Undertaking a whole house retrofit: First steps Green Building Store

Undertaking a whole house retrofit: First steps

We get approached almost every day by people wanting to undertake an energy efficient retrofit of their homes and finding it difficult to know where to start. It can be difficult to work out the order to do things in and which professionals to bring in and when. We are currently starting work advising on the radical retrofit of a weaver’s cottage here in West Yorkshire. We’re going to blog about it through all the different decision-making stages to help others in a similar position.

From the clients’ point of view, undertaking a radical retrofit can be daunting, with practical, financial and technical challenges along the way. So we hope these blogs will help would-be retrofitters’ through the process.

The project

Weavers Cottage radical retrofit projectThe project we are advising on is a small 50m2 weavers’ cottage. It is a ‘doer-upper’ in need of general repair and improvement so it is in a perfect position for a retrofit. The clients are keen to undertake a radical retrofit, using Passivhaus principles, which is likely to involve insulating floors, roofs and internal walls. The house is made up of a double storey solid wall cottage dating from the 1800s and a newer single storey cavity wall extension. The smallness of the project and the different wall types will make this an interesting and challenging project, which will draw on all our retrofit building experience to date.

Below we outline our suggested approach for undertaking a retrofit project of this type. Of course, each retrofit project is different and other strategies may work better so there is no definitive prescribed process that will work every time. Our approach is always inspired by Passivhaus. We believe we should be applying Passivhaus principles to radical retrofits because Passivhaus reduces the ‘performance gap’ on buildings so that they actually perform, in terms of energy consumption and comfort,  as they are designed.

Step 1: Find out what you’re dealing with


To get the ball rolling on a retrofit like this we would suggest that the first things you need are:

  • Floor plans (views of the house from above)
  • Elevations (views of the house from the outside)
  • A section through the house (a sideways view of the house, a bit like a slice all the way through the house which shows the wall make up, etc.)
  • Basic information about wall make-up and other key elements.


The best person to provide those would be an architect or architectural technologist, who will usually provide the drawings in CAD.


These drawings are needed to get the project modelled in PHPP or DesignPH software (more on which later). The drawings will also be really useful to help get accurate quotes for building work and quantities of building materials.

Step 2: Model the project in Design PH

PHPP DesignPH imageWhat?

All the data collected in Step 1 will now need to be fed into  PHPP (Passivhaus Planning Package) software. We would go for the simplified Design PH version of PHPP (which we also describe as Energy Performance Modelling) at this stage.


You would need to find a Passivhaus Designer/ Consultant experienced in using PHPP software. This could be the same person who undertook the drawings in Step 1, if they also have that knowledge, or it could be a separate person. In this project, we are working with a local Architectural Technologist to undertake the drawings but are using our in-house Passivhaus Consultant Paul Smith to work on the DesignPH modelling.


Design PH is an invaluable tool at the early stage of a project as it can help with early practical and financial decision-making. It should be used early on in a project as it can really help steer the direction of a retrofit. Once the basic details of the house are inputted, you can play around with different possible insulation and airtightness strategies to see what their effect is on overall space heating needs of the project – the key metric of kWh/m2/ year. For example, you could add more insulation to the roof or floor and then see if less internal wall insulation (IWI) is needed (or vice versa).

Modelling the retrofit using Design PH will help you understand how the retrofit will perform and what the resulting energy savings will be as well as offering a model of the house as a whole system.

It really is important to undertake Design PH modelling of a retrofit at an early stage. Too often, people ask us to provide this at a later stage, after all the key building decisions have been made.  

In the next blog, we will share concrete modelling examples from the weavers’ cottage project to illustrate how Design PH can really help make key decisions on your radical retrofit project.


Bill Butcher, Director, Green Building Store

Bill Butcher, Director, Green Building Store

7th February 2020

3 responses to “Undertaking a whole house retrofit: First steps”

  1. Philip Jordan says:

    Please note:

    I’m wanting to show this to the new Dorset Council in response to their “Call for Ideas”* – which I’ve already suggested a focus on Enerphit for all existing buildings (both homes & other) by 2030 – in regards to positive action to deal with the Climate & Ecological Emergency they declared in their 1st ever full Council Meeting in May 2019

    I hope it’s ok with you to make this submission of this Blog link – & also would be glad to include any similar examples to also show

  2. This is excellent, look forward to following the journey of this project. Thanks, as ever, for sharing your knowledge and expertise so openly.

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