Mill office radical retrofit: A ‘box within the box’

Office overspill

Green Building Store is outgrowing its existing office space at Heath House Mill (in Bolster Moor on the hills above Huddersfield), Fortunately, the Heath House Mill former woollen mill complex where we’re based does have some spare warehouse units that we can spill over into.

However, there are two challenges with this: we only rent the space and the space tends to be very basic, so to create office space will require investment that our landlord would not be prepared to undertake. We managed to negotiate a solution whereby our building team undertakes a retrofit of one of the former mill’s warehouse spaces in exchange for the landlord keeping our rent at the warehouse levels going forward (as opposed to a higher office rent level).

Pragmatic low-cost approach

We have therefore decided to undertake a cost-effective and pragmatic radical retrofit of 200m2 of groundfloor warehouse space (Unit 4 of Heath House Mill) into a cosy and energy efficient office space for up to 26 MVHR staff. Unit 4 has a separate industrial unit above, so our retrofit is just dealing with the ground floor.  Having another unit above gave rise to two particular issues, fire separation and noise transmission, both of which require careful thought, design and installation.


Heath House Mill retrofit

The warehouse building was probably built around 1900 with stone outer leaf and brick inner leaf, with some rubble fill between. Effectively, we would therefore treat the wall as a solid wall. We decided that the easiest retrofit solution for this space was to replicate our retrofit strategy at the Cre8 Barn at Stirley Community Farm by creating a timber frame ‘box within a box’. This approach makes it easier to deliver the airtightness strategy and to minimize thermal bridging, without costly IWI solutions. As at Stirley Farm, it is important to have a ventilated cavity between the outer stone wall and the inner timber frame, so we will be inserting 60 X  75mm diameter louvered vents to the external walls at intervals of approximately 2.4m  Between the brickwork and the timber frame breather membrane we have a 50mm ventilated cavity to allow air movement.

Modelling and moisture mitigation

For once, we haven’t modelled the retrofit in PHPP or WUFI but are just going on our experience from the Stirley Farm project.  However, we may require an SBEM model for Building Control which will give us a benchmark as to the level of energy improvement. Though, like SAP, SBEM is not as detailed as PHPP. We know that the ‘box within a box’ approach we have chosen has worked well at Stirley Farm and has had moisture sensors monitoring it, so we are just replicating the approach just in a less exposed hillside location.

Airtightness strategy for ceiling and external walls

Whilst there is an industrial unit above our space, we know that it is very leaky. Therefore for the ceiling, we used Constavap 2.3 taped with Tescon Vana at the joints which we have installed over the top of ceiling joists in order to create a 100mm depth service zone between the top and bottom of the joists. The advantage of this is that we have an airtightness layer that doesn’t have any service penetrations going through it.

The walls have an 18mm OSB layer over the timber studs which has a reasonably good level of airtightness (though in hindsight we would have been better using Constivap under the plasterboard). The OSB board joints are taped and where the OSB meets the floor it is taped to the smooth concrete surface. This gives us a reasonably good ‘airtight box’.

Timber frame walls & ceiling

Timber frame panels at mill office retrofitExternal Walls U-Value 0.148 W/m2K

  • Timber stud walls at mill office retrofit500mmm solid wall, stone outer and brick inner
  • 50mm vented cavity with vents to the outside
  • Pro Clima Fronta Solitex WA breather membrane taped at joints with Tescon Vana
  • 140mm timber stud, filled with 140mm Knauf Frametherm 35 mineral wool insulation
  • 18mm OSB taped at joists with Tesco Vana
  • 62.5mm PU laminated plasterboard
  • Plaster skim finish

Surfaced fixed dado trunking for power and data to desks


Ceiling U-value 0.158 W/m2K

  • Wall ceiling detail at mill office retrofitExisting timber floor structure above
  • 100 x 50mm timber structure @400mm centres
  • 240mm Knauf FactoryClad Roll 40 mineral wool insulation 0.040W/mK for thermal and acoustic insulation
  • Pro Clima Constivap VCL, taped at joists with Tesco Vana
  • 2 layers 12.5mm fire-rated plasterboard, staggered joints
  • Plaster skim finish

Lights and MVHR ducting surface mounted on ceiling.


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


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


Adam Harper, Green Building Company

1st March 2022

One response to “Mill office radical retrofit: A ‘box within the box’”

  1. blank Ronan says:

    Hi, looks like a great way to retrofit and bring new use to an old stone building. I have a stone building that will need a warm roof and the external ground level is 1m above floor, but dry. Would this method work here as I’m doing all the work myself

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