Holmfirth farmhouse radical retrofit: Internal wall insulation (IWI) strategy

Our building team have been working away over the last few months on an adaptation and deep retrofit of some 19th century farm buildings to a single dwelling on the hills above Holmfirth, West Yorkshire. The project draws on the experience of all our radical retrofit projects up here on the hills in Yorkshire, while also making sure that we build on lessons learnt and with some variations and evolutions.

Holmfirth Farmhouse before retrofit

Holmfirth Farmhouse before retrofit






The original building comprised of three distinct sections, a barn, a two-storey dairy and a single storey milking parlour built at different stages over the 19th century. Construction is very traditional for this area of West Yorkshire and Peak District, 450 mm sandstone coursed rubble-filled walls with heavy stone slate roofs. The retrofit/conversion is being applied to the two-storey and single-storey buildings, while the barn is being left unimproved.


The project was well underway when lockdown was announced. Thorough COVID risk assessments and precautions are in place at the site, with the small building team managing to keep themselves socially distant and safe during construction. The isolated rural location and ease of transport to the site has made this easier to manage. A big thanks to all the team, who were determined to carry on with the project.

Project team

The client brief was for a retrofit and conversion similar to our previous Lower Royd project. We recommended Architect Sue Heywood at A+DP Architects, Holmfirth to prepare the plans for the planning stage, as we have worked previously with Sue on our Stirley Farm project. After planning consent was received, we took over to project manage under a Design and Build contract, as is normal for us on previous projects. Architectural technician Ashley Bamford was then employed to prepare working drawings for Building Control approval and for the building team to work from. Dan Sawyer is our Structural Engineer again on this project.

Holmfirth Farmhouse Retrofit plans 2Planning constraints

The buildings are in the curtilage of a Listed Building so we have been very constrained as to what we can change, including number of window openings. This means that the amount of passive solar gain in the house is sub-optimal and maximizing glazing is an important consideration for the project.  Using external wall insulation is obviously not possible either, due to planning constraints.

Terrible form factor

The retrofit is working on an L shaped building at different heights, which is a terrible form factor (the heat loss form factor is over 4.5). The heat loss form factor is the ratio of thermal envelope surface area (that can lose heat) to the floor area (that gets heated). It is a useful measure of the compactness of a building. Generally, the more compact a form, with few ‘sticking out’ features, the easier it will be to make it energy efficient. The lower the number of the Heat Loss Form Factor the more compact a building is. Ideally, a detached single dwelling Passivhaus would aim to achieve 3.5 or less. Our building is penalised not only for its ‘L’ shape but because of the large single-storey section.

Holmfirth Farmhouse Retrofit plans 3Towards EnerPHit standard

We would love to be able to get the project to EnerPHit certification levels but the form factor and low solar gains mean that the only way it could be achieved is through the EnerPHit component method. I suspect that many EnerPHit projects in the UK are certified this way, because of similar constraints. If the airtightness result goes the way that we hope (under 1 ach/hour @50Pa) we could try for EnerPHit certification. We haven’t made a commitment to achieving this in the contract, but we’re keeping our fingers’ crossed.

One of the first decisions we’ve had to make on the project was what is our internal wall insulation strategy.


With internal wall insulation (IWI) we are aiming for a wall U value of 0.35 W/m2K. This is the elemental EnerPHit standard and is the accepted ‘rule of thumb’ limit for IWI to limit risks from interstitial condensation. This is exacerbated by the potential of rising damp and rain-driven moisture from being in a high exposure region using a relatively porous sandstone construction. We have been wrestling with this problem for many years and have dealt with it in various ways on our previous similar projects as readers of our previous blogs will know.

Exploring a hybrid approach – Diathonite & Wood Fibreboard

We have used Diathonite Evolution IWI at our Lower Royds project, which is a very similar building and location. Diathonite is a cork and lime insulating plaster which has very good thermal performance relative to other similar products.  Just as importantly, it is vapour and capillary open allowing the wall to dry out naturally, airtight, inhibits mould growth, robust and looks good! 

For this project, we were originally thinking of a different approach by using Diathonite as a flattening coat and then wood fibreboard because we thought that this might bypass the slow drying issues we experienced at the Lower Royd project and because it might potentially be quicker to install….and more economic.  We were exploring having around 40 mm of Diathonite and 60 mm of wood fibreboard. Niall Crosson at Ecological Building Systems modelled this option in Wufi for us and it would have been fine, even though the house was in exposed location at the top of a hill. WUFI deemed that the 40mm of Diathonite to be a good buffer against any potential moisture issues .


However, when I went back to Jude our site foreman and said this is what we’re going to do he said “No it isn’t – If we’ve got to get a machine in for the Diathonite we might as well carry on and do the whole lot in Diathonite (and not use wood fibre as well). It’s going to be far, far cheaper labour-wise”. So that’s what we decided.

