Insulating a Suspended Timber Ground Floor

Surveying a Floor Nobody Set Out

Modern joists are laid to a spacing because a span table and a sheet material both demand it. A Victorian floor was laid to whatever the carpenter had on the cart. Expect nominal four hundred centres and find three hundred and eighty at one end, four hundred and forty at the other, a doubled pair where a partition used to stand, and one bay half its neighbours' width against the chimney breast. Each is a different cut, and a quantity worked from one assumed spacing is short in the direction that matters.

Do the take-off in two passes. The first is arithmetic: room length, joist span, nominal spacing, and out comes a baseline joist count and the lineal run of timber that the netting, the battens and the perimeter tape all follow. The second is a tape in every bay, recorded on a sketch, with the odd ones circled. The first pass tells you what to order; the second tells you how much of it will be cut waste, and on a period floor that difference is not a rounding error.

Deciding What Number You Are Aiming At

Two regimes describe the same floor from opposite ends, and it is worth being explicit about which one the specification is written in. British and European practice states a target as a maximum U-value in W/m²K, calculated to BS EN ISO 6946, Building components and building elements — Thermal resistance and thermal transmittance — Calculation methods. North American practice states a minimum R-value, and insulation is labelled that way at the merchant. They are exact reciprocals of each other within a unit system, and they are not interchangeable across unit systems: a metric U in W/m²K and an imperial U in BTU/hr·ft²·°F differ by a factor near 5.68, which is quite large enough to make a wrong answer look plausible.

Where the target comes from depends on why the work is happening. Approved Document L, Conservation of fuel and power, sets limiting standards for thermal elements being renovated as well as for new ones, and a ground floor being taken up is squarely in that territory — subject to what is technically and functionally feasible, which is exactly the argument a two hundred millimetre joist and a fixed threshold height forces you to have. The International Energy Conservation Code sets floor insulation minima by climate zone for floors over unconditioned space. Neither is a number to carry in your head between jobs; both are confirmed against the edition the jurisdiction has adopted, local amendments included.

One provision is worth quoting in spirit because it kills more floor insulation jobs than any thermal figure: the IECC requires floor insulation to be installed so that it maintains permanent contact with the underside of the subfloor. Insulation drooping in a sling with a thirty millimetre gap above it has an air layer on its warm side, and that air layer will convect. The declared performance was measured on a specimen in full contact — to ASTM C518, Standard Test Method for Steady-State Thermal Transmission Properties by Means of the Heat Flow Meter Apparatus — and nothing about a sagging batt resembles that specimen.

Adding the Layers Back Up

Resistances add along the path heat takes; transmittances do not. Total every layer of the assembly first — deck, insulation, any board under the joists, and the surface air films at each face — and take the reciprocal at the very end. Summing the U-values of individual layers is the error that produces a floor apparently performing several times better than physics allows, and it survives review because each individual number in it is correct.

The honest layer list here is short, and the deck contributes less than people expect. Twenty-two millimetres of softwood is a real but small resistance; the insulation is almost the whole answer; and the ventilated void counts as outdoor air rather than a still cavity, so it earns you nothing. Note which way that cuts at the boundary: BS EN ISO 6946 has you disregard the void and every layer beyond it, then close the assembly with an external surface resistance for still air rather than the low, windswept figure an exposed wall is given. The void is ventilated, not weathered, and a sub-floor U-value quoted with a wall's exposed film on it is pessimistic rather than safe. ASHRAE Handbook—Fundamentals is the reference to defend the film and cavity figures you used, and BS EN ISO 13370, Thermal performance of buildings — Heat transfer via the ground — Calculation methods, is the one that puts the whole suspended ground floor together, void ventilation and ground included, rather than treating it as a wall lying down.

What Actually Goes Between the Joists

The material choice on a ventilated timber floor is not a free one. Whatever sits between those joists spends every winter with cold, moving, sometimes near-saturated air on one face, and has to tolerate that indefinitely without slumping, without holding water, and without stopping the timber either side of it from drying. Mineral wool — glass or stone — does that: vapour open, non-wicking, and forgiving enough to friction fit a bay nineteen millimetres wider at one end than the other, which is the ordinary condition here.

Specify by declaration rather than by trade name. Factory-made mineral wool products are covered by BS EN 13162, Thermal insulation products for buildings — Factory made mineral wool (MW) products — Specification, and in North America by ASTM C665, Standard Specification for Mineral-Fiber Blanket Thermal Insulation for Light Frame Construction and Manufactured Housing. The declared thermal conductivity is on the label and it varies meaningfully between product lines at the same nominal thickness, so the R or lambda you design with should be lifted from the declaration of the product being delivered, not from a generic table.

Density and format matter more here than in a wall. A semi-rigid slab holds its own shape in a bay and can be cut ten to fifteen millimetres oversize to grip on both sides; a soft roll of the same conductivity will sag out of a wide bay within a season unless the support underneath is genuinely continuous. Slabs also handle the awkward geometry better — around a sleeper wall, past a doubled joist, into the tapering bay at the chimney breast — because you can trim them square and they stay square.

