Guidance Notes on Room Sealing

1. Background

Successful extinguishment of a fire by a gaseous extinguishing system is critically dependent upon the extinguishing concentration being maintained for a specified period after discharge. Excessive leakage of extinguishant through the construction or ventilation system may result in reignition.

The criterion used to determine whether the amount of leakage is acceptable is the retention time. In most circumstances, extinguishant is lost via leakage through low level apertures to be replaced by fresh air entering at high level, thus creating a descending interface. The retention time is the time it takes for the interface to reach the highest item requiring protection. A minimum retention time of ten minutes applies in most cases.

The only ways to determine the retention time are either to discharge the gas and measure concentration or, universally preferred, to undertake an enclosure integrity (or fan) test. This utilises a door-mounted fan to pressurise the room and the air flow is measured. From this an equivalent leakage area is derived and the retention time predicted.

2. Commentary

The retention time is influenced by the type of gas, the height of protected equipment and the number, size and positions of the leakage paths.

As a broad principal, smaller rooms require a better standard of sealing than larger rooms. This is because the total leakage is surface area related whilst extinguishant concentration is a function of volume.

In general the degree of sealing of the enclosure has to be significantly better than 'normal' building practice. Considerable care needs to be taken in sealing leaks.

The basic room construction has a noticeable bearing. Solid blockwork, concrete and the like are found to give inherently better results than lightweight plasterboard-type designs. Among the least favourable are designs where partitioning runs between suspended ceiling and raised floor rather than slab to slab. Subsequent compartmenting of floor and ceiling voids presents many difficulties.

It should always be remembered that leakage rarely occurs in straight lines. A frequent mistake is to seal the top of stud partitioning on one face and the bottom on the other so permitting a tortuous but nonetheless effective leakage path. Sealing is best done on the inner face of the enclosure.

The largest proportion of leaks occur at the position of service penetrations. Cabling in particular is often a problem. Ideally, penetrations should be via a proprietary sealing system. However, remedial sealing is often done by the use of suitably fixed boards and/or mineral wool and/or mastic and/or in-situ foamed sealant.

It should be noted that mineral wool itself is not impervious to gas, even when compressed. Whilst tight packing will reduce leakage, it is much better to seal the surface by means of mastic sealant or foil tape. There are available semi-compressible, intumescent faced mineral boards that are well suited to this purpose.

In-situ foam is widely used as it is frequently seen to be a simple sealing method, expanding as it does to fill the gap. There are however a couple of problems associated with it. Firstly, it has a tendency not to adhere to all the surfaces so that what might superficially seem to be a good seal has, when examined closely, a significant leakage path running around it. Secondly, care should be taken to ensure that its use does not breach any fire resistance requirement.

Constructional joints are another feature that require sealing. This may include board joints and the junctions between wall and floor elements.

Any profiles in ceilings/walls should be sealed at the junction with adjacent walls/ceilings. Even small open profiles will pose a leakage problem if there are many of them.

Doorsets should incorporate flexible 'smoke' seals and frames should be backfilled or mastic sealed. Rolling shutters and louvered doors are a particular problem and should, if possible be avoided. If drop curtains are used to reduce leakage through these items it is important that they run in channel guides to reduce edge leakage and unwanted displacement.

Air supply and extract ducting will usually be dampered closed on extinguishant discharge. There will inevitably be some leakage past louvered dampers. Whilst this will not normally pose a problem, it will become significant in enclosures where there are extensive areas of dampers in relation to the enclosure size. Under these circumstances it should be ensured that the dampers are well adjusted to minimise leakage.

Thought should also be given to other penetrating elements. Cable ducts may be well sealed externally but are likely to provide a leakage path unless packed internally at the point of penetration.

Finally, it should be remembered that, despite the need for good practice, some leakage is not only inevitable but necessary to help relieve initial overpressure.

It is hoped that the above comments are of assistance to those responsible for sealing protected rooms. Clearly successful sealing can only be determined by fan testing. However, provided the above points are carefully and fully applied there is no reason why the enclosure should not satisfy the retention time requirement.