April update

We have spent the last month continuing to tweak and refine the design. It seems to be what we were doing last time I posted but the process is iterative and a few days ago we got to the point where we felt we could submit our planning application. Window placing and sizing, both hard to change after planning, have a big effect on the performance of a Passivhaus, so we had to be sure this was right before submitting planning.

Next, we are working out where all the ductwork for the heat recovery ventilation (MVHR) will go. The MVHR is a small unit, typically a metre and a bit square by half a metre. It needs to be located very near an external wall for maximum energy efficiency and, although barely audible, away from bedrooms and living areas. In our refurb, it will be going into the loft. The principle of MVHR is to extract stale air from kitchens, bathrooms and WCs through extraction ducts back to the MVHR unit where it passes through the incoming air whilst remaining physically separate. In this way, most of the heat (at least 75% in a Passivhaus) is transferred to the fresh incoming air. This new air then supplies the living areas and bedrooms through a separate network of supply ducts. Air is moved very slowly through large, smooth sided ducts, with silencers between rooms to eliminate perceptible noise. Choosing a good MVHR unit, designing the ducts and the room valves, and configuring the whole system once installed are vital parts of achieving a Passivhaus. The MVHR ensures that the internal air is always fresh, even when the windows are closed during the heating season. It means that the building is continually being aired - something we normally have to do briefly as a daily ritual. It gives a much better indoor air quality than virtually all other modern and many older buildings without sacrificing the building's energy efficiency.

The other great thing about using MVHR in a Passivhaus is that you can use it to distribute heat around the house; but only if your building needs very little heat. The air can only carry a very small amount of heat because the air moves through the ducts quite slowly (to avoid wasting energy or creating air noise by running the fans at a higher speed) and because the air can only be heated to about 30C above room temperature. These physical constraints translate (once you do the physics) into a maximum of 10W/m2 maximum heat load that can be conveyed by the ventilation system. For a typical UK home of around 100m2, 10W/m2 translates to 1000W (or 1kW). Compare this with the output of many UK gas boilers: 25kW or more. If your building can be heated with such a small heat input, you can use the MVHR as your heating system as well as your fresh air system, saving upfront capital costs and reducing on-going maintenance. In fact, an MVHR is a very simple system, not to be confused with heat pump systems or air conditioning systems. The only regular attention it needs is a change of air filters, something you can do yourself - there's no need for any servicing. The units use very little energy, many times less than they save.

Floor wall thermal bridge, the plot thickens
We have revisited the floor-wall thermal bridge problem. In a new build it is possible to design out thermal bridges but in our refurb, there is no economic way to do so. When you get a building as energy efficient as a Passivhaus, any remaining weaknesses, such as thermal bridges, can become quite significant sources of heat losses. In the PHPP, it is possible to include figures for the thermal bridge losses, however the calculations you need to get the figures are complicated. One good reason to design them out if you can.

This time I used another free, if infuriating, piece of software called Therm, which is intended for window designers but which can be adapted for other purposes. It allowed me to calculate a "U-factor" (as distinct from a U-value) for the external floor-wall junction, something which Heat 2, which I mentioned in an earlier post, doesn't do.

Once you have successfully calculated your "U-factor" in Therm, a further manual calculation is needed to gives you a psi-value for the junction: the psi-value is the linear equivalent of the U-value. Multiplying the psi-value by the length of the junction gives you the number of Watts the junction will lose for each degree centigrade difference between inside and outside.

The convention in Passivhaus is to use the U-value and the external dimensions to calculate heat losses through a wall or other area, rather than the internal dimensions normally used here in the UK. The Passivhaus convention of using external dimensions means that, at corners, the area is double counted, resulting in a slightly over conservative figure for the building's overall heat loss. In a well designed new build Passivhaus, this allows for any minimal thermal bridging that might remain. In our refurb, the thermal bridge looks like it will be significant and I don't know yet whether it will stop us achieving the Passivhaus standard. The Passivhaus Institute are planning to relax the standard for refurbishments because in part, I suspect, of the extra heat loss caused by these unavoidable thermal bridges.

