U-value of an uninsulated suspended timber floor

Bare floorboards over a ventilated void, before any insulation goes in.

A suspended timber floor with bare floorboards over a ventilated void has a U-value of about 0.81 W/m²K for a typical 30m perimeter / 50m² floor (P/A 0.6), calculated the full BS EN ISO 13370 way with the void, its ventilation and the ground beneath. Larger floors do better (0.58 at P/A 0.3), smaller ones worse (0.94 at P/A 0.9). 100mm of mineral wool between the joists brings the typical floor to about 0.30. The U-value leaves out draughts through the boards, which are a separate loss.

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The build-up (outside → inside)

The pre-filled link opens with U-Monkey’s default void and exposure settings and 30m / 50m² (P/A 0.6), as an existing floor judged against Approved Document L’s 0.70 threshold (2021 edition, Table 4.3) — edit the perimeter, area and void height to your floor.

Typical U-values for this construction

SpecificationU-value (W/m²K)Notes
Large floor (P/A 0.3)≈ 0.58Better than the 0.70 threshold
Typical floor (P/A 0.6)≈ 0.81As pre-filled
P/A 0.75≈ 0.88
Small floor (P/A 0.9)≈ 0.94
+ 100mm mineral wool between the joists (P/A 0.6)≈ 0.30See the insulated suspended timber floor page

Figures computed to BS EN ISO 13370 with BR 443 conventions, 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 and the footprint and depth shown: your products, thicknesses, perimeter, area and ground will move the number, and that's what the calculator is for.

What changes the number

The boards are a small part of the resistance; the ventilated void and the ground under it provide most of it, which is why the floor’s perimeter-to-area ratio and the void’s ventilation openings move the result. Keep the airbricks clear whatever you do: the void’s ventilation protects the joists. Insulating between the joists from above or below, with a breathable membrane and the boards sealed, is the standard retrofit, and it deals with the draughts as well.

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 uninsulated suspended timber floor?

A suspended timber floor with bare floorboards over a ventilated void has a U-value of about 0.81 W/m²K for a typical 30m perimeter / 50m² floor (P/A 0.6), calculated the full BS EN ISO 13370 way with the void, its ventilation and the ground beneath. Larger floors do better (0.58 at P/A 0.3), smaller ones worse (0.94 at P/A 0.9). 100mm of mineral wool between the joists brings the typical floor to about 0.30. The U-value leaves out draughts through the boards, which are a separate loss.

Is an uninsulated suspended floor worse than a solid one?

For the same floor, a little better on paper: about 0.81 W/m²K against 0.98 for an uninsulated slab at P/A 0.6, because the ventilated void adds resistance. In practice it often feels worse, because air leaks through the gaps between the boards and under the skirting, and that is not in the U-value. Sealing the boards matters as much as the insulation.

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 in solid brickwork where BR 443 §4.2 requires them) and air-gap corrections, BS EN ISO 13370 for ground floors, suspended floors and basements, and ISO 13788 for the condensation screening check.

Other common build-ups

Floors

Walls

Roofs

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.

Your build-up won't be exactly this one.

It never is. Open this one pre-filled, then swap the insulation, change the thicknesses and watch the U-value move — free, in your browser, with a Building Control-ready PDF at the end.

Open this build-up pre-filled or call 01202 623236