Open-cell spray foam insulation, 30 to 35 cm thick

According to manufacturers, open-cell foam usually has a thermal conductivity of roughly 0.036 to 0.040 W/m·K. That puts it close to good mineral wool. A layer of 30 to 35 cm gives a thermal resistance of roughly 7.5 to 9.5 m²·K/W, depending on the product and the final thickness. Designers often aim for this level in the roofs of low-energy and near-zero-energy houses.

This thickness is most often used in these situations:

• A loft conversion where you want the roof insulated to new-build standard
• A new house whose design calls for a low U-value in the roof
• A ceiling under a cold loft, sprayed from above, where there is no height limit
• Replacing a two-layer mineral wool build-up (between and under the rafters) with one continuous layer without joints

Why is the usual 20 cm not enough? In a well-insulated house the roof becomes a major route for heat loss, because warm air rises. Every extra ten centimetres also shows in summer, as a thicker layer slows down the heat coming through from a hot roof covering. Loft bedrooms stay noticeably cooler. A thick layer does, however, place higher demands on the structure, on moisture control and on the spraying method. That is what the rest of this article deals with.

The price list has two other spray foam items, each with its own page. Here is a short comparison so you can pick the right one:

• Open-cell foam, 20 to 25 cm: usually fits within the depth of standard rafters without extending them. It is normally sprayed in one or two passes, and there is less trimming. It suits roofs where a standard level of insulation is enough, or where there is no room to make the roof build-up deeper.
• Open-cell foam, 30 to 35 cm (this variant): almost always deeper than the rafters, so the rafters need extending or a separate frame is needed. It is sprayed in several passes with cooling breaks in between, and it produces more offcuts. The job takes longer, and the moisture assessment of the roof matters more.
• Closed-cell foam: a denser foam with lower thermal conductivity. It is almost impermeable to vapour and water and adds some stiffness to the structure. It goes on in thin layers, so it is used mainly on foundations, steel buildings, metal roofs, cellars or tight spaces. Nobody normally sprays closed-cell foam 30 to 35 cm deep into a timber roof: it would seal the timber on both sides and cost far more than needed.

In practice it often happens that a customer orders the thick layer when 25 cm would have met the target for their rafter depth. The opposite happens too: a thin layer goes into a loft that becomes the main bedroom, and the room is hot every summer. Base the decision on a thermal calculation, or at least on a conversation with your designer or energy assessor.

A thick foam layer buries rafters, cables and every service passing through the roof. Anything forgotten has to be cut out later. Careful preparation pays off.

Roof and underlay

The roof must be watertight. Fix leaks, damaged tiles or slates and missing flashings before spraying. Foam holds water for a long time, and you might not notice a leak for years. Under the roof covering there should be a vapour-permeable breather membrane or another build-up that the foam manufacturer approves. Open-cell foam should not go onto old bituminous roofing felt over sarking boards without an assessment. Vapour would condense under the felt and the timber would get damp.

Rafters and extending them

Rafters in typical houses are roughly 14 to 20 cm deep. To spray 30 to 35 cm evenly, and to have something to fix the plasterboard frame to later, the rafters get extended. Timbers or boards set on edge are fixed to their underside, or spacer brackets with battens are used. The carpenter or builder must finish this before spraying day. The timber should be dry: foam manufacturers usually ask for a moisture content no higher than about 18 to 20 %.

Electrics, chimney and penetrations

Run all cables in the roof plane in conduit beforehand, and set back boxes so they stay accessible after spraying. This is work for a qualified electrician, not the foam crew. Foam must never be sprayed directly onto a chimney. A clearance is normally left according to the chimney manufacturer and the chimney inspector, and the gap is filled with non-combustible mineral wool. Soil vent pipes, ventilation ducts and roof windows must already be fitted and flashed.

Regulations and flats

Insulating from inside does not change how the building looks. It is often part of a loft conversion or roof renovation, though, and those can fall under local building regulations. Do not assume nothing is needed: check with your local building authority what your particular project requires. In a block of flats the loft and roof are shared areas, so agree the work with the building management or owners' association first. In some countries surveyors and mortgage lenders also look closely at spray foam in roofs, so keep every document the installer gives you.

The crew arrives with a van carrying a two-component machine, heated hoses and drums of the foam components. Usually two or three people work on site: the sprayer, a helper handling the hose and someone trimming and cleaning up.

1. Cover windows, floors, stairs and anything else that must stay clean. The foam atomises, and fine overspray can travel several metres.
2. Check the temperature and moisture of the surfaces and the air. The components and the substrate must be within the range the manufacturer specifies. In winter the loft is often heated beforehand.
3. Spray the first pass tight against the membrane or boards, sealing joints, edges and corners at the wall plate.
4. Let it cool. Foam gives off heat as it cures, and a pass that is too thick can overheat inside, crack or turn brown. Manufacturers therefore limit the depth of one pass, usually to roughly 15 to 20 cm for open-cell foam.
5. Spray further passes up to the required thickness, with a little extra beyond the edge of the extended rafters.
6. Trim the excess with a long saw or a special foam trimmer, flush with the rafters, so the frame can be fixed to them.
7. Check the thickness with a probe, top up any thin spots and clear away the offcuts.

At 30 to 35 cm there are noticeably more offcuts than with the thinner variant. The extended rafters matter just as much: they give the sprayer a clear guide plane. Without them it is very hard to keep the thickness even.

