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Stone Barn Conversion Thermal Insulation (Brainstorm)

Stone Barn Conversion Thermal Insulation (Brainstorm)


I was recently consulted about options for thermally insulating a barn conversion project.

It raises a number of issues, and here we look at not just the best practice for the situation, but the reality of working with clients, builders and suppliers where other factors, such as budget, come in to play.

The question was how best to advise the client who has said that the cost of the specification is too high, and the builder is advising the use of phenolic insulation (plastic foam).

If the specification of wood fibre is too expensive, what, realistically, can the client install which carries least risk to the building fabric?

As this is a problem which many architects face, it is worth considering in some detail.

This balance between the ‘real world’ of domestic projects and the difficulty of getting independent advice on more sustainable solutions is hard to achieve.

This is where building physics meets the High Street.


A traditional stone barn, part of which is being converted from agricultural to domestic use. The existing construction is as follows:

Floor: concrete on bare earth; the two small end barns were used originally for housing stock, so the floors are at various levels, with stalls and drainage. The floors are to be grubbed up in their entirety, so we will not consider them in this exercise; they will have a new, insulated concrete, ground-bearing slab.

Walls: Approximately 500mm thick overall, with two ‘leaves’ of gritstone and rubble fill. Externally, the walls are coursed, although the coursing is informal in places. The intention is to completely repoint externally, and to rebuild internally where loose or unstable masonry is found. Some internal remodeling is required, with new steel structure being inserted as required. It is understood that wind-driven rain may be an issue, even with the repointing.

Roof: The original stone slate roof covering remains, but years of patching and adapting internally mean that the roof has had to be replaced entirely. This work has already been carried out, so we have a new roof structure, insulated over rafters with wood fibre, with a breather membrane and battening, and the lovely stone slates replaced. Insulating between the rafters will form part of the current works.



Existing floor grubbed up and levelled. 100mm compacted hardcore, with 25mm sand blinding, DPM and then 100mm Styrozone insulation.

100mm reinforced concrete slab and 50 – 75mm screed (depending upon spec.) with underfloor heating pipes in screed.

(A structural engineer has agreed the loading for insulation and slab.)


Internally, once the walls are stabilized, Speedfil (filling mortar) dubbing out (to provide approx. even surface), with Pavadry woodfibre insulation – thickness to be determined, but available in 52mm, 72mm and 92mm, then plasterboard and skim finish.

(This has been agreed with NBT, who helped with the specification.)


Already installed is: 60mm Pavatherm Plus over rafters, with Kloeber breathable membrane (which was probably not needed, given the ventilation provided by the stone slates, but its also a rainwater drainage layer so we better keep it), battened and counter battened (25 x 50mm treated sw) and stone slates.

To complete the installation, specification is 140mm overall thickness Pavaflex tightly fitted between rafters, with Pro Clima Intello DB3.5 airtightness membrane and plasterboard and skim finish.

Building Physics:


Hygroscopic insulation is usually any plant/tree based insulation.

  • If you use hygroscopic insulation you are using vapour-open construction: (ATL) air tightness layer on the inside, and (WTL) wind tightness layer on the outside.
  • If you move away from hygroscopic insulation, you are using vapour-closed construction: (VCL) vapour control layer on the inside, and (BM) breather membrane on the outside.

  • ATL and WTL are usually done well using trained, approved installers
  • VCL are generally not and will need thorough spec, supervision and checking of itself and any service or structural penetrations for competency before sign off and covering up.
  • Plugging air gaps with insulation spray from a can is risky since the surfaces are not smooth, cleaned or primed and the foam will shrink away leaving gaps (smaller than the original gaps but gaps all the same).
  • Plugging gaps does not solve the moisture-through-timber problem.
  • Moisture through timber is bad news for risk of high moisture content, rot and structural failure.
  • Moisture through air gaps is bad news for risk of heat loss or coolth gain with the air leakage and moisture passage.
  • There is no real solution to this problem, hence plastic insulation should never be used between the rafters if you want a long safe building life.
  • Placing the insulation on top of the rafters raises the roof tiles, where the roof structure is being retained.
  • (Planner and Conservation Officers usually object).
  • In this example, the roof structure was replaced, so the roofline was maintained, and the ceiling heights reduced internally to suit.
  • Placing the insulation below the rafters is a potential solution but inevitably this leads to thermal bridges and moisture bridges through insulation by roof timbers.

