Sprayed Closed-Cell Foam Insulation

Sprayed PU foam is made by mixing two liquid components, a polyol and an isocyanate. They only meet inside the spray gun, foam up straight away and set within seconds. The recipe and density decide what kind of cells form in the foam.

• Closed-cell (rigid foam): The cells are sealed and filled with gas. The foam is firm, holds its shape and lets almost no water or water vapour through. Its density is usually above roughly 35 kg/m³, and the share of closed cells is around 90 % or more.
• Open-cell (soft foam): The cells are connected. The foam is light and flexible, lets vapour through and dampens sound better, but it does not cope well with moisture. It is used mainly in dry interiors.

Why do people also choose rigid foam to save space? According to manufacturers, its thermal conductivity (λ, a measure of how easily heat passes through) is usually about 0.025 W/(m·K), while soft foam sits at roughly 0.037 W/(m·K). So you need a thinner layer for the same insulating effect. This matters in a loft with shallow rafters, where you do not want to lose headroom.

Rigid foam suits places where you need insulation that resists water and is mechanically strong:

• Pitched roofs and lofts: The foam fills the space between the rafters and also seals tricky details around chimneys and roof windows.
• Flat roofs: The foam is sprayed on from outside and then protected with a top coating or another layer.
• Foundations, plinths and substructures: These are areas in contact with soil or splashing water.
• Industrial halls, warehouses, farm buildings and cold stores: Here large areas of metal sheet or concrete need insulating quickly.

On a pitched roof, the spray foam is only one layer of the whole build-up. It works together with the roofing and sheet metal work: the breather membrane or sarking boards, the battens and the roof covering. A designer should specify the roof build-up. Rigid foam on its own is almost vapour-tight, so a separate vapour barrier is often not needed. What decides this is the specific system, and the rule that timber rafters must never end up sealed in with moisture. Spraying foam directly onto the underside of tiles or slates is not recommended, because any later roof repair then becomes very difficult. In practice, some surveyors and mortgage lenders are also cautious about spray foam in roofs, especially where it hides the timber. If you plan to sell or remortgage, ask about this before you commit.

If you are insulating a facade or a flat roof, or making any other change to the building, check with your local building authority what the local building regulations require. In a block of flats, always speak to the building management or owners' association first, because the roof and external walls are usually shared parts of the building.

This job is always done by a specialist company with spraying equipment. During spraying, the isocyanate irritates the airways. The operator therefore needs a respirator or breathing apparatus, and everyone else must stay out of the area while the work goes on. This is not a job for a layperson.

1. Inspect the structure, set the layer thickness according to the design and check that the roof or wall is dry and free of leaks.
2. Cover windows, floors, electrical installations and anything else that must not be sprayed.
3. Clean the substrate of dust, grease and loose material.
4. Check the air and substrate temperature and humidity against the manufacturer's technical data sheet.
5. Spray the foam in several thinner passes until the required thickness is reached.
6. Trim off any excess foam and check the thickness in several places.
7. Ventilate the area and keep to the re-entry time stated by the manufacturer.
8. Add covering layers where needed, such as plasterboard in a loft or a protective coating on a flat roof.

How demanding the work is, and so what you end up paying per square metre, varies from building to building. The job is simpler with:

• Large, flat and easily accessible surfaces
• A dry, clean substrate that needs little preparation
• An empty space where not much needs covering

The job is more demanding with:

• A thicker layer that has to be sprayed in several passes
• Complex roof structures, low lofts and lots of details around chimneys, pipes and windows
• A small job, where bringing the equipment to site is spread over only a few square metres
• The need for a fire-protective or other protective covering layer

In practice it often happens that cheaper open-cell foam is chosen for a damp location, such as a plinth or a flat roof. It soaks up moisture and loses its effect over time. Other common problems:

• Poor substrate preparation: Dust, grease or moisture weaken adhesion, and the foam may start to peel later.
• Spraying in unsuitable weather: In cold or very humid conditions the foam expands and cures unevenly.
• A thinner layer than agreed: The insulation then falls short of its designed performance.
• Foam left in the sun: UV light slowly breaks down PU foam, so outdoors it must always be covered.
• Missing fire protection: Standard PU foam usually falls into reaction-to-fire class E under EN 13501-1. In a habitable loft it therefore normally has to be covered, for example with plasterboard. Treat claims of "non-flammable foam" with caution and ask for the actual classification.

Before you order, ask for the technical data sheet, the declaration of performance and the reaction-to-fire class. Have the layer thickness stated in the quote itself. Only compare quotes for the same area and the same thickness.

• The layer is continuous, with no holes, cracks or spots where the substrate shows through
• The thickness matches what was agreed, checked by probing or measuring in several places
• The foam is firmly bonded to the substrate and does not peel when pressed
• The surface is even, without large bubbles or soft, sticky patches
• Details around the chimney, windows and penetrations are carefully sealed
• Windows, floors and installations are free of foam overspray
• No strong smell remains in the room after ventilation
• You have the technical data sheet, the reaction-to-fire class and the warranty terms