Topping up is the cheapest way to improve a loft. You roll a new layer across the joists, the depth goes up and the bedrooms below get warmer. Most of the time it is exactly the right thing to do.
But in a good number of the lofts we visit, what is already up there is the problem. Laying fresh insulation over it hides that problem for a while and then lets it carry on underneath. This guide explains how to tell which kind of loft you have.
The Quick Test
Go up with a torch and look at the existing insulation in several places, not just near the hatch. If you can answer yes to all of these, topping up is usually fine:
- It is dry: no dark stains, clumping or damp feel.
- It is clean: not grey with dust, and no droppings, tunnels or nests.
- It smells of nothing: no mustiness, mould or ammonia.
- It is evenly spread: no big bare patches, and not crushed flat under old storage.
- It is ordinary rolls or level loose-fill: not pebble-like granules and not foam stuck to the roof.
If any answer is no, read on.
When You Can Lay New Insulation Over the Old
Clean, dry mineral wool that is simply too thin is the ideal candidate for a top-up. The usual approach is to make sure the spaces between the joists are filled, then lay a second layer across the top of the joists, at right angles to the first, to reach a total of around 270mm. Running the top layer across the joists covers the timber and the joins in the lower layer, which is where a lot of heat escapes.
Before you start, fill any bare patches around the hatch, tank and light fittings, pull insulation back from the eaves so air can still get in, and lift cables so they sit on top of the new layer rather than underneath it. Our guides to the eaves and to downlights, cables and water tanks explain why.
When the Old Insulation Must Come Out First
| Problem with the old layer | Why topping up fails | What to do |
|---|---|---|
| Damp or water-stained | Moisture is trapped under the new layer and spreads into it | Fix the leak or condensation, remove the wet insulation, refill when dry |
| Mould | Mould keeps growing in the dark, damp layer beneath | Remove, find the cause (often ventilation), refill |
| Rat or mouse droppings and urine | Smell and contamination stay under the new roll and return on warm days | Pest control first, then strip out, disinfect and refill |
| Heavily dusty or grey | Dust keeps falling through the hatch and the old layer underperforms | Remove and refill |
| Crushed flat | You pay for new depth on top of old depth that is barely working | Usually remove; a light top-up only if the old layer is otherwise good |
| Clearing for boarding, a conversion or roof work | The joists need to be clear for the work | Removal only, then refill as the work allows |
Rodent-soiled insulation is the clearest case
Once rats or mice have been living in a loft, the insulation is usually soiled through. It cannot be cleaned where it lies. Rolling a new layer over it leaves droppings, urine and sometimes dead rodents underneath, and the smell tends to come back every time the loft warms up. The soiled material has to be stripped out, the loft disinfected and the entry holes sealed before new insulation goes in. Our page on rodent-damaged insulation covers the process.
Stop and Check: Vermiculite and Spray Foam
The Hidden Benefit of Starting From Clear Joists
Removing the old layer is more work, but it gives you something a top-up never can: a clear view of the ceiling, the joists and the underside of the roof. With the insulation out, you can see staining that points to a leak, gaps where cold air comes in around pipes and cables, entry holes that rodents have been using, and any timber that needs attention. New insulation then goes onto a clean, dry, sound loft and performs at its full depth.
Removing It Yourself or Getting It Done
Clearing a loft is dusty, physical work in a cramped space, and a typical loft fills far more bags than people expect. Some homeowners are happy to do it themselves; many would rather not. Our step-by-step guide to how to remove loft insulation explains the kit, the method and the disposal rules, so you can decide.
If you would rather hand it over, our loft insulation removal service bags everything in the loft, carries it out under dust sheets, vacuums the loft back to the joists and takes the waste away. We can then lay new insulation to 270mm as part of the same job, covered on our insulation replacement page.
Frequently Asked Questions
Can you put new loft insulation on top of old?
Yes, if the existing insulation is clean, dry, evenly spread and in decent condition. Lay the new layer at right angles across the joists. If the old layer is damp, soiled, mouldy or badly flattened, remove it first.
Is it worth removing old loft insulation?
It is when the old insulation is not doing its job or is causing a problem: damp, mould, droppings, heavy dust or compression. Removing it lets you check the ceiling and timbers, fix leaks and lay new insulation that performs at its full depth.
Does old loft insulation need to be removed for a loft conversion?
Usually yes. A conversion moves the insulation from the ceiling joists to the roof slope and new floor, so the old layer between the joists normally comes out to give the builder a clear, clean floor to work from.
Will topping up over damp insulation work?
No. The damp stays trapped under the new layer, the new insulation gets wet too, and the timbers and plasterboard stay damp. Find and fix the cause, remove the wet insulation and let the loft dry before laying new.
Can I mix different types of insulation?
Generally yes. A new mineral wool roll can go over older mineral wool, or over loose-fill that is level and in good condition. Avoid laying anything over vermiculite or spray foam until it has been checked.
Related Reading
Need a Hand With Your Loft?
Send a couple of photos from the loft hatch and we will tell you honestly whether yours can be topped up or needs to come out, with a written price either way.
Or email info@loftrestore.co.uk. Based in Cheshire, working across the UK.