Overlay vs Screed Underfloor Heating: Which Should You Choose?

Why most people end up choosing overlay

On day-to-day renovation work across Devon and Cornwall, the most common reason people choose overlay over screed is simple: they do not want to dig up the existing floor.

That can be inconvenience, cost, or time. Digging out, rebuilding the base and pouring screed takes the room out of use. Traditional cement screeds are often discussed with a rough “1 mm per day” drying rule of thumb. A standard 65 mm pour can mean two months-plus before you treat it as fully dry for sensitive finishes — you can often walk on it sooner, but full cure is the planning constraint. Liquid screeds are typically thinner (often in the 40–50 mm region for many domestic pours) and dry faster, but they still need a real programme, not a weekend.

If the kitchen or living room has to stay usable, or trades cannot wait on a long dry, overlay becomes the default: retrofit on top of the existing floor and connect to the boiler or heat pump, provided there is proper flow control and thermostatic zoning.

That is not an anti-screed argument. It is a programme argument. When the floor is open anyway, screed is still an excellent system.

Screed systems in detail

Insulation goes on the slab (or structural deck designed for the load), pipe is clipped or held on a carrier at the design spacing, and screed is poured over. After drying and curing, the finished floor is installed to the screed manufacturer’s guidance.

Strengths

• •High thermal mass — steady heat delivery, good “flywheel” once up to temperature
• •Well-understood output tables for heat-pump design temperatures
• •Often competitive material cost per m² when the floor is already open
• •Familiar to many builders on new builds and extensions

Watch-outs

• •Build-up height and door thresholds
• •Dig-out cost if existing levels cannot accept the build-up
• •Dry and cure time before finishes
• •Disruption if the household is in occupation

Best fit: extensions, new slabs, full dig-outs, projects with a clear empty programme before flooring.

Overlay systems in detail

Low-profile systems — gypsum-type panels, XPS boards with pipe grooves, and similar — are fixed to a prepared existing floor. The pipe sits in the board layer; the finish follows the system’s rules.

Strengths

• •Much less structural disruption than a dig-out
• •Lower finished build-up than many screed routes
• •Faster path to a usable finished floor
• •Easier to phase room by room

Watch-outs

• •Base must be sound, dry and flat enough — budget levelling on uneven historic slabs
• •Less thermal mass than a thick screed — faster response, less stored heat
• •Floor finish resistance must match the design (thick carpet changes output)
• •Still needs proper manifold balancing and commissioning

Best fit: occupied renovations, height-critical solid floors, phased works.

Floor height reality

Measure before you order anything:

• •Door clearances and ironmongery
• •Stair nosings and the top step
• •Kitchen units and fixed joinery
• •Level access and threshold requirements

Overlay still adds height. Screed after a dig-out can reset levels but spends time and money to get there. Low doors and period joinery in South West cottages regularly force the thinnest viable overlay — or a hybrid strategy room by room.

Performance with heat pumps

Both system families can work very well with air source heat pumps. The test is whether each room’s heat loss is met at a low flow temperature — often in the mid-30s to mid-40s °C when design is honest — not whether the floor can be forced hotter to feel like old radiators.

• •Screed: excellent mass; slower warm-up; strong steady-state comfort when spaced for the loss
• •Overlay: typically faster response; still needs correct spacing and finish assumptions
• •Deep setbacks fight both systems — especially high-mass screed
• •Boilers can drive UFH via blending; heat pumps prefer a pure low-temperature design

Flow temperature & efficiency · Not warming fast enough · HP + UFH worth it?

Cost comparison (how to read quotes)

Supply-only and fully installed figures are often mixed in conversation. Keep them separate.

As a rough industry feel (not a quote): traditional screed supply for main UFH equipment on standard domestic work is often discussed in a broad band around the low tens of pounds per m² for materials depending on specification; installed totals are much higher once labour, prep and manifolds are in. Overlay supply is typically higher per m² for the board systems themselves. Always compare like-for-like scope.

Standard systems are typically installed across a wide £/m² band depending on scale and complexity — get written scope. More numbers: UFH costs 2026.

The mixed-floor problem

Many South West homes have solid ground floors and joisted first floors. One product rarely fits both without compromise.

A common successful pattern:

• •Ground floor: overlay on solid (or screed if open)
• •Upstairs: between-joist plates, deck overlay, or structural boards
• •One heat source, coherent controls, proper balancing on every manifold

Do not pour wet screed onto ordinary domestic joists without structural design. UFH upstairs renovation.

Side-by-side summary

Common mistakes to avoid

• •Picking a system from marketing before measuring doors and stairs
• •Assuming one board type suits solid and joisted floors
• •Ignoring screed cure time while the family stays in occupation
• •Designing for high flow temperature “just in case” on a heat pump project
• •Skipping manifold balancing — then blaming overlay boards for cold spots
• •Changing to thick carpet after design assumed tile
• •Not insulating joist voids on upstairs plate systems

Cold spots — causes and fixes.

Devon & Cornwall notes

• •Uneven historic slabs — overlays need proper base prep
• •Low doors and ceilings — height is often the binding constraint
• •Mixed solid ground + joisted first floor is normal
• •Solid stone / cob — floor heat will not fix a naked envelope alone
• •Oil/LPG to heat pump projects — UFH becomes part of a whole-house low-temperature plan
• •Conservation limits — floors are often freer to upgrade than external walls

Which system should you choose?

1. Map solid vs joisted rooms and level changes
2. Measure height at doors, stairs and fixed joinery
3. Decide programme and which rooms stay occupied
4. Confirm heat source (boiler, heat pump, or staged)
5. Pick system family per floor type
6. Design output at realistic flow temperature and finishes
7. Price supply, prep, install and controls — then brand within that family

Best system for renovations · Systems explained · UFH for renovations.

Honest bottom line

Frequently asked questions