8 tools

Building envelope

A building loses heat through whichever part you thought about least. The glass, the frame and the wall are three calculations that people do in three different places with three different assumptions, and the answer that matters is the one that puts them side by side. These tools finish the set: the glass and the window are already here, and this is the wall they sit in.

Four tools, and they chain: work out the wall, check it does not get wet inside, price the junctions, then add the lot up into a heat load in kilowatts. Same rules as everywhere: the formula and the standard on the page, no sign-up, nothing uploaded, all of it computed in your browser.

Wall and roof U-valueBuild the construction layer by layer to EN ISO 6946 and see which one is actually doing the work — it is almost never the one that costs the most. Also tells you the insulation thickness you need to hit a target U.Example render + brick + 100 wool + block + board → U 0.274Calculate the U-value

0.27 W/(m²·K)
338 mm thick
5 layers

How they fit together

The wall passes. The house still needs 3 kW.

A wall at 0.274 looks like a job well done, and it is — but it is only 74 of the 133 watts per kelvin that size the heat pump. The rest is ventilation and the junctions nobody drew, and one of those junctions is sitting at 82 % humidity. Three calculations usually done by three people who never compare notes.

Measure and buy

Floor, wall and plot areas in feet or metres, with the waste, the boxes and the paint to buy.

Square footage calculatorRooms, L-shapes and plots in feet and inches or metres, less what is not covered. Then the waste, the whole boxes of flooring or tile, the cost, and the walls with the paint.Example three rooms: 407 ft², 23 boxes of 20 ft²

Countertop calculatorStone and quartz worktops: area, slabs, weight, polished edge and cut-outs.Example Worktop area: 3.53 m²

Heat

From one wall to the whole house: layers, junctions and the design heat load in kilowatts.

Roof pitch calculatorDegrees, per cent and X in 12 from any one of them, with the rafter length and the roof area.Example 6 in 12 → 26.57° · 50 % · factor 1.118

Insulation thickness for a target UHow much insulation a wall, roof or floor needs to reach the U-value you need, rounded to the board you can buy.Example U 0.25, mineral wool → 130 mm · U 0.233

How to calculate a U-value1 over the total resistance, layer by layer, for a wall; then how a glass unit and a whole window are worked out instead.Example the same unit: glass 1.10, window 1.30

Thermal bridgesWhat the lintels, corners and floor junctions give back of the insulation you paid for. Turns ψ values and lengths into a y-value, compares it with the penalty default — and checks whether the junction will go mouldy, which happens long before it gets wet.Example a junction at the legal minimum still hits 82 % surface RH

Whole-house heat lossThe design heat load in kilowatts — the number a heat pump is actually sized on. Fabric, junctions and ventilation added up, with a breakdown that shows which one is really costing you and what the single best fix is worth.Example the default house: 3.07 kW, and the biggest line is the air

Radiator sizeEach room’s heat loss as the radiator output to buy at ΔT50, in watts and BTU, for a boiler or a heat pump, and what an existing radiator gives.Example a room losing 1,200 W on a heat pump at 45/40 °C: 3,595 W at ΔT50

Average heat loss of a housePublished figures per m² and per house, each with its source, and how the design heat load becomes a heat pump size.Example the 100 m² house: 3.07 kW at −2 °C

Moisture

Whether the wall gets wet inside, and the vapour control that fixes it.

Interstitial condensationThe Glaser method to EN ISO 13788: temperature and vapour pressure through every layer, and exactly where the wall gets wet. Then it tells you the vapour control layer that would fix it — and which side it goes on.Example the default wall condenses behind the insulation by 416 Pa

Why this collection exists

Because the three parts of an envelope are usually calculated by three different people who never compare notes. A wall at 0.25 W/(m²·K) sounds excellent until you notice the window beside it is at 1.30 and covers a quarter of the facade — at which point the window is losing more heat than the entire wall around it. Having the glass, the window and the wall in the same place, with the same rules and the same honesty about what they leave out, is the point.

And because the same three questions keep coming back in a different order. A wall that passes on U-value can still get wet inside. A house full of good U-values can still lose a third of its heat through junctions and open windows. Only when all of it is added up does a number in kilowatts come out — and that number, not the floor area, is what the heating should have been sized on.