Floor Assemblies

Acoustic requirements for multi-family dwellings became more stringent in the 1995 National Building Code of Canada. Sound transmission class (STC) ratings between units increased to 50, from 45 in the 1990 code.

The new requirements would, it was realized, necessitate changes in traditional construction methods, some of which might alter the fire-resistance ratings of floor assemblies. These changes for acoustic reasons and changes in the materials themselves created doubt about the veracity of existing fire-resistance data for common floor assemblies.

The National Research Council’s Institute for Research in Construction assembled a consortium of industry partners to support a wide-ranging study to determine whether sound transmission class and fire-resistance ratings (FRR) of insulated and non-insulated floor assemblies protected by gypsum board would be affected by the code changes. The consortium was an excellent example of public and private sector collaboration through which it was possible to test a large number of constructions. The work was carried out in full-scale fire and acoustics facilities.

As a result of this project and the related acoustics project, a table with STC ratings for more than 600 floor assemblies and fire-resistance ratings for more than 150 floor assemblies was drafted. The information has been accepted in principle by the relevant technical committee of the National Building Code and will be published as a table in Part 9 of the next edition of the NBC, expected in 2003.

Following are only some of the key results of the large study.1

Screw spacing

The research revealed that the location of screws used to fasten gypsum board edges to resilient channels could have a significant influence on the fire-resistance of floor assemblies. A resilient channel is a long strip of thin steel, or flexible furring, placed between the joist and the gypsum board and attached to both. Its purpose is to provide discontinuity so that sound is less able to be transmitted through the assembly. While current construction practice places screws 10-12 mm from board edges, the Institute for Research in Construction’s tests showed that a spacing of 38 mm increased the fire resistance by a surprising 50%.

Type of insulation

Three types of insulation were tested in a variety of floor constructions to determine their effect on the fire resistance. The insulations used were glass fibre batts (90 mm thick), rock fibre batts (90 mm thick) and wet-sprayed cellulose fibre (59 mm to 122 mm thick).

For floor assemblies with solid wood joists and a single layer of gypsum board, it was found that the installation of glass fibre insulation above resilient channels reduced the fire resistance by 20% compared to an assembly with no insulation. Glass fibre batts melted two to three minutes after the gypsum board fell off and, subsequently, the joist sides and subfloor were exposed to furnace heat. The glass fibre was unable to compensate for the earlier failure of the gypsum board.

By sharp contrast, rock fibre batts and cellulose fibre, sprayed wet on the joist sides and underside of the subfloor, remained in place longer and increased the fire resistance by 33% and 31%, respectively. Overall, the assemblies with one layer of gypsum board and either rock or cellulose fibre insulation installed in the cavity provided a 45-minute fire rating; assemblies containing glass fibre provided less than a 45-minute fire resistance rating.

When tested with a double layer of gypsum board and solid wood joists, all three insulations reduced the fire resistance of the assembly. The reductions, compared to a non-insulated assembly, were 16% for glass fibre, 10% for rock fibre and 7.5% for cellulose fibre. Due to longer heat exposure, the deteriorated glass, rock and cellulose fibre were unable to compensate for the earlier failure of the gypsum board; thus, all insulations had a negative effect on fire resistance. However, the assemblies all provided a one-hour fire resistance rating.

The insulations were also studied in constructions with wood I-joists. With a single layer of gypsum board, rock fibre batts installed above resilient channels increased the fire resistance by 10% compared to an assembly with no insulation. Cellulose fibre, wet-sprayed on the joist sides and on the underside of the subfloor, increased the fire resistance by 24%. Overall, the assemblies with wood I-joists, one layer of gypsum board ceiling finish and either rock or cellulose fibre insulation provided a 45-minute fire resistance rating.

When a second layer of gypsum board was used with wood-I joists, the installation of glass fibre insulation reduced the fire resistance by 7% compared to an assembly with no insulation. Rock fibre increased the fire resistance by 7%. Nevertheless, overall the assemblies with wood I-joists, a double layer of gypsum board ceiling finish and glass, rock or cellulose fibre insulation were able to provide a 1-hour fire resistance rating.

Glass fibre batts installed in the floor cavity above resilient channels were tested in assemblies with steel C-joists and a double layer of gypsum board. This produced a fire resistance 8% lower than that of an assembly with no insulation. Overall, these assemblies provided a one-hour fire resistance rating.

Other Key Findings

Two layers of gypsum board. The research revealed a significant benefit to using two layers of gypsum board ceiling finish. This practice increased the fire resistance by 78% for assemblies with solid wood joists and by 71% for those with wood I-joists, compared to constructions with a single layer.

Resilient channel spacing. Widening the spacing between resilient channels for floor assemblies might not be desirable from a fire resistance standpoint. For example, increasing the spacing from 400 mm o.c. to 600 mm o.c. in assemblies with wood I-joists decreased the fire resistance by 12%.

Type of subfloor. It was found that subfloors made with oriented strand board (OSB) and plywood provided similar fire resistance.

Concrete topping. Adding a concrete layer on top of a lightweight floor assembly — an approach for reducing sound transmission — was found to reduce the fire resistance by 12%. The concrete increases the thermal resistance of the assembly, and thus reduces the heat transfer across it.CCE

1 More detailed results are available in NRC/IRC Internal Report No. 764: Sultan, M.A., Seguin, Y.P. and P. Leroux. Results of Fire Resistance Tests on Full-scale Floor Assemblies, May 1998.

The fire tests were carried out in accordance with CAN/ULC-S101-M89 “Standard Methods of Fire Endurance Tests of Building Construction and Materials,” which is similar to ASTM E119 “Standard Test Method for Fire Tests of Building Construction and Materials.”

IRC acknowledges the technical and financial contributions of several partners from government and industry in this joint research project.

Drs. M.A. Sultan, N. Benichou and V.R. Kodur, P.Eng. are researchers in the fire risk management program at NRC/IRC.