Should loft insulation cover joists?

Author: Darlene Considine  |  Last update: Wednesday, May 7, 2025

Choosing loft insulation The first layer is laid between the joists – the horizontal beams that make up the floor of the loft – then another layer is laid at right angles to cover the joists and make the insulation up to the required depth.

Should attic insulation cover joists?

In unfinished attic spaces, insulate between and over the floor joists to seal off living spaces below.

Where not to put loft insulation?

One important thing to remember is that insulation should never be pushed right into the eaves because it will interfere with airflow. If you're going for a warm loft then you'll need much more insulation to be able to cover the underside of the roof along with the gable and party walls.

Should loft insulation go into the eaves?

For cold ventilated loft spaces, be sure to maintain the ventilation at the eaves. This is ensured by maintaining a minimum 25mm gap between the insulation and the membrane. Better still – terminate the loft insulation at the point where it meets the rafters (Fig. 26).

Should I leave a gap between insulation and floorboards?

Make sure you have a good 50mm clear space between insulation and boards or you'll get mould under the boards.

The the wrong and the right way to insulate a loft .

Is it OK to put insulation between floor joists?

Insulating in between your floor joists is one of the best ways to upgrade your home comfort and energy efficiency.

Does loft insulation need an air gap?

It's important to leave a ventilated air gap between the insulation and the boards to prevent condensation on the underside of the boards. Make sure you don't squash the mineral wool when you fit the boards on top, as this will reduce its insulation value.

Should loft insulation be in between joists?

Remember that insulating between the joists will create a cold roof space as the heat is kept underneath the insulation like a blanket. Therefore, if you require a warm space for your storage it's better to insulate between the rafters in the roof rather than the joists.

Where should you not put insulation in the attic?

Also, when laying down additional insulation, work from the perimeter toward the attic opening. Never lay insulation over recessed light fixtures or soffit vents. Keep all insulation at least 3 inches away from recessed "can" lights, unless they are rated IC (Insulated Ceiling).

Can you have too much loft insulation?

If insulation is installed too thickly or packed too tightly into the loft, it can block air flow and prevent moisture from escaping. This can lead to condensation and dampness, particularly in areas where moisture tends to accumulate, such as bathrooms, kitchens, and laundry rooms.

Should I remove old loft insulation before laying new?

Should I remove old loft insulation before laying new? Older insulation material will typically need to be removed before adding new insulation. However, in some cases the material may be in good condition, meaning you can simply top up the existing insulation material for a fraction of the cost of a full replacement.

Should I insulate the underside of my roof?

Do you have any plans for furnishing/fitting out your attic in the future? Then you could simply insulate the pitched roof. But the best solution is actually to insulate both the attic floor and the underside of the roof.

Should you board over loft insulation?

Boards for loft flooring can be installed on top of insulation using plywood, wood, or plastics. Installing loft flooring and boarding over insulation helps save space, prevent heat loss, and increase home value.

Is it OK to cover attic insulation with plywood?

Can I put plywood over insulation in the attic? Yes you can install plywood subflooring directly over the floor joists and insulation on your floor. You need to install rigid insulation boards on top of the attic flooring joists and then install plywood over the top.

Is it OK to walk on attic insulation?

The Golden Rule of Attic Safety

These are the bones of your attic, offering the firm support needed to bear your weight. Ignoring this rule and stepping on the drywall or insulation is a fast track to an unplanned ceiling inspection from below, thanks to our unforgiving friend, gravity.

Does attic insulation need a vapor barrier?

In Most Cases, Attic Insulation Does Not Need To Provide A Vapor Barrier. The good news is that blown-in insulation should not need a vapor barrier.

What is the rule for attic insulation?

Insulation is graded by its R-value—the higher the R-value, the greater the insulating power. If you live in a mild climate, your attic should have a minimum grade of R-38, or about 13–14 inches of insulation. If you live in a colder climate, R-49 is the minimum recommendation, or about 16–18 inches of insulation.

Can you put too much insulation in attic?

Too much insulation will leave humidity and moisture to accumulate in your attic space. This can lead to mold growth on your walls and insulation. There might also be water damage to the support beams in your home's roof.

Do you need 2 layers of insulation in attic?

Overall, the second layer of insulation plays a crucial role in improving insulation performance, reducing heat loss, and maintaining a comfortable and energy-efficient living space.

Should you insulate between rafters in a loft?

Insulating between rafters can help to improve the energy efficiency of your home or building by reducing heat loss through the roof. This can lead to lower energy bills and a more comfortable living or working environment.

Do you need an air gap between roof and insulation?

Yes, an air gap between the roof and insulation is needed, especially for traditional materials like fibreglass, mineral wool, and foil insulation.

How close to the eaves should loft insulation be?

(For more on walk boards, visit our 'How to work safely in the loft' article.) Lay the first loft roll at the eaves, leaving a 25mm gap away from the eaves for ventilation to help prevent condensation.

What happens if you compress loft insulation?

Squashing your insulation reduces its effectiveness by 50%

Numerous studies on the topic have indicated that there's a huge number of Britons who don't understand why doing this is so problematic. Mineral wool type insulation (the most common loft insulation) needs to retain its full depth in order to work effectively.

Do you need membrane under loft insulation?

A vapour barrier is a damp-proof membrane used to create a perfect seal between the surface you want to insulate and the rest of the loft. This should prevent condensation from the home rising into the roof which would end up degrading the insulating materials.

Previous article
How close can a stove be to a cabinet?
Next article
Why are four blades better than three?