Undertaking a whole house retrofit: Exploring the options Green Building Store

Undertaking a whole house retrofit: Exploring the options

In this blog series on the Weavers’ Cottage project, we are trying to approach a whole house retrofit from the clients’ perspective and looking at the order in which decisions are made and how they are made. 

Get advice as to which retrofit measures are suitable

Weavers Cottage radical retrofit projectThe weavers’ cottage project that we are advising on has now been modelled in Design PH.  Before you can model your retrofit project in Design PH you obviously need to get some advice as to what retrofit measures would be suitable for the type of building you are improving. For this project, the clients and builder chose to have a Design Review with us to go through what measures and insulation types we thought made sense. Any Passivhaus Consultant or Retrofit Coordinator will have this knowledge to feed into the Design PH model.


The groundfloor has concrete floors throughout. The clients do not want to go to the effort or expense of digging them up so the idea is to add insulation on top of the existing floor. We will also need to add or make sure that the floor has a damp proof course (dpc). Going down this route means that door thresholds will need raising and the bottom step to the stairs will need altering.

 The two insulation options we are considering to balance achieving reasonable thermal performance and maintaining a practical head height through doorways are :

  • PUR rigid insulation
  • VIPs insulation (Vacuum Insulated Panels), a space-saving insulation suitable for floors.

Key considerations

VIPs are twice as good thermally as PUR, for example  a 25mm VIP has the same performance as 50mm of PUR. Therefore to achieve the same overall U value and performance the clients will need to choose which option depending on:

  • Ceiling and door heights
  • Cost (the VIPs panels are more expensive and potentially prone to damage during installation)



Original building

Weavers cottage whole house retrofitThe clients were at one point considering having a warm roof to allow for a Mezzanine space. Head height issues and building control rules mean that this is not a possibility now, which in a way was a relief to the clients as it would have entailed complete re-roofing and a lot more cost.

Key considerations

The main decisions for insulating the roof will be how much insulation to use and which type, which will ultimately come down to cost and performance, as demonstrated in the Design PH model.

Extension with flat roof

We have proposed to the clients that they replace the existing flat roof with a warm roof, like the one we installed at the Kirkburton Passivhaus.  This means that the insulation is above the timber decking, The lower decking is acting as the airtightness and vapour control layer and this design means that we are taking the risk of interstitial condensation away from any timber in the roof. Without this sort of design, many flat roofs fail as the timber decking starts to rot if there is not an adequate and continuous vapour control layer.

The new flat roof we are suggesting comprising of (from the outside in):

  • Waterproof flat roof membrane
  • Plywood decking
  • Rigid PUR insulation (at a thickness to be determined in Design PH)
  • Plywood decking, taped for airtightness

Key considerations

  • Head height in the extension, especially as the extension will be getting additional floor insulation.
  • Whether raising the height of the flat roof slightly will be an issue with the new flashings into the abutting/adjoining first floor main house stone wall
  • Cost and performance of the insulation



Original building

Weavers cottage whole house retrofitThe walls of the original wall are uneven, so we are proposing the use of Diathonite 37 on the internal exterior walls and maybe some internal partition walls. The clients are concerned about the drying time of Diathonite, when applied at 100 mm, so one option we are exploring is first applying a flattening coat of Diathonite to even out the walls and then applying a wood fibre insulation board system, such as Gutex.


Space is very limited in the extension so we suggest the only viable IWI in this small space would be laminated PUR of XPS insulation on walls 65mm 50 mm PU 12.5mm plasterboard 3 mm finish on top. The laminated board, if carefully taped and glued behind, would act as the vapour control and airtightness barrier. This would be stuck directly onto the existing plaster. We have modelled this scenario in WUFI and know that it will not create moisture risks because the extension is built in a modern cavity wall construction.

Key considerations

  • Cost of the different IWI options and their relative thermal performance and moisture risk
  • Drying times of Diathonite insulation and whether the Diathonite and Gutex wood fibre insulation can be used together to reduce drying time
  • How much difference additional thicknesses of insulation will make – eg would 50mm of IWI insulation be enough. PUR is considerably better than Diathonite and the woodfibre in thermal performance but not as effective in dealing with any potential moisture issues.



Weavers cottage whole house retrofitIn the future, the clients are likely to want to change the windows but for now, they are considering keeping the recently installed UPVC double glazed windows.

The windows are fixed and probably screwed through the frame.  If we get the detailing right on the inside around the window frames, then potentially replacement windows can be installed from the outside which would not disrupt the detailing around the window frame on the inside.

Key considerations

So the key decision for the clients will be whether to take the plunge now and replace the windows with triple glazing or do that at a later stage.

In the next blog, we will look at how all these measures perform in Design PH.


Bill Butcher, Director, Green Building Store

Bill Butcher, Director, Green Building Store

30th April 2020

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