Holmfirth farmhouse radical retrofit: Nearing completion

The building team has practically finished our retrofit project at the Holmfirth Farmhouse now and it is close to being ready for the client to move in.

To wrap up, here is an overview of the other key detailing elements of the retrofit that we haven’t yet covered in the blog.

Ground floor

Floor insulation detail at Holmfirth Farmhouse radical retrofit

The floor detail is exactly what we did at our previous retrofit projects at Cumberworth and Lower Royd:

  • Sub floor of 150mm concrete slab
  • 150mm of polyurethane with 100mm upstands around the edges.
  • 75mm screed with underfloor heating pipes

Intermediate floors

Intermediate floor detail to allow for continuity of insulation and airtightnessWe’ve also done our typical IWI retrofit intermediate metal web floor detail, leaving a gap packing off the wall using Compacfoam at the edge of the intermediate floor so that the internal wall insulation can be applied continuously from the ground floor to the first floor.

This time we’ve parged that area carefully between the floors and we’re going to make sure the Diathonite is applied carefully in that gap (learning lessons from Lower Royd where it was an airtightness weak spot). We really didn’t want to get caught out again!

The metal web joists of the intermediate floor have also housed some of the MVHR ducting


There were several roofs to deal with, all requiring different strategies:

Two-storey building

Replacing the roof at the Holmfirth farmhouse radical retrofit project

The walls and roof on the two-storey building were in a really bad state (and were in fact dangerous).

So, we completely re-built the top part of the walls and dropped in a modern roof, made with modern laminated trusses with steel tensioners. It is a warm cathedral roof similar to what we did at our Stirley Farm EnerPHit or Golcar Passivhaus projects.

This comprised of (from the inside out)

  • Intello airtightness membrane
  • Timber I-joists (302mm) There is always a potential problem with I joists in that the insulation does not fill the webbing area of the joist properly.Previously we’ve got around this by filling the web of the joists with polyurethane sheet insulation to minimize thermal bridging. As a simpler alternative approach, this time instead we spaced the I joists at 570mm centres rather than 600mm which meant that the 570mm mineral wool batts fitted perfectly, minimising any gaps in the insulation and around the I-joists.
  • 60mm Gutex wood fibre insulation sarking board on top
  • Solitex Plus windtightness roofing membrane on top
  • Slate roof covering

Airtightness detailing in the roof at the Holmfirth farmhouse radical retrofit

Barn roof and single storey building

Single storey roof and new flat roof at Holmfirth farmhouse radical retrofitBy contrast, the roofs on the barn and single storey building were in a good state and didn’t need replacing. The client understandably did not want to go to the expense of replacing them.

This meant that we were not able to fully implement all the windtightness strategies on these roofs that we would normally do. We insulated the roofs as warm cathedral roofs and used Pro Clima Intello and Tescon Vana tapes on the underside BUT because we had to leave the existing roof coverings  we had to leave a ventilation gap under the existing slating felt, to avoid moisture build-up. If we had been able to replace the roofs and use Pro Clima’s vapour-open Solitex membrane we wouldn’t have needed to leave a ventilation gap . So although we have good airtightness on the roofs we don’t have our usual windtightness measures,  which potentially could affect thermal bypass around insulation.

New detail at the junction between the houses

Adjoining roofs detail at Holmfirth farmhouse radical retrofitWhere the single-storey building abuts the main house, there was a big cast iron gutter running the whole length of the building, which, as you can imagine, was a bit of a nuisance to clean twice a year. So, rather than have that detail, we replaced the gutter with a 1 metre wide flat roof section, laid to falls in both directions, The rainwater is collected now at each end of the sloping flat roof. This new roof section also allowed us to put in a quadruple glazed lantern light from Fakro, which lights up a corridor below.

To create this detail, we had to add triangular stonework between the wall of the two-storey building and the roof of the single-storey building. It looks really neat and from a builder’s point of view that makes it far more robust with fewer maintenance issues going forward.


The team did lots and lots of pointing for this project to stop rain entering the wall, using lime and grit sand. the pointing really contributes to making this a very handsome building.

Treating the walls

We have applied StormDry water-repelling masonry treatment to the west-facing gable wall of the single-storey building to help the Diathonite  IWI dry out, as the Diathonite was drying more slowly in that area.