External insulation
We have managed to reduce the thickness of the external from 300mm to 180mm. Originally, I'd hoped to use an insulator made from a tongue-and-groove wood fibre board product, like Pavatherm. The insulation would have had to be more than 300mm thick to achieve a wall U-value of 0.1. As this would have been applied to an existing wall of 300mm, the external walls would have been excessively thick. The weight of the insulation would probably have posed structural issues too, possibly requiring reinforcement of the footings. Instead, we are using Phenolic foam, which will deliver the same performance for half the thickness as wood fibre. Even though Phenolic foam is made from fossil fuels and has a higher embodied energy, our solution is overall less energy intensive than the wood fibre alternative. At 180mm, Phenolic foam costs £40/m2 or £100/m2 including installation, so not cheap. However, the thickness of the insulation has turned out to be more than just an aesthetic matter. The deeper the insulation, the deeper the windows are inset into the walls. This cuts down the amount of solar energy the windows can capture and negates some of the additional benefit of the thicker insulation.

Part of our refurb is a new build extension. Here we are using wooden I-beams in-filled with a cellulose insulation like Warmcel. I-beams are so called because they have the shape of a capital letter I when looked at end on. This design, using engineered wood, gives high strength with less thermal bridging because of the thinner central section. We will also be using I-beams in-filled with cellulose in the roof. An I-beam construction can become a source of thermal bridging unless the wall is carefully designed so that the "I" of the beam does not run through the entire thickness of the wall.

As all Passivhaus projects have to manage thermal bridging between and within walls, floors and roofs, a very helpful group of people have produced a compendium of building drawing and materials details that architects and Passivhaus Designers can copy, rather than always trying to re-invent the wheel. The "Passivhaus Bauteilkatalog" is in English and German. It costs about £81, so not cheap, but cheaper than doing all the work yourself.

A lot has happened since the last post. I have been working on the Passive House Planning Package (PHPP), the very sophisticated Excel based spreadsheet used to model how your proposed building will perform. I had spent a lot of time getting to grips with the basics of entering our proposed building's parameters - areas, materials, glazing, window frames; together with the climate and any shading - which is a bit tedious but which has to be done accurately if you want the PHPP to predict how the real life building will perform. It was only when I started getting initial estimates of how the building would consume, especially whether we were going to make it down to the magic 15kWh/m2/annum, that the power and usefulness of the PHPP really became apparent. Since then, I have been refining the information going into the PHPP and it has allowed us to pinpoint the problem areas and tweak the design further to optimise performance. The floor is still our biggest obstacle. It definitely does not not make sense to remove the slab, as this would also mean removing all the internal supporting walls, all of which we would otherwise want to keep. My earlier modelling of the floor-wall junction may have been too optimistic, as I had only assumed a temperature of zero outside, when -10C would be a more realistic worst case. Also, the outside walls will likely perform better, as they will have insulation outside, right down to the footings. The inside walls won't have this insulation and are therefore probably a bigger thermal bridge. We have added south facing glazing in the roof, following a visit to Ecobuild earlier in the month. Velux now do a triple glazed window with an overall U-value of 0.82. The window has an integrated external blind (essential in a south facing roof window) with remote controlled operation - even though I am not keen on excessive complexity and gadgetry built into the fabric of the house, when the window is 4m or 5m up, I'm willing to make an exception. I had wanted to avoid using roof windows, as I think it is very hard to avoid thermal bridging around the frame. How a window is installed makes a huge difference to its performance. Even with good double glazing, the way it is installed can boost or degrade its rated performance a lot. In a Passivhaus, as well as avoiding thermal bridging, the window has to be fitted to create an air-tight seal around the frame and set in the wall so that the wall reveals and overhang do not create additional shadowing on the window. In a standard block or brick built house, this means wrapping the external insulation around the window frame on the outside so that the window frames will look quite slim from the outside. Also, the rough opening, into which the window is to be installed, needs to be plastered accurately and smoothly, and let to dry out fully, before the window is installed. This allows the specialist tapes, used to make the building airtight, to be fixed between the window frame and the plasterwork to form a good air-tight seal. This changed sequencing of work is quite different to conventional builds but is essential in a Passivhaus. These tapes should be used around the frame, even when air-tight bands are placed between the frame and rough opening. Such bands are designed to expand once in place, so making a good seal but the tape provides a second layer of airtightness and it is easier to patch it if the building fails it's airtightness testing during the build (more of this later). Visiting Ecobuild, it was possible to compare triple glazed windows. Many only have a single rubber gasket to maintain the airtight seal between the opening section and the frame. Although it may perform adequately when the Passivhaus is first commissioned, it is likely to fail over time. Better designs have double or treble gaskets and use engineered wood, which does not warp nearly as easily as its non-engineered counterpart. On airtightness testing, we will need to test at two points during the build: after the windows go in but before the 'first fix' and again after first fix. This is essential because there is not much point in identifying any failures in the building's airtightness after it is all finished. It is much easier and cheaper to resolve problems before all the finished surfaces and fittings are put in. On a wider note, new passive houses are between to get formal certification from the Passivhaus Institute. One of the latest was a house in Kent featured on Grand Designs. Congratulations to them! We will kick off the process for getting certification after we have got through planning. We want to get our planning application off this week. In a conventional build, we could have sent our application off some time ago but in a Passivhaus this is much more risky as we had to do a lot more detailed design work, aided by the PHPP, to get the building working right first, before we could apply for planning permission. Otherwise we might have found ourselves unable to change something that is crucial to the building's energy performance. This might be a real barrier for some people, as it means spending more on architect's fees while the project is still not certain to go ahead.

