Energy saving: choosing internal wall insulation

Attention to detailing

25 February 2016

Internal wall insulation offers opportunities for energy saving, but the choice of product requires care, says Philip Smith

To begin with, a few statistics: around 30% of buildings in the UK have uninsulated solid walls – more than eight million – and space heating contributes around 75% of energy use in buildings.

Given that around 45% of heat loss in the average solid-walled property passes though the walls, if roof/loft insulation and double-glazing have been installed, solid wall insulation appears the next logical step to reduce heat loss, promising huge savings in energy and carbon emissions.

Insulating the walls externally may be the more obvious choice, but external wall insulation is not generally permitted to front walls in conservation areas and normally requires costly alterations to roofs and reveals. The alternative is internal wall insulation (IWI), although this requires careful consideration of detailing and thorough preparation to avoid pitfalls.

Product range

The range of products, systems and materials available reflects the varied technical and economic requirements of the marketplace.

Rigid insulation boards formed of phenolic foam, polyurethane, polyisocyanurate or wood fibre are fixed mechanically to studwork or by dot and dabs. Aerogel Spacetherm claims to have the highest thermal performance (measured in U-values and R-values), albeit at considerably higher cost than comparable rigid-board products (around £150).

Some products cope better than others with damp. Expanded and extruded polystyrene products (EPS and XPS) can be installed in locations where condensation is an issue and vapour permeability is not required, such as window reveals.

Window insulation

Windows require particular consideration

Alternatively, mineral wool batts can be inserted between studwork. Knauf provides an integral system that uses thermally engineered studs with XPS bonded to oriented strand board (OSB), which can be prepared off-site. For larger-scale insulation projects, wet-sprayed cellulose also offers considerable economies of scale and can fill gaps otherwise inaccessible.

Those wishing to address the broader sustainability agenda of low embodied energy might prefer natural, less processed materials such as sheep’s wool, hemp, cotton or natural fibre, or wood fibre. Advocates claim the new finishes provide a healthier environment due to greater thermal comfort and the radiant heat of the surroundings. However, slightly thicker insulation layers might be required to achieve the target U-value.

Insulating the inside of solid walls results in a marginal reduction in floor area, so it is worth checking the thickness required to achieve the target U-value. Whether or not this is significant enough to affect the property value depends on its location. However, the visual impact can often be mitigated by design. For example, the splayed corners of the window bay and bookshelves can conceal insulation that is returned along the back of the alcove to prevent thermal bridging.

Not all insulation products on the market provide significant improvement in thermal efficiency, so it is always worth checking the U-value would comply with Building Regulations for improved external solid walls before selecting. Part L1B stipulates a U-value of 0.3W/m2K, and similar criteria apply under Part L1A.

Bookshelves, bay window and insulation

Splayed corners to a window bay conceal insulation that runs behind a bookcase

Thermal bridging

The effectiveness of insulation depends on how well it integrates with other building elements that conduct heat more efficiently. Insulation must overlap at junctions and not be compressed excessively. Ill-conceived detailing at junctions may compromise its performance and lead to condensation and the ensuing problems.

Particular consideration should be given to window reveals, unvented joist ends, partition walls, the eaves junction and penetrations for building services. Cold bridging can also occur through the insulation’s fixings and some systems provide thermally broken screw fixings. BRE’s Good Practice Guide 183 Minimising thermal bridging provides a set of recommended section details.

Internal insulation reduces the temperature of the masonry wall to which it has been applied. Academic research indicates the increased risk of frost damage in UK building stock is not significant. However, the insulation causes a shift in the dew point, potentially resulting in interstitial condensation. This may damage external wall finishes.

Opinion is divided about how to manage moisture when insulating walls internally. Some systems incorporate a vapour barrier to the inside face. Other so-called breathable systems allow vapour to pass through both insulation and brickwork. Where a vapour-closed method is used, vapour barriers need to be continuously sealed and taped at edges to prevent moisture bridges. Adequate ventilation to internal spaces is then all the more important to avoid problems associated with condensation.

The installation process

Careful consideration should be given to the practicalities of installing each product and selecting the system that best suits the circumstances. For example, one might think twice about specifying heavy 2.4m rigid boards to upper floors in a high-rise block.

One manufacturer has produced an automated installation system with laser measurements, which allow batts to be pre-cut off-site.

Time is paramount, given the cost of labour and decanting owners or tenants, but allowance is needed for preparing walls, for example, hacking off plaster and sealing damaged brickwork. Personal protective equipment is particularly important for installing mineral wool and some rigid boards that produce fine dust. Electrical cables will be exposed and may require upgrading for fire safety.

Studwork must support the load of fixtures such as radiators, curtain rails and shelving, which must be refitted, along with light fittings and sanitary appliances. Many owners choose to refit decorative coving, dado rails and the like.

Close supervision of installation work (or ‘upskilled’ personnel) is necessary to ensure high standards of workmanship. Careful measurement and precision fitting is required to reduce thermal bridging and avoid breaks in vapour barriers. Insulation should be refitted where necessary to avoid small gaps that would make a big difference in thermal performance.

Regulatory compliance

Multiple tenure remains the greatest legal hurdle, because occupiers often lack a long-term interest in the building and generally require freeholder consent to undertake alterations.

In addition to Building Regulations Part L1B or L2B requirements, consideration should be given to Part B, appendix A relating to the spread of fire and minimum required periods of fire resistance, Part E (sound resistance) and Part F (ventilation). Part M (access) restricts the minimum width of new stairs to 1,200mm, which could limit insulating stairs internally.

Listed building consent and the Party Wall Act may apply in some instances. In one recent case (Kelliher v Ash [2013]), the judge concluded the particular method used to remove plaster was notifiable under the act.

The regulatory body Solid Wall Insulation Guarantee Agency provides guarantees for solid wall insulation.

The greatest economies of scale can be achieved by more efficient installation processes. It is the labour that costs the most, not the materials. Precise financial payback periods will be unique in each case. Naturally, better cost savings can be achieved when installing IWI as part of an existing refurbishment project, and for larger-scale insulation schemes.

Until recently, the upfront cost of IWI provided a strong disincentive to private owners. Funding from the Green Deal and grants such as the Energy Company Obligation removed this barrier. However, the Green Deal did not achieve the anticipated uptake after its release in 2013, with its over-complex and lengthy implementation process seemingly its greatest weakness.

The scheme relies initially on automated reports generated from fixed data entry on site. Installing IWI clearly requires careful consideration of the technical issues summarised above, in particular, with regard to thermal bridging and moisture management.

I am sceptical as to whether the Green Deal’s profit margins permit the detailed consideration or have the capacity to calculate and inform owners accurately of the cost benefits of each measure. In any event, the current government has withdrawn funding with no promise of reinstatement on the horizon.

The potential benefits of IWI are clear from a statistical perspective, even though the improved thermal performance may not be immediately apparent to the occupier following installation. Installers must be aware of the inherent pitfalls and hurdles, and the level of quality control required to achieve the full benefits of improved thermal efficiency.

But given the great opportunity it offers to make huge savings in carbon emissions and to reduce the nation’s dependency on imported fuels, sooner or later its value will fall under the spotlight. The ball is now in our court. 

Philip Smith is a building surveyor at Watts

Further information