Roof and floor insulation: thickness and quantities
Separate thermal thickness from purchase quantities. A pitched roof needs its slope area; a floor needs its actual insulated area and assembly conditions. The calculators and examples here distinguish plan area, pitch, waste and homogeneous thermal resistance.
Reviewed:
Calculator
Pitched roof area
Aslope = Aplan / cos(θ); Apurchase = Aslope × (1 + waste / 100)
For a simple roof whose whole horizontal projection has the same pitch, 100 m² at 30° gives 115.47 m² of slope area. With an illustrative 10% allowance the purchase area is 127.02 m². Include overhangs in the projection and treat differing pitches separately. Dormers, valleys and irregular shapes need separate measurements. This formula does not design rafters or roof structure.
Waste, overlaps and package rounding
There is no universal roof waste percentage. Derive the allowance from cuts, layout, valleys and product instructions. Effective coverage can already include overlaps: do not add the same loss twice. Divide the required area by usable pack coverage, round to whole packs and report both the net and ordered areas. Keep the illustrative 10% separate from a project-specific estimate.
Roof insulation thickness
For a homogeneous insulation layer, d = λ × RSI. An illustrative RSI = 5.0 with λ = 0.040 needs 200 mm. Rafters, gaps and fasteners can reduce the complete assembly's resistance. Decide whether the insulated boundary is the attic floor or the roof slope; counting both as the same area gives the wrong quantities. Preserve the designed ventilation and air-control details.
Floor insulation and ground-contact limits
A floor above unheated air differs from a ground-bearing slab. Ground heat flow depends on geometry and perimeter as well as layers; a simple 1/R sum is not a ground-floor compliance calculation. For the insulation layer alone, 120 mm / 0.035 gives RSI = 3.429. Check compressive load, moisture protection, joints and the product's approved use before selecting a floor board.
Flat-roof area and usable coverage
Aplan = length × width; Aslope = Aplan / cos(pitch); Apurchase = Aslope × (1 + allowance/100)
A rectangular 10 m × 6 m projection is 60 m². At zero pitch it remains 60 m²; an illustrative 10% purchasing allowance gives 66 m². A real flat roof still has designed drainage falls: enter the actual pitch or measure its surfaces. Treat upstands, edges, outlets, overlaps and tapered insulation separately. Use usable product coverage and supplier details; this area calculation cannot establish waterproofing, drainage or structural loads.
Garage insulation: separate the boundaries
Establish whether the garage is heated, whether the room above is heated and where the thermal boundary runs. Measure walls, ceiling, door and windows separately; subtract openings from wall quantities. A 6 m × 5 m garage at 2.5 m height has 55 m² gross wall area and 30 m² ceiling area; a 9 m² door reduces net walls to 46 m². At 150 mm, those walls geometrically contain 6.9 m³ of insulation before openings other than the door. This is quantity arithmetic, not a recommended thickness. Door rating, air leakage, ventilation, fire separation and moisture control need separate checks.
Worked examples — illustrative inputs
| Aplan [m²] | θ [°] | Aslope [m²] | +10% [m²] |
|---|---|---|---|
| 100 | 0 | 100.00 | 110.00 |
| 100 | 30 | 115.47 | 127.02 |
| 100 | 45 | 141.42 | 155.56 |