Project Update - September 2010

We are still awaiting a decision from the planners, although lots of work continues in the interim, there's nothing Passivhaus specific to report on this month. Watch this space...

New Passivhaus Refurb Standard announced

The Passivhaus Institute is finally to announce a long trailed standard for refurbishments. This new standard, to be known as EnerPHit, will be easier to achieve. The headline targets are:

25kWh/m2.a - annual heat demand
1.0 - air changes per hour - airtightness standard

There'll be more detail on what assumptions lie behind these standards in the coming weeks I hope. Refurbishments to near Passivhaus standard will now be recognised with this new standard. The less stringent standard will make Passivhaus thinking and methodology more accessible to those refurbishing by giving them a challenging but achievable standard to aim for. I imagine that the procedure to gain EnerPHit certification will be similar to that for the full Passivhaus standard. Certification adds value by enforcing a certain discipline to the design and build process. This benefit will now to open to the refurb sector.

We will still be sticking with the full Passivhaus standard for our refurb. Being based in the south west of England has made it feasible to aim for. If our project had been located in Manchester, the colder climate there would make the full Passivhaus standard unrealistic practically and financially for us. The new EnerPHit standard would be within reach though.

Before we all relax too much, a quick comparison with other UK building standards reminds us that this new PH refurb standard is still far ahead of others; it will still be a huge challenge to get EnerPHit certification.

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.

Windows revisited

We have made a few changes to the windows.

We decided to use Internorm some months back for various reasons discussed in a previous post. We had planned to use their "Varion" triple glazed range in the north and south windows and Varion 4 range - with triple glazing and a fourth pane that encloses integral, external blinds - in the east and west windows, of which we have too many; this is to compensate for constraints on the south side of the site. An ideal Passivhaus has lots of south facing windows - ca. 25% of the south façade area; modest amounts on east and west façades and minimal window area on the north façade. There's more on our east west windows and the Varion 4 dilemma further down this post. First, something about skylights...

Internorm do not make skylight windows and so we looked at Velux and Fakro, who do. We had planned a large Velux window that the top of the south facing side of the roof. But this window has now been ditched. It would have added more light in our stairwell but we are already improving the daylighting dramatically, so the extra provided by the Velux didn't seem essential. We had also thought it would provide nice stack-effect cooling in summer, being at the top of the house. We (I) have been put off by Velux's poor communication, their incomplete technical data and by a lack of confidence that we would be able to detail and execute the installation well enough. In the PHPP, a Velux would only have brought a net benefit in winter solar gain if the window spec and installation had been good enough. Putting in conservative estimates into the PHPP (in lieu of verifiable data) for the Velux window/installation resulted in no net benefit in heat gain from the window - the additional losses resulting from installing the Velux roughly balanced the solar gains through the window. The window we were looking at was one aimed at the Passivhaus market, although they don't market it in the UK. Trying to get the key information needed for the PHPP from window manufacturers seems to be quite hard. The U-values for the frame and the psi-values (the linear equivalent of U-values) for the spacers (the bits that hold the panes apart around the edge of the sealed double/triple glazed units) seem always to have eluded us! These two factors have a significant impact on the whole window's performance and tend to be the poorest performing parts of a window. Also, it wasn't clear whether the window had a single or a double, all round gasket - the latter being essential for reliable, durable airtightness in an openable window or door. Velux's figure for the "g-value" of the triple glazed unit was not great either, 0.46, meaning that only 46% of the sun's heat energy is let through the glass. 0.50 is the minimum for a Passivhaus; 0.60 or higher is worth aiming for.

The other manufacturer that makes a triple glazed skylight-style window is Fakro. I didn't investigate their products in as much detail as the Velux because, by that stage, the whole idea of a skylight in our project was beginning to look less appealing. The other issue with a south facing skylight high up in a stairwell is that you need an electrically powered and electronically controlled mechanism to open and close the window and its external shuttering. Without these, the window would be a source of summer over-heating. With them comes over complexity and problems if (when?) the mechanism fails.


