EPS/XPS insulation

What do the EPS grade numbers actually mean?

EPS 70, EPS 100, EPS 150: the number is not a thermal figure but the compressive stress at ten per cent deformation, expressed in kilopascals. EPS 70 takes roughly 70 kPa, EPS 150 more than twice that. An unloaded facade is perfectly served by EPS 70, while a board under a screed, on a flat roof or in the plinth zone needs a higher grade because it will be walked on, loaded or backfilled against.

Thermal conductivity across the white grades sits in a single band of 0.036 to 0.040 W/(m·K), so a higher grade number does not buy better insulation. The real thermal difference comes with grey graphite EPS: the graphite additive reflects thermal radiation inside the foam and lowers conductivity to roughly 0.030 to 0.032 W/(m·K), about a fifth better. For the same U-value that means a noticeably thinner build-up, which counts in a retrofit where the window reveal or the width of a cornice caps the thickness available. Graphite boards have one site drawback: they heat up strongly in sunlight and can distort before the render goes on, so they are fixed behind a shading net and without unnecessary delays.

What is the difference between EPS and XPS?

The two materials are both polystyrene foams, but they are made differently and they belong in different places. EPS is expanded from beads that are then fused, leaving a partly open structure at the bead boundaries. XPS is extruded as a continuous closed-cell board, which makes it stronger, almost impervious to water and resistant to freeze-thaw cycling.

The plinth is where the distinction becomes practical. The band of wall from the ground up is splashed, frozen, thawed and backfilled against, and an EPS board that would be entirely adequate two metres higher will take up water there. XPS, or an EPS grade explicitly made for perimeter use, is the correct answer, and the same logic puts XPS under a foundation slab where load and ground moisture meet.

Why does polystyrene dominate Slovak facades?

On an external thermal insulation system, EPS is the Slovak default, and the reason is price per unit of U-value achieved. In the renovation of both apartment blocks and single-family houses, including work funded through the Obnov dom programme, it accounts for the great majority of installed area.

Practical properties reinforce the price argument. The boards are light and dimensionally stable, can be cut with a hot wire or a knife, sand flat easily and do not take up water under normal site handling. Installation is therefore faster and less demanding than with mineral wool boards, and a smaller crew can cover more facade in a day. On a low house with a flat substrate and no fire constraint, it is difficult to argue against.

Where does polystyrene not belong?

• In permanent contact with ground water, unless it is a grade made for that exposure.
• Under high point loads, beneath a pad, a post or any concentrated support.
• In contact with solvents and bitumen primers, which dissolve polystyrene.
• Against elements at permanently high temperature, a flue for example.
• On buildings where the fire report rules out combustible insulation.

The fire case is the most common item on that list. A polystyrene board is reaction to fire class E to EN 13501-1, while an external system built with it and finished in mineral render is normally classified as a system in class B-s1,d0. Slovak fire regulation restricts combustible facade insulation on taller buildings, the governing standard is STN 92 0201, and the fire authority signs off the specific project. Where combustible insulation is ruled out, mineral wool is the established substitute.

How does polystyrene behave on an old masonry wall?

Polystyrene has a much higher vapour resistance than mineral wool or a wood fibre board. On a new build with a thermal clay block that is rarely a problem, because the wall is dry and the moisture load is ordinary. On old solid masonry, whether brick, stone or mixed walling with a damp plinth, the wall turns into a closed build-up, and vapour entering the masonry from inside has to be able to leave it.

Such a build-up is therefore checked by calculation for interstitial condensation rather than by judgement. Thicker insulation actually helps: the warmer the outer face of the masonry, the less vapour condenses within it, so thin layers and unfinished details cause more trouble than generous ones. Where a wall carries built-in moisture, salts, or has no working damp-proof course, the vapour-open route is safer, meaning mineral wool or wood fibre with a lime plaster. That decision belongs in the design stage, not on site.

How should the three common insulants be compared?

Read that way, the choice becomes a question rather than a preference: is this position governed by cost, by fire, by water, by acoustics or by summer comfort? Polystyrene wins the cost question convincingly and loses the fire and summer questions, which is exactly why the same house often carries three different insulants in three different places.