John Palmer from Enhabit explains what happens if you choose not to go all the way to Passivhaus standard, including how this impacts on cost, energy efficiency and comfort. He also outlines a best practice approach if you are going to build with Passivhaus principles.
Interview with John Palmer
John is the Managing Director of Enhabit, a company that helps to improve building performance, efficiency and comfort levels. John is a qualified Passivhaus consultant, SAP and BRE Assessor, systems engineer and experienced project manager. Prior to joining Enhabit, he ran his own low energy design consultancy.
First understand the implications of Passivhaus for your project
If you are trying to set an energy target, John’s advice is to start by reviewing your plans in line with Passivhaus standards. This will allow you to fully appreciate what it might take to achieve Passivhaus. Once the implications are understood, you may take the necessary steps or make a conscious decision to compromise and aim slightly lower.
Invest in professional advice at the design stage
John says that Passivhaus is not so much more complicated than a normal build, it just requires more thought at the beginning. One of the key thermal bridges to eliminate or mitigate is where the ground floor meets external walls, which will require careful detailing and potentially some specialist materials. Paying for some extra time with your architect or Passivhaus consultant is a worthwhile investment.
When clients commission a design review with Enhabit, they are shown a Passivhaus model of their design and given graduated options to see what’s possible with their house and how close they can get to Passivhaus, with indicative costs for extra requirements.
Focus on a fabric first approach, not eco technology
John says it’s easy to get bamboozled by all the technology available. Ground source heat pumps, solar panels, boilers, etc. all have a design life of around 20-25 years and will need to be replaced. However the walls of your building will be there for 80-100 years, so the extra investment you put into them at the start will repay itself time and time again.
Architectural aspirations may make Passivhaus harder to achieve
John explains that in Passivhaus terms, the shape of your house is the ratio of your liveable area to outside heat loss area. So you may need to compromise on an ambitious design if you want to target Passivhaus.
If your plans have already incorporated professional advice, and you intend to follow a very high-performing fabric first approach, it may be you just need to add in a MVHR (mechanical ventilation with heat recovery) system.
Alternatively you may find you haven’t gone far enough with the fabric of the building and need to change the specification of insulation, glazing, etc. This is typical of retrofits, where John says it’s quite hard to achieve the levels required for Passivhaus with the existing fabric.
Heating a Passivhaus costs a very small amount
The annual space heating demand in a Passivhaus should be no more than 15 kilowatt hours per metre squared per year. John comments that this is a very specific number and you may decide this is unachievable.
The key is to understand what number you can get down to, and what that means. For example, a target of 30 kWh will cost you twice as much to heat your property in any given year than it would have done if it were a Passivhaus. But heating a Passivhaus costs very little, so doubling very little isn’t that much and is likely to be better than other new build properties in the UK.
Every design decision will impact on comfort levels
Doubling your space heating demand to 30 kWh will impact on comfort but the nature of this impact will depend on why you haven’t achieved the Passivhaus target of 15 kWh. John gives a few examples:
| Issue | Implications | Impact On Comfort |
|---|---|---|
| Insufficient insulation in walls | Heat loss from walls | Feeling cold off the walls |
| Poor detailing where walls meet ground floor | Thermal bridging | If bad, noticeable cold spots and possible condensation/mould |
| Window specification isn’t triple/ high-quality double glazing | Cold will radiate off the glazing | Feeling cold in rooms at certain times of year |
Thermal comfort is all about constant temperatures
Many homes, especially Victorian buildings, tend to have rooms that are hot near the ceilings but cold at lower levels due to drafts and a lack of insulation below the ground floor. John explains that although the average air temperature in the room might be ok, having cold feet and a hot head will feel uncomfortable! Paying attention to thermal bridges and insulation will help to achieve what John calls good temperature stratification.
Building regulations rely on people opening windows for required ventilation
Although our awareness of temperature is quite good, it’s harder to detect poor air quality. We may notice smells and detect when rooms are a bit stuffy, but that’s about it.
Humidity, VOCs (volatile organic compounds), odours and nitrogen oxide levels are all examples of pollutants within buildings. John advises we need about 30 cubic metres of fresh air per person per hour moving through a property to deal with pollutants, but this isn’t achievable with trickle vents and extractor fans unless you start opening windows.
An MVHR system will deliver the required level of ventilation, but there are other mechanisms out there superior to extractor fans that will give extra ventilation but perhaps not cost as much.
Insulation, glazing and ventilation will cost more, but not much more
These three aspects are typically more expensive than a normal new build, as a Passivhaus requires:
- More insulation – but you’ll only be paying more for materials (approx. 0.5% increase on overall cost); the labour costs are the same
- Triple glazing, for thermal performance and to avoid condensation and mould – the gap is closing between triple and double glazing prices
- An MVHR system – for an average sized house this may cost £9-10k for supply and installation
Of these, the greatest return on investment will be insulation.
Some builders are willing to learn
Hiring the right people to build a Passivhaus is another factor to consider. Although it’s still relatively new to the UK, John has found that builders relish the challenge of doing something that requires more detail and thought; they appreciate the ability to measure how well they’ve done, and revel in achieving the standard. He adds, “You have big, hairy builders hugging each other on site when they get their final air test and they’ve got down to the right levels.”
Five helpful tips from John
1) Visit a Passivhaus or speak to someone who lives in one, to see what it can deliver from a living experience
2) Find out what would be involved in achieving Passivhaus for your project
3) Understand how each decision affects the final design, floor space, efficiency and comfort
4) Put an MVHR system in, regardless of your airtightness target; ventilation and air quality are worth doing as standalone
5) Have some form of quality control in place, even if you’re not targeting Passivhaus, to get the results you’re expecting
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Transcript
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