Topping up loft insulation before boarding

If you are boarding your loft, it is the ideal time to check the insulation. Once boards are down, insulation is harder to reach. This guide explains what to look for and how boarding and insulation work together.

How much insulation is recommended?

The Energy Saving Trust recommends a minimum of 270mm of loft insulation, and advises not to squash mineral wool, as that reduces how well it insulates. It also advises leaving a small ventilation gap between the insulation and the boards to help prevent condensation.

Checking what you have

Measure the depth of insulation in a few places, as it is often uneven. Look for gaps, areas that have been pushed aside, and patches that have been squashed by stored items. Joists in many lofts are shallower than the recommended depth, so if the insulation is level with the top of the joists, it is probably below the recommendation.

How a top-up is laid

The Energy Saving Trust describes laying a first layer between the joists and a second layer at right angles across them to reach the required depth. That second layer sits above the joists, which is why boards cannot then be fixed straight on top without squashing it.

Boarding over deep insulation

Where insulation is deeper than the joists, raised loft boarding lifts the boards on supports, keeping the insulation at full depth with an air gap above. An alternative is to board only a small area near the hatch and leave the rest fully insulated and unboarded.

Ventilation

Topping up insulation keeps more heat in the house, so the loft above becomes colder. Good ventilation helps prevent condensation. Keep eaves vents clear, leave the gap above the insulation open, and fit an insulated, draught-sealed hatch to reduce warm, moist air getting in.

Tanks, pipes and cables

• Do not insulate directly under a cold water tank; the warmth from below helps protect it from frost. Insulate the tank and pipes instead.
• Check cables before covering them. Ask a qualified electrician if you are unsure.
• Keep walkways to the tank and any services you need to reach.

What we do

At the survey we measure the existing insulation and agree any top-up as part of the quote. We then top up, fit raised supports where needed and board over, keeping ventilation clear. See loft boarding and our cost guide.

Common questions

How deep should loft insulation be?

The Energy Saving Trust recommends a minimum of 270mm of loft insulation.

Can I board over loft insulation?

Yes, but not by squashing it. Where insulation is deeper than the joists, raised boarding keeps it at full depth.

Should I insulate under the water tank?

No. Leave the area under the tank uninsulated so warmth from below helps protect it, and insulate the tank and pipes instead.

Get a free quote

Areas we cover

Bromley | Orpington | Beckenham | Chislehurst | West Wickham | Hayes, Bromley | Petts Wood | Biggin Hill