January update

I have not written anything on the blog for few weeks now. This has not been due to lack of activity!

We are working on the design. I don't see it as a problem that we are taking plenty of time to get the design right. It is much cheaper to change things at this early stage! The architect pointed out another thermal bridge problem that I'd failed to notice before. Part of the building, currently a garage, is attached to the neighbouring house. We need to re-look at our solution to eliminate a significant thermal bridge at the corner with the neighbour. I also want to minimise any party wall issues if possible, as I've never met the owner; he lives 12,000 miles away.

I am also starting to enter data into the PHPP - Passivhaus Planning Package - the huge Excel-spreadsheet based tool used to model proposed Passivhaus developments. The data entered, and any assumptions made, all need to checked very carefully. It is very easy to overlook something that will give you an over optimistic picture of how the house will perform. It is vital to use the tool during the design phase, as correcting problems that it flags up, like the potential thermal bridge I missed above, is often very difficult and expensive.

We have chosen our favoured window provider. Unfortunately, there are no manufacturers of Passivhaus Institute accredited windows in the UK. Until the Passivhaus market grows, this is unlikely to change. Using windows with the PHI accreditation is important because it means that technical performance of the windows has been independently scrutinised and tested. It gives confidence to Passivhaus builders that the windows will do what they say they do on the tin.

[Edit on 23/Jan/10 - there does appear to be one UK manufacturer, Greenspec, who produce a range known as "ecopassiv", that claims a whole window U-value of 0.75, which would be low enough to meet the Passivhaus standard. They do not appear to have certification from the Passivhaus Institute but do meet the AECB Carbonlite Gold Standard. There is more information here: http://www.greenspec.co.uk/html/product-pages/ecocladwindow.php - click on the product brochure for the ecopassiv range.]

Because we are forced by the constraints of the existing building to have quite a lot of east and west facing glazing - not ideal because unshaded east/west windows cause summertime overheating - we wanted a window with integrated external Venetian blinds. This will allow us to keep the daytime heat out without plunging the room into darkness. I also much prefer "tilt-and-turn" windows that open inwards and can be operated with one hand. They are so practical in many ways; once you have lived with them, any other type of window seems unsatisfactory.

The other vital task now is choosing our builder and deciding how we are going to work. A very formal arrangement of tendering, backed up by copious documentation, will not give us the flexibility and the type of working relationships we need. It is very easy to get focussed on the hardware of a Passivhaus, and on modelling it in the PHPP, but one of the most important decisions in the project is choosing your two key partners - the architect and the builder - building a trio that co-operates well as a team. This is, of course, very important in all builds but especially so in a Passivhaus where there is very little room for winging-it with last-minute workarounds to correct mistakes. In Britain I think we are great at winging it and less good at following a proscribed methodology. I am no exception to this. And both approaches have their pros and cons of course. The other vital ingredient is very good communication between the trio of client, architect and builder, with all of us checking that the others have the same understanding of how to proceed. It also means all parties have to be be engaged, really to want to achieve the Passivhaus standard. I'll write more about this later I'm sure.

Designing the layout and choosing windows

I haven't posted for a while but the project has been continuing. We have been busy working with our architect to design the internal layout of the house; focussing on stuff that clients have to consider in the design stage of every such project. We are trying to make the best use of the space for our current and expected future needs and, to a lesser extent, for the needs of a potential future buyer - although we are not planning to sell for many years. One point to consider during the design of the layout is how to minimise the hot water runs from the hot water tank or thermal store and the various point of use.

