Sprayed Foam Insulation (Open Cell PUR, 20-25cm)

A trained company with proper spray equipment does this job. It is not suitable for DIY. The components have to be dosed precisely and heated to the right temperature, and the installer needs protection from the fumes. A typical job runs like this:

1. Survey and measuring: the installer checks the roof structure, the depth of the rafters and the condition of the roofing membrane, then proposes a thickness.
2. Preparation: dust and dirt are removed, and windows, floors and anything else that must stay clean are covered.
3. Spraying: the foam goes on in several passes until it reaches the agreed thickness.
4. Trimming: once it has cured, any foam that has grown past the edge of the rafters is cut back so the ceiling boards can be fixed.
5. Thickness check: the installer measures the layer in several places.

The foam feels firm almost straight away. During spraying, and for several hours afterwards, nobody should be in the space without protective equipment, and the room needs good ventilation. The foam manufacturer sets the exact time. A typical house loft is usually finished in one working day.

Open cell foam insulates slightly less per centimetre than closed cell foam, so it is sprayed in a thicker layer. A thickness of 20 to 25 cm is a common standard for pitched roofs and the ceilings of a habitable loft. On many roofs this fits between the rafters, sometimes with a small batten added underneath.

Some people go for 30 cm or more because they are aiming for a low-energy house or want noticeably better comfort in both summer and winter. That uses more material and takes longer. Rafters are often not deep enough for it, so you either add battens underneath or accept a lower ceiling height in the loft.

The right thickness for you is not just a question of centimetres. What counts is the thermal resistance of the whole roof, which depends on the thickness and on the thermal conductivity of the specific foam, shown on its technical data sheet. Minimum values for roofs are set by your local building regulations. On a new design it is sensible to aim a little above the minimum, because nobody wants to open the roof again a few years later.

Why use open cell foam in a loft at all? Its cells are connected to each other, so it is soft, light and lets water vapour pass through. The roof copes better with small amounts of moisture, and the foam also deadens the noise of rain well. Closed cell foam is harder and more water resistant, and it suits flat roofs, plinths and foundations better.

The foam is only one layer of the roof. For the whole roof to work, the rest of the build-up has to be right:

• Under the roof covering there should be a breathable roofing membrane and a ventilated gap. Foam is not sprayed directly onto the tiles.
• On the warm side, facing the room, a vapour barrier or vapour control layer is fitted as designed. A designer should decide the type and position.
• If the whole roof is being redone, counter battens and tiling battens go above the insulation.

Polyurethane is combustible, so in a habitable loft it must be covered by a ceiling lining, for example plasterboard. Around the chimney the installer keeps the clearance the manufacturer specifies. If you are insulating a loft in a block of flats, talk to the building management or owners' association first. A loft conversion or other work on the roof may need approval, so check with your local building authority before work starts rather than assuming it is not required.

How demanding the job is depends on conditions on site. These make it easier:

• A large, continuous area with few openings or pipes passing through
• A dry, clean, open roof structure with good access
• Rafters deep enough for the chosen thickness

These make it harder:

• A complex roof with dormers, valleys and roof windows
• A thicker layer, which means more material and more passes
• Old insulation to remove or a damaged roofing membrane to repair
• Cold or damp surfaces, which mean waiting or heating the space first
• Difficult access, with hoses running through a lived-in house

Most problems come from preparation and planning, not from the material. In practice it often happens that:

• A thinner layer is chosen so the quote looks cheaper, and the roof then falls short of the thermal resistance it needs
• Foam is sprayed onto damp or frozen timber, so it bonds poorly and can come away
• The vapour barrier or vapour control layer is forgotten, and moisture builds up in the roof
• Foam is sprayed straight onto the roof covering with no ventilated gap
• Quotes are compared only by the rate per square metre, even though each one assumes a different thickness

When you compare quotes, always check the thickness and type of foam each company lists. Only quotes with the same thickness, or the same resulting thermal resistance, can be compared fairly. Ask for the technical data sheet of the foam and for references from similar roofs.

Look over the work before the insulation is covered with the ceiling lining. Once it is boarded over, you cannot get to it again.

• The layer has been measured in several places and matches the agreed thickness
• The foam is continuous, with no holes or empty corners at the wall plate, the ridge or around penetrations
• The foam is firmly bonded to the rafters and does not come away anywhere
• Excess foam is trimmed flush with the rafters or battens
• The ventilated gap under the roof covering is still clear
• The area around the chimney follows the manufacturer's instructions
• The vapour barrier or vapour control layer is fitted as designed and sealed to the surrounding structure
• The installer has handed over a record of the material used and the actual thickness, as a basis for the warranty