Is it worth topping up loft insulation that is already there?

A great many UK lofts still have the 100mm that was fitted decades ago, laid between the joists and stopping level with the tops of them. That is a substantial improvement over nothing and a long way short of current practice. The question is not whether more is better — it is — but whether the extra depth is worth the day and the cost, and that depends on what is already up there.

Safety

What you'll need

• Mineral wool rolls of the right thickness
• Particle mask, goggles, gloves, long sleeves
• Crawl boards
• Head torch
• Sharp knife or a long blade
• Rigid eaves trays (optional)
• Loft caps for downlights (optional)

Step by step

1. Measure what is there.Push a rule down through the existing layer to the ceiling in several places. Depths vary hugely across a loft, and a loft that is 250mm over the middle is often 40mm at the edges where somebody has walked on it.
2. Identify the existing material before touching it.Mineral wool in rolls is straightforward. Loose granular fill, especially anything grey and vermiculite-like in an older house, must not be disturbed until it has been identified — this is an asbestos question and it is answered by a licensed assessment, not by a look.
3. Understand the diminishing returns honestly.Each additional layer resists a smaller share of the remaining heat flow. Nothing to 100mm is transformative; 100mm to the current recommended depth is worthwhile but much less dramatic; beyond that the gains become small. Check the current recommended depth and any current funding against up-to-date guidance rather than a published figure.
4. Note the reason that is not about depth.The existing layer stops at the tops of the joists, so every joist is a timber cold bridge straight to the ceiling. Cross-laying a second layer over the joists eliminates those, which is why ghosting lines on a ceiling disappear after a top-up.
5. Lay the new layer at right angles to the old one.Across the joists, not between them. That is what covers the timber. Butt the rolls tightly against each other with no gaps at the ends.
6. Do not compress it.Insulation works because of the still air trapped in it. Squashing it — by walking on it, by boarding straight onto the joists, by stuffing it into a tight space — removes most of its value.
7. Keep the eaves clear.Stop the insulation short of the eaves so the ventilation path from the soffit into the roof void stays open, and fit rigid eaves trays if the depth would otherwise choke it. Blocking this is the classic cause of a loft that streams with condensation.
8. Deal with the electrics and the downlights.Lift cables above the new layer rather than burying them, because a buried cable runs hot and its safe rating drops. Recessed downlights need either a fitting rated to be covered or a proprietary loft cap — never insulation packed around them.
9. Do the tank and the pipes at the same time.Insulation goes round the sides and over the lid of a cold water tank and never underneath it, because the heat rising through that patch of ceiling is what stops it freezing. Lag the pipes in the loft while you are up there.
10. Do the hatch too.A loose uninsulated hatch is a hole in the middle of the surface you have just improved, and it leaks warm moist air into the loft continuously.

Tips

• Buy the insulation in the thickness that makes up your shortfall in one layer rather than stacking several thin ones — fewer joints, less handling, and less to compress.
• Do it on a cool day. A loft in summer is genuinely dangerous to work in for any length of time.
• If you need storage as well, plan the boarded area first on raised loft legs above the full insulation depth, and insulate around it. Boards laid straight on the joists destroy the value of what you have just paid for.

Common mistakes

• Laying the second layer between the joists again — It leaves every joist uncovered, so the timber continues to conduct heat straight to the ceiling. Cross-laying is what makes the second layer worth doing.
• Pushing insulation hard into the eaves to get right to the edge — It blocks the ventilation path from the soffit, and the roof void then collects the moisture that leaks up from the house. Condensation, damp timbers and rotting rafter feet follow.
• Boarding over the top for storage — Boards on the joists compress the insulation to joist depth. Raised loft legs are the answer and they are not expensive.

Questions people ask