How to achieve energy efficiency in your passive house self-build
Indoor and outdoor temperatures at Woodlands Malvern B&B during 2023
Most new-build, and many existing, houses can be made highly energy efficient. The gold standard for this is Passivhaus (passive house), providing such buildings with top quality indoor environments and minimal heating and cooling costs.
We have now lived in a certified, self-build Passivhaus for five years. We would say that the continuous thermal comfort (irrespective of the time of year — see the graph above), coupled with a constant supply of 100% fresh air, and a total absence of draughts, providing a tranquil, relaxing, bright and open yet somehow also ‘cosy’ environment, is its most important — and pleasurable — aspect. The fact that our heating costs are an estimated £273/year at January 2024 prices is an added bonus.
Research passive house
Whether your project is a new-build or a retrofit, Passivhaus standard, or ‘near Passivhaus’, you’ll need advice from a suitably qualified professional. But, we suggest, first undertake some research of your own so that you have an idea of what it’s all about and what’s likely to be feasible in your circumstances. This will help you in choosing your specialist adviser as well as in your subsequent discussions with them.
In various posts in Our Passivhaus Story and the Blog sections of our website you can learn about what we did, and find many links to further information. In this post, we particularly look at airtightness and ventilation as key components of a home’s energy efficiency.
Energy efficient design: the Passive House Planning Package
We’ve previously described how many different building designs can achieve the Passivhaus standard. Whether your project is a new build or a retrofit, there will be a big advantage in your specialist adviser using the Passive House Planning Package (PHPP) as a design aid. This will enable your architect/consultant to model varied design options, for example, the impact of different levels of insulation in various parts of the building, the effect of the size and positioning of windows, and the effect of differing standards of materials on the overall performance of the building.
Using the PHPP has three main benefits: (i) it avoids guesswork and ‘hoping that design features will work properly’ because it’s an evidence-based planning tool of proven effectiveness, (ii) you can avoid over-engineering, for example, saving money by not fitting top-specification windows and doors if the PHPP shows that they are not necessary (as was the case for us); and (iii) if you then build to the PHPP specification your house will perform as intended and do so for many years. For example, in the video opposite, Prof Wolfgang Feist describes how the first ever Passivhaus is still performing as designed 30 years later.
Airtightness and mechanical ventilation are key features
Everyone knows that proper insulation is a key feature of energy efficient buildings, but so is minimising/eliminating draughts through adequate airtightness. And with this comes the requirement for a proper ventilation system. We look at these two aspects below.
Airtightness
In traditional buildings there are often air gaps in the (usually inadequate levels of) insulation; and almost always gaps in the building’s fabric and at joints (for example, between window and door frames and the walls, and in the mortar joints of bricks and building blocks). These gaps create draughts which both let out heat in winter and allow in unwanted heat in summer. Traditional double-glazed windows are likely to have ‘trickle vents’, and bathrooms and kitchens may have extractor fans. Whilst these will provide some degree of ventilation (if they are used!), (i) their performance is affected by the weather and wind, that is, they are both uncontrolled and varied, (ii) air going out takes heat with it as well as creates draughts, and lets in excess heat on hot summer days and nights, (iii) ventilation is uneven throughout the house, and (iv) you are still likely to get condensation.
In the video, opposite, Anton Dobrevski, a certified Passivhaus consultant and trainer, discusses airtightness in more detail.
The advantages of having an airtight building are that it increases the effectiveness of the insulation and prevents draughts thus substantially increasing energy efficiency. However, with good levels of airtightness you will require a good ventilation system in addition to opening windows occasionally.
Mechanical ventilation heat recovery
Both new build and retrofit properties with high levels of airtightness will need, ideally, mechanical ventilation with a heat recovery (MVHR) system. These provide a constant supply of 100% fresh, filtered air from outside to all living areas whilst simultaneously extracting stale air from areas such as kitchens and toilets. We liken this to having a window open in every room, all of the time, but without any draughts or noise (if the system is properly designed, installed and commissioned). In colder months, heat from the outgoing air is extracted to warm the incoming air; with Passivhaus Institut-certified MVHR units this can be 90% or more so heat loss is minimal and is very substantially less than occurs when opening windows.
In the video, opposite, Anton discusses ventilation systems for airtight buildings.
You can read about our MVHR system in this post, and, in another, the constancy of our indoor temperature and relative humidity: the house’s insulation and its airtightness, coupled with an MVHR system, maintain excellent indoor comfort. (The graph at the top of this post shows the indoor and outdoor temperatures at Woodlands Malvern B&B throughout 2023.)
There are two things we’d strongly recommend when planning an MVHR system. The first is to get it designed by a company experienced in Passivhaus and low energy buildings. As shown in the diagram opposite of the air flow calculations for our ground floor, there are different air supply, extraction and flow rates required in different places. Designing such systems needs an expert.
The second is to get an MVHR unit that is Passivhaus Institut certified so you’ll get one that recovers at least 90% of the heat from the outgoing air — non-certified units can extract as little as 75% or worse.
Green Building Store design for Woodlands Malvern B&B MVHR system air flow rates in and between different ground floor rooms
Other passive house design considerations
The other key features of a new-build and retrofit passive house design are:
very high levels of wall and roof insulation, as well as under the floor;
minimisation of ‘thermal bridges’ at the design stage to reduce/avoid reducing the benefit of the insulation layer (this is more difficult to achieve in a retrofit); and
suitable shading to minimise overheating in the summer.
