Week 4 ends

Windows finally ordered on Wednesday. I managed to get the window spec selectively trimmed using PHPP to test what effect this would have on the building's performance. The PHPP paid for itself many times over just in this one week of 'optimising' the spec.

Getting to grips with Therm (also after a long struggle). The model of the external floor-wall junction I created, has produced a provisional psi value of 0.15W/mK, lower than the 0.2W/mK estimate I had previously. Putting that more accurate figure has trimmed another 0.5kWh/m2.a off our Annual Heat Demand. Helpful, as it allowed me to trim a bit more off the window spec.

I have a sense of having achieved quite a bit this week.

Meanwhile, the scaffolders have been busy by this time next week, the whole house will be shrouded in a tarp and tin weather shield that will protect the existing structure from the rain and provide a working environment for the site operatives more conducive to accurate and quality work. It also much reduces the risk of weather-related project delays. We have been lucky in that we have missed most of the bad weather affecting the rest of the country, although night time temps have dropped to as low as -10C. Even though we have not had much snow, it has not been nice weather to be working outdoors.

Week 4 begins ... the latest on windows and on thermal bridges

Windows

Week 4 starts and we Still Haven't Placed Our Window Order. The process of placing the order is taking a lot longer than I'd anticipated. I am optimistic that we will be in a position tomorrow, finally, to press the Go button.

This has all come as surprise to me, as I'd thought naively that we could spend time fine tuning our window spec, then present our carefully worked out window schedule to our helpful, local Internorm distributor who would then be able to process it quickly and simply ... no probs, job done.

I realise now that we should have sat down and gone through our schedule with the distributor and talked prices and options at least two months ago, despite not having planning permission or completely finalised window dimensions at that stage. The order process is slow because, while it is easy to spec out the windows for their required energy performance, there are so many other variables to consider and requirements to fulfil. One is the thickness of each pane of glass. If the glazed unit is over a certain area, 4mm glass has to become 6mm. For a given width of bead, this means 4mm less space between the panes, which has a significant impact on the U-value of the glazing. None of these are problems in themselves but overcoming them adds to the cost of the windows unnecessarily.

Better to use slightly less exacting window performance figures in the PHPP. That way, you can order standard products and get a much more cost effective solution. It seems obvious to me now that I am writing this and, in fact, I have used been using conservative figures throughout the PHPP but I think I got carried away by all the exciting "Leading Edge" or best case figures that the different window manufacturers banded about. Assuming your chosen window provider is in the business of manufacturing windows with near Passivhaus performance, the conversation needs to be about what their standard spec is on those windows. It also needs to be about whether window sizes or other variables will make it harder to achieve the energy performance you are planning for in the PHPP.

Here are my rules of thumb:

	My "Leading Edge" Spec Assumptions this time	More conservative PHPP assumptions I will use next time
Glazing "g"-value	0.6 or 60%	0.5 or 50%
Glazing U-value	0.5W/m2K	0.6 or even 0.7W/m2K
Frame U-value	0.94W/m2K	0.94W/m2K
Spacer psi-value	0.038W/mK	0.05 or even 0.1W/mK

I got it right with the frame U-value, because I knew that the "leading edge" spec was a lot more expensive than their standard Passivhaus spec.

These figures aren't set in stone and I'm sure that each year what is considered standard "Passivhaus suitable" spec will improve.

Thermal bridging

On Thursday, I attended a one day course on how to use Therm, together with a very helpful Excel spreadsheet developed by Peter Warm, to calculate the psi-values of thermally bridged building junctions.

Therm is not at all intuitive but does have the advantage of being the only free software that can be used to derive a psi value for many types of thermal bridge. Therm can only model in two dimensions and more complex (and very costly) software needed to model certain types of thermal bridge junctions.

I have been grappling with this for some months but am now finally biting the bullet and getting to grips with Therm, so that we can replace the conservative (I hope) thermal bridge psi-values (0.2W/mK) we have used in the PHPP to quantify the additional heat loss through the floor-wall junctions of the existing house: this type of thermal bridge is unavoidable in a refurb but can be designed out in a new build.

Likewise, I want to model the junction between the existing house walls and the window jambs (sides), heads (top) and cills.

Meanwhile, the first insulation is being installed around the base of the walls of the existing building and in the base of the new build side extension.


We had to buy double quantities of the insulation in half thicknesses because, even though the thicker sizes do exist, the suppliers will only sell them in very large quantities, unsuitable for a project of our scale. I asked our builders to stagger them slightly to reduce possible thermal bridging in the inevitable, if tiny, gaps between each piece. So instead of this...

As we got around the corner, we started adding a 50mm overlap, like this...

