Often abbreviated to PU, Polyurethane is usually applied as a foam insulation sprayed onto the underside of roof rafters.

Description
A complex manufactured compound, polyurethane has traditionally been manufactured from petrochemicals and so has a few green credentials except for those gained from the energy it saves. Recent developments toward the use of polyols derived from vegetable oils to make polyurethane may yet redress this and shift this technology toward the green part of the insulation rainbow.

Features

• Manufactured from petrochemicals so not sustainable and toxic when burnt.
• High embodied energy.
• Low k-value.
• Moisture & fire resistant.
• Adds structural stability to a roof, seals small gaps etc. – Can be a viable alternative to a complete re-roof.

Performance
Polyurethane has a thermal conductivity or K value of 0.02 W/m.K.
(Watts per meter Kelvin ~ a lower value is a better result)

Cost
Per meter costs are difficult to derive as Polyurethane is usually supplied as an all-in service including installation. Typically, it will cost 4 to 5 times more than mainstream alternatives such as mineral wool with a total cost of £2-3,000 for an average UK home.

Reference:
For the science see the very thorough Wikipedia article: http://en.wikipedia.org/wiki/Polyurethane