ASTM C1320, Standard Practice for Installation of Mineral Fiber Batt and Blanket Thermal Insulation for Light Frame Construction, is worth following even where nothing enforces it: fill the cavity completely, no compression at the edges, no voids at the ends of the bay, and cuts made around obstructions rather than the obstruction buried. A batt tucked past a pipe instead of split around it leaves a gap either side, and gaps scale badly — a few percent of a floor left uninsulated costs far more than a few percent of the performance.

The Rigid Board Route, and What It Costs You

Where joist depth is the binding constraint — a shallow floor, a threshold that cannot move, a client who will not accept a step at the door — rigid polyisocyanurate does more with less depth than anything else you can push into a bay. Products to BS EN 13165, Thermal insulation products for buildings — Factory made rigid polyurethane foam (PU) products — Specification, declare conductivities well below what mineral wool reaches, and on a floor with a hundred and fifty millimetres to play with that difference can be the whole argument.

The catch is that rigid board only performs when it fits, and nothing in a period floor fits. Every bay is a different width, every cut has to be within a millimetre or two, and the honest fix for the gap you cannot avoid is foam or tape at every edge of every board, on a floor that may hold two hundred board edges. That is slow work, and slow work gets skipped in the middle of a room nobody will see. A three millimetre air path around a board convects, and unlike a compressed batt it does not announce itself by looking wrong.

The other consequence is moisture. Foil-faced PIR is close to vapour closed on both faces, so it becomes the vapour control layer whether you designed it that way or not, and the timber either side of it now dries in one direction only. That is workable — it is how thousands of floors are built — but it has to be a decision, with the seal deliberately on the warm side and the cold side left able to breathe, rather than something you discover afterwards. Rigid board also needs mechanical support: battens fixed to the joist sides at a set-down that puts the board's top face hard against the underside of the deck, never friction alone.

The Perimeter Band Where the Heat Leaves

The edge of the floor loses heat out of proportion to its area and is the part most often left out of the take-off entirely. Where a rim board or perimeter joist exists — standard in North American framing and in newer suspended floors here — it is a continuous band the height of the joist running the full perimeter of the building, quantified as perimeter multiplied by joist depth rather than as part of the field.

In construction with a rim board, that band takes rigid foam cut to fit and sealed at every edge, and it is a defensible detail because there is a board there to seal against. In a period floor with joist ends built into solid brickwork, it is not. Those joist ends rely on air movement in the pocket around them to shed the moisture that comes through the wall, and foaming them in trades a small thermal gain for a rot mechanism you cannot inspect. Insulate up to the bearing and stop; leave the pocket ventilated.

The same restraint applies to the sleeper walls in the middle of the floor. They are honeycomb — laid with deliberate gaps — for the sole purpose of letting air cross the void from one elevation to the other. Packing insulation into those gaps to tidy up the edge of a bay closes the cross-flow at the exact point the whole ventilation strategy depends on, and it is invisible from the moment the boards go down.

Which Face the Moisture Sees

The layer order is settled by one fact: in a heating climate the vapour drive runs outward, from the warm room towards the cold void, for most of the year. Vapour control therefore belongs on the warm side, above the insulation and immediately under the deck, and the cold side stays open so anything that does get through can leave. Reverse them and you have built a moisture trap onto the underside of the floorboards. BS 5250, Management of moisture in buildings — Code of practice, is the document to argue from here; the vapor retarder class provisions of the International Residential Code do the equivalent work in North America, with classes defined by permeance measured to ASTM E96/E96M, Standard Test Methods for Water Vapor Transmission of Materials.

Permeance is the number to specify by and the number nobody checks. A polyethylene sheet, an intelligent membrane and a coated building paper are three very different materials that all look like a roll on a pallet, and only the declared permeance separates them. On the cold side the sling must be vapour open — breather membrane or netting, never a second sheet of poly, which sandwiches the insulation between two barriers and leaves condensate nowhere to go.

The oversite is worth a sheet in its own right on a floor where the ground is visibly damp or the void smells of earth. A membrane laid across the soil, lapped and dressed up to the sleeper walls, cuts the vapour load reaching the timber at source, and unlike blocking the airbricks it reduces the moisture without removing the mechanism that deals with the rest. It is also the change that makes the biggest difference to how the void reads on a return visit two winters later.

Rolls are the unit these come in, and the wastage is in the laps rather than the cuts. A generous overlap at every seam, taped rather than spot-fixed, is what turns a set of sheets into a layer — and the roll count has to allow for it before the van is loaded.

Ordering for a Floor Where No Two Bays Match

A wall take-off can run on a nominal bay width because the framing was set out to one. This floor cannot, and the waste allowance is where that difference gets priced. Ten percent is the habitual number and it is roughly right for a plain rectangular room with regular joists; it is optimistic the moment the room has a chimney breast, a bay window, a hearth, a doubled joist under a former partition, or a run of pipework crossing three bays at an angle.

Take off the field area of the room, apply a waste factor chosen from what the sketch shows rather than from habit, then add the awkward areas as their own small quantities. Under-ordering costs a second delivery and a lost morning; over-ordering by one pack costs the price of one pack, and mineral wool stores dry in a garage indefinitely. The asymmetry is not close.