It means a little bit of extra labour but it is essentially a free way to get the best from the insulation you are using. The images above are of the insulation around the base of the existing building, 2 x 60mm thick, which we are fitting from DPC level down about 400mm.

Installing dual layer of Foam Glas, under the toe of the concrete slab, where the weight of the walls of the new build will bear down - Foam Glas can take much heavier loads than other insulation  - we are also taking the opportunity of staggering the two layers to minimise unnecessary thermal bridging.

Later this week, the scaffolders will be here to start setting up the shroud that will cover the building for the next three months while the external wall insulation, the windows and the new roof are put in place.

PHPP and planning

We are still in planning, having revised our design in July to reflect the planners' concerns. The new design is currently out to 21 days' consultation (ending 25 August). The change meant we had to go through most of the data we had entered into the PHPP, as well as re-visiting some of the airtightness and thermal bridge detailing, to make sure that we could still meet the Passivhaus standard.

During that process, I discovered I'd made a silly but easy-to-make error in how I had entered data into the PHPP. I hadn't assigned all the areas I had defined to the correct building elements. Luckily, this error was in our favour. After I'd corrected the PHPP, our annual heat load figure dropped considerably so that we should be within the 15kWh/m2.a without having to include any particularly expensive options. It has taught me a lesson about how to minimise the risk of a similar mistake in future. It really is worth calculating the treated floor area separately and entering the total figure into the PHPP. It makes it easier to spot errors of the type I made.

I hope that we will have some good news about the planning soon, so that we can start on the build as soon as possible. Watch this space!

Joyous buildings

Recently, I had the opportunity to meet Richard Hawkes, whose very attractive house Crossways appeared on an episode of Grand Designs last year. His project has just gained Passivhaus certification, which is great. Congratulations!

When I met Richard and on his blog, he expressed his belief that the PHPP is a block to creating "joyous" buildings and, now that he has certification, that the PHPP is redundant. Indeed, the PHPP seems to be generally a Bad Thing; as well as interfering in the creative process, it is antithetical to collaborative working.

I am amazed that the PHPP, which is after all just an Excel spreadsheet, albeit a very sophisticated one, has such power over architects, builders and others working in the built environment that it can prevent them from producing beautiful designs or working collaboratively. My experience as the client and Passivhaus designer during the design phase of our project does not bear this out. We have found the PHPP to be a very useful tool in helping us to optimise our design, for instance by not over spending on unnecessary insulation or by focussing our attention on detailing critical areas correctly. However, the PHPP does not design the building for you. It is still up to the architect to use their creativity and experience to deliver a building that uplifts, that is "joyous". It is still essential to work collaboratively on the project, indeed collaborative working between client, architect and the builder (who should be identified and involved early in the project) is probably the most important success criterion in a Passivhaus project. The PHPP is merely one of the tools in the design tool-kit.

We have been more constrained by other factors, primarily by the fact we are refurbishing an existing property, rather than building from scratch. Also, our house is on an estate with a strong architectural character and it is this, other site constraints, financial constraints and the requirements of the planners that are driving our design, not the PHPP!

I think that Richard is right when he talks about experience. The Passivhaus design process uses a lot of rules of thumb, derived from many years' collective experience, to get the initial design broadly correct; this is something that an experienced, certified Passivhaus Designer can bring to a project. Having produced an initial draft of the plans informed by those rules of thumb, the PHPP is there to help to highlight potential problems in the design. It is still up to the architect, Passivhaus Designer, client and builder if they want to aim to use the PHPP to achieve Passivhaus certification. They may decide that a particular aspect of the design is more important than reaching the full certified Passivhaus standard. Using the PHPP properly means this decision is a concious one, rather than an unintended error. In that scenario, the PHPP can sometimes help identify other areas where the shortfall can be made up elsewhere in the design. In a world where Passivhaus design is the norm and understanding of Passivhaus design and methodology has been widespread for decades and is embodied in the DNA of all the professions working in the build environment, the PHPP would probably become considerably less useful. However, particularly here in the UK, where we are very far from this, the PHPP will remain a useful tool for a long time.

I would add that in a refurb, where nearly every project needs a bespoke design, the need for the PHPP is even greater. In a new build, provided that client, architect and builder have that "Passivhaus DNA", whether or not they would recognise it as that, and particularly in the less challenging (meteorological) climate of southern England, Richard Hawkes has shown that it is possible to reach certification. However, I wonder whether he could not have trimmed some elements of his design* and saved himself a bit of money by using the PHPP tool earlier on.

* I am referring to those relevant to the building's energy performance, not those that make it "joyous"

[edit 27 Jul] This recent article on the Centre for Alternative Technology's new WISE building continues the debate.

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.

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.