What is blown loft insulation?

Blown insulation solves a specific problem — lofts that are hard to reach or full of obstructions. Here's how it's installed, where it's the right choice, and the points to check before going with it.

How blown insulation is installed

Blown, or loose-fill, insulation is exactly what it sounds like: small fibres or granules of material that are blown into the loft using a specialist machine rather than laid as rolls. The two common materials are shredded mineral wool and recycled cellulose (treated paper fibre). An installer feeds the material through a hopper and a hose, distributing it evenly across the loft floor to build up a consistent depth — typically to around 270mm to match the standard performance target.

Because it needs the right equipment and even distribution to perform, blown insulation is a professional installation, not a DIY roll-out. The upside is speed and even coverage: a competent installer can fill a loft quickly and reach into spaces that would be slow and fiddly to fill by hand.

Where it beats rolls — and where rolls win

The clearest case for blown insulation is an awkward loft: low headroom, irregular shapes, lots of pipework and cabling, or restricted access that makes manoeuvring rolls difficult. The loose material flows into gaps and around obstructions, giving more even coverage than rolls that tend to bridge over uneven surfaces and leave cold spots. It's also useful for topping up an existing layer in a hard-to-reach space.

For a standard, open, accessible loft, though, mineral wool rolls are usually the cheaper and perfectly effective option — there's no advantage to blowing material into a loft you can simply walk into and roll out by hand. So the choice is led by access and shape more than performance.

Indicative comparison for guidance; the right choice depends on access and use. Source: Energy Saving Trust roof and loft insulation guidance.

The cautions before you choose blown

A few practical points are worth knowing before settling on blown insulation:

• Storage: a loose, fluffy layer is harder to board over for storage than a firm rolled layer, and you still mustn't compress it. If you want a usable storage area, raised boarding is needed, and rolls are sometimes the simpler base.
• Movement and draughts: very lightweight loose-fill can be disturbed by strong draughts or by people moving around the loft, potentially thinning in places over time. Quality installation and the right material reduce this.
• Topping up later: mixing loose-fill with later roll top-ups is possible but needs care to keep even depth.
• Certification: as with any product, look for a BBA certificate or equivalent so the performance is independently assessed.

Used in the right loft — particularly a hard-to-access one — blown insulation is an effective, even and quick solution that reaches the same target depth as rolls. In an easy, open loft, it rarely justifies its cost over standard mineral wool rolls. As ever, the honest comparison is on the same scope: the same target depth, the same eaves ventilation, and whether storage boarding is included.

One thing often overlooked when weighing blown against rolls is how each behaves over the long term in a working loft. Rolls, once laid, stay put and are easy to read at a glance — you can see the depth, spot a gap, and lift a section to run a cable. A blown layer gives a more even initial fill but is harder to assess afterwards: depth has to be checked with a probe or marker pegs, and any thinning from foot traffic or strong draughts is less obvious to the eye. That is not a reason to avoid blown insulation, but it does mean a loft with a loose-fill layer benefits from a quick visual check every few years and from keeping any foot traffic to designated boarded walkways so the fill is not scuffed aside. If you expect to go into the loft often — to reach a cold-water tank, store seasonal items or run cabling — that ongoing access argues quietly in favour of a firm rolled base with raised boarding, while a loft you rarely enter is an ideal candidate for a quick, even blown fill you can leave undisturbed for decades.

Mineral-wool blown versus cellulose blown

The two materials that get blown into lofts behave a little differently, and it is worth understanding the distinction rather than treating “blown insulation” as one thing. Shredded mineral wool (glass or rock) is the loose-fill cousin of the standard rolls: it is non-combustible, doesn't absorb water readily, and shares the same long, fit-and-forget lifespan as roll wool when kept dry. Cellulose is made from recycled paper fibre treated with mineral salts to resist fire and pests, which gives it a strong environmental story and very good gap-filling behaviour, but it is more sensitive to moisture than mineral wool, so leaks and ventilation matter even more, and it can settle over the years — a reputable installer accounts for this by blowing in slightly more depth to allow for settlement.

Neither is universally “better”; they suit different priorities. If fire performance and moisture tolerance are top of your list, blown mineral wool is the safer default. If recycled content and a tight, gap-filling fill are what you value, cellulose is a credible choice provided the loft is sound and dry. Whichever you pick, the same fundamentals decide whether it performs for decades: it must reach a genuine even depth of around 270mm, it must not be compressed, the eaves ventilation must stay clear, and the product should carry independent certification. The material is a secondary decision once those basics are right — a well-installed layer of either will quietly do its job, while a poorly installed layer of the “best” material will not.

It is also worth knowing what to ask an installer so the quote is comparable with a rolls-based one. Confirm the target settled depth (the depth after any settlement, not the freshly blown depth), the material and its BBA certification, how the eaves will be kept clear (baffles or trays), and whether they will fit depth markers so the layer can be checked later. A good installer will also explain how they handle downlighters, the loft hatch and any recessed fittings, since loose fill must be kept clear of heat-producing fittings unless they are rated to be covered. Get the price on the same basis as a rolls quote — same depth, same ventilation provision, same allowance for boarding if you want storage — so you are comparing like with like rather than a cheap thin fill against a proper one. Done this way, blown insulation in the right loft is a genuinely good outcome: fast, even, and reaching the same standard the rest of this site recommends, while in an easy open loft the simpler economics of rolls usually win.

Frequently asked questions

Sources & further reading

• Energy Saving Trust — roof and loft insulation
• British Board of Agrément (BBA) — product certification

Figures on this page are typical UK ranges drawn from published sources and depend on your specific loft. They are guidance, not a quotation.