Systems and specification
Open-cell spray foam: for hybrid systems, sound, and build-ups that breathe.
A light, flexible foam whose cells are open to each other. It absorbs sound, lets water vapour pass, and costs less per millimetre than closed-cell. HOLVEM uses it in hybrid systems with closed-cell foam, where sound absorption is needed, and where the build-up has to stay breathable. Not directly on a cold metal sheet.
- Best in hybrid systems
- Acoustic absorption
- Vapour-open, breathable build-ups
01What open-cell foam is
Open cells, and everything that follows from them.
The same two-component chemistry as closed-cell, blown with water so the cells burst open as the foam expands. Air fills them instead of a sealed gas, and every difference between the two systems comes from that.
01 · Soft
Light, flexible, easy to trim.
Open cells make a low-density foam that gives under a thumb, moves with timber as it dries and shrinks, and is trimmed flush with a saw or a blade. It expands many times more than closed-cell as it is sprayed, so it fills deep voids quickly.
02 · Vapour-open
Moisture passes through it.
Water vapour moves through the open cells almost as freely as through air. The build-up has to be able to dry, and in most UK assemblies that means a vapour control layer on the warm side and a surface on the cold side that can shed what gets through. It is never itself the vapour barrier.
03 · Acoustic
It absorbs airborne sound.
The open structure that lets vapour through also traps sound. Sprayed into a ceiling void, a partition or under a floor it reduces the sound passing through, which closed-cell foam, being rigid and sealed, does very little of.
04 · Per millimetre
Lower cost, higher λ.
Air conducts heat better than the gas in a closed cell, so open-cell foam has a higher thermal conductivity and needs more depth for the same resistance. Its price per millimetre is lower; its price per unit of resistance is the comparison that matters, and depends on the void being filled.
02Why not on a cold sheet
On a metal roof, the vapour has to be stopped before the steel.
A profiled metal sheet is the coldest surface in the building on a winter morning, and the room below it is full of moist air. Whatever is sprayed on the sheet either stops the vapour at its face or lets it through to the steel. That is the whole difference, drawn.
Room vapour passes through the open cells to the sheet, which is below the dew point. It condenses there, inside the foam, against the steel.
Section, not to scaleThe same sheet, the same room air, on the same cold morning.
03On commercial buildings
Three places open-cell earns its keep, and one where it does not.
Open-cell foam is usually best in a hybrid system with closed-cell foam, where sound absorption is needed, or where the build-up has to stay breathable. The condensation risk analysis says whether a vapour-open layer belongs, and it is run before either system is specified.
01Hybrid systems
Inside a closed-cell layer, as the second insulation.
The build-up HOLVEM reaches for most often. Closed-cell does the vapour and air-sealing work against the cold face; open-cell fills the depth behind it.
- A closed-cell layer on the sheet or deck first, stopping the room's vapour at a face that stays above the dew point and sealing every lap and fixing.
- Open-cell inside it, to the depth the U-value target sets, at a lower cost per millimetre than carrying the whole depth in closed-cell foam.
- The condensation risk analysis sets the split: enough closed-cell depth that the interface between the two foams stays warm and dry.
02Sound absorption
Where sound absorption is the objective.
The open structure that lets vapour through also traps airborne sound. Closed-cell foam, rigid and sealed, does very little of this.
- Ceilings, soffits and partitions in offices, studios and mixed-use buildings, where the brief asks for a quieter room, not only a warmer one.
- Rain noise on lightweight roofs, where an open-cell layer inside the closed-cell layer takes the edge off the drumming.
- Under floors and in voids between occupied spaces, reducing the sound passing through the structure.
03Breathable build-ups
Where the assembly has to stay vapour-open.
Some build-ups are designed to dry outwards through the insulation. A vapour-resistant foam there would trap moisture; an open-cell layer lets it move.
- Timber-framed roofs and walls with a vapour control layer on the warm side, where the analysis shows the assembly drying every year.
- Deep timber voids and irregular cavities, which the foam's expansion fills completely at a lower cost per millimetre than a rigid foam.
- Behind an existing vapour control layer, where a second vapour-resistant layer would trap the first between two closed faces.
04Not here
Not directly on a cold metal sheet.
The sheet sits below the dew point and open cells carry the room's vapour straight to it. The condensation moves into the foam; it does not stop.
- The underside of profiled metal roofs and walls: closed-cell's vapour resistance is what protects the steel, and it is what HOLVEM specifies there.
- Fibre-cement and asbestos-cement encapsulation, which needs a rigid, bonded, vapour-resistant layer that stiffens the sheet.
- Anywhere the layer must seal laps and fixings or add stiffness. A soft, open foam cannot do either.