Improved thermal performance and space-saving

Applying Diathonite at Holmfirth Farmhouse 1Since our Lower Royd project, a new Diathonite product has been introduced. At Lower Royd we use Diathonite Evolution (with a lambda value of 0.045 W/mK).  On this project, we are using Diathonite Thermactive (with a superior lambda value of 0.037 W/mK). The better insulation qualities of the Thermactive mean that we only need to use 85mm of IWI (as opposed to the 100mm of Diathonite Evolution we would have needed to achieve the same U value). Even though Thermactive is more expensive than Evolution we’ve ended up saving a bit of money on materials because of the reduced depth and better performance with the Thermactive. Hopefully, the reduced depth needed with the Thermactive will result in quicker drying times as well.

The 85 mm will be applied in 4-5 layers (a maximum of 25 mm in each layer) and there will also have to be a dubbing coat to level the wall to start with.


Diathonite spraying machineWe had a search for various installers around the country to install the Diathonite but there was no one really nearby. We also started looking at local alternatives, including a friend of Jude’s but unfortunately, their existing spraying machinery just couldn’t cope. We’re very excited because we are now proud owners of a G4 3 phase larger spraying machine, suitable for applying Diathonite. Big thanks to Jasper Kirkman at Border Lime in Cumbria for his help and support to help us take this step. The team have had some training from the machine manufacturers in Rotherham and are raring to go. We hope this will help to keep costs down for our clients and will be an excellent long-term investment for the building team. This way, at least you’re not paying for travelling and hotel fees for installers.  We are exploring whether we could offer Diathonite IWI sprayed on walls for low energy retrofits in the West Yorkshire area! (Warning: we already have a queue!)


In the next blog, I will look at the new approach we’ve taken to window detailing, as well as looking at the floor strategies at the project.


Bill Butcher, Director, Green Building Store

8th September 2020

9 responses to “Holmfirth farmhouse radical retrofit: Internal wall insulation (IWI) strategy”

  1. blank james fisher says:

    do you use thermactive for the dubbing coat as well? and if so how do you apply it?

  2. blank james fisher says:

    I’m doing a similar project and have insulated the floor with Geocel partly to reduce the risk of rising damp in the solid walls – time will tell if it works!

  3. blank james fisher says:

    I’m repointing the outside with Lime obviously, but the dilemma is whether to apply a water repellent. Obviously reduces the absorption of rainfall is it counterproductive in making the outer surface of the pointing hydrophobic and therefore inhibits the capillary action bringing moisture to the surface to evaporate. What approach are you adopting?

  4. blank bill says:


    All great questions and comments. Since writing the blog we have decided to flatten the walls with Lime Green Duo plaster , far cheaper than Diathonite Thermoactive. Which then takes us to whether to install a minimum of 85mm or work to an average thickness. We know the area of a wall and the No of bags it should take to reach a given U value, should we accept as PH people the variance in thickness. We now think this is the case. I have just confirmed with our friends Mike Rowe at WARM and Mark Siddall, they are in agreement.

    We have the luxury in a rural situation of being able to install land drains either inside or outside the building sometimes both where ground conditions are particularly wet. This should take away the pressure of rising damp in the walls when installing impervious floor build-ups.

    Both from WUFI calculations and IWI material suppliers brick creams or vapour open clear cladding applied to the external face of masonry walls is recommended. We at GBS have over the years shied away and not relied on brick creams, because when is the process renewed? I believe the materials are more robust now-a-days and 25 years is being talked about. Good flush lime pointing to the masonry is crucial as we all know.



  5. blank Ingrid says:

    I have been recommended to use a product called cerabran,
    Branelit reg by a specialist in Finland.
    (mineral dammputz system 0.055-0,077 W/mK )

    Haven’t been able to locate this product or skilled tradesmen in UK who use this product.
    Applied by a large ‘squirter’ which sounds quite similar to the one you are trialling.

    Happy to find our more about what you are doing,
    Current project is an L shaped Arts and Crafts solid walled house , in Oxfordshire,
    any thoughts or comments welcome. P.s I really value your blog!

  6. blank bill butcher says:

    Ingrid, What is the Cerabran made of? I’m not not familiar and I haven’t managed to translate the technical. The Lambda value isn’t as good 0.55 to 0.37, and so on the face of it would take more material to reach the same U value. Another question would be around its ability to deal with moisture…………..and of course cost?
    Having said all of this it looks similar in application

  7. blank Chris Frith says:

    Thank you seems an effective efficient answer.
    Does it shrink when it dries? As 4-5 25mm layers adds up to 100-125mm not 85mm or have I misunderstood you?

  8. blank bill says:


    Please see my reply to James above. Diathonite does not shrink to my knowledge and so passes at approx 20mm thickness would be 4 or 5 passes. We have used temporary timber battens to screed off resulting in the use of more material than I hoped.

    Hope this helps.


  9. blank james fisher says:

    I decided to have my walls dubbed with Thermactive, the rationale being that it was a lot quicker and therefore saved labour and also contributed to the overall thickness of insulation. I think it was a first for the installers! but seemed to go well.

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