How to Insulate Your Loft
One of the best investments you can make in your home. Proper loft insulation can save you around 300 per year on heating bills and pays for itself within two years.
Difficulty
Medium
Time
1-2 days
Cost
200-400
Safety First
Always wear a dust mask (FFP3 rated), safety goggles, gloves and long sleeves when handling insulation. Mineral wool fibres irritate skin, eyes and lungs. Lay boards across the joists to walk on. Never step between joists or you will go through the ceiling. If your loft has old wiring, get an electrician to check it before you start. Watch out for wasps nests in summer.
Loft insulation is one of the simplest and most cost-effective energy improvements you can make to your home. Around 25 per cent of heat is lost through the roof in an uninsulated house. The government recommends a minimum thickness of 270mm of mineral wool. If your existing insulation is less than 100mm, topping it up is a no-brainer. Most homeowners can do this themselves in a weekend, though spray foam insulation should always be done by a professional.
What You Will Need
Tools
- - Dust mask (FFP3)
- - Safety goggles
- - Heavy-duty gloves
- - Tape measure
- - Stanley knife or insulation saw
- - Loft boards (to walk on)
- - Head torch or inspection lamp
- - Staple gun (for vapour barrier if needed)
Materials
- - Mineral wool rolls (270mm total thickness)
- - Second layer rolls (if topping up)
- - Loft legs and boards (if you want storage)
- - Pipe lagging for water pipes
- - Jacket for cold water tank
- - Draught excluder for loft hatch
- - Vapour barrier (if not already present)
Step-by-Step Instructions
Clear the loft
Remove any stored items and lay boards across the joists to create a safe walkway. Set up a good light source. Check for any electrical cables running between joists and clip them to the side of the joist so they sit above the insulation.
Measure existing insulation
If you already have some insulation, measure its depth. Anything under 100mm is worth topping up to the full 270mm. If you have nothing, you will lay two layers: 100mm between the joists and 170mm across the top at right angles.
Choose your insulation type
Mineral wool rolls are the cheapest and easiest for DIY (about 6-8 per roll from B&Q or Wickes). Sheep wool is more pleasant to handle but costs more. Rigid boards are better for limited headroom. Spray foam is the most effective but must be professionally installed and can affect your mortgage valuation.
Lay the first layer between joists
Unroll the mineral wool between the joists, cutting to fit with a sharp knife. Do not compress it. It needs to be fluffy to trap air. Tuck it snugly against each joist but do not force it. Cut around pipes and cables rather than laying over them.
Add the second layer across the top
Lay the second layer at right angles across the tops of the joists. This eliminates cold bridges through the timber. Use 170mm rolls to bring the total to 270mm. Again, do not compress it.
Insulate pipes and tanks
Any water pipes in the loft must be lagged with foam pipe insulation. Fit an insulation jacket to your cold water tank if you have one. Do NOT insulate under the cold water tank as it needs some heat from below to prevent freezing.
Draught-proof the loft hatch
Fit draught-seal strip around the loft hatch frame. Consider adding a piece of insulation to the top of the hatch itself. This is often the single biggest air leak in the loft.
Ventilation check
Make sure you do not block the eaves ventilation. There should be a gap at the eaves where air enters the loft. This prevents condensation building up. Use eaves ventilator trays if needed to maintain airflow where the insulation meets the rafters.
Cost Estimate
DIY mineral wool insulation for a typical 3-bed semi costs around 200-300 in materials. Professional installation runs 300-600 but you may qualify for the Great British Insulation Scheme which can cover the full cost if you are on certain benefits or in council tax bands A-D.
Roll vs Board
Mineral wool rolls are best for open lofts with regular joist spacing. Rigid PIR boards (like Celotex or Kingspan) are thinner for the same insulation value, so they are better if you want to keep usable loft space. A 100mm PIR board insulates the same as 170mm of mineral wool.
Loft Storage Tip
If you want to use your loft for storage after insulating, install loft legs that raise the boarding above the insulation. Never compress insulation by laying boards directly on top. Compressed insulation loses most of its effectiveness.
Common Mistakes to Avoid
- - Compressing insulation by putting boards directly on top (halves its effectiveness)
- - Blocking eaves ventilation (causes condensation, damp and mould)
- - Insulating under the cold water tank (the tank needs heat from below to prevent freezing)
- - Leaving gaps around pipes and cables (cold bridges let heat escape)
- - Using spray foam without checking your mortgage lender accepts it (many do not)
- - Forgetting to insulate the loft hatch (one of the biggest heat loss points)
When to Call a Professional
Call a professional if you have spray foam requirements, flat roof areas that need insulating from above, or if your loft has a lot of old or unsafe wiring. Also get a professional if your roof has any signs of damp or damage that needs fixing first. If you qualify for government grants, a registered installer must do the work.