Knowledge centre
Loft Insulation and Boarding
Depth, materials, condensation and storage
Most lofts are used for storage whether they are boarded or not, and most are under-insulated. These guides cover how deep the insulation should be, what the materials actually do, and how to board a loft without ruining the insulation underneath.
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In short
Depth, airflow and the right order
Loft insulation works by holding still air, so what matters is depth and the absence of gaps rather than the name on the packaging. The Energy Saving Trust recommends at least 270mm of mineral wool in a loft where that is suitable. A great many UK lofts still have 100mm or less, fitted when that was the standard, which is why topping up is often the cheapest worthwhile improvement available to a homeowner.
Two things get done wrong more than anything else. The first is compressing it. Insulation that has been squashed, usually by boards laid straight on top of it, no longer does its job, and the loss is not small. If you want to use the loft for storage, the deck needs to be raised clear of the insulation on a sub-frame so the full depth stays intact underneath. The second is blocking the airflow. A cold loft relies on ventilation at the eaves, and insulation pushed hard into the eaves closes that path off. It is one of the most common causes of a loft that suddenly starts showing damp after an insulation upgrade.
Condensation
In winter a loft is cold and the house below is warm and full of moisture from cooking, washing and drying. Where that warm air gets up into the loft, it condenses on the first cold surface it meets. The fixes are usually unglamorous: seal around the loft hatch, deal with gaps around pipework and downlights, make sure extractor fans are ducted outside rather than into the loft void, and keep the eaves clear. Boarding does not cause condensation by itself. Boarding that blocks ventilation, or that traps moist air under a sealed deck, will make an existing problem worse.
Do it in the right order
If the loft needs both, insulate and board as one job rather than boarding now and insulating later. Doing it the other way round means lifting a deck you have just paid for. The same applies when spray foam is being removed: the loft is open and accessible at that point, which is the sensible moment to put the insulation back to full depth and raise any boarding above it.
On cost, loft insulation supplied and installed qualifies for zero rated VAT in certain circumstances, which is worth checking against your own situation when you are comparing quotes.
01 Insulation
Getting the insulation right
Depth and material are most of the job.
Depth
What depth should my loft insulation be?
The recommended depth and why topping up is usually worth it.
Materials
Fibreglass or mineral wool: the difference
What the two terms mean in practice.
Contractors
Loft insulation contractors
What to expect from the people doing the work.
02 Condensation and boarding
Using the loft without wrecking it
Boarding over insulation is where most lofts go wrong.
Myths
The truth about loft boarding and condensation
What causes it, and what does not.
Damp
How do you reduce condensation in your loft?
Ventilation, insulation and the things people get wrong.
Example
Loft insulation and boarding in Bournemouth
What a combined insulation and boarding job looks like.
Options
Four roof insulation options
Where loft insulation sits against the alternatives.
Our services
The work itself
Insulation, boarding and removal, all by our own teams.
Insulation
Loft insulation
Knauf Loft Roll mineral wool fitted to the recommended depth.
Removal
Loft insulation removal
Old, damp or contaminated insulation taken out.
Free no-obligation survey
Want the loft
sorted properly?
We look at the depth, the joists and the hatch, then quote the insulation and any boarding together. Call, email, or fill out the contact form.
