Can You Have Too Much Loft Insulation?
“Can you have too much loft insulation?” is a fair question once people hear about 270mm targets. Here is a plain answer for UK cold lofts.
Quick answer
For a typical cold loft with mineral wool between and over joists, aiming around the commonly cited 270mm depth is the usual target for energy performance — see our 270mm loft guide. Piling on much more still adds some benefit, but with diminishing returns, extra cost, and risks if you compress the wool, block ventilation, or bury services and lights unsafely. More is not always better if it is badly detailed.
Diminishing returns
The first loft insulation you add saves the most heat. Each extra chunk of thickness helps less than the one before. That is why topping up a bare loft is usually excellent value, while doubling an already deep quilt is less dramatic. We will not invent £ savings — check the Energy Saving Trust for independent figures.
When “extra” becomes a problem
- Blocked eaves vents — moisture risk in the roof.
- Compressed wool under boards or stored boxes — thickness lost where you squash it.
- Downlights and cables — follow fitting and wiring rules; see our downlights safety guide once you need it.
- Hatches and tanks — insulate the hatch and lag tanks/pipes as advised; do not trap moisture against cold tanks without following good practice.
What to buy
Measure the existing depth, then top up with loft roll (e.g. Knauf Loft Roll 44) to reach the target. Use how many rolls you need and the step-by-step loft guide.
Frequently asked questions
Is 400mm better than 270mm?
It can reduce heat loss a little more, but check depth at eaves, hatch clearances and ventilation before buying a huge extra layer.
Should I remove old wool first?
Only if it is wet, damaged or contaminated. Dry, even wool can usually stay and be topped up.
Do you install insulation, do surveys or help with grants?
No. Insulation Warehouse Direct sells insulation materials online. We do not install insulation, carry out surveys or handle grant applications. If you want it fitted, use a qualified installer, and for grants check the official GOV.UK energy schemes. We are happy to supply the materials your installer specifies.
| Where | What we deliver | Delivery charge |
|---|---|---|
| England | All ranges | Free on insulation, plasterboard and drylining from £400 ex VAT. Celotex: £400 ex VAT minimum, free delivery. Access panels: £15 + VAT under £300 ex VAT, free from £300 |
| Mainland Wales and mainland Scotland | British Gypsum (including Gypframe), Celotex, Knauf and access panels | £400 ex VAT minimum plus £40 + VAT (access panels: £15 + VAT under £300 ex VAT, free from £300) |
| Scottish islands | Not delivered | – |
| Northern Ireland | Not delivered | – |
Company: Bay Drywall and Insulation Supplies Ltd (company no. 14473556), Unit 3 Babbage Court, Totnes Industrial Estate, Totnes, Devon TQ9 5JE.