Mill building office radical retrofit: Floors & windows

The timber frame ‘box within a box’ approach that we are undertaking for our mill office retrofit was described in our previous blog.

In this blog we look at the floor and windows strategy we have undertaken for the retrofit. In particular, we look at how continuity of insulation and airtightness were dealt with in relation to the floors and window detailing.

PERFORMANCE triple glazed windows at Mill office retrofit

FloorMill office retrofit Pre-existing floor

The mill unit had a pre-existing concrete floor. In many places this was not level and there was a severe slope at one end. As we’ve explained, cost is a big factor on this project as ultimately the unit does not belong to us. The landlord has made a deal with us so that after the retrofit we can still rent it out at the industrial rate on a 10 year tenancy agreement. Even so, we are working to a tight budget for the retrofit as we shall need to ‘amortise’ the capital costs over the 10 year tenancy, offsetting these against the low industrial rent.

Mainly for cost reasons we have therefore decided NOT to dig up the floor but instead to add a ‘floating floor’ on top of it.  It is a pragmatic decision – we don’t own the building and have sufficient floor-to-ceiling height to make this a practical solution. As with our recent Marsden bungalow retrofit, we could also consider the embodied energy in the concrete floor and the balance of benefits.  Although we have not modelled this, clearly this would be significant, particularly given the potential 10 year tenancy and the marginal difference in overall performance of the office space. But ultimately, cost was the biggest factor.

We’ve had some levelling to do as the concrete was so out of level with one end really dipping down, so we had to flatten a bit off.

The ‘floating floor’ strategy for the floor involved installing 75mm PU insulation boards (with thermal conductivity of 0.022W/mK) on top of the existing slab. We have taped the PU insulation board joints for airtightness and damp prevention. We have laid 18mm carefully glued together tongue and groove chipboard on top of this, which also acts as an airtightness layer taped to the 18mm OSB wall boarding. The insulated floating floor joins the insulated timber framed walls ensuring there is continuity of insulation, minimising thermal bridging at that junction.

Floor U-value 0.26 W/m2K

  • Existing reinforced concrete slab
  • 75mm PU insulating board
  • 18mm Chipboard
  • Recycled polyester carpet or PVC-free floor tiles


Insulation around window frame to minimise thermal bridgingGreen Building Store’s PERFORMANCE GBS78 triple glazed timber windows have been specified, with a nominal whole window Uw value of 0.85 W/m2KThe critical issues to consider in fitting windows in deep retrofit are the installation psi values and airtightness.

Ideally, we would want to install the windows in line with the insulation, in this case within the timber frame.  This would minimise the installation psi value, and is what we did on the Stirley Barn retrofit.  However, it means changing the look by pushing the windows into the building and also means that we would have to fit expensive aluminium closers to seal between the window and the stone wall, together with exceptionally deep aluminium sills.  The alternative is to fit the windows in the stone wall and insulate the internal reveals, as well as we can at a sensible price.  We have used a combination of Compacfoam and PU insulation to minimise thermal bridging at this detail.  We know that we have done the best we can with the psi value, without resorting to either moving the windows into the timber frame or utilising very expensive insulation materials such as Aerogel or Vacuum Insulate Panels, which offer even better thermal conductivity (lambda) values.

PERFORMANCE triple glazed timber window at Mill office retrofit

The windows were installed using Compacfoam at intervals where frame fixings fix the frame to the masonry wall, this is because the Compacfoam gives a solid fix for the screws. Between the Compacfoam we have wedged 50mm thickness PU insulation board and filled the gaps with expanding foam, to ensure continuity of the insulation. An alternative would be to use Compacfoam all the way around the edges, but a more cost-effective pragmatic approach is to utilise PU insulation board between them. Airtightness was dealt with through careful installation of Pro Clima Solitex WA membrane from the OSB wall boarding through the window reveal under 50mm PU insulated plasterboard taped to the window using Tescon Vana and Tescon Profil. The window edges externally will be finished in due course with burnt sand mastic.

We are developing a bespoke insulated window sill to bridge between the solid brick outer leaf and our 140mm timber frame inner leaf, this will have insulation underneath the timber where it sits on the solid brick in order to reduce the thermal bridging and reduce the risk of condensation degrading the window sill.

Window U-value 0.85 W/m2K


In our next blog, we will look at the ventilation and heating strategies for the retrofit.

Bill Butcher, Director & Adam Harper Construction Manager, Green Building Store


Adam Harper, Green Building Company

28th March 2022

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