If the government is to make good its pledge to act on the latest UK Green Building Council report which calls for urgent action to improve the country’s energy-deficient housing stock, the thermal efficiency of every aspect of the building envelope will need addressing. External and internal wall insulation has a major part to play in determining whether a new-build home will attain a favourable U-Value rating. Ben Warren, Managing Director at global building materials manufacturer, Baumit, looks at current EWI/IWI options and how each compares in terms of performance.
External or internal wall insulation? That is the tantalising question, particularly when it comes to property construction. The need for new housing to keep pace with the UK’s increasing population is without question and identifying the ideal insulation for each project will ensure today’s new development doesn’t become tomorrow’s retrofit nightmare.
In terms of external wall insulation systems in new-build properties, acrylic top coats are very much in vogue, particularly with social housing projects. It’s an option primarily based on cost. Local authority funding has been cut to the bone across the board – every pound is being accounted for. It means, although an insulation system specified for a council-owned building might fit the budget, it’s not necessarily best for the property or its occupants’ long- term well-being.
Through everyday living, residents create water vapour. Baths, washing, cooking…it all creates steam – even breathing. Now, consider how much vapour is generated in a multi-occupancy building containing, say, 150 tenants. All that vapour will pass through the building’s elements, but in cases where the EWI contains a non-vapour permeable acrylic topcoat, the vapour will be trapped. This can lead to condensation and the dreaded “d” word – damp, which once inside a property, can take an awful lot of time and expense to remove.
Therefore, an EWI solution which might have appeared cost-effective in the immediate-to-short-term, could potentially result in unnecessary and unforeseen expenditure.
Applying non-breathable EWI to a building causes what might be referred to as the “plastic bag effect” – walls can become cloaked in condensation from non-escaping vapour. To create a breathable outer layer for buildings private or public, a silicon-based finish render is preferable. Water-repellent, stain and weather resistant, the system provides a robust white or coloured façade. It’s easily applied to mineral renders old and new, providing a decorative, vapour-permeable topcoat that makes for an attractive exterior, whilst optimising occupants’ living environment.
For a damp, moisture-free, healthy-living interior, lime-based, thin-coat plasters offer an excellent option. Vapour permeable and low in emissions, these systems make an ideal environmentally-friendly, breathable plaster for interiors of all types. We spend 90% of our lives indoors, therefore providing a quality finish for interior walls would appear paramount to our wellbeing. There are a couple of very good reasons for choosing EWI over interior insulation, however.
Interior space invader
Unlike the interior option, floor space isn’t compromised with exterior insulation. A few millimetres’ less living room in a building containing multiple flats and potentially hundreds of walls, for instance, amounts to a sizable loss of space, which for private landlords could mean a reduction in income.
Another benefit of ‘going external’ when it comes to wall insulation? EWI removes the dew-point – the temperature below which water droplets condense – from the inside of a building to prevent condensation forming. A high-performance EWI system lets go of moisture and retains airtightness, while those comprising Expanded Polystyrene (EPS) façade insulation boards ensure high vapour permeability, resulting in the release, rather than the entrapment, of water vapour. This system has the added benefit of having no effect on a building’s airtightness.
In short: EWI improves the aesthetics on the outside and the building’s thermal performance without affecting the interior space. A case can be made for specifying exterior and interior insulation in semi-detached buildings. The EWI can be applied to two sides of the property, with an internal system installed to the front wall. This method helps maintain the road’s overall look, whilst improving the thermal performance of the home.
There can be no doubt the government is right to address the alarming statistic provided by the Green Building Council report which stated 25 million homes across Britain will not meet insulation standards by 2050. However, airtightness can mean as well as retaining heat; our buildings are also retaining moisture – which creates problems of its own. Insulation applied whether internally or externally, but most importantly, correctly, will do much to ensure the buildings we so desperately need to fulfil our future housing needs perform with the utmost thermal efficiency, thus prolonging the life of the property and enhancing the health and wellbeing of those living within its walls.