What to consider when specifying a window seal for high-rise construction

This blog post will help you to:  

• Learn how building regulations have changed for high rise construction in the UK
• Understand why membranes and seals are treated differently by the regulations
• Get to grips with the confusing rules for window seals
• Learn how construction products are tested for fire safety

The Hackitt Review, which investigated the tragic Grenfell Tower fire, described the UK’s building fire regulations as “not fit for purpose”. The review led to major changes in how construction products are treated by the regulations. These included:  

• A switch to the European fire testing standard EN 13501 for determining the fire performance of building materials, moving away from the old British system 
• A new rule that all products used in the external cladding of tall residential buildings  — defined as those over 11 metres high — must achieve a fire classification of A2-s1,d0 or better. This is commonly known as the “ban on combustible materials”.  
• However some products, which must be flexible in order to function effectively, are exempt from this rule. Membranes must meet a less onerous classification of B-s3,d0. Other products, such as seals, gaskets, fixings and sealants do not need an improved fire rating as they make up such a small area of the building envelope.

Window connection products: membrane or seal?

However, there has been some confusion about where window connection products, i.e. materials designed to join a window to the surrounding construction, fit into these rules. Should they be classified as a membrane or a seal?

In September 2020, the Centre for Window and Cladding Technology (CWCT) sought to clarify the difference between a membrane and a seal.

Their conclusion?

• If the material is less than 250 mm wide and is used to seal windows to the surrounding construction, it should be deemed a “seal”, and thus be exempt from the need for an improved fire rating.
• If the material is wider than 250 mm it should be classified as a membrane, and thus require a B-s3,d0 classification.

The conundrum over window seals

So, what does this mean for you as an architect or engineer who wants to specify products with the best performance possible?

“We get a lot of calls from architects asking for A or B rated window seals, even though these products don’t require an improved fire rating,” says Stan Admiraal, SIGA’s deputy head of application engineering for West Europe. 

Flexibility or fire-rated?

“Traditional window seals that provided flexibility, and protection from rain and wind, were not fire-rated,” says Admiraal. “EPDM has long been used as a window seal because it is flexible and durable, but it is also combustible.”

Admiraal points out that there are no A-rated window seals on the market, and many B-rated seals may not be durable enough for long-term use. Indeed, the Centre for Window Cladding and Technology states: “Several Class B membranes are now on the market, but they tend to lack the physical flexibility provided by materials such as EPDM.”

Flexibility and stretchability are important because they allow for the movement of building elements over time.

Introducing Trestard

says Admiraal. “Thankfully SIGA now has a solution to this conundrum.”

SIGA recently launched Trestard, the first fully self-adhesive window connection tape with a Class B fire rating. Trestard is an ideal product if you’re looking for a fire-rated, flexible and durable window seal.

What you need to know about SIGA Trestard:

• It has a fire classification of B-s1,d0. This means that it makes very little contribution to fire, and produces little or no smoke, and no droplets
• It is flexible enough to allow for movement of the building elements over time
• It is resistant to driving wind and rain

How are building materials fire tested anyway?

There is a new primary system for fire testing of construction products in the UK. The Euroclass standard EN 13501 includes five different tests, and these determine the fire rating of a product, on a scale of A to F, as well as its classifications for production of smoke and droplets.

Read on to find out more about these tests.

1. The single flame source test (EN 11925)

During this test, a building material is placed in a small chamber and exposed to a small gas flame. The test measures the ignition time and how fast the flame spreads through the material. Depending on the results, the material can be given an E (pass) or F (fail) rating. If the material achieves a 30 second exposure to the gas flame, it can be subjected to the next test.

2. The single burning item test (EN 13823)

This takes place in a full-size test room. The building material is exposed to a fire with a heat release of 30kW, similar to a waste paper basket that has caught fire. The reaction of the material is evaluated over 20 minutes. Heat release and smoke production are both measured, while production of flaming droplets and lateral flame spread are also observed. Depending on its performance, the material can be rated Class B, C, or D. If an A rating is sought, the material must be subjected to two further tests.

3. The gross heat of combustion test (EN 1716)

During this test, the building material is ground to a powder and ignited in a pressurised oxygen container, surrounded by water. The temperature change in the water is measured during burning, and the gross calorific potential is calculated based on the temperature rise, the mass of the sample, and other factors. Depending on the results, the product may be eligible for an A1 or A2 rating.

4. The non-combustibility test (EN 1182)

During this test, a sample of the product is placed into a small furnace at about 750C, and temperature changes in the sample are monitored using sensors. The sample is also observed for signs of flaming, and its mass is measured before and after burning. Depending on the change in furnace temperature, the duration of flaming, and the average loss of mass, the product may be eligible for an A1 or A2 rating.