Peter Thompson, General Manager UK of ISO-Chemie, explains the best practice and products for sealing External Wall Insulation.
Improving the thermal efficiency of buildings in the UK is an increasing trend, for both existing domestic and commercial properties and new build projects, utilising alternative construction methods. Much of this work is being carried out using an assortment of external wall insulation (EWI) systems, which has seen an explosion in demand and has led to a number of suppliers significantly increasing their installation teams.
The minimum requirement of any EWI system is to add additional thermal insulation to the outside of the structure, then seal the insulation with a suitable weather tight finish to protect both the extra insulation layer as well as the underlying fabric.
Different systems use different types of thermal insulation, but the one thing they all have in common is that the insulation must be kept dry. In my experience, it appears to be a little known fact that most insulation layers suffer significantly due to the addition of a very small amount of moisture. It is a general rule of thumb that a 5% ingress of moisture into this layer will reduce its thermal insulation capacity by 50%. This may be one of the factors behind the Zero Carbon Hub report, highlighting the difference between ‘As Designed’ and ‘As Built’ performance when EWI systems are used in new build properties. The potential factors mentioned in the report however are vast and varied.
The report tends to point the finger at both site installation practices and the need to use products that give guaranteed on site performance. What this highlights is the importance of reliably sealing the insulation layer of any EWI system against the weather, in both new build and refurbishment projects.
The vast majority of any EWI system is protected by the final weather proof layer, but the risk areas are the small junctions where the EWI system terminates against other building components. This could be abutments against windows, doors and under parapet walls etc. Different manufacturers and installers have different ways of dealing with the detail, but a well-used and increasingly favoured method is employing expanding impregnated foam sealing tape that offers larger expansion capabilities, constant movement throughout its life and guaranteed weather resistance; as well as thermally and acoustically insulation the sealed joints.
As mentioned above, different EWI manufacturers and installers finish joints and intersections in different ways. Some will simply butt joint the insulation up to the termination area, such as a window, then cover this with a render type finish. However, there will be movement in the area where the two materials meet, as they each have different coefficients of movement. This will cause a crack where the water moisture can creep through into the insulation layer.
Other installers recommend the EWI system is terminated using a stop bead, and the window, door or wall to be butt jointed up against it. However, the difference in surface levels between the stop bead and window, door or wall can result in small gaps for the weather to penetrate.
Using advanced impregnated foam sealing tapes in these areas will ensure a weather tight seal. The tape will take up any construction tolerance between the materials and accommodate constant differential movement between the two components caused by the different coefficients of expansion.
The nature and structure of foam tapes also means additional thermal and acoustic insulation is an added benefit. If used in a simple butt joint of insulation against the existing construction element, sealant tapes are best placed on the outer edge of the insulation layer, between the two materials, and rendered over in the usual way. It is then good practice to trowel the joint area to cause an induction crack that will form in a straight line, as it breaks along the area of least resistance.
When used in combination with a stop bead, assuming the render system is sealed correctly to the bead, the sealing tape is placed between the stop bead and the window, door or wall to seal the joint. If there is a potential that the render finish could break away from the stop bead, the sealing tape can be used on both sides of the bead.
The optimal size of specialist sealing tapes used for EWI systems are those designated to fit a gap of between 2m and 6mm. It is also beneficial that tapes can be compressed further to 1 mm, giving a site tolerance of 5mm+. Optimal tape performance allows an expansion beyond the stated 6mm size, but as they do so, the level of weather resistance starts to reduce accordingly.