Insulating a wall is two decisions wearing one coat. The first is how much area you are covering, which decides how many boards you buy. The second is how thick they are, which decides what the wall ends up performing at — and the second decision is the one that is usually made by habit rather than by arithmetic. Doubling the thickness does not halve the U-value, because the wall you started with is already resisting heat flow, and every extra millimetre is fighting a losing battle against the resistance that is already there. This gives you both numbers and shows the law of diminishing returns for what it is.
Area, board and thickness
Thickness decides the U-value. Area decides the boards. Both matter.
Wall, floor or ceiling. Do not subtract openings here.
Result
Resistance adds, U-value does not
This is the one piece of building physics worth knowing before choosing a thickness. Thermal resistance — the R-value, in square metres kelvin per watt — adds up: the resistance of the brick, plus the plaster, plus the board, plus the surfaces, is the total. The U-value is one divided by that total. So adding resistance always helps, but the U-value it produces falls more and more slowly.
On a solid brick wall with an existing resistance of about 0.45, the first 50 mm of PIR board takes the U-value from 1.61 down to 0.35 — a seventy-nine percent improvement. The second 50 mm takes it from 0.35 to 0.19, which is a further forty-five percent of the reduced figure but only eleven percent of the original heat loss. The board costs the same both times.
Where the diminishing returns actually bite
The practical question is not whether more insulation is better — it always is — but whether it is worth the space and the money. Every millimetre of internal insulation comes out of the room, and on a small room with a fitted wardrobe along one wall, 75 mm can mean the wardrobe no longer fits. It also moves the dew point, which is why internal insulation needs a vapour control layer and careful detailing at every junction.
And the money is not linear either, because the board is not the whole cost. The battens, the plasterboard, the skirting that has to come off and go back, the window reveals that have to be re-formed, the sockets that have to be extended — most of that cost is the same whether the board is 50 mm or 100 mm. The board itself is the part that scales.
Insulating a wall does nothing about the junction where that wall meets the floor, the ceiling, another wall, or a window reveal. Each of those is a path straight through the insulation layer.
Reveals are the worst on internal insulation, because a 100 mm board turns a window reveal into a 100 mm deep rebate where cold air sits and condensation forms.
Choosing the board
PIR — polyisocyanurate, the foil-faced board — has the lowest conductivity of the common boards at about 0.022 watts per metre kelvin, which is why it is used where space is tight. Phenolic is slightly better again and considerably more expensive. EPS — expanded polystyrene — is much cheaper and needs roughly half again the thickness to do the same job. Mineral wool is cheaper still, fire-resistant, and much thicker for the same performance.
The lambda figure on the data sheet is what matters, and it is worth reading carefully: some boards quote a value that holds for the lifetime of the building and some quote an initial value that degrades as the blowing agent diffuses out. A board at 0.022 and a board at 0.028 differ by nearly thirty percent in thickness for the same result.
| Board | Conductivity | For 0.30 W/m²K | Note |
|---|---|---|---|
| PIR, foil faced | 0.022 W/mK | About 55 mm | Best performance per mm |
| Phenolic | 0.020 W/mK | About 50 mm | Best, and the most expensive |
| EPS, standard | 0.035 W/mK | About 90 mm | Cheap, needs thickness |
| EPS, graphite | 0.031 W/mK | About 80 mm | Better, still cheap |
| Mineral wool | 0.038 W/mK | About 100 mm | Fire performance, acoustic |
| Wood fibre | 0.040 W/mK | About 105 mm | Breathable, for old walls |
A worked example
- Boards30 m² at 2.88 m² a board, plus ten percent, is 12 boards.
- Resistance50 mm of PIR at 0.022 adds 2.27 to the wall's existing 0.45, plus the two surface resistances: 2.89 in total.
- U-valueOne over that is 0.35 W/m²K, against 1.61 before.
- SavingThat is a 79% cut in heat loss through that wall, for $504 of board.
- The next stepDoubling to 100 mm would reach 0.19 — another eleven percent of the original loss, for another $504.
Eleven percent for a doubling of the material cost is the honest trade, and it is sometimes worth making — where the wall is large, where energy is expensive, or where the work is being done once and never again. It is not worth making on autopilot.
Fitting it
Insulation board is fixed either on battens or directly with adhesive and mechanical fixings, and the board must be continuous: gaps between boards, gaps at the top and bottom, and gaps around services are all paths for air to bypass the insulation entirely. Tape every joint on a foil-faced board, because the foil is the vapour control layer as well as a facing.
Then it gets covered. Internal insulation has to be lined with plasterboard, which is a separate quantity with its own layout arithmetic — the drywall sheet figure sizes that, and the wall area figure gives the surface being covered in the first place. Where the ceiling is being done at the same time, the ceiling area figure handles that surface separately.
Do
- Read the conductivity off the data sheet, not off the product name.
- Tape every joint on a foil-faced board — the foil is the vapour layer.
- Detail the reveals, the floor junction and the ceiling junction.
- Compare thickness against what the room can actually lose in depth.
- Remember the fixed costs do not scale with thickness.
Avoid
- Assuming doubling the thickness halves the U-value.
- Leaving gaps between boards, which let air bypass the whole layer.
- Forgetting the window reveals become deep cold rebates.
- Insulating a wall that has an untreated damp problem first.
- Comparing board prices without comparing conductivity.
Where the numbers stop being true
The U-value here is a simple one-dimensional calculation through the middle of a wall. It ignores every cold bridge: the junctions with the floor, the ceiling, other walls, window reveals, and any structural timber penetrating the insulation layer. On a real installation those bridges can add ten to twenty percent to the heat loss, and they are the reason a calculated U-value and a measured one rarely agree.
The existing wall resistance is a choice from three options, and it is an estimate. A solid brick wall varies with the brick, the mortar and how wet it is — a damp wall conducts considerably better than a dry one, which is why fixing a damp problem before insulating is not optional.
And nothing here covers ventilation. A well-sealed, well-insulated room with no ventilation is a condensation problem waiting to happen, and any insulation job of this kind should be accompanied by a look at how the room actually breathes.
How the figures are produced
Boards are the area divided by the board size, with the cutting allowance applied and the result rounded up. Resistance is the internal surface resistance of 0.13, plus the existing wall resistance, plus the thickness divided by the conductivity of 0.022, plus the external surface resistance of 0.04. The U-value is one over that total, and the saving compares it with the U-value of the wall before the board was added, using the same surface resistances.
Questions people ask
Is 100 mm twice as good as 50 mm?
No. It takes the U-value from 0.35 to 0.19 — better, but only about eleven percent more of the original heat loss.
How many boards for 30 square metres?
Twelve 1200 x 2400 mm boards with a ten percent cutting allowance.
PIR or EPS?
PIR where space is tight and performance per millimetre matters. EPS where there is room and the budget is tight.
Can I insulate internally?
Yes, but it needs a vapour control layer, careful detailing at every junction, and it costs room depth. External is better where it is possible.
What U-value should I aim for?
Around 0.3 for a wall, which is roughly 50 to 55 mm of PIR on solid brick. Deeper than about 0.2 is rarely worth the money on a retrofit.
Do the gaps between boards matter?
Yes, a lot. An untaped gap lets air bypass the insulation entirely, and the board might as well not be there.