Insulating a building with polystyrene - step by step manual

Polystyrene insulation

Polystyrene is the most popular insulation material for exterior walls. However, popularity does not mean that contractors cannot commit serious errors. Beneath we try to describe the most essential insulation stages.

System selection

The first stage should be always selecting a particular insulation system. Unfortunately, even designers can provide in the documentation e.g. “Polystyrene and thin-layer plaster of X company”. And each company has a few system! Each such a system, or its version, is a defined package of compliable products comprising the following:

Mixing various systems, e.g. to save some money is a high-risk action. Not only do we lose warranty automatically, but also it may turn out, that the products are not compatible. We need to remember that although both plasters are acrylic, but of other manufacturers, they can differ significantly. No wonder why all available strawberry yogurts in stores are of different flavour.

Substrate preparation

The substrate, i.e. our walls must be even and their surface clean and solid, i.e. free from any flaking plasters, greasy stains etc. Mortar pieces sticking out of the joint must simply be removed, cracks must be filled with mortar, walls must be dusted off and washed if necessary (then they must dry!). Remember that polystyrene holds limited elasticity towards walls, e.g. in the event of protruding mortar the boards will not adhere well. Rough walls in new houses, and these type of walls we are dealing here with, must be primed to level the substrate absorption and strengthen them and bond all tiny particles.  A lack of priming is a seeming saving of time and money, as durability of the entire system depends primarily on good substrate preparation.

Starting track

A starting track is a kind of base for the first sheet/board. In addition, it prevents penetration of cold air between the wall and the insulating boards, and - which is very important - forms a barrier for rodents and insects, which can nest in the insulation.
The track must be ideally levelled, otherwise the error will be increasing as we move up with the next boards.

Installation and anchoring

Boards are installed with joints displacement with a half lenght, just like bricks in the wall. Adhesive is applied on the entire length of the edges and at least 6 mounds in the middle. Applying only mounds does not guarantee stability and additionally causes gaps occurrence between the boards and the wall.
Anchors are used when the adhesive is cured, usually after 2 days. Earlier anchoring may result in tearing the adhesive joint. Even if the system does not require anchoring on the entire wall surface, generally boards are anchored mechanically in specially exposed areas - around doors and windows and wall edges.

Correct distribution of anchors in the polystyrene system.

Filling cracks and gaps

All gaps between the boards exceeding 2 mm and other cracks must be filled with polystyrene, or a low expansion foam. Failure to do so may result in thermal bridges, such areas can also be visible on the facade. Any excessive amount of foam and protruding polystyrene pieces must be carefully cut off, and irregularities must be abraded with a float polystyrene rasp. Otherwise they can be visible on the facade.

Adhesive and mesh

A very important and underestimated stage is application of adhesive and immersing in it a fibreglass mesh. This layer forms a foundation prior to plaster, facade tiles and must be strong enough. Its cracks or irregularities can also be visible on the facade, destroying the whole effect. A part of performing this reinforcing layer is strengthening edges of the insulation layer - corners, areas near windows and doors etc. These are the areas where stress is concentrated and may be subject to cracking if sufficient strength is not provided.

Correct mesh installation on adhesive. First adhesive is applied and then the mesh is immersed in it.


The reinforcing layer must be covered with an appropriate priming agent. It improves plaster adhesion and levels absorption, in addition is available in various colours, in order to use a similar colour to the plaster. Thanks to it we can minimize a risk of discolouration occurrence and prevent the substrate from being visible.

Plaster or paint

A thin-layer plaster or facade paint is the final finish layer of the entire insulation system. Its type has not only an aesthetical value. There are various types of plasters from resistant to dirt and algae to vapour permeable. Contrary to the common belief, even the latter feature must be considered when using polystyrene.

Blatant error. No adhesive around the edges.
Difficult areas. The boards must be precisely cut at correct angle.
Performing a reinforcing layer with mesh.
No starting track here.