Saturday, 19 December 2009

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.

Wednesday, 25 November 2009

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.

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 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.

Sunday, 22 November 2009

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 clich├ęd. 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!