Loft Insulation

Mineral Wool Between Ceiling Joists

If your home has an accessible attic, insulating your loft could be a simple and cost-effective job. You should aim for a 300mm thick blanket of insulation, installed in two layers - the first between the ceiling joists and a second laid perpendicular over the top. Many homes already have loft insulation between the joists but this is often only 100mm thick, in which case adding a second layer will provide significant additional benefits.

Knauf Loft Roll is perfect for this job. Two 150mm thick layers of this will achieve the Building Regs stipulated U-value of 0.15. Click here to purchase on Amazon.

Wool-type insulation is best for this job because it’s flexible so can be pressed into awkward spaces without leaving gaps. Wool insulation is available in several varieties including natural materials such as sheep’s wool and hemp, but the most common are mineral or glass wools which are both cheap and effective.

There’s a “shopping list” at the bottom of the page with links to recommended products.

Before You Start…

Access - If you don’t have a fixed ladder to get into the attic it’s worth investing in one because you’ll be up and down a lot. In the attic, avoid stepping on the ceiling below or you might well put your foot through it. A few MDF panels will come in handy as moveable walkways.
Lighting - If you don’t have good lighting in the attic perhaps now’s the time to upgrade? Battery powered lamps suspended from the ceiling can also be good but you’ll struggle if you rely on torchlight.
Electrics - Many attics have old wiring which could be a fire hazard if covered with insulation. If in any doubt, consult an electrician before proceeding.
Obstructions - It’s very easy to bang your head on a roof timber or trip on an obstacle. It’s worth wearing head protection and possibly wrapping any particularly in-the-way timbers in padding.
Dust - The average attic has years of accumulated dust. It’s worth wearing a face mask and hoovering the attic at the earliest opportunity to reduce the amount that ends up in your lungs.

Step 1 - Preparation

Before starting you’ll need to remove any items stored in the attic and tidy up. It’s often worth hoovering at this point to reduce the amount of dust you’ll inhale during the project.

Take stock of any of the following features -

Ceiling joists - Measure the distance between them so you can purchase the right width insulation rolls (these are usually 600mm apart).
Downlighters - The holes for these lights are rarely airtight and will need to be sealed to prevent air leakage through your insulation. If you don’t already have LED bulbs it’s worth upgrading because these are more energy efficient and will generate less heat.
Water tanks - Insulation should pass over any cold water tanks rather than under, otherwise these could be at risk of freezing during the winter. Both hot and cold water tanks should be wrapped with thermal jackets (“lagging”) if not already.
Vents - Roofs are often ventilated at the eaves. If this is the case, you will need to ensure that the vents aren’t blocked during the works otherwise you could get damp issues in the attic.

Step 2 - Cover Lights

If you tightly wrap a light fixture with insulation you will trap the heat and potentially create a fire hazard, especially if you still have older filament bulbs.

Rather than leaving gaps in the insulation around downlights, covers can be used to avoid contact between the light and insulation. The covers also make it easier to seal around the lights in the next step.

Downlight covers are sold as Thermahood. We strongly recommend installing these over all downlights before proceeding any further.

Step 3 - Seal Gaps

Wool insulation isn’t airtight, and it won’t perform as well if warm air from your home can simply pass through it. Therefore, before insulating it’s important to make the ceiling below as airtight as possible.

Some installers recommend installing a membrane across the whole floor of the attic. A simpler alternative is to seal any gaps with flexible sealant. Smaller gaps can be sealed with decorator’s caulk which is available for as little as £3 per tube. Larger gaps can be filled with expanding foam. We like the Soudal Genius range which come with built-in application nozzle and are perfect for smaller jobs.

Areas you might find gaps include:

Around downlights
Around water pipes or electrical cabling
At the junctions between timber/masonry/ceiling
Around access hatches

Remember you’re only trying to seal gaps underneath where the insulation will go - it’s not necessary (or wise) to seal gaps in the eaves or roof which are important for ventilating the attic.

Step 4 - Eaves Ventilation

After insulating, your attic will be colder than before because less heat will be transmitted through the ceiling below. This increases the risk of damp and rot, so it’s essential to ensure ventilation is maintained.

The roof space is usually ventilated by air gaps at the eaves (the bottom corner where the roof overhangs the brick walls). We recommend installing eaves vent trays so you don’t accidentally block the eaves ventilation when you install the insulation.

Eaves vent packs are available in several forms but most are ridged plastic sheets which form several air channels between the insulation and the roof.

A simpler alternative is simply to stop the insulation short of the eaves gaps, but you could be left with a cold bridge at the edge strip unless you’re careful.

Step 5 - Tank and Pipe Lagging

It's important to insulate any water tanks and their connected piping. Once the attic is insulated it will be much colder in the winter. This could cause cold water tanks to freeze, and means hot water tanks will lose more heat than before. Both types should be wrapped with a “lagging jacket” before proceeding. Connected pipework should also be wrapped with foam insulation.

In the following steps, if you have a cold water tank you should insulate around it and up the sides, but not underneath it. This allows a bit of heat through the ceiling to prevent freezing. Hot water tanks aren’t likely to freeze so you can insulate all around them.

Loft insulation - water tank and pipework lagging

Step 6 - Hatch Insulation

It would be a shame to go to all this effort and still have cold draughts from the loft hatch.

A 75mm sheet of rigid foam insulation cut to size and glued to the top of the hatch will bring it up to the same standard as the rest of the attic. Don’t forget to also seal around the edges with draughtproofing strips to make it as airtight as possible.

Loft insulation - hatch insulation and draughtproofing

Step 7 - First Layer of Insulation

Once lights are covered, gaps are sealed, tanks are lagged, and your ventilation is protected, you’re ready to insulate.

300mm of loft insulation is typically required to meet Building Regs requirements, but you should check the thermal properties of whichever product you purchase to ensure it achieves the required U-value.

Usually two layers of are used - the first is placed between the ceiling joists and the second over the top at 90 degrees. The thickness of the first layer should therefore match the depth of the ceiling joists. If the joists are 200mm deep you'd want 200mm thick insulation for the first layer and 100mm for the second layer to make 300mm in total.

The first layer should also be the right width for your joist spacing. If you can’t find the exact width don’t worry - buy the insulation too wide and squash it into the gaps. For any particularly narrow spaces the insulation can be cut (it’s easiest to saw the whole roll while it’s still in the packaging.)

When installing, be careful not to leave any gaps around the edges. Allow the insulation to “puff out” to its full thickness. It’ll be more effective the more air is contained in it.

Step 8 - Second Layer of Insulation

The next layer of insulation goes on top of, and perpendicular to the joists. This arrangement makes gaps less likely.

At this stage you’ll almost certainly have to cut some of the insulation rolls to fit around roof timbers and other obstructions but again focus on filling all gaps.

Step 9 - Finishing Touches

If you still need to access the attic, or if you use it for storage, you’ll probably want to build a walkway/platform above the insulation. If you don’t have convenient roof timbers to support the platform you can use Loft Legs as supports. There are little stilts that are screwed to the ceiling joists (before you install the insulation, of course!) to which sheets of flooring can be attached.

The loft legs should be aligned with the insulation joints to avoid needing to cut holes. The image shows the insulation transparent so the arrangement is clear.