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Holmfirth Farmhouse Radical Retrofit: Window Detailing

Internal wall insulation update

Since the last blog we’ve had a few updates on our internal wall insulation strategy.

We were worried that we were using up too much of the Diathonite to flatten the walls, adding significant cost to the project, so we have decided to flatten the walls first with Lime Green Duo plaster, which is far cheaper than Diathonite Thermoactive. We have then had to decide whether we should work to a minimum thickness of 85mm or work to an average thickness. Working to a minimum thickness would mean that we know we would achieve our target U value of 0.35 W/m2K everywhere, but of course we would be better than this in places.  Alternatively, working to an average 85 mm would mean that there was some variation of U value with some areas at a little worse than our target and some a little better.  Having discussed this with our friends Mike Rowe at WARM and Mark Siddall, we have now settled on the mean U value approach. 

Window detailing

We’ve gone a new approach to installation of the windows, compared with our previous retrofit projects.

Irregular window openings

The window openings in the farmhouse project are not necessarily square, in fact, they are all over the place.

We are installing our PROGRESSION triple glazed ‘frameless’ rangeat the project, with a whole window U value of Uw 0.68 W/m2K. In addition to its usual Thermowood cladding option PROGRESSION is now available with a new GRP (fibreglass or glass reinforced plastic) cladding option all the way around the frame.  We wanted to trial this on one of our own projects but it has also been very useful for the particular detailing on this project..

This means we can have some of the frame showing, which disguises the irregularity of the existing masonry openings.  The great thing is that with the fibreglass we can leave as much as we want showing so it copes with the uneven window openings.

If we’d used our ULTRA window in this position we would have had a lot more frame visible  and in this project we need to maximise the glazing, daylight and passive solar gain. The PROGRESSION window looks neater and lets in more light because of the ‘frameless’ (concealed sash) design. So, we’re still benefiting from PROGRESSION’s key features.

‘Framing’ the window

We also used new Compacfoam Eco rigid insulation (soon to be available from Green Building Store) as a sort of ‘subframe’ or grounds for the windows internally. Compacfoam Eco is not as strong as Compacfoam but is perfect for this situation and also has the benefit of being made from recycled polystyrene. This subframe of Compacfoam Eco helps minimize thermal bridging around the window. It also means we can take the Diathonite internal wall insulation right into the window as the frame is protected somewhat from the Diathonite spray. This means that we’re maintaining the full depth of Diathonite right round into the windows. An added benefit is that the windows could be easily removed and replaced simply by unscrewing them without damaging all of the plasterwork reveal.

We could have bracketed the windows straight off the back of the stonework and taken the Diathonite right into the stone jambs, surrounding the window with it but we were concerned about moisture and the effect it would have on the window frame. It is not ideal for the window frames to be in wet plaster while it dries out. We know from our Lower Royd job that it takes some time for it to dry out.

Timber formwork

We also used some temporary timber formwork around the windows. This helped us bring the Diathonite neatly around into the Compacfoam and helped us form the reveals. This worked quite well but was hard to remove afterwards, without having to break the formwork apart. If we were doing it again we’d make the formwork more easily detachable and re-useable.

Locating the windows in the insulation zone

The original windows in the farm building were positioned towards the outer face of the wall.   When we were planning the retrofit we knew that the window was never going to be installed that way because that would have been bad for thermal bridging and would have made the PSI value even worse.

Putting the window further back into the IWI insulation zone also protects the frame a bit more from the weather.  The client also likes the detailing of the deep window reveals.

Airtightness taping around the windows

We have used various Pro Clima airtightness tapes at the window junction. We used Contega Solido SL tape as an airtightness seal between the walls and the window/ Compacfoam Eco. Pro Clima Contega Solido SL-D was also taped over the join between the windows and the Compacfoam Eco. We also used Contega Fiden Exo expanding tape and Extoseal Encors underneath the aluminium sill, for external weather tightness. 


We’re really please with the crisp contemporary look of the PROGRESSION windows within the soft beautifully-pointed Yorkshire stone farmhouse walls. It is a similar aesthetic to that at our previous Stirley Farm project

In the next blog, we’ll look at the floor and roof strategies at the project.


Bill Butcher, Director, Green Building Store

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