At this early stage in the project, it is so easy to add to our original requirements, so-called project scope creep. So far, I've added a smallish (3.5m x 9.7m) green or living roof on what would have been a plain, flat roof! Nothing to do with Passivhaus but it will give us a bit more green space and a nice view from two of our bedrooms. It should also help to moderate temperatures in the summer.

We have not yet specified exact window sizes, just their approximate positions, which will often be where the existing windows are located. I am also looking at window and door manufacturers. This is easier in one respect in a Passivhaus project because there are relatively few products that have been certified by the Passivhaus Institute as meeting the necessary performance standard. The U-value of the whole window, i.e. the glazing and the frame, must be 0.8W/m2K or lower. Most new windows in the UK are in the range 1.5 to 2 and are double glazed. Passivhaus certified windows are always triple-glazed but triple glazing alone is not enough to reach the PH standard. The design of the frames and the spacers (the bit between each pane in the window). Frames and spacers must not create any thermal bridging between inside and outside. Triple-glazing has three properties relevant to Passivhaus: a very low U-value for the glazed area, an inner surface temperature within two or three C of room temperature (in winter), this helps with thermal comfort; lastly, triple glazing lets less sunlight through than their double glazed equivalents. The best Passivhaus windows try to address this last point by using glass that has the highest "G-value" - a measure of solar transmittance. Passivhaus windows are also designed to ensure that the 0.6 air changes per hour standard is not compromised. They have multiple seals to ensure this.

Even if you find your 'perfect' Passivhaus window, their real world performance will be determined by how well they are installed. Passivhaus window manufacturers are based in the countries where there is a significant PH market and only the larger ones have a UK presence. Two of these are Internorm and Nordan. Internorm have an agent for our part of the country and their installation teams have been trained in a two day course by Internorm. However, I doubt that their installation teams have ever installed windows for a Passivhaus project, where airtightness and avoidance of thermal bridging are both so important.

First steps

Passivhaus is a voluntary, international standard developed by the Passivhaus Institute, who have developed Excel-based software known as the Passivhaus Planning Package (PHPP), to help Passivhaus designers to model and predict how a proposed design will perform. We have been using it to work out how best to insulate our concrete slab floor.

U-values
The rule of thumb when designing a Passivhaus is that all the exterior building elements, except the windows, have a "U-value" of 0.15 W/m²/K or less. The U-value measures how well a wall, floor or other building element acts as an insulator. The lower the U-value, the better the insulation performance. For any given material, doubling the thickness halves the U-value. Of course, some materials act better as insulators than others; this property is measured by the k or lambda value. For those who are unfamiliar with U- and k values, there is a helpful, non-technical explanation of them at theyellowhouse.org.uk. Wikipedia, also have this to say on the subject. This is probably all a bit too much info for some but it is important to take a little time to understand how insulation performance is measured; it really helps in understanding what works and what doesn't.

The floor
We considered removing the concrete slab to create the space needed for the depth of insulation needed to achieve 0.15. Doing this would have left us with very little of the original building, added additional cost and potentially risked damaging the structure of the remaining building, so we wanted to avoid it if at all possible. The existing floor (from the finished floor level down) consists of 25mm of pine floor boards, felt underlay, 70mm of screed, assumed 150mm of (reinforced?) concrete slab and a hardcore base. We are planning to replace the screed with 60mm of the highest performance insulation we can find, 40mm of wood fibre insulation and a wooden flooring with a total thickness of 17mm. This will increase the finished floor height by about 20mm and give us a floor U-value of 0.26.

The walls
To compensate for the underperforming floor, the walls will need to overperform! On top of the existing 100/50/100mm outer-block/filled-cavity/inner-block wall, we are adding 300mm of high performance external insulation with a rendered facade. This gives a wall U-value of 0.08 with a overall thickness of 58cm! This is about double the thickness of a typical post-war built house. We can't say for certain yet whether this will give us the overall building U-value we want. That will have to wait until many other factors about the new building are decided, particularly the overall dimensions of the structure and of the windows (as well as U-values for the windows). Some dimensions are unknown because we are planning to change the roof and extend the top floor.

Why insulate on the outside?
Insulating externally keeps all of the building's structural elements on the warm side of the insulation, or within the "thermal envelope". This virtually eliminates the risk of condensation building up within walls and roofs behind internal insulation. Condensed water soon rots wood and damages the building structure. External insulation keeps the building structure more temperature stable and this helps to prolong its lifespan. In renovations, external insulation will often improve the building's "thermal mass"; the concrete or brick walls act as a thermal store, making it easier to maintain a more constant internal temperature. Also, external insulation does not shrink room sizes - internal insulation of 300mm thickness that we are planning would compromise the usefulness of many rooms in a typical UK home. Finally, where a house is still being lived in, fitting external insulation does not result in nearly so much disruption, making good and re-decoration.

