Tips for Building
With blown insulation, the count comes from the installed density and the bag weight, not from area alone. 50 m² of cold loft at 300 mm is 15 m³ of wool; at 12 kg/m³, the density of a common glass wool, that is 180 kg, or 12 bags of 16 kg with 5% waste. Stone wool or cellulose, two to four times denser, needs that many times more bags. The depth to aim for is not chosen here: it comes from the Building Regulations, the product's certificate and the criteria of any grant you apply for.
The assumptions behind this calculation
- Blown wool: the volume is the area times the target depth, the weight is that volume times the installed density given on the product datasheet or certificate, and the bags are that weight, waste included, divided by the weight of one bag. The depth entered is the final depth, after settling: if the datasheet states a settlement, it also gives the depth to blow on the day, and that is the one the depth pegs check.
- The manufacturer's coverage table comes first: many bags carry a table of bags per 100 m² by depth or by thermal resistance. Where it exists, it is the reference; the calculator is there to cross-check it and to scale it to your area.
- Rolls: the number of layers is the target depth divided by the roll thickness, rounded up to a whole layer, and the depth reached is shown. The insulation area is the slope area times the number of layers, turned into rolls of the area printed on the label, waste included. Two layers of different thickness, the first between the rafters and the second under the frame, are worked out one after the other.
- Not counted: the vapour control layer and its tape, eaves trays, depth pegs, the lining frame and hangers, plasterboard, and the hatch insulation.
What this calculator does not replace
It gives quantities, not a performance. No thermal resistance and no minimum depth are offered here: the Building Regulations (Approved Document L in England) set the level to reach, the product certificate or declaration of performance says which depth, and for blown fill which weight per m², gives which resistance, and a grant scheme sets its own criteria, often an approved installer. Check them before you buy. The values in the fields are examples to replace with your product's.
Bag coverage varies from brand to brand and from one wool to another: that is why the calculator asks for the density and the bag weight instead of assuming a pack size. Clearances around the flue and recessed downlights follow Approved Document J and the manufacturers' instructions, as the job page explains.
The steps of the job, in order
Frequently asked questions
How many bags of blown insulation for 100 m²?
It depends on the depth and on the wool. At 300 mm, 100 m² is 30 m³; at 12 kg/m³ that is 360 kg, or 23 bags of 16 kg with no waste and 24 with 5%. The same depth of blown stone wool at 30 kg/m³ weighs 900 kg. The table printed on the bag gives the exact figure for that product.
How thick should loft insulation be?
As thick as it takes to reach the thermal resistance the regulations and any grant ask for, read on the certificate of the product you choose. Two products do not reach the same resistance at the same depth, which is why this calculator offers no depth.
How many rolls for a sloping ceiling?
The slope area, times the number of layers, divided by the area of one roll printed on the label. For 40 m² in two 100 mm layers with 4.5 m² rolls, that is 18 rolls with no waste and 19 with 5% offcuts.