Can underfloor heating be laid directly on the insulation?
Yes, that is the rule: the pipes are fixed on the insulation layer, then embedded in the screed. The insulation must be flat, resist compression and be covered with a membrane.
Should you insulate an upper floor above a heated room?
Thermally, hardly: heat does not leak between two heated rooms. Upstairs, acoustic insulation against impact noise is what matters, except above an unheated space.
How long between sprayed PUR and the screed?
Sprayed PUR sets in under 24 hours. The screed can follow as soon as the foam is sanded level and covered with a membrane, often within a few days depending on the site schedule.
Is sprayed PUR affected by moisture in the base?
Once in place, closed-cell PUR absorbs little water. But it must be sprayed onto a dry base, and it does not replace treating moisture rising from the ground.
How do you measure the height available to insulate a floor?
Start from the finished floor level set by thresholds and adjoining rooms, measure down to the highest point of the slab, then subtract the floor finish and the screed: what is left is the space for insulation.
How long does a screed last, and what guarantees apply in Belgium?
A properly executed screed lasts the lifetime of the building. In Belgium, ten-year liability covers serious defects for ten years from handover.
What flatness must a screed achieve?
Measurement is taken with a 2 m straightedge. Common practice allows a 5 mm deviation for an ordinary screed and 3 mm where the covering demands it.
Can a poorly levelled screed be ground down or repaired?
Yes: high spots are ground down, low spots filled with levelling compound. The limits are the remaining thickness and, on a heated screed, the pipe positions.
Why does a screed sound hollow?
On a floating screed it is normal: it sits on insulation. On a bonded screed, a hollow sound signals that it has come away from the substrate.
Why does a screed crack and what can be done?
Usually because of drying that was too fast or missing joints. Before repairing, you need to establish whether the crack is stabilised or still active.
When can you walk on fresh screed?
After 24 to 48 hours for careful access, and 3 to 7 days for normal traffic. Walkable does not mean able to carry a load.
How do you protect a screed while it dries?
By preventing it from drying too fast in the early days: no draughts, no direct sun, no frost, and no loads placed on the surface.
Can the house be heated while the screed dries?
Yes, to hold a stable room temperature of 18 to 20 °C. No for the underfloor heating, which only starts after the minimum waiting time and in stages.
Can the drying of a screed be speeded up?
Yes, through ventilation and a dehumidifier. Not through heat: blowing hot air onto a screed dries the surface and cracks it.
How do you know a screed is really dry?
Only a carbide bomb test counts, on a sample taken through the full depth. A screed that feels dry can still be very wet at its core.
Do you need to move out while screed is laid?
Rarely overnight, but the rooms concerned are out of use for 24 to 48 hours. In an occupied renovation you need a refuge zone and a clear way out.
How much does a screed weigh on the structure?
About 20 kg per m² for each centimetre of thickness. A 6 cm screed therefore weighs 120 kg/m², before insulation and the final covering.
Why does a screed need movement joints?
Because a screed expands and shrinks. Without joints it cracks wherever it likes; with joints it breaks where you decided it should.
Does a screed need reinforcement mesh?
Not always. Mesh is needed on a floating screed, over underfloor heating and on large areas. On a thin bonded screed it is often unnecessary.
How many m² of screed can be laid in one day?
From 50 to 80 m² a day laid by hand, 150 to 250 m² with a pump, and several hundred m² in liquid screed. Site access matters as much as the technique.
Is a vapour barrier needed under a screed?
A decoupling film is almost always needed. A genuine vapour barrier or damp-proof membrane is only required where moisture can come from the ground.
Can a new screed be laid over an existing screed?
Yes if the old screed is sound and well bonded, and if the available height allows at least 4 cm. Below that, a levelling compound or removal is needed.
Can screed be laid on a damp substrate?
No, not without identifying where the moisture comes from. A screed does not stop rising damp: it hides it, and then the floor covering lifts.
Can screed be laid on a timber floor?
Yes, but only as a floating, decoupled screed, and only after checking that the floor can carry the load. A traditional screed is often too heavy.
Can screed be laid in winter or in cold weather?
Yes, provided the temperature never drops below 5 °C while the screed sets. Below that, the mixing water freezes and the screed loses its strength for good.
