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Written rules, not rules of thumb
The thermal resistance of insulation comes from its thickness and its thermal conductivity. Ventilation airflow for a home is set out in official guidance. What an appliance uses is its power multiplied by the time it runs. Each calculator names the document or data sheet it applies.
Thermal resistance by thickness and conductivity
The thermal resistance R of a layer of insulation is its thickness in metres divided by its thermal conductivity λ (lambda), in W/(m·K). The higher R is, the better the layer insulates. Conductivity is printed on the label and in the product’s declaration of performance; the values in the table are worked examples, not those of a particular product.
| Thickness | R at λ = 0.032 | R at λ = 0.035 | R at λ = 0.040 |
|---|---|---|---|
| 60 mm | 1.88 | 1.71 | 1.50 |
| 100 mm | 3.13 | 2.86 | 2.50 |
| 140 mm | 4.38 | 4.00 | 3.50 |
| 200 mm | 6.25 | 5.71 | 5.00 |
Results in m²·K/W. Two layers on top of each other add their resistances: 100 mm plus 60 mm of insulation at λ = 0.035 gives 2.86 + 1.71 = 4.57 m²·K/W.
Which rules apply in the UK
In England, insulation and energy use are covered by Approved Document L, ventilation by Approved Document F and electrical safety in homes by Approved Document P, with wiring to BS 7671, the IET Wiring Regulations. Heat loss for a heating design is calculated to BS EN 12831. Notifiable electrical work is done by an electrician registered with a competent person scheme, or checked by Building Control. Scotland, Wales and Northern Ireland have their own guidance.
What these calculations do not replace
Sizing a heating system properly takes a room-by-room heat loss calculation using the location and the real walls, floors and windows. The calculators give an order of magnitude and the basic rule. For a new system or a major renovation, have the calculation done by a professional.