Home Contractor's Corner(Quite Interesting) Insulation Facts Correct Water Flow Reducing Layer (WFRL) installation guidance
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Correct Water Flow Reducing Layer (WFRL) installation guidance

by Matt Downs

Correct Water Flow Reducing Layer (WFRL) installation

WFRL is a fairly new term widely adopted for a product that contractors may call thermal sheet, paper or by a trade name such as MinK, Aquazone, etc. WFRL stands for Water Flow Reducing Layer and is the official term adopted by standards and guidance writers for the layer that is installed on top of the thermal insulation in an inverted roof construction.

The WFRL performs an important function, and whilst not being a waterproofing layer it does act as a water barrier that significantly reduces the volume of rainwater reaching the waterproofed roof deck, assisting the thermal performance of the insulation board and reducing the thickness required to meet a target U-value.

As such an important part of the thermal design of the roof, the WFRL should neither be excluded from the installation process or viewed as non-critical. In many ways it is as important as the waterproofing layer if optimal roof performance is to be achieved.

The correct installation methods for a WFRL are:

1. Unroll and loose lay over the insulation, unrolling across the slope/direction of fall at the bottom of the slope next to the parapet wall or upstand.

2. Overlap the next roll by 300mm creating an unsealed overlap joint in the downward direction of the roof slope/fall. When doing runs longer than the roll, offset the 300mm wide end laps roll to roll in a brick bond fashion.

3. Temporary ballast as you go, checking the side and end laps remain at 300mm wide.

4. Cut separate strips for use at upstands and penetrations ensuring they are wide enough to provide a 300mm overlap onto the flat roof at the base and high enough to terminate at the level of the finishes. At the bottom of the slope/fall tuck beneath the first flat sheet installed.

5. At drainage outlets star cut the WFRL and turn down into the insulation board.

6. At square or rectangular penetrations cut strips of WRFL wide enough to overlap 300mm beneath the flat WRFL and reach the level of the finishes.

7. At soil vent pipes or round penetrations, the 300mm base of the WFRL should be star cut and tucked beneath the flat WRFL. A separate piece of WFRL should then wrap the pipe to the height of the finished to aid continuity.

The Liquid Roofing & Waterproofing Association (LRWA) have issued Guidance Note No. 14 relating to this best practice guidance.

It’s also worth remembering that the WFRL is excluded from the fire performance requirements within the Building Regulations under Regulation 7(3)(g).

 WFRL is a fairly new term widely adopted for a product that contractors may call thermal sheet, paper or by a trade name such as MinK, Aquazone, etc. WFRL stands for Water Flow Reducing Layer and is the official term adopted by standards and guidance writers for the layer that is installed on top of the thermal insulation in an inverted roof construction.

The WFRL performs an important function, and whilst not being a waterproofing layer it does act as a water barrier that significantly reduces the volume of rainwater reaching the waterproofed roof deck, assisting the thermal performance of the insulation board and reducing the thickness required to meet a target U-value.

As such an important part of the thermal design of the roof, the WFRL should neither be excluded from the installation process or viewed as non-critical. In many ways it is as important as the waterproofing layer if optimal roof performance is to be achieved.

The correct installation methods for a WFRL are:

1. Unroll and loose lay over the insulation, unrolling across the slope/direction of fall at the bottom of the slope next to the parapet wall or upstand.

2. Overlap the next roll by 300mm creating an unsealed overlap joint in the downward direction of the roof slope/fall. When doing runs longer than the roll, offset the 300mm wide end laps roll to roll in a brick bond fashion.

3. Temporary ballast as you go, checking the side and end laps remain at 300mm wide.

4. Cut separate strips for use at upstands and penetrations ensuring they are wide enough to provide a 300mm overlap onto the flat roof at the base and high enough to terminate at the level of the finishes. At the bottom of the slope/fall tuck beneath the first flat sheet installed.

5. At drainage outlets star cut the WFRL and turn down into the insulation board.

6. At square or rectangular penetrations cut strips of WRFL wide enough to overlap 300mm beneath the flat WRFL and reach the level of the finishes.

7. At soil vent pipes or round penetrations, the 300mm base of the WFRL should be star cut and tucked beneath the flat WRFL. A separate piece of WFRL should then wrap the pipe to the height of the finished to aid continuity.

The Liquid Roofing & Waterproofing Association (LRWA) have issued Guidance Note No. 14 relating to this best practice guidance.

It’s also worth remembering that the WFRL is excluded from the fire performance requirements within the Building Regulations under Regulation 7(3)(g).

Contact Quantum Insulation:

T: 01895 456018 / E: sales@quantuminsulation.com / W: quantuminsulation.com

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