Ground source heat pump

The project uses a ground source heat pump (GSHP) for heating the building’s underfloor heating system.

A GSHP is more energy efficient than an air source heat pump (ASHP).  It can deliver constant temperatures because the underground temperature is more constant than the ambient external air. A system’s heating efficiency is measured using the Coefficient of Performance (COP) which is calculated from a ratio of heat produced over electrical energy input.  The higher the COP, the more efficient it is.  The GSHP at this project was recently measured as having a COP of 6, which is significantly more efficient than the typical ASHP system at 3 to 4.

The downside of a GSHP system is the fact it uses up land space, with large trenches needing to be dug for the system, and is therefore more expensive than an ASHP system.

Our client has chosen an optimized electric supplier which feeds in when electricity on the grid is cheap. The GSHP will be linked to a software programme and will only come on when electricity is cheapest. Such a system is quite common in the commercial world but not in residential properties and is nimble to what is being generated on the grid. The house will cope well with intermittent heat because it will retain the heat effectively and underfloor heating is very steady and slow.

There is a 250 litre hot water cylinder that is heated by the GSHP, that is carefully insulated with a 100mm polyurethane-insulated metal jacket to minimize heat loss to the building and potential overheating.

We are confident that the house won’t overheat because there are minimal – and small-sized – windows and the building has high thermal mass.

MVHR mechanical ventilation with heat recovery

MVHR ductwork in intermediate floors at Holmfirth farmhouse radical retrofitThe Zehnder ComforAir 350 MVHR unit is housed in a plant room, with much of the rigid steel ducting concealed in the intermediate floor in the 2-storey building and underneath the flat roof section in the single-storey building. The root and branch system was, of course, designed by our brilliant MVHR design team here in Golcar.  

Addressing low humidity with an enthalpy exchanger 

The MVHR system has an enthalpy exchanger, which is a moisture-transmitting heat exchanger which is a useful precaution in living spaces with low humidity. It provides additional humidity into a space, by recovering moisture from the extract air and putting it back into the supply air.

The enthalpy exchanger was used for this project because the large space and low occupancy levels would result in low amounts of human-generated humidity. In the winter the lower temperature external air contains less moisture and so in winter incoming as it warms up air can have very low relative humidity. Unlike a normal MVHR heat exchanger an enthalpy exchanger doesn’t have a condensate drain, some of the moisture passes through the membrane of the heat exchanger while the remainder is expelled to outside in the exhaust air.

Modelling, testing and certification

Paul Jennings undertaking the airrtightness test at the Holmfirth Farmhouse radical retrofit projectWe were honoured to have Paul Jennings  (aka ‘Door Fan Man’ and all round airtightness guru) to undertake the air testing at the project. The whole team was delighted to achieve an airtightness result of 0.69 ach per hour @ 50 Pa. A massive well done to our building team Jude Wilson, Andy Nuttall, Danny Read, Stevie Gruschka and Isaac Straker who worked really hard to achieve this great result.  

Because of the form factor issues that we’ve mentioned previously the PHPP is saying the house has an overall space heating need of 50 kWh/m2/ year. With the airtightness result that we achieved, we could definitely aim for EnerPHit certification for the project (using the component method) or the AECB retrofit standard but the client is not looking to go down the certification route and is simply confident that they have a super-efficient  home.

Sharing our experiences

As ever, we like to share an honest account of the lessons we’ve learnt on our retrofit projects. In this project, again, we have found the Diathonite internal wall insulation drying process quite time-consuming, something that needs to be factored into projects considering using Diathonite. It is a brilliant product but the slow drying times, at this level of thickness and application, do need to be considered when working on project timelines and timing in decorating etc.

We are pleased to have completed another whole house deep retrofit project and pleased that we are continually improving our understanding of moisture management and internal wall insulation.


Bill Butcher, Director, Green Building Store

Bill Butcher, Director, Green Building Store


27th August 2021

3 responses to “Holmfirth farmhouse radical retrofit: Nearing completion”

  1. blank Alex BROWN says:

    Great information Bill thanks for sharing about to undertake an ENERPHIT of a school and will look at using some of your learning.

  2. blank Andy M says:

    Great project
    Why do you use steel ducting not semi-rigid?

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