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When installing loft insulation using batts or rolls it is essential that they are cut to size perfectly to achieve a fit that is safe and effective. This requires the insulation being tightly butted together to avoid thermal bridging occurring within the thermal envelope. Making a decision when it comes to cutting loft insulation rolls varies depending on your chosen material. There are a number of ways that loft rolls can be cut to size from specialised tools to simply using a bread knife.

What is thermal bridging?

Thermal bridging or cold bridges are weak points within the building envelope. These week points within the envelope allow heat to pass through the building and in some case escape. This is generally cause by a gap within the insulation layer. Thermal bridges can cause up to 30% of heat loss. The heat lost from the property will increase your monthly energy bills as a result.

In addition to the heat loss, the cooler areas within the property can cause condensation which could encourage mould growth. Mould growth can present health issues or damage to the property.

How to Cut Loft Insulation Rolls by Length

The majority of loft rolls are available partially cut in order for the rolls to fit between the joists without much cutting required. Although this is an option, some rolls are still 1200mm in length and are not partially cut. The easiest way the cut the length with minimal fuss is to simply keep the rolls within the original packaging and cut the rolls whilst they are tightly wrapped. Using a tape measure, measure the width of the roll whilst in the plastic and mark the plastic with the chosen width. Due to the compression of the rolls, adding a few cm to the measurement is recommended. For example, if the joists are 600mm in length, mark the plastic wrapped rolls at 620mm. Then using a long blade or fine-toothed saw, cut through the plastic. Turn the roll as you cut if required.

How to Cut Loft Insulation Rolls by Width

Use a Fibreglass material will blunt blades quickly, be sure to use a sharp knife for a more accurate and quicker cut.
When cutting the insulation, be sure to the cut the material slightly bigger than required. Fibreglass or mineral wool is very flexible and can be tucked into tighter spots if slightly too long. If too short, it will create an air gap which is when thermal bridging can occur. The extra length will also ensure the rolls will stay in place.

To cut the insulation use a sharp knife. Put a board on each side of the insulation, use the board on top as a guide to make a straight cut. Run the knife along the edge of the board. If you need to cut the rolls for trickier spots such as around pipes, fixtures, or fittings, simply use a large pair of scissors. You can also use a spirit level or a ruler to ensure an accurate cut

If using a smaller blade, hold at an angle and keep running the blade along the same cut until detached.

See how to install Loft insulation rolls in the video below.

Tips for Cutting Loft Roll Insulation

Protective Wear

Loft insulation materials are known for their rough texture that can cause irritation to the skin eyes, and airways. Wearing protective gear such as a long length clothing, dusts masks, and safety goggles, is recommended. This is to avoid inhaling the fibre produced when cutting the roll and to prevent the fibres getting into your eyes.

Utilise The Packaging

Only remove the packaging of the rolls you wish to fit. Cut the rolls to size and install and then move onto the next roll. If all the rolls are removed from compressed packaging materials at once, it will reduce the space within the already confined area. It will also make the installation process longer and more difficult. When unrolling the required package, unroll just enough to make the necessary cut.

Fluff It Up

Once cut, lifting the roll and giving a slight shake will fluff out the material to fill the joist depth and width. The plumped-up roll will achieve a better fit and therefore better-quality performance.

Get the most out of the roll

By fitting the roll the opposite way that it has been packaged in will help to achieve the maximum length of the roll.

Use a Board

When installing loft insulation be sure to use a board and place it across the joists to kneel on when installing loft insulation. This will reduce the risk of damage caused to the ceiling below, or worse going through the ceiling altogether.

Get into Every Corner

If the insulation space is tight, use a long object such as a broom to push and tuck the insulation roll into the joists. Lay the insulation rolls tightly together and go right to the edge of the loft space. This will maximise efficiency.

Keep existing Layers

When installing new insulation, removing your old insulation isn’t always necessary. Most insulation will retain its thermal properties. By keeping the existing layer, you will still benefit from the thermal quality whilst also having an additional layer of insulation on top.

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