This page covers:
-Passivhaus in the UK
-Why PHPP and not SAP?
-Passivhaus Design Principles
Passivhaus in the UK
1) PassivHaus can be any style and any construction method
2) It’s not expensive
Generally 10-15% extra to build, though there are some projects that are the same as average UK prices to build. There’s also a development in Dublin that cost no more. Getting these price improvements is easier with larger developments. Having a proportion of Passivhauses in a development can help with planning.
3) It adds value to developments
Passivhaus is at the top end of performance, it adds typically ~10% to the build price and adds >10% to the sale price. For example, Stroud is in an area where improving houses from SAP D to B typically adds £16k to the value of the house, see a report and map published by the Government. The survey was of 300,000 houses. In the home counties there is no reliable increase in house value from retrofit, probably because house prices are already very high. For the developer a greater incentive might be that projects of this standard are more likely to be accepted.
4) It’s being done successfully at large scale
There are about 800 Passive House units in the UK (dwellings or commercial buildings), over 70 homes planned for 2018 with support from the Welsh Innovative Housing Programme. Passive House is also widely used in commercial buildings such as offices and schools
112 at Hansard close
105 at Goldsmiths Street, near completion
120 at Generation Park
187 at Carrow Quay for Broadland Housing (out of 250)
10 at Hansard Close (council’s first scheme)
14 at Carrowbreck Meadow, now complete
Total: 548 Passivhaus on the plans already but according to this article they plan to have 1000 within 10 years. In the writeup for Goldsmith Street it was mentioned that the £1875/m2 construction cost is average for the UK.
Large scale projects
41 homes: Lancaster Cohousing, the Cohousing group also has this website.
68 homes: Saffron Lane
360 homes: Camden Council: Agar Grove London
150 homes: Archihaus Herefordshire
23 (eventually 72) homes: Primrose Park Plymouth
Hastoe Housing Association: 3 projects in Norfolk Wimbish, Ditchingham, Burnham Overy Staithe
Broadland Housing Assn Norfolk, 75%PH
Gentoo Housing Association, Sunderland
Portsmouth City Council Passivhaus retrofit of flats (now finished!)
Exeter City Council Knights Place
Why PHPP and not SAP
We use the Passivhaus Planning Package (PHPP) for energy calculations because of the performance gap in SAP calculations. PHribbon REALcosting gives reliable energy performance using Passivhaus software and produces indicative build costs as well. For long term energy use accurate energy calculations do make a difference, small percentage errors end up being quite large.
In the UK energy calculations are widely available using SAP based methods because they are required by Building Control for new houses, however the construction industry recognises and frequently discusses a ‘performance gap’ where buildings do not meet the energy performance intended.
Four studies are shown below, the performance gap can be seen in the first 3 studies. No Performance gap is seen in the Passivhauses in the last two studies.
1) NHBC Study found actual energy could be double.
2) The Green Construction Board found that actual energy use could be double that of the EPC certificate.
3) LEAP and Newcastle University found that there was quite a performance gap using conventional software, but hardly any for Passivhauses. This is partly also due to the extra rigour of the Passivhaus certification process. Every Passivhaus must have a calculation in PHPP. There are 60,000 Passivhaus Projects worldwide and these save 80-90% of the heating energy.
4) The passivhaus Institute also found that even though there was a range of results the average measured consumption matched the PHPP prediction well.
SAP methods are quite basic and do not usually include climate data, measurements of windows (in rdSAP) or accurate u-values for the windows. Most energy assessors use SAP based software for their energy calculations, however SAP was not intended to accurately represent the energy use of a building in use, only to allow reasonable comparison of consumption for home buyers and in any case quite a lot of the energy consumption is specific to the occupier, and what temperature they like to live at.
Passivhaus Design Principles
The UK has almost 1000 Passive House homes/commercial buildings. There are simple principles for good low energy design. The highest performing building standard is Passivhaus, which has been around for over 25 years. For eco-refurbishment (retrofit) a major consideration is also moisture issues, which is a much more complex topic.
1. Efficient shape
The building should be reasonably compact, a complex shape increases the surface area and makes the design more expensive. Good, compact shapes are, for example, flats and terraced houses. Semi-detached houses are less compact and the least efficient shape is detached houses. On the other hand, detached houses tend to be simpler from the point of view of planning and some of the calculations.
Insulation needs to be thick and continuous all the way round. Usually there are lots of breaks in the insulation called thermal bridges for example at the eaves or wall to floor junction. Typical houses have over a dozen different types of thermal bridge, Passivhauses are usually built with none.
Passivhauses are better insulated, as shown by U-values. A typical detached house would need an average U-value of around 0.1 W/(m2.K) for wall, roof and floor elements. Windows almost always need to be triple glazed in this country to get anywhere near this. The best triple glazed windows have a U-value of around 0.5 W/(m2.K), a typical double glazed window would be 2.8 and a single glazed 5.8.
This all-round insulation ensures that internal surface temperatures are no less than around 17C, preventing temperature related draughts.
3. Airtightness and Mechanical Ventilation & Heat Recovery (MVHR)
Passivhauses use mechanical ventilation because it allows the heat to be recovered from the ventilation air. They are almost 20 times less leaky than normal houses. Reduced leakiness (airtightness) is needed for 3 reasons:
- to ensure that most of the ventilation air goes through the heat recovery unit (example shown right)
- reduce draughts both for comfort and energy reasons and
- prevent condensation
4. Orientation and Shading
New buildings are optimised to minimise the heating and cooling needs and a large part of that is the orientation and shading of the building. Trees, buildings and hills to the south should be avoided if possible to allow winter sun to warm the building through the windows. Passivhauses are so efficient that they get a lot of their heat this way, the rest is from a smaller heating system. Both winter sun and shading work better if the main windows are within 30 degrees of South.
During summer shading devices above the windows may be needed to avoid overheating. Windows can be opened, as in a normal house, if it is too warm.