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Should I be insulating a solid wall internally or externally?

Thinking about insulating a solid wall? Unsure whether to opt for internal or external wall insulation, or both? This article looks at the pros and cons of each approach and lists some of the potential problems encountered when insulating a solid wall.

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Internal wall insulation

Pros

Internal glasswool wall insulation between studs. Timber itself can be a thermal bridge in a super-insulated home.

  • may be easier to accomplish, especially one wall at a time
  • ideal to do when decorating
  • cheaper than external insulation.

Cons

  • there is a loss of internal space
  • it is disruptive to residents
  • the heat storage value of the wall is lost, so heat will leave quickly with the air
  • it is necessary to move electrical sockets and light switches
  • character features such as skirting boards, door frames, coving, panelling and picture rails present a problem
  • compatibility with door fittings must also be checked.

Challenges

Internal wall insulation

Applying internal wall insulation in between studwork. A vapour control layer is vital.

  • do not cover electrical cables with insulation as they may overheat:use cover strips or place in ducts
  • thermal bridging may occur where ceilings, floors and internal walls join the main outside wall, so insulation should continue along internal walls for up to a couple of metres
  • an intelligent vapour control layer must be installed (see below)
  • a single nail driven by a residents into the dry lining or vapour control layer behind the plasterboard can ruin it. One solution is a service zone separate from the plasterboard but this takes up more space. Can you guarantee residents will always locate and bang nails into the vertical studs?

External wall insulation

External wall insulation has improved the appearance and added weather protection besides making this home more energy efficient.

External wall insulation has improved the appearance and added weather protection to the house on the left making it more energy efficient.

External wall insulation involves applying an insulating layer and a decorative weatherproof finish to the outside wall of a building. The external cladding or render is generally the major cost, so you want to maximize the amount of insulation to maximize the benefits.

Pros

  • improves weather protection
  • provides noise insulation

    Cross section of external wall insulation

    Cross section of external wall insulation.

  • it’s easier to take care of thermal bridges such as exposed concrete frame or window sill
  • preserves the value of the thermal mass of the walls in regulating temperature inside
  • depending on thickness, any value of insulation can be achieved
  • no inconvenience to occupants
  • internal or interstitial condensation and damp is banished
  • any gaps and cracks in the wall, or poor rendering, are covered up
  • the detailing around windows and doors is more easily managed
  • airtightness can be better preserved
  • you can insulate a whole block or terrace at once
  • any dew point, where water vapour leaving the inside condenses onto a cold surface in the wall, will be nearer the outside.

Challenges

Installing external wall insulation

Installing external wall insulation

  • if there is not sufficient roof overhang, the top of the extended wall must be sloped, weatherproofed, and guttering fitted
  • may need planning permission – ask the planning department
  • downpipes and other projections, or service entry points, must be dealt with
  • the damp proof course and window trickle vents must not be covered
  • may not be possible with listed buildings or in conservation areas¹
  • it might be more expensive (£9,500 to £13,000), especially if scaffolding is factored in, unless carried out at the same time as other external work.

General points

The Steico external wall insulation uses interlocking waterproofed wood fibreboard over studs infilled with wood fibreboard batts, protected by a proprietary lime render system called Steico Protect.

The Steico external wall insulation system uses interlocking waterproofed wood fibreboard over studs infilled with wood fibreboard batts, protected by two applications of a proprietary lime render.

  • When taking insulation into window and door reveals, whether internally or externally, use the maximum width allowed by the amount of frame.
  • External insulation is covered by some kind of render to protect it from the elements. Lime render is preferable as it is breathable. Acrylic renders are also widely used, but these, being plastic, are not breathable. Both are flexible to a point and can be coloured in any number of tints.
  • The path of the airtightness barrier around the skin of the building should be mapped during planning of the work, so that it joins up around all edges of the wall.

Intelligent membranes

Airtight membranes with variable vapour resistance are known as intelligent membranes. These have the almost magical property of being able to resist vapour migrating into structural elements, particularly in timber frame constructions, to fight interstitial condensation over the lifetime of the building. They are recommended for solid walls whether the insulation is internal or external.

They consist of a sheet with various layers that allow or obstruct moisture-laid air from passing through it according to the relative humidity, temperatures and pressures on either side. For example, if a room has high temperature and humidity and it is cold on the outside it would prevent the moisture from travelling outwards and risk condensing on cold surfaces.

Conversely, when it is cooler on the inside and there is less humidity, moisture can gradually return to help dry out the interstitial space.

Andy Simmonds' Grove Cottage

Andy Simmonds’ Grove Cottage, Hereford, the first Passivhaus standard retrofit in the UK. Andy is CEO of the AECB. The thermal image shows the benefit. Andy used a foot of expanded polystyrene to achieve this effect, which is literally “outstanding” from the houses either side.

Conclusion

Walls should have a U-value of less than 0.30 W/m2K to meet Building Regulations. Around 120mm of insulation will usually achieve this. Half of this value, i.e. double this depth, should on average be aimed for to achieve around Passivhaus standard.

Insulating external walls can save on average between £450 and £500 a year on heating costs, and around 1.8-2 tonnes of carbon dioxide emissions for a typical dwelling.

The payback on internal insulation is around 10-11 years, and on external insulation around 15-17 years on average, but this really depends on the amount of insulation applied, the way the building is occupied and the state of the rest of the dwelling, so this figure should be confirmed in individual cases.

In an earlier article, I compared and ranked the best thermal insulation materials. Of relevance to this topic is that walls should be able to ‘breathe’ in order to prevent condensation. This means that the best choice is natural materials, including wood fibreboard, mineral wool batts and rolls. Polyurethane, polystyrene and phenolic foam boards do not ‘breathe’.

So, is it best to insulate on the inside or the outside of an external wall? There is no one definitive answer to this question, as it depends on the particular case. However, all other things being equal, external will give better results than internal insulation.

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© David Thorpe, Manager of Green Deal Advice and author of Sustainable Home Refurbishment: The Earthscan Expert Guide to Retrofitting Homes for Efficiency

1.  In conservation areas it is important to preserve the original facade of buildings. This rules out external wall insulation at the front. However, it may still be possible to install it to the rear or the side. Quite a few refurbished Victorian SuperHomes have both internal and external wall insulation installed in order to maintain the original facade – take these examples [1] [2] [3].

Also see:
External and internal wall insulation at free SuperHome open house events in Mar/Sep
Best insulation
Cork insulation
Insulate a floor
Interstitial condensation

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