In terms of options, here are some to consider:

  • Vacuum Insulated Panel (VIP) will be thinner more expensive than wood fibre, and will not stop overheating from the heat of the sun (worse than plastic insulation).
  • Aerogel will be more expensive and thinner.
  • Glass or stone wool will be cheaper, thinner or similar thickness to dense wood fibre but will be thicker than plastic insulation to meet the same U value.
  • Hemp-lime between the rafters
  • Cellulose flake recycled newspaper between the rafters
  • Expanded/Extruded Plastic insulation will be vapour resistant, so any inadequacy in the VCL will concentrate moisture passage through the timber or the insulation’s poor fit gaps.

External Walls:

  • Plastic insulation is fundamentally wrong for old, breathing buildings. Think of it like the difference between going out in a plastic mac or a Goretex jacket.
  • The stones and the mortar wick water from outside and below into the wall and it would normally breath outwards and/or inwards depending upon relative conditions.
  • Putting any barrier in the way will prevent this evaporation and hold the water in the stones and the mortar and any embedded timbers.
  • By adding insulation to the inside, this stops internal heat driving the moisture outwards, leaving the wall saturated and cold.
  • The outer face becomes more vulnerable to frost damage and any embedded timbers become prone to rot.
  • By adding vapour tight plastic insulation, the barrier to inward evaporation is greater, keeping the inner stones damp.
  • By adding battening between lumpy walls and flat board, you create a void across which water cannot transfer.
  • Moisture at the surface of the void in the stone will attract and trigger growth of ever-present spores in the air (present before closing-up).
  • Mould will start on the damp surface of the stones, grow in the void, spread to fill the void, creep through any gaps below or between the boards, travel in the service/fixing void/hollow skirting/dado paneling.
  • Ultimately, it will grow through the plasterboard and onto the surface; asthma, toxic mould, loss of possessions and home, death in the worst cases.

With regards to filling the voids on the internal face of the stones:

  • The rubble-filled wall core will be prone to air movement and eddie currents in the open interstices, which will enable dissipation of heat over a large area/volume of the wall..
  • Mortar pointing on the outside should make the wall wind-tight, so whilst the interstices makes air passages through the wall to the inner leaf possible, the outside air and internal air should not be connected so not making drafts. Stone, itself, will have imperfections, enabling air and wind-driven rain to penetrate, so pointing alone will not, necessarily, prevent these occurring. Don’t forget also that air may be entering and leaving in many other places at the perimeters of the walls.
  • Air movement means heat loss and coolth gain, and risks of surface condensation anywhere in its path.
  • If the outside face of the insulation is cooled by colder air or drafts in voids, heat will be drawn from the insulation.
  • Badly fitted insulation will have heat loss through joints and under lining boards.
  • If a void is created behind the board, you will need to use many mortar dabs or dots to make a continuous barrier at top, bottom and openings and close the gaps above and below, to prevent longitudinal movement of air.

Options to consider:

  • Diasen Diathonite Evolution; one build up internally acts as dubbing-out, levelling and insulating, all in one.
  • Gypsum plasterboard is relatively moisture vapour resistant and should not be added to create a barrier to inward movement and evaporation of moisture from a solid wall; note, however, that products such as Pavadry can work with plasterboard – seek advice.
  • Lime plaster instead of gypsum plasterboard and check if Pavadry or its replacement is suitable.
  • Clay plaster instead of gypsum plasterboard and check if Pavadry or its replacement is suitable.
  • Clay board instead of gypsum plasterboard.


  • Here are some things to consider: there is no getting away from trawling the market place and finding alternative costs for any project.
  • Plastic insulation is not cheap; you are paying for thinness (for a set U value).
  • Aerogels are not cheap and they are thinner still.
  • VIP Vacuum Insulated panels are thinner still but not cheaper.
  • Calcium Silicate Insulation was developed for solid wall internal insulation, probably not cheap.
  • Cheap comes from glass and stone wool; it will be thicker (for a set U value).
  • Cheap comes from expanded polystyrene (middle of the road k value).
  • Hemp-lime internal insulation (damp sprayed onto surface between framing set off wall, for clay boards).
  • Cellulose flake recycled newspaper internal insulation (dry blown through holes in clay lining board).

Cellulose flake recycled newspaper internal insulation (damp blown onto surface between framing set off wall for clay lining board).

Self-Insulating earth/cork render: e.g. Diasen Diatonate Evolution (probably not cheap).

© GBE NGS ASWS BrianSpecMan aka Brian Murphy & JM
25th January 2017 – 27th January 2017

Stone Barn Conversion Thermal Insulation (Brainstorm)
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© GBE NGS ASWS BrianSpecMan aka Brian Murphy
25th January 2017 – 27th January 2017

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