Loft insulation installation
Loft insulation installation in London
Mineral wool fitted to the 270mm depth current building regulations recommend — the single most cost-effective energy improvement available to most London homes, and the one most likely to have been done badly the first time.
- Fitted to the recommended 270mm depth
- Cross-laid to bridge the cold bridge through joists
- Eaves ventilation path kept clear
- Hatch insulated and draught-stripped as standard
- Old material removed and disposed of where needed
Why 270mm, and why it matters that it is 270mm
Roughly a quarter of the heat leaving an uninsulated house goes up through the roof. Heat rises, the loft is cold, and the ceiling between them is often nothing more than plasterboard and a few centimetres of ageing wool. Insulation at that boundary is the cheapest thermal improvement in the building, which is why it is almost always the first one worth making.
The recommended depth has climbed steadily over the decades. Homes insulated in the 1980s typically got 100mm; before that, 50mm or nothing. Current guidance sits at 270mm. What this means in practice is that a great many London lofts contain insulation that was entirely adequate when it went in and is now doing roughly a third of the job it should be. If you have not looked in your loft with a tape measure, that is the first thing to do.
The cross-lay, and why it is not optional
Joists are timber, timber conducts heat, and a layer of insulation sitting only between the joists leaves a repeating cold bridge across the entire ceiling. The correct method is two layers: the first laid between the joists to their depth, the second laid across them at right angles, covering the timber completely. It is a small difference in method and a significant difference in outcome, and it is skipped often enough that we mention it explicitly on every quote.
What we check before anything comes off the van
- Existing depth and condition. Measured, not estimated. Whether it can be topped over or has to come out is the single biggest variable in the price.
- Joist depth and spacing. Determines the first layer's thickness and whether boarding will need battening.
- The eaves. Is there a ventilation path, and is it clear? If it is already blocked, that is dealt with as part of the job.
- Signs of moisture. Staining on the felt, damp timber or rusting nail points all indicate an existing condensation problem that adding insulation would make worse. That gets diagnosed first.
- Services in the loft. Tank, pipework, cabling and recessed downlighters all need handling as part of a proper installation.
How the day goes
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Protection
Dust sheets on the landing and stairs, boards over the hatch surround. Mineral wool fibres travel, and we would rather they did not travel through your house.
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Strip out, if needed
Old material bagged and removed. This is the slowest part of any job where it applies, and it is why it is priced separately and honestly.
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First layer, between the joists
Cut to fit snugly, teased into the corners without being compressed, and stopped short of the eaves so the airflow path stays open.
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Second layer, across the joists
Laid at right angles to full depth, covering the timber and eliminating the cold bridge. This is the layer that takes you to 270mm.
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Services and hatch
Tank and pipes lagged, downlighters cleared or hooded, hatch insulated and draught-stripped, then a walk-through with you and the paperwork handed over.
If you are also planning to board
Say so before we insulate, not after. Boarding and insulation have to be designed together — 270mm of wool is deeper than a 100mm joist, so the boards have to be raised on battens or loft legs. Doing them in one visit is cheaper than doing them in two, and laying boards over a finished insulation job that was not planned for them is how people end up crushing what they just paid for. See loft boarding.
What it costs
Price is driven by the loft's floor area, how easy it is to move around in, whether the old material has to come out, and what else is happening at the same time. A straightforward top-up in a mid-terrace is a very different job from stripping out a vermin-fouled loft in a large detached house. Every quote follows a free survey and is fixed once given. Our London cost guide explains the variables in more detail.
In the loft
Mineral wool installation
The standard job, done to the standard it should be done to.
Questions & answers
Loft insulation installation — common questions
What depth should loft insulation be?
270mm of mineral wool is the depth current building regulations guidance points to for a cold loft. Many London homes still have 100mm, which was the standard a couple of revisions ago, and a surprising number have 50mm or nothing at all. Topping up from 100mm to 270mm is among the highest-return improvements available to a UK homeowner.
Do you take the old insulation out?
Only when it needs to go. Dry, intact mineral wool can be topped over — we cross-lay the new layer at right angles to the joists, which also bridges the cold path through the timber. Material that is damp, vermin-fouled, contains old loose-fill, or has collapsed to nothing comes out and is disposed of. The survey tells you which, with removal costed separately.
Which material do you use?
Mineral wool — glass or rock — in rolls, which is the standard product for a cold loft floor and remains unbeaten on cost per unit of thermal performance where depth is available. Where a roof calls for something else, we specify something else, but for the overwhelming majority of London lofts this is the right answer.
How long does it take?
A typical three-bedroom house is a half-day. Larger, awkward or heavily cluttered lofts take longer, and removing old material adds time. We tell you the expected duration when we quote.
Will it make my house damp?
Not if the ventilation is right, which is why we check it before we start. Insulating the loft floor makes the roof space colder, so the moisture that rises out of your home has a colder surface to condense on. The eaves airflow path is what carries it away — block it with insulation and you create the very problem people blame insulation for. See our ventilation guide.
Does it improve my EPC rating?
Loft insulation depth is a direct input to the assessment, so an increase generally helps and often noticeably. We hand over the product documentation so your assessor can record what is actually there instead of defaulting to a pessimistic assumption.
What homeowners say
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Written and fact-checked by the Loft Insulation London survey team, who assess loft spaces across the capital every working week. Last reviewed .
Find out what is actually up there
A free survey, a tape measure and an honest answer — including, sometimes, that your loft is already fine.