Insulating a suspended timber floor with 100mm mineral wool between the joists brings the floor deck itself to roughly 0.40 W/m²K (joist bridging included) — but the floor’s U-value is meaningfully better once the ventilated void and ground resistance are counted to BS EN ISO 13370: about 0.31 on a typical semi footprint (perimeter 30m, area 50m²), and 0.27–0.32 across most house footprints. U-Monkey runs the full suspended-floor method, which is why its answer differs from simple calculators — and is right.
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The build-up (outside → inside)
- Floorboards / chipboard deck
- 100–150mm quilt between joists (~12% timber)
- (Ventilated void + ground via ISO 13370)
The pre-filled link opens with default void and exposure settings — edit perimeter, area and void height to your floor.
Typical U-values for this construction
| Specification | U-value (W/m²K) | Notes |
|---|---|---|
| 100mm quilt between joists | deck ≈ 0.40 → floor ≈ 0.31 | The standard retrofit |
| 150mm quilt between deeper joists (netted) | deck ≈ 0.28 → floor ≈ 0.24 | Worth it if access allows |
Figures computed with the BS EN ISO 6946 combined method and BR 443 conventions — bridging, mortar joints in blockwork and the air-gap correction included — exactly as the pre-filled calculator works them out, with any pass or fail judged on the unrounded U-value. Indicative for the generic build-up shown: your products, thicknesses and (for floors) footprint will move the number, and that's what the calculator is for.
What changes the number
Support the quilt tight to the boards (netting or battens) — sag creates air paths that wreck the number. Keep the airbricks clear: the void must stay ventilated to protect the joists, and the 13370 method accounts for that ventilation honestly.
Next: check the junctions
A U-value covers the flat area of an element — but heat also escapes where elements meet, and at modern U-values those junctions can be 20–30% of a dwelling's fabric heat loss. Now you know your U-values, model the junctions with the free ΨMonkey psi value calculator (no subscription; PDF reports £25 +VAT a junction):
Or read thermal bridging explained to see how junction Ψ-values feed your SAP calculation.
Frequently asked questions
What is the U-value of an insulated suspended timber floor?
Insulating a suspended timber floor with 100mm mineral wool between the joists brings the floor deck itself to roughly 0.40 W/m²K (joist bridging included) — but the floor’s U-value is meaningfully better once the ventilated void and ground resistance are counted to BS EN ISO 13370: about 0.31 on a typical semi footprint (perimeter 30m, area 50m²), and 0.27–0.32 across most house footprints. U-Monkey runs the full suspended-floor method, which is why its answer differs from simple calculators — and is right.
Why do different calculators give different answers for suspended floors?
Simple tools treat the floor like an external wall and stop at the element value. The proper method (BS EN ISO 13370) adds the ventilated void and the ground, which improves the effective U-value. U-Monkey implements the full method — the differing answers are the other tools’ shortcut.
Is this calculator free?
Yes — building the element, reading the U-value, the compliance targets (including the Future Homes Standard), the condensation-risk screening check and the thermal-mass figures (decrement factor, time lag, κ) are all free in your browser, and the Building Control-ready PDF report is free too. No sign-up.
Does it follow the official calculation method?
Yes — the combined method of BS EN ISO 6946:2017 with BR 443 conventions, including timber bridging, mortar joints in blockwork and air-gap corrections, ISO 13370 for ground and suspended floors, and ISO 13788 for the condensation screening check.
Other common build-ups
- U-value of a cavity wall with full-fill insulation
- U-value of a cavity wall with PIR partial-fill insulation
- U-value of a timber frame wall
- U-value of a solid brick wall with internal insulation
- U-value of a one-and-a-half-brick (317.5mm) solid wall
- U-value of a solid wall with external insulation
- What is the U-value of an uninsulated cavity wall?
- U-value of 270mm loft insulation
- U-value of insulation between rafters (loft conversion)
- U-value of a warm flat roof
- Flat roof insulation thickness — about 150mm PIR for 0.15, 140mm for 0.16
- U-value of a cold flat roof (cold deck)
- U-value of an inverted flat roof
- U-value of an insulated concrete ground floor
- U-value of a beam and block floor with PIR
Different build-up? See the full floor U-value calculator, or start from scratch in the main U-value calculator.
Need the wider compliance picture? U-values feed straight into SAP calculations — or let our New Build Compliance Pack handle the lot. Background reading: what U-values actually are, and why two calculators can give different answers for the same wall.