blocks (perforated clay blocks) have high mass, high specific heat capacity and store large quantities of heat. Standard masonry also works well although it does release heat rather quicker. These materials and systems absorb internal heat gains, buffering internal temperatures changes and making for a more comfortable environment.

Lightweight insulation is not necessarily a problem in high performance buildings but you are then more reliant on an environmental system that can regularly adjust temperatures, either by heating or cooling. Again, this is perfectly possible but the whole point of building at this high standard is that you reduce your energy consumption by not relying equipment so much. When you build at this level of insulation it really is amazing how much heat we produce, both from appliances and from simple chores like vacuuming.

One final point is about simplicity. The more simple the design is the easier it is to build and the more likely you are to achieve your targets. Another benefit of using natural materials is that you can build simple, solid, unventilated structures without any concern about moisture. Here is one of our most popular and simple timber frame designs, it is a slender wall and roof but has low U-values, very good air tightness and is arguably the most simple way to build with timber frame.

So, when you are deciding on what sort of materials and systems to use in your Passive House building just remember, simplicity is the key.

Let me know what you think by commenting below.


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How-to Videos

On the new site we will be adding how-to videos in the ‘Resources’ section for our customers. Initially this will be for the Unger-diffutherm products  but we will slowly add for the our other systems too.

Our first two videos are on plinth insulation for external wall insulation and how to instal it. This is a very important area as it gets a lot of rain, mud and also impact over the course of it’s life.

This video shows installing a full plinth insulation down to ground level.

This video shows plinth insulation being installed in a tray.

For all of our systems we use a high density enhanced expanded polystyrene (EPS 200) which is very tough but also has a very low conductivity (0.031W/mK) and does not absorb water. This can be installed into a plinth tray, down to ground level or taken below ground level with drainage.

If you have any suggestions for videos please let us know.

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