THREE LESSONS FROM CURRENT PASSIVHAUS PROJECTS
- The PH requirements only affect a small proportion of the project.
- The effect on the budget is therefore commensurately small.
- Most construction methods can be accommodated, so PH construction doesn’t have to be treated as “unusual”.
- Introduction – Who, Why, What & How
- Who: Buildings are designed by people for people. However absorbed we may become in the technicalities we must remember that people come first.
- Why: Most decisions are based on client’s preferences and the particular circumstances so every project is different. Most decisions have nothing to do with PH but the great advantage of using PHPP as a design tool is the ability to evaluate and explain the consequences of decisions. Successfully meeting the client’s requirements requires effective collaboration with the many participants. Most preferences can be accommodated although some are more cost effective than others. Experience from my clients indicates that their “Why?” for choosing PH is primarily the guaranteed comfort - followed by freedom from energy price rises and a general desire to save the world. This raises the question “Why not?” which generally boils down to “Does PH construction have to cost more?”
- For a definitive answer it would be necessary to build identical buildings simultaneously. This is never going to be possible with individual houses. For the school building (currently under construction) it is easier to compare the standard budget with the contract sum and this PH project came in 15% lower overall including external works (18% for the building excluding externals and furniture) .
- See the PH School Case Study tab for more details about this project.
Fig 1 Although this school building is 17.8% below the standard budget (15% including external works), this is not just due to Passivhaus improvements which account for about 6.8% of the savings. The total value of all the elements that are relevant to PH is less than 18% so the scope for making any effect either plus or minus on the budget is very limited.
- For individual houses the main variables are: Site Location, Footings, Finishes, Fixtures & Builder’s Margin.
- Of these, only the builder’s margin is affected by PH and this can be addressed by tailoring the construction to the builder’s preferences and minimising additional supervision and “unknowns” (see construction details below). To make a meaningful comparison the PH elements need to be separated from the other variables as shown on Fig 2. Note that the elements of significance to PH represent only about 22% of the total on these averaged bespoke house projects and the PH enhancements affect only about half of that i.e. about 11%. Since their influence on the budget is correspondingly small, whether they cost more or less is not very significant to the cost of a project.
Fig 2 The PH elements are such a small proportion that they cannot have a significant effect on the total budget.
FIg: This standard prefabricated house (Anchor "The Suburban) is constrained by the dimensions and structure needed transport it. It is designed to face the sun and benefit from passive solar gain. Only a series of small modifications are needed to overcome the thermal bridges of the steel floor, the air leakage and general heat loss to bring it up to Passivhaus standard.
FIg: This is an instructive example of taking an existing design (not by 5cSBD) and converting it from "Passive Solar" to "Passivhaus". It is easier and cheaper to achieve the desired results with Passivhaus than with just passive solar design although the general principles are compatible.
- The main PH elements are Insulation, Windows, HVAC, Airtightness and Certification.
- INSULATION: Insulation is such a small % that it is difficult to overspend on this but the chart below shows that for most of my projects the cost effectiveness reaches a peak at a Heating Demand of about 7kWh/m2/annum.
- Fig 3 Spending an extra $2K on insulation can halve the Heating Demand but it requires $10K to halve it again.
- Why aim for 7kWh/m2/annum rather the Passivhaus heating demand limit of 15kWh/m2/annum?
- At this level it is easy and cheap to generate more solar electricity than the building uses and reach Passivhaus Plus or carbon positive in use. The small extra spent on insulation is compensated by the lower cost of the photovoltaic array. At this level it is also possible to stay pretty comfortable without using any heating as shown by the Sunstone temperature monitoring data.
- Fig: See the Residential PH Case Study tab for more information about Sunstone
- WINDOWS: We are lucky that it is now possible to source excellent windows and external doors at very good rates compared with standard windows. On most of my low budget projects the windows are costing less than the Rawlinson’s standard rate. Even on high budget projects they are costing less than the nearest equivalent local windows.
- HVAC: There is now a good choice of cost effective HRV systems. PH design provides savings on heating systems, cooling equipment, extract fans, ceiling fans, gas connections, opening windows and window control equipment. Of course PH design also delivers the design temperatures and air quality which the CSIRO survey indicated that few standard designs were able to do. The installed cost of PH whole building HRV systems on my projects is averaging between 2% and 4% of the budget which is usually about half the cost of the standard heating/cooling/ventilation equipment they replace.
- AIRTIGHTNESS: All insulated construction should include a vapour membrane so it is a small extra cost to make this into an airtight vapour membrane if it is detailed to keep it simple. The Rawlinson’s rates for Building paper and orange polythene are virtually the same as the rates for “intelligent airtight vapour membranes”. The additional cost is only in taping at junctions. This is typically costing less than 0.5% of the budget.
- CERTIFICATION: Although I am confident that might designs will comply, I consider certification and air testing to be essential because they prevent the temptation to let standards slip. Together they usually represent less than 1% of the budget.
- Construction choices – Why, What, How
- The PH requirements do not dictate any particular form of construction as is demonstrated by the wide array of techniques that have been successfully used around the world. Where cost is an issue, simple variations on techniques familiar to the builder are likely to be most economical. As previously mentioned, minimising the “unknowns” and the need for additional supervision helps to keep the builders costs down. The variations used on these projects are due to specific project requirements, in particular client’s preferences for finishes/fixtures and builder preferred construction methods.
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All methods are equal but some are more equal than others!
A great variety of construction methods have been used for Passivhaus around the world but some are easy and some require more ingenuity.
In Australian domestic building, whatever methods the builder prefers will probably be the most cost effective.
Two basic approaches:
- Standard timber construction detailed to eliminate the usual thermal bridges and air leaks
- Thermal bridge free systems
- Conclusions / Summary
- I have three occupied Passivhaus projects including the first Passivhaus Plus buildings outside Europe, half a dozen more due for completion this year (including the first Passivhaus school building in the southern hemisphere) and another dozen in consultancy stages for builders, schools and private clients. Experience from these projects indicates:
- Although the Passivhaus elements can cost less than the standard equivalents, the proportion of the budget they represent is too small to influence the budget significantly.
- Achieving Passivhaus minimum standard is cost effective and reduces the heating to very little, but spending two or three thousand on insulation above the Passivhaus minimum can make a house comfortable with almost no heating and allow a cheaper p.v. array to produce more than the building uses.
- Each of the elements makes a contribution but to reap the full benefits they need to be implemented holistically.
- There is a wide variety of construction methods used around the world for PH. Designing to whatever methods the builder is comfortable with and making it as “normal” as possible is likely to produce the most cost-effective construction.
- There are many clients and builders eager to implement Passivhaus design once they understand the minor variations necessary. Any good building requires careful construction. Any careful builder can work to a PH design.
- Clearly PH design demands more care, effort and time which might deter some designers. However most of a designers’ time is still spent on all the usual non PH processes so PH represents a small part of the design effort. Fundamentally Passivhaus Design is easier to achieve than Passive Solar design. Doing anything well requires commitment but it becomes easier with experience which is why I share my experience on www.passivhausdesign.com.au and many of my clients/builders generously open their projects on Sustainable House Day or PH open day.
- Passivhaus design gets easier and quicker with experience. In the period I have been designing these projects, suitable products and components have become increasingly available which makes the design process easier and the construction more cost effective.
- Now I know that the great benefits of PH design can be provided cost effectively I cannot ethically offer anything less.