How is the thermal insulation of exterior walls correctly done?
Why thermally insulate exterior walls?
Insulating layers ensure superior thermal comfort and good protection against humidity, both necessary conditions for providing a pleasant indoor space.
Based on a simple principle, removing, as much as possible, the impact of permeating heat and humidity on the inside face of the space to be protected, much like choosing thicker clothes and more layers in colder weather or a rain coat, insulation minimizes the impact temperature and humidity have on the space we need to protect. Our goal is to comply with EU directives regarding thermal comfort – a 2°C temperature difference between the indoor air temperature and the interior surface of the wall temperature.
Outdoor insulation ensures a healthy environment and protects the building. It eliminates thermal bridging, keeping structural materials in a warm zone – a relatively constant environment compared to the variation of external factors like temperature, humidity, wind, etc. This improves a building’s performance in time.
XPS boards are not only suited to new builds but are also recommended for existing buildings, there being obvious advantages in insulating: an increase in comfort, mold and mildew proofing, reduced heating and cooling needs and costs leading to a positive environmental impact.
Last but not least one must consider the definite improvement in the look of a building brought about by insulating it.
What must we consider?Advance training and previous knowledge will save you time and money. Thermal enveloping is a substantial investment with long-term effects which makes the need to choose the right materials all the more important. Whether you are doing it yourself or contracting a specialist, we recommend that you always consult an expert and stick to the execution stages of your project.
The most important factors to consider when dimensioning your particular thermal insulation system are: building placement and specific environmental factors (having different behaviours in a seaside building compared to a mountain dwelling), but also the materials it was built with.
Another condition which must be satisfied prior to beginning installation is a surface check. The recommended deviation is no more than 1-2 cm from the flatness of the surface the board is being installed on, both vertically and horizontally.
For deviations larger than these, a rough plaster is normally applied, making the fastening of the boards faster and easier.
The building front must be covered and protected prior to installation. The building’s structure must be completely dry in order to get the best results from your thermal insulation.
What makes up a thermal system?
1.Adhesive plaster / adhesive acrylic or mineral layer applied to facilitate the polystyrene board installation
3.Fixing dowels for anchoring the boards to the structure
4.Exterior plaster including a structural mesh
5.Decorative plaster applied on top of a primer that binds it to the structural plaster
What are the steps in the installation of a thermal system?
Priming the surface before starting the enveloping.
If a check on the surface that the thermal system will be applied to highlights layers that have no adherence (peeling plaster, especially in old buildings), these will be removed and the surface will be rectified accordingly. Ensuring verticality and horizontality (flatness) on all building fronts. If the surface flatness is inadequate, with deviations greater than 1-2 cm, a rough plaster will be applied, thereby ensuring a lesser need for adhesives when installing the XPS boards, a faster work rate in bonding the thermally insulating boards and better fixing conditions for the dowels. If the surface flatness is adequate, to ensure the appropriate bonding of the thermally insulating board it is recommended to apply a mineral or acrylic adhesive layer.
Installing the footing
Correctly installing the initial footing is of outmost importance as it determines the uniform mounting of the insulation oboards. The footing profile width must match XPS extruded polystyrene boards indicated in the project and will be fixed using special dowels 20 cm below the level of the first floor.
The baseboard profile must match the surface it is mounted on. Any gaps that may occur will be filled with plastic wedges in order to ensure the profile’s linear stability.
Installing XPS boards using continuous or strip adhesives
The next step in installing the insulation is attaching the boards. This can be done in one of two ways, however, it is important that in either situation the adhesive does not overspill after being applied and bonded. Remove any excess immediately.
1. Applying the bonding mortar (the adhesive) to the XPS polystyrene board as spots or strips is done as follows: strip adhesives are used along the edges while spots of adhesives are applied in the middle of the board.
The adhesive should cover at least 40% of the surface of the board. The recommended dosage is of about 4-5 kg of adhesive per m2.
Fitting the boards
Once the adhesive has been applied to the back of the XPS board, it is ready to be placed in position on the footing profile and fixed to the wall. Repeat for the next board, taking care not to leave gaps. Should gaps appear, gently push the boards against one another until the gap is filled.
Flatness checks and levelling adjustments must be done using a ruler before the adhesive cures.
Anchoring polystyrene boards with the help of dowels
Other than the fixing with adhesives, the anchoring of the XPS boards is required in order to provide durability and performance. Mechanical fixings must be used, dowels in this case. The dowels are fitted 24 hours after the curing of the adhesive. The building height and the building front features are the factors that determine the appropriate type of dowel.
Holes are drilled in the XPS boards which have been fixed with adhesives. Holes are then drilled into the wall, at a depth of 1 cm or more than the length of the dowel. The drill bit diameter must match the diameter of the dowel. Reinforced concrete surfaces require a drill bit diameter 1mm greater than that of the dowel.
Applying exterior plasters
The first plastering will be done using a steel trowel at least 24 hours after the fixing of the boards. The structural net is lightly pushed into this layer, followed immediately by a second layer applied with a 3x3 mm notched grout spreader. This layer can either be applied horizontally or vertically, making sure the layer has the same thickness throughout. So called weak spots (such as window and door edges) require an additional layer of structural meshing to be applied. This additional mesh must not be part of the first layer of plaster, nor can it intertwine with the first mesh.
This is why it’s important to allow the first layer of plaster to slightly dry up (waiting time depends on weather conditions). Once the weak spots have been enveloped in the mesh the second layer of plaster can be applied.
Inferior layer with high mechanical resistance
The footing and the ground floor in buildings (up to a height of 1-1.2 m) need a thermal insulation system comprising of materials that can hold up against external factors. The building’s footing is particularly subjected to extreme factors such as: humidity due to soil permeability, water, rain, mechanical strains or pressures, humic acid, etc.
In order to ensure adequate resistance a steel mesh is fixed to the wall using spacers and fastenings. A layer of classic or precast plaster is then applied, plaster layer which must be at the same level with the layers on the higher floors of the building (where the structural mesh can be found), thereby levelling the front of the building. This is why the XPS boards installedin this area must be about 1 cm thinner than the rest of the boards.
Applying the final decorative layer
Before applying the final layer (the decorative layer may be mineral or acrylic) you must allow the reinforced plaster to completely dry up. The drying up of the decorative layer must not be accelerated by adverse weather conditions (strong winds or high temperatures) as these will cause cracks in the material which will erode it and decrease its performance.
The application of the decorative layer must be continuous for large building fronts in orderto prevent unaesthetic marks. This condition is satisfied by assigning the appropriate number of workers for the respective surface.
A protective strip must be applied where endings are required (between the different floors of a building, etc.)
Decorative mineral or acrylic plaster layers are applied using a trowel, the thickness of the layer being dependent on the type of plaster used and the size of the area being applied to. A textured look is created on this layer before drying up, using a plastic trowel.
The acrylic plaster has specific colours (own colour). The decorative mineral plaster is white and in order to get to the desired colour a coat of solvent-free paint needs to be applied, only 4-7 days after plastering.