Siding is fixed to battens, at a certain distance from the wall. That gap can be left empty or filled with insulation — and this is the moment when a facade renovation quietly becomes a thermal upgrade of the whole house. The opportunity will not come back for decades, because nobody takes the panels down just to add mineral wool. So the question is not whether to insulate, but with what. In practice it comes down to two materials: mineral wool and expanded polystyrene, commonly known as EPS. Both work, but in completely different situations. How a siding facade is built Before comparing materials, it helps to understand the layers, because everything else follows from them. Going outwards from the wall: Structural wall — masonry, concrete or a timber frame Insulation — wool or EPS, placed between the battens Wind barrier — a membrane protecting the insulation from air movement; mandatory with wool Ventilation gap — 2 to 4 cm of empty space Battens and siding panels That gap is the crucial part. It is what separates a ventilated facade from ordinary insulation under render. Air enters at the bottom, leaves at the top under the eaves and carries away the moisture migrating out of the wall. The insulation therefore works in dry conditions — and dry insulation performs the way its manufacturer promises. Remember one thing: if the gap is closed off, or the insulation is pushed all the way to the panels, the whole system stops working. Moisture has no way out and accumulates inside the wall. This is the most common and the most expensive mistake made with siding. Mineral wool Wool is vapour permeable, meaning it lets moisture pass outwards instead of trapping it. In a ventilated facade this is a fundamental advantage, since the entire system relies on moving moisture out of the wall. It is also non-combustible — classified in the highest reaction-to-fire class — which is sometimes required for taller buildings and is always an argument in favour when the wall itself is timber. Facades call for a suitably rigid grade of wool, not the soft rolls used in lofts. Its thermal conductivity typically falls between 0.035 and 0.040 W/(m·K). When to choose wool Timber, frame or log walls — here there is effectively no alternative Older masonry without a damp-proof course, which needs to release moisture Buildings where fire safety matters Houses near the coast, where humidity stays high for most of the year Expanded polystyrene (EPS) EPS is cheaper, lighter and easier to cut — one person can install it without help. It also offers slightly better thermal performance for the same thickness, especially in the graphite version, where conductivity drops to roughly 0.031 W/(m·K). In practice this means the layer can be 2–3 cm thinner for the same result. It has two drawbacks. It is vapour tight, so it performs poorly on walls that need to breathe. And it is combustible, in a far lower reaction-to-fire class than wool. When to choose EPS Masonry walls in good condition, with no moisture problems Outbuildings, garages and carports Situations where the budget decides whether insulation happens at all Side by side Property Mineral wool EPS Vapour permeability High — the wall breathes Low — traps moisture Reaction to fire Non-combustible Combustible Thermal conductivity 0.035–0.040 W/(m·K) 0.031–0.038 W/(m·K) Sound insulation Noticeably better Average Material cost Higher Lower Wind barrier Required Not needed Timber walls Recommended Not advised How thick should it be Polish building regulations require external walls of residential buildings to reach a thermal transmittance no higher than 0.20 W/(m²·K). For a typical block wall this means an insulation layer of 15 to 20 cm, depending on the wall material and the type of insulation. Thinner layers, of 5 or 10 cm, appear on outbuildings or where the insulation merely supplements an existing layer. On a house, going below 15 cm rarely makes sense — the difference in material cost is small, while the difference in heating bills stays with you for decades. Four mistakes that ruin the job No ventilation gap. Insulation pressed against the panels blocks the airflow and moisture stays inside the wall. Compressed wool. Wool insulates thanks to the air trapped in its fibres. Forced between battens, it loses part of its performance. Skipping the wind barrier. Without a membrane, air moving through the gap draws heat straight out of the wool. Blocked inlets and outlets. A sealed bottom edge at the plinth or a closed top under the eaves leaves the air nowhere to go. The effect is the same as having no gap at all. What we usually recommend in Tricity Given our climate — high humidity and steady winds from the sea — we more often suggest mineral wool. Not because EPS is a bad material, but because walls near the coast carry more moisture to release, and wool makes that easier. For dry masonry buildings further inland, EPS is a perfectly sensible choice and keeps the budget lower. We make the decision after inspecting the wall. We look at what it is built from, whether it has a damp-proof course, whether damp patches appear on it, and how much room the eaves overhang leaves — because that determines how thick a layer can be fitted without rebuilding the flashings. Planning a siding facade? We will inspect the wall, select the insulation and calculate the complete material list — panels, trims and insulation. The quote is free of charge. Request a free quote Boryszew siding — panels and colours You will find more about the material itself on our Boryszew siding page, and about the finishing profiles under siding trims. Both pages are in Polish — call us on 602 216 960 if you would prefer to go through it in English. ← Back to the blog