The floor-wall junction
One of the most important concepts in building an energy efficient building is thermal bridging. Heat is a bit like sound. If you are trying to keep it in, it will always find the weak points in your defences and make a bid to escape to the outside world. A thermal or cold bridge is a weak point, often a line along a join between two building elements where there is a gap in the insulation. In a Passivhaus, the design needs to eliminate any significant thermal bridging. In nearly all existing buildings, the join between the walls and the floor is a significant thermal bridge. In ours, the inner leaf of the wall, which is within the thermal envelope at floor level goes down to the foundations, which are outside it. In a new build, this problem can be designed out but in a renovation this is virtually impossible; all we can do in minimise it.

To help us do this, we are using another piece of software called Heat 2 - available free on the internet - to create a picture of how the floor-wall junction will perform. Click on the image at the top of this post to see how the software predicts how the junction will perform. It shows the temperature at different points in the structure and the different materials we want to use. It shows that the lowest interior temperature will be on inner wall, just above the skirting board which will be between 17C and 18C. This should be fine, if the model describes reality accurately.

A radical renovation

Welcome to my blog! I hope it will be informative and entertaining. If you are planning any changes to your home, whether or not motivated by environmental concerns, there should be something of interest here for you.

We are about to embark on a very challenging journey to convert our leaky, draughty, unrenovated 1970s home into a certified Passivhaus: a house that needs no active heating or cooling systems to maintain a comfortable indoor temperature and a healthy indoor air quality, all year round.

The project and this blog are intended to be about more than just eco-renovation. In fact, I want to avoid using the "E" word and the "C" word (carbon) as much as possible, as they have both become clichéd. Even if you are one of those who apparently doesn't "believe" in climate change or peak oil - both of which are significant motivators for me to take on this project - most of you would still want to live in a comfortable, cheap to run home, so read on. That said, implicit in this project is an understanding that we are moving into an era where energy is much more expensive and increasingly less freely available than most of us have been used to.

In the blog, there will inevitably be a certain amount of jargon. I'll try to make sure I explain any technical terms when I first use them.

What do we want to achieve?
Put simply, we want to create a comfortable home that is very cheap to live in and to maintain; a house that will meet our needs for the whole of our lives. It will also be a house with a low environmental impact.

Why a renovation and not a new build?
Here in the UK, building plots, particularly where we live, are scarce and expensive. It is a practical choice for us, given our other constraints. Renovations are important because most of the buildings we will be using in 2050 already exist today; renovating our existing housing stock is inevitable. Each renovation is an opportunity to reduce the building's future running costs by reducing its energy use.

What is a renovation?
The word renovation covers a wide spectrum of repair and modernisation work. It could just mean fitting new cupboards and appliances in your kitchen, re-painting inside and out and re-fitting the bathroom with a new suite from the DIY centre. All superficial changes intended to make the property more attractive. Here we are looking at a much deeper renewal, replacing and augmenting elements of the building that many renovations leave untouched. In our house, the roof, windows, external walls, doors and all services need replacement or repair.

Where have we got to so far?
I guess the project started a couple of years ago, when we started looking for a house or a plot. We chose with a keen eye on what type of property we thought would lend itself to Passivhaus renovation. We decided to live in the house as is, partly to get to know it before making any changes. There is quite a lot about the house that we like and it often takes a while to tease out what changes really are needed.

A Passivhaus is different from most building projects, as a lot of thought must go into some detailed design considerations at the beginning of the design stage. Because of this, it is very hard to convert a non-Passivhaus design into a Passivhaus one later on in the design process. We have been working with our architect on these: the two areas that are most challenging are the floor and exterior walls.

The construction of our house is typical of many built in the early 1970s. It has two leaves of dense concrete blockwork and a concrete slab of concrete (reinforced?) covered by about 70mm of screed. The ceiling heights are not particularly generous, so we cannot increase the finished floor height significantly. The 50mm wall cavity is filled with mineral wool. We will be cladding the walls with external insulation and a rendered finish to match what we currently have. This is because the house is on an estate of similar properties and we want our house to retain its "group identity", as will the local planning officers!