Ideally, in the northern hemisphere, the house’s main elevation should face south, or south east/west to maximise solar gain in the colder months. If this is not possible because of aspects of the building plot for a new build, or if retrofitting an existing building, all is not lost! Again, the Passive House Planning Package will enable you and your designer to work out what is achievable in your circumstances by tweaking different aspects.
Building to the Passivhaus standard
Based on our new-build experience, and this would apply to retrofitting as well, you’ll need a builder who applies meticulous attention to detail and is willing to find out about the Passivhaus approach and to employ it with your project. We’d strongly recommend seeing several building companies and two or three of the houses that they have each built, inside and out, to judge both how good is their attention to detail and also to talk with the people who engaged them to find out what they are like to work with.
It’s then not particularly difficult. Maintaining the airtightness layer, using special ducting and airtightness tapes where there are joints, or where the layer must be breached, is very important (there are examples about airtightness in this post); installing the windows and doors properly is vital (triple-glazed windows need a different approach to the standard one for double-glazed ones (discussed in the same post); the MVHR system needs to be properly designed and commissioned, whilst installation is pretty straightforward; and all of the insulation needs to be installed properly (which principally means without there being gaps or breaches, with necessary ones being properly sealed).
Is passive house worth it?
Would we choose to move into a traditionally-built house having lived in a passive house? Absolutely not! The unparalleled indoor comfort, air freshness and tranquillity throughout the house at all times is just too good to give up. We’ve described living in our passive house in another post in some detail, and the novelty and pleasure of it have not worn off.
Is the passive house standard more expensive?
There will be some additional costs if you want to achieve the Passivhaus standard (noting that it’s not necessary to have your house formally certified, although this can provide good external quality assurance of both its design and build). This additional cost has been calculated by the Passivhaus Trust to be about 8%, and to be just 0.9% for a new-build Passivhaus and 0.04% for a retrofit according to research by University College London (UCL) Estates.
And there are savings to be made beyond having only minuscule heating costs throughout the time you live there — lower up-front costs for your minimal heating system, for example; and, most importantly, avoiding over-engineering and thus the use of unnecessarily expensive items by using the Passive House Planning Package.
The likely areas for increased costs over a traditional new-build or retrofit are:
additional design fees — it will take a bit longer to design out things like thermal bridges and the higher insulation requirements, but an experienced Passivhaus designer will know about these from the outset, and the PHPP will need to be used, which also creates some extra work but delivers benefits absent from a traditional design approach;
higher levels of insulation (see the section Installing the encapsulation system in this post), including insulation under the foundations; and
triple glazing rather than double-glazing, but the price difference is dropping and is now only some 10-20% more.
There are two other increased costs, but these will be incurred for any higher energy efficient building:
draught-proofing and an MVHR system — these will both be necessary in any building if you want to avoid draughts and their consequential heat loss in winter and heat gain in summer; and
external shading — if you don’t have this in any higher energy-efficient building then you run the risk of overheating in summer because of increased insulation and draught-proofing trapping excess heat.
Against this, in a properly designed and built passive house, you’ll have:
unequalled levels of indoor comfort — there’s a pervading peacefulness: a relaxing quietness and stillness throughout the house at all times;
an absence everywhere of draughts, and of surface and interstitial condensation (which protects the building’s fabric and reduces maintenance costs and likely extends the life of the building);
no sense of cold when right up against triple-glazed windows even if it’s -10℃ or lower outside — so there is more usable floor space available; and
heating costs that are 90% lower than those in average UK houses and 75% lower than current building regulations and new-build ones.
Will I be too hot or too cold in a passive house?
Indoor and outdoor temperatures at Woodlands Malvern B&B during 2022
In 2022 there was both a heatwave in July and a ‘cold snap’ in December when outdoor temperatures reached extremes. The graph opposite shows the indoor and outdoor temperatures we experienced in 2022 — very similar indoors to those in 2023 shown in the graph at the top of this post.
Below this, you can see in greater detail what happened to our indoor temperatures in the July heatwave and the December cold snap: the ‘Passivhaus difference is clear — you can expect thermal comfort indoors whatever is going on outdoors.
Indoor and outdoor temperatures (℃ ) at Woodlands Malvern B&B during the July heatwave (above) and December cold snap (below) in 2022
So the answer is, no, you will be neither too hot nor too cold in a Passivhaus’. Indeed, in terms of indoor comfort, a passive house will always provide you with a Goldilocks environment: one that’s just right.
How to achieve energy efficiency in your passive house self-build
Based on our experience, you will achieve energy efficiency and unsurpassed year-round indoor comfort by doing your own research, using a passive house specialist, designing your new-build or retrofit using the Passive House Planning Package, and engaging a builder who has a meticulous approach to their work. The additional passive house cost is very small compared to the whole project, and the benefits are enormous.
Experience the passive house difference at Woodlands
Come and stay at Woodlands Malvern B&B and experience the Passivhaus difference yourself, and enjoy a break in the outstanding natural beauty of the Malvern Hills National Landscape. We’ll be pleased to discuss our experiences of self-building and of living in a passive house with you; see what our guests have been saying about staying here and try it for yourself!