Essential checks before drilling: secure the ventilation gap and avoid condensation
Identify the underlay: HPV membrane, non-HPV membrane or no membrane (impact on ventilation)
Before any roof-slope insulation by blow-in, open up an inspection point (eaves, ridge, access hatch) and identify the underlay. Check the marking and its condition. A breathable HPV membrane tolerates closer proximity to the insulation more easily. A non-breathable membrane or the absence of a membrane means you must preserve a continuous ventilated air gap between the roof covering and the insulation.
Check the continuity of the air gap under the membrane or battens: free air intake and exhaust at the bottom and top of the slope
Your priority is the air gap. Check that air can enter at the eaves (soffit vent, grille, comb strip) and exit at the top (vented ridge, roof vents). Make sure nothing blocks it (existing insulation, dust, timber, sagging underlay). A visual check or borescope inspection at several bays avoids drilling “blind”.
Identify obstacles and singularities: purlins, bracing, downlights, ducts, electrical boxes and penetrations
- Map out the purlins, noggins, bracing, loft openings and valleys so you can compartmentalise by bay.
- Locate downlights and electrical boxes. Secure the clearances and avoid any fill in contact with a hot spot.
- Identify ducts and penetrations. Maintain fire clearances and plan for airtight sealing after drilling.
Create a cavity that is truly airtight to loose-fill insulation: the foundation of blow-in insulation in a closed roof slope
Choose and install the cavity lining on the inside: airtightness to insulation + mechanical resistance to blowing
When insulating roof slopes by blow-in, do not rely on the existing lining. Create a continuous lining: an air-tight membrane or a reinforced fleece designed for blow-in, fixed back to the rafters with counter-battens. The goal is straightforward: withstand blowing pressure and prevent any insulation escaping into service voids.
Ensure airtight joints: stapling, tapes, sealant, treatment of edges and hard points
Work as you would on an airtight building envelope. Staple tightly, overlap the sheets, then tape them with a substrate-compatible adhesive. At perimeters and on uneven supports, run a bead of sealant. Systematically treat edges, purlins, valleys, loft openings, ducts and boxes to prevent leakage paths from starting.
Compartmentalise by bay: closures at the eaves and ridge end of each slope to prevent migration and settlement
Close off each bay at the eaves and at the ridge with a rigid piece or reinforced netting fixed to sound timber. That prevents the insulation from migrating toward the eaves or ridge and stabilises the fill. Pre-drilling checkpoint: the under-roof air gap must remain clear, with baffles if needed. For guidance on best practice, also see roof-slope insulation.
Carrying out the blow-in: density, drilling pattern and even filling without settlement
Set the fill density: aim for denser than a flat loft to hold on the slope (and why)
In roof-slope insulation by blow-in, the rule is the “dense pack”. On a slope, loose-fill insulation is subject to gravity. At “lost loft” density, it slides and settles at the top of the slope. Set the machine to achieve a higher density, which locks the fibres together and stabilises the volume.
| Configuration | Target density (kg/m³) |
|---|---|
| Flat loft (blown) | 25 to 35 |
| Closed roof slope (blow-in) | 55 to 65 |
Define the drilling grid and hole diameter: spacing, zone sequencing and fill progression
Drill to the diameter suited to your nozzle and hose, then work bay by bay. Keep a regular spacing (generally 60 to 80 cm), and sequence from bottom to top. Fill one area, plug it, then move on to the next.
Monitor during the work: material return, back pressure, cavity behaviour and treatment of “voids”
Watch the material returning at the hole, the back pressure in the hose and any deformation of the cavity. If an area “sounds hollow” or remains cold on the thermal camera, re-drill locally and top it up. Track the installed mass (bag tally) and compare it with the treated area. To compare with best practice on loft insulation by blowing, keep in mind that densities and fill checks differ between flat blown insulation and blow-in insulation in roof slopes.
Sealing, patching and finishing: close everything up properly without weakening the cavity
Seal the blow-in holes: plugs, reinforcement pieces and durable airtightness
After roof-slope insulation by blow-in, close each drilled hole with a rigid plug of the right diameter, installed over a reinforcement piece to prevent the lining from tearing out. Recreate the airtight layer with a membrane or a substrate-compatible tape. The objective is zero insulation leakage and a continuous barrier.
Patch and restore the lining: boards, plaster, joints and aesthetic continuity
If the lining has been cut, reinstall a board patch on supports, screw it off, then tape and finish it as you would for a standard joint. Treat the joints around penetrations to achieve a continuous lining. Finish with a visual check for cracks, and a local smoke test if you suspect air leakage.
Special cases: hatches, service access, recessed downlights and fire safety requirements
Prepare hatches with a rigid frame and compressible gasket. For downlights, follow the luminaire manufacturer’s instructions and avoid direct contact with insulation if the fitting is not designed for it. If necessary, fit a protective hood or a built-out enclosure, while maintaining fire safety and the airtightness of the cavity.
Completion checks and site evidence: validate the installed mass per m² and document your work
Calculate and verify the installed mass: treated area, number of bags, total mass and consistency per m²
In roof-slope insulation by blow-in, sign-off starts with the calculation. Record the actual treated area by bay (excluding openings). Count the bags used, check the unit weight, then calculate the total mass and the value per m². Consistency should be judged against your target density, not by feel.
| Treated area (m²) | Bags used (u) | Unit weight (kg) | Total mass (kg) | Mass per m² (kg/m²) | Target value (kg/m²) | Difference (%) |
|---|---|---|---|---|---|---|
| … | … | … | … | … | … | … |
Check the result: thermal imaging (weather conditions), spotting gaps and targeted corrections
Carry out a thermal camera check when the indoor/outdoor temperature difference is sufficient, with no direct sun on the slopes and limited ventilation. Look for linear cold spots (rafters), local “holes” around the drill points and air leakage at the bottom of the slope. Correct them with targeted touch-ups, not by re-blowing the whole area.
Formalise sign-off: bag tally, self-inspection sheet, photos and traceability in 2026
Archive the bag tally and proof of batch, your self-inspection sheet (area, mass per m², drilling grid, sealing), and a photo report before, during and after. In 2026, favour timestamped photos, geotagged if possible, and a single site file so you can respond quickly to an inspection, a dispute, or a request for certification; see also preparing your sites and documents.
