Blown-in or sprayed insulation: which technique should you choose?

Beyond material choice, installation technique plays a decisive role in your final insulation performance. CAE operators in Spain mainly use two methods: dry flake blowing and wet spraying of an insulation-water-binder mix. Each addresses different architectural constraints. We explain here the concrete difference between the two approaches, their respective advantages and how the technician selects one or the other during your free assessment.

The blowing technique principle

Blowing consists of mechanically projecting dry insulation flakes (defibred mineral wool or cellulose) into unused attics using a carding-blower machine. The material is drawn from a bale located in the intervention truck and routed via a hose up to the attic.

The technician directs the jet to spread material uniformly over the attic floor, ensuring the target thickness calculated during the technical assessment. Graduated stakes are regularly planted in the material to verify the effective height of installation.

The operation is fast: 4 to 8 hours for a standard 80 to 150 m² floor home. It requires no roof dismantling, the technician working from the existing access hatch.

The wet spraying principle

Wet spraying is used mainly for rafters (inside the inclined roof) and walls. The material, generally polyurethane foam or a cement-fibre mix, is mixed with water or a binder in the lance and sprayed under pressure onto the surface to insulate.

The wet mix adheres immediately to the substrate and expands to fill all interstices. Once dry, it forms a continuous envelope, without interruption, that eliminates thermal bridges. This continuity is what creates the technical performance of spraying.

Compared advantages: performance and accessibility

Blowing offers several advantages for unused attics:

• Fast execution on large flat surfaces.
• Lower material cost than wet spraying.
• No drying time, so the attic can be used immediately.
• Possibility of local top-up in case of future settlement.

Wet spraying for its part offers:

• Perfect adhesion on vertical or inclined surfaces.
• Superior airtightness (useful in windy zones).
• Higher thermal resistance R at equal thickness (for sprayed polyurethane).

Typical use cases in Castile and León

For an unused attic in a single-family house or urban building with hatch access: blowing mineral wool is almost always the recommended technique. This is the most frequent situation in Castile and León (1960s-1980s buildings in Valladolid, Salamanca or Burgos, and rural stone houses).

For converted attics with rafters and inclined ceilings: wet spraying (generally polyurethane) is preferred because it follows the complex geometry without thermal bridges. This configuration mostly concerns recent suburban chalets.

For listed buildings or old stone houses where breathability is essential: blown-in cellulose is preferred, regulating humidity better than closed-cell polyurethane.

Limits and precautions to know

Blowing has a few limits to anticipate. It can cause slight material settlement (5% to 15%) during the first year. Serious operators compensate by over-thickening initial installation.

Some old attics also contain technical conduits (electrical sleeves, ventilation, roof outlets) that must be protected before blowing by dedicated casings. This preparation generally takes 30 minutes to 1 hour and is part of the CAE service.

Wet spraying for its part requires variable drying time (12 to 48 hours for polyurethane, longer for hydraulic mixes). During this period, attic occupation is limited. Forced ventilation is sometimes needed to evacuate initial volatile organic compounds.

Frequently asked questions

Can blowing damage my ceilings?

No. Blowing is carried out from the attic floor, with no contact or pressure on the ceilings below. Technicians wear specific footwear and walk on distributing planks so as not to point-load the joists.

Which technique offers better acoustic insulation?

Mineral wool blowing is slightly superior to wet spraying for airborne sound insulation. Its fibrous structure absorbs sound waves better. For impact noises (rain on metal roofing for example), both techniques offer significant improvement.

Can material be blown on top of existing insulation?

Yes, provided existing insulation is not degraded, wet or contaminated. The technician evaluates the existing layer during assessment. If healthy, the new material is blown on top to reach target thickness. Otherwise, prior removal is planned and billed separately.

Which technique is favoured in the CAE programme?

Blowing represents about 85% of CAE operations in Castile and León, mainly because it applies to unused attics, the majority configuration in the region. Wet spraying is used for about 15% of files, on specific configurations.