EXTERIORS
25
Why should windows breathe? T
he provision of a weather and airtight seal around the perimeter of windows and doors in a facade is typically achieved by correctly specifying and installing a suitable membrane before the fi nal cladding is attached. There has been considerable focus on the specifi cation and use of these membranes on construction sites since the recent updates to Approved Document B (ADB). Approved Document B requires that any membrane used in an external wall construction has a minimum reaction to fi re Class B-s3, d0 in accordance with EN 13501- 1. As well as achieving the requisite fi re performance, it is critical that the appropriate level of weather protection is also provided, as this is the most common cause of warranty claims within the construction industry. Perimeter sealing membranes should always be assessed for both water resistance and weathertightness. Both properties sound very similar but are measured in different ways.
The water resistance of sealing membranes is tested in accordance with EN 1928 where the membrane is subject to a static head of water at a pressure of 2 kPa over an extended 24 hour period. The requirement is that no water penetrates the membrane over this test duration in order to achieve a Class W1+ performance. Weathertightness is determined in accordance with EN 1027, which although originally intended as a means of testing assembled windows and doorsets, is now commonly used to test the performance of perimeter seals. Using this method, the perimeter seal is applied around a window before being subjected to water sprayed under a pulsating pressure to simulate wind driven rain, where a minimum performance of 600 Pa is required. In addition to the fi re and water/weather resistance performance of the sealing membrane, there is a further critical factor which should not be overlooked, relating to vapour and moisture control through the external wall and joint. With a UK climate, the transmission of
warm, moist air will be prominently from the inside to the outside of a building and therefore window and door interfaces should be designed to allow the escape of unwanted moisture or condensation to the external environment. The basic principle of the inside of the joint being ‘tighter’ than the outside should always be considered and appropriate membranes specifi ed to satisfy this approach. When two membranes are used in the wall or around the window, the internal
membrane must be more vapour tight than the external one. The recommended ratio is that the external membrane should be at least ten times more vapour open than the internal one. This will create a positive pressure gradient which in turn ensures that moisture movement is always from the inside to the outside of the wall construction. If this orientation is reversed or the ratio not observed, for example by installing a vapour tight membrane as the external seal, interstitial condensation could occur within the joint, causing mould growth and potential structural damage. This principle applies to the whole facade design and is not just specifi c to the window interfaces, as the potential for damage is relevant to both and the principle is based on basic building physics and endorsed by CWCT in their technical publications, it is also the most recognised approach followed widely throughout Europe. Traditionally, EPDM membrane has been
specifi ed for this type of sealing application, but with a surge in the availability of fi re classifi ed membranes to the market, specifi ers, main contractors and warranty providers are now favouring these new membrane products especially for use on ‘relevant’ or Higher Risk Buildings (HRB’s). EPDM continues to be used as a generic term for any window sealing membrane, which unfortunately often leads to confusion and poorly specifi ed solutions. A membrane which has a fi re performance of Class B or better is not by defi nition an ‘EPDM’ because true EPDM is actually a particular grade of rubber which has a fi re performance of Class E and has a relatively high vapour resistance.
Therefore EPDM or so-called EPDM alternatives do not typically comply with the ‘inside tighter than outside’ principle as outlined above nor reaction to fi re and therefore can compromise the performance of the window interface.
In conducting
‘due diligence’ when
considering the specifi cation of window sealing membranes, the following criteria should provide the focus: •
• •
• •
Be subject of recognised third party accreditation.
For ‘relevant’ or ‘HRB’s’ be a minimum Class B-s3, d0 or better to EN 13501-1.
Be tested as a system, including when tested to EN 13501-1, with the relevant specifi ed adhesive if required.
Should be as vapour tight as necessary but as breathable as possible.
Follow the 10:1 inside to out rule for vapour transmission.
• Water resistance to Class W1+ when tested for 24 hours.
• Weathertight to minimum 600 Pa. •
• •
Permanently UV resistant when used in open rainscreen cladding applications.
Airtight where the air control layer is required on the external side of a building.
Compatibility confi rmed with all relevant substrate types and associated facade membranes.
Tremco CPG UK have been supplying a wide range of facade and window sealing membranes and associated products for over 75 years. Our philosophy for best practice in specifying and applying window membranes is in line with the above guidance and our illbruck FR Window & Door Sealing Membrane (Black) – ME010 meets all the listed criteria. The product’s performance is proven by its success since its introduction to the market, with current sales to-date of over 3 million linear metres! This has now been complemented by ME007 FR Window & Door Sealing Membrane HD, which provides enhanced strength.
Approved Document B has created a huge demand for appropriately fi re classifi ed facade and window membranes and whilst the basic performance characteristics for the former are well defi ned, the same basic principles can be applied to window sealing membranes, such as reaction to fi re, weathertightness/ water resistance and vapour transmission. illbruck FR Window & Door Sealing Membrane (Black) – ME010 and FR Window & Door Sealing HD – ME007 Membranes are the products of choice to satisfy all of these requirements.
hello@cpg-europe.com
www.tremco-europe.com/en-gb
WWW.BUILDINGCONSTRUCTIONDESIGN.CO.UK
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28
