How the blown loft calculation works
Blown (or blowing) wool is loose mineral wool fibre that an installer pumps through a hose into the loft, building up an even layer across and between the joists. The calculator models Knauf Supafil Loft 045, which has a certified thermal conductivity of λ 0.045 W/m·K.
The target
By default the calculator aims for U-value 0.16 W/m²K, the Approved Document L value for upgrading an existing roof (Table 4.3). Switch to New-build standard for the stricter 0.15 that applies to a new roof (Table 4.2).
The U-value of the loft is:
U = 1 ÷ (Rsi + Σ thickness/λ + Rse)
With the surface resistances and the ceiling taken off, the insulation in a loft needs to supply R 5.68 m²K/W to reach 0.16. The calculator values any existing mineral wool at λ 0.040, as RdSAP 10 does, allowing for the joists that run through it. New blown wool fills between the joists first, so the calculator allows for the timber there too, the same way it does for the first layer of loft roll.
Depth to any millimetre
Rolls come in fixed thicknesses, so a roll stack often overshoots the target. Blown wool can be installed to any depth, so the calculator gives the depth this model needs for the target, to the millimetre.
The installed price
Because it's an installer job, the price is an installed cost estimate, not a materials price, and no waste allowance is added. It's derived from the Energy Saving Trust's figure of about £750 to insulate a typical loft from 0 to 270 mm. Assuming a typical loft is 40–55 m², that works out to £13.60–£18.75 per m² at 270 mm, which the calculator scales to the depth you need. It's a rough guide for checking quotes, not a quote.
Worked example: a 45 m² loft
Take a 45 m² loft, for example 9 m × 5 m, with Building Regs (Part L) selected.
| Empty loft | 100 mm already there | |
|---|---|---|
| U-value now | 1.75 W/m²K | 0.40 W/m²K |
| Blown depth to add | 275 mm | 168 mm |
| Estimated installed cost | £625–£860 | £382–£526 |
| U-value after | 0.16 W/m²K | 0.16 W/m²K |
For comparison, doing the same lofts yourself with Loft Roll 44 needs 270 mm (100 mm + 170 mm) when empty, for £369–£1,021 in materials, or a single 170 mm layer over the existing 100 mm, for £234–£693. Run the loft insulation calculator to compare for your own loft.
The Energy Saving Trust's general recommendation is a minimum of 270 mm of mineral wool. Treat the calculator's depth as a check on the U-value, and ask the installer what depth and coverage their product needs to reach it.
When blown insulation makes sense
- Hard-to-reach lofts. Low headroom, a shallow pitch or narrow eaves make rolling out insulation slow and awkward. A hose reaches where you can't.
- Obstructions. Lots of cross-bracing, pipes and wiring are easier to blow around than to cut rolls to fit.
- Uneven old insulation. Blown wool fills dips and gaps in a patchy existing layer.
- You don't want to do it yourself. A crew with the right machine does the work, and you don't handle the material.
In an open loft with good headroom, loft roll is usually the cheaper route if you're happy to do it yourself.
Before the installer arrives
- Clear the loft so the crew can reach every corner, and decide where you'll want storage. Raise any boarded area on loft legs so the insulation isn't squashed.
- Keep the eaves ventilation clear. Ask how the installer will stop the wool blowing into the eaves and blocking the airflow that keeps a cold loft dry.
- The hatch. Ask for the hatch to be insulated and draught-stripped, and for a dam around the opening so the wool doesn't spill out when you open it.
- Water tank and pipes. The ceiling under the cold water tank is usually left uninsulated, with the tank and pipes lagged instead.
- Downlights and cables. Point out recessed downlights so they aren't buried, and any cables that need to stay clear.
- Vermiculite. If there's pebbly loose fill up there already, tell the installer. It may be vermiculite, which can contain asbestos, and shouldn't be disturbed until it's been checked.
Afterwards
Ask how you can check the installed depth, for example with depth markers fixed to the joists, and keep the paperwork, including the product and depth installed. To see how depth relates to R-value for each product, read loft insulation thickness, and for the targets behind the numbers see Part L U-values.
Sources
- Approved Document L, Volume 1: Dwellings (2021 edition incorporating 2023 amendments)
- ACERMI certificate 04/D/016/378 — Knauf Supafil Loft 045
- Energy Saving Trust: Roof and loft insulation
- RdSAP10 Specification (BRE, February 2024)
- BRE BR 443: Conventions for U-value calculations (2019)
- Knauf Insulation: UK & Ireland Insulation Solutions Guide (June 2026)
- HSE: Asbestos — location of materials (loose-fill)