How long it takes

With a thinner layer, a typical house loft often fits into one working day. With a thick layer, allow more time, possibly two days, because there are more passes and breaks between them. The foam is usually firm to the touch within a few minutes and can normally be trimmed within an hour. It cures fully in roughly 24 hours. During spraying, and according to the manufacturer for several hours up to about a day afterwards, nobody without protective equipment should stay in the house, and the space must be ventilated.

With a thick layer, the difference between a simple roof and a complicated one shows more strongly, and so does the effort the job needs.

Things that make it easier:

• A simple pitched roof with straight rafters at regular spacing
• Rafters extended accurately to the required depth, giving the sprayer a gauge
• An empty loft without furniture, stored materials or old insulation
• Easy hose access from the van
• Dry timber and a suitable temperature, so no heating is needed

Things that make it harder:

• A complex roof with dormers, valleys, hips and many roof windows
• Lots of penetrations and boxes to work around and protect
• Old insulation that has to be removed and carried out first
• Uneven or twisted rafters that need more trimming
• A tall, steep loft where the crew works from scaffolding or a mobile platform
• Winter, when the space and the components need longer heating

Open-cell foam lets water vapour through, which is why it is called vapour-open. That is an advantage, because any moisture in the timber can slowly dry out. At 30 to 35 cm, though, it matters where in the build-up the vapour condenses. In winter, warm moist air from inside moves outwards and can condense at the cold membrane under the roof covering.

Manufacturers disagree on this. Some do not require a vapour control layer in a suitable build-up. Others recommend one, especially for thick layers and humid rooms such as bathrooms. The reliable route is a condensation risk assessment of the roof build-up following the relevant EN standards. The designer or energy assessor calculates whether you need a vapour control layer and which type. If one is used, it goes onto the trimmed foam, under the plasterboard frame, and all its joints and connections to the walls are taped.

Do not forget ventilation under the roof covering. The foam must not block the ventilation gap at the eaves or the ridge, otherwise moisture under the tiles has no way out. On a roof with sarking boards and a membrane, the ventilated layer is above the boards. On a roof with only a membrane over the rafters, the crew must make sure the foam does not push the membrane up into the counter-batten zone.

Plasterers and drylining fitters can usually start the next day, once the foam has cured and been checked. They fix the frame to the extended rafters, never to the foam. Cables stay in their conduit, and back boxes should be marked so nobody boards over them.

• One pass too thick: the foam overheats inside and cracks. You notice a smell or voids that are not visible after trimming.
• Spraying onto damp timber: the foam bonds poorly and gaps open along the rafter edges. The timber under the foam dries slowly and can go mouldy.
• Uneven thickness: corners at the wall plate and around roof windows tend to get less foam. In winter cold spots appear there, followed by dark patches on the plasterboard.
• Foam pulling away from the rafters: foam shrinks slightly as it cures, and on a poor surface a gap opens along the rafter. A thermal camera shows it as a cold stripe.
• Foam on an unsuitable membrane or felt: condensation and wet boards, often found only when the roof is repaired.
• Buried boxes and light fittings: the electrician then cuts the foam out and the insulation is no longer continuous.
• Foam right against the chimney: a fire risk and a serious defect at the chimney inspection.
• Blocked ventilation gap at the eaves: moisture under the roof covering, frost and, in the worst case, damaged battens.
• Trimming too deep: some bays end up several centimetres short of 30 cm, which only measuring reveals.

Spraying the foam itself is not a job for a layperson. It needs a special machine, respiratory protection and experience with the chemical reaction. You can, however, do the preparation yourself and save the crew time. Leave the wiring to an electrician, and keep a safe footing when working under a steep roof.

1. Clear the loft completely, including stored items and leftover materials.
2. Remove old insulation and carry it out in sealed bags. Wear a respirator, goggles and gloves. If you suspect asbestos, leave it untouched and have a specialist assess it.
3. Sweep and vacuum dust off the rafters, wall plates and boards.
4. Inspect the roof from inside during or after rain and mark every leak.
5. Check the timber moisture with a moisture meter on several rafters and write down the readings.
6. Check that the extended rafters are firmly fixed and line up in one plane.
7. Photograph all cables, boxes and penetrations before the foam covers them.
8. Arrange access for the van and hoses, and agree the power supply the crew needs.
9. Plan where the household, children and pets will stay during spraying and for about a day afterwards.

Inspect the foam before the vapour control layer and plasterboard go on, while everything is still visible. Ask the sprayer to measure the thickness with a probe in front of you in several places, especially in corners, at the wall plate and around roof windows. If the handover is in winter, a thermal camera survey is worth doing, as it reveals cold stripes. In a new house with an airtightness target, the air test follows later, after the vapour control layer is fitted.

• Foam thickness matches the agreed 30 to 35 cm, including corners and around penetrations
• Foam is continuous, without cracks, voids or brown overheated areas
• No gaps along the rafters or at the walls
• Surface is trimmed flush with the extended rafters
• Ventilation gaps at the eaves and ridge are clear
• Clearance around the chimney is kept and filled with non-combustible material
• Electrical boxes and penetrations are accessible and marked
• Windows, floors and stairs are clean of overspray
• Offcuts are removed, or it is agreed who disposes of them
• You have the foam's technical data sheet plus the thickness and area, recorded in the handover document