While insulation is hidden from view, it still affects a wide range of issues including heating bills, damp, infestation proofing, environmental impact, wall thickness and overall cost of an extension or conversion.

Polyisocyanurate Board

The standard for the majority of extensions, new builds and conversions by Marble Construction and are easily recognizable from the foil outer coating on each side.

Sold under the brand name Celotex, it regularly out performs rival boards and its space efficiency makes it a common choice where space is at a premium.

Marble Construction offer a unique prefabrication service for extensions, conversions and new builds which allows for the use in factory based heavy machinery to increase accuracy and therefore energy savings.


Courtesy of Mary and Angus Hogg


This super eco-friendly alternative boasts natural fire resistant, infestation proof treatment, high damp and condensation resistance. On top of this, it actually cools the house during summer and heats it during winter through its hydroscopic nature.

Multi-Layered Foil

By bonding or sewing together multiple layers of insulating materials multi-layer  foil is the thinnest of the insulation options at as little as 25mm (though it does require an air gap for optimum performance)

It is only suitable for new builds or extensions as it does not breathe (as necessary in older buildings). The thermal requirement differs greatly by brand so it is best to investigate different suppliers and options.

Mineral Wools

This covers both rock, glass and slag wool from igneous rock, recycled glass and steel industry waste respectively. While this is much more efficient than the natural options, it cannot be recycled, is irritant and has a high embodied energy.

Hemp and Cotton

While costly, this is easy to install, non-irritant, very eco-friendly and retardant to fire and pests. It is also highly efficient in nearly all applications.


This loose insulation made from recycled newspaper requires a whopping 225mm of insulation to achieve the target U value for loft ceilings. On the plus side it is non irritant, resistant to fire and rodents and has a very low embodied energy.


Due to its water resistance, polystyrene is very suitable in places known to suffer or be at risk of damp. However it is by far the most environmentally damaging as it is toxic when burnt, does not bio degrade and can even react and damage electrical cabling.


Courtesy of the Architects Journal


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