Back to the east west windows. On a visit to Internorm's shiny new showroom last week, we learned that the only way we could get Varion 4 triple glazed with a fourth pane for integral, external blinds, was to use krypton gas, instead of much cheaper, more abundant but less highly performing argon to fill the sealed units; this is due to space constraints. You can make krypton units much slimmer for the same performance and their main market is in listed buildings, where a high performance double glazed unit can be designed to look like a traditional single glazed window. The external blinds are important to keep overheating well below the modest Passivhaus target of less than 10% of days annually where internal temperatures rise above 25C. If we had been able to have less east-west glazing, we could have managed with the standard Varion windows and more ad hoc shading arrangements. The other problem with a total of four panes, is that our g-value for the window would have been less favourable than with the standard triple glazed Varion windows.

It is possible to fit shutters or non-integral external blinds but using these created other difficulties because of other constraints arising from this being a refurb project.

So now, we have decided to go for the double glazed Varion 4 windows with a third pane to enclose the integral blinds for most of the east west windows. It seems that, although they are not true triple glazed units, the third pane provides a measure of additional insulation, a bit like secondary glazing can do, with the U-value coming in at around 0.9 to 0.95.

Putting these windows into the PHPP did have a small effect on the annual heat load. The last concern was about the temperature on the interior surface of the window. One of the key principles of Passivhaus design is to ensure that no interior surface has a temperature more than 4C lower than the ambient temperature - a bigger temperature gradient reduces thermal comfort and risks creating draughts. Using a U-value of 0.95 and a worst case scenario of a winter night of -10C gives you a temperature of 16.3C in the calculation below, still just warm enough.
In practice, I am sure that these windows will be fine and even though they appear to be ok, just, Passivhaus wise, I suspect that it may make certification more problematic. Watch this space!

p.s. Have just realised that it is possible to insert pictures anywhere in the text and to insert more than one per post, so will definitely use more in future.

Passivhaus and the planning system

Well, our plans are currently in planning and we are living in limbo, uncertain whether the months of work we have put into the design and the PHPP calculations will translate into the home of our dreams.

The UK planning system does not fit well with the Passivhaus approach, which involves much more upfront, detailed design work than a traditional build. Normally, an architect produces an outline design with just enough detail to satisfy the planners; the point being not to commit more resources than necessary until after planning permission has been given. In any Passivhaus project, but particularly in ours, which is much more challenging because we are new to Passivhaus in the UK and because it is a refurb, more work is needed to be sure we would meet the Passivhaus standard before the planning application can be submitted. If we had submitted our plans earlier, we would have locked in window sizes and other variables that have a significant bearing on the building's energy performance. We have found getting down to the key Passivhaus standard for heating of 15kWh/m2 per annum quite difficult, without throwing silly money at some exotic materials.

Similarly, although we are stuck with some thermal bridging - at the floor wall junction of the existing house - we have managed to eliminate most other potential bridges. If we had submitted our planning application earlier, we would almost certainly have inadvertently designed in thermal bridges into other junctions.

All this means that we have had no choice but to stick our necks out and spend a lot more before planning permission has been secured. The alternative would have been probable failure to achieve the Passivhaus standard and unnecessary design work and building costs later in the project.

As a newcomer to the construction sector, it strikes me that more detailed design work, early on, pays dividends. The build costs should be easier to tie down accurately before contracts are signed and the build starts and there should be much less risk of unexpected costs. Of course, being a refurb, there are still some unknowns which could throw a spanner into the builder's cost estimates. By the time we submitted our plans, we were only a week or two away from being ready to submit plans to building control.

Maybe, in these times of looming public expenditure cuts, it would make sense to combine the full planning application and building control application into a single approval process, with a much simpler initial approval in principle procedure for new builds and larger schemes or where a building is listed or in a conservation area. Whatever the solution, the planning system needs to give clients and architects enough certainty about a project early on to allow them to commit the resources necessary to do the more detailed design work required for a Passivhaus.

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.