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.


  1. Adam,

    Great to see your project under way. I will be following in your footsteps soon - we move into our 1950s house next Friday.


  2. Hi Adam
    we have just followed this fantastic blog
    Please may we have permission to put your progress on our website and blogs
    under your name