How to Insulate a Loft

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Planning & preparation

Loft insulation is an effective way to reduce heat loss and save on your fuel bills. Here we will show you how to fit loft insulation rolls, as well as how to insulate pipes and cold-water tanks
This guide is based on installing glass mineral loft insulation wool rolls. The minimum recommended depth of this type of insulation material is 270mm. This equates to a Thermal Resistance R-Value of 6.1
If your loft has existing loft insulation in between the joists, measure the depth of it. Ideally, this base layer needs to have a depth of 100mm and come up to no more than 25mm above or below the joist. A second layer of at least 170mm can then be laid over the top of the joists to get you to the recommended building regulations guide of 270-300mm
The gap between joists is most commonly 400mm or 600mm, so loft insulation rolls tend to be available in a width of approximately 1140mm. Most come with pre-cut perforations so they can easily be cut into halves or thirds
Remove any stored items from the loft so you can work in a clean and uncluttered space. Try to work in good lighting and use a portable battery powered light if necessary
Measure your cold-water tank so that you can fit an appropriate tank jacket. You'll need to insulate tanks and pipes before you start to lay the loft insulation
To calculate how many rolls of insulation you need, work out the size of the area in m2 (length x width = number of metres squared) and check the coverage of the rolls. You can deduct around 10% to allow for ceiling joists

Do it right

Before starting, make sure your loft has ventilation gaps at the eaves or is fitted with loft ventilation. Be sure to maintain this ventilation gap during and after installation
If you want to use the loft space to store items, install raised floor loft legs that you can fix boards to, above the insulation. Leave a minimum air gap of 50mm beneath the boards. Alternatively, cover an existing boarded area with insulation boards rather than insulation rolls
Insulation shouldn't be laid over any electric cables. Ideally, cables should be clipped to the roof joists or gently lifted out of the way and then laid over the insulation once it is in position
Do not fit insulation underneath the water tank unless it is in an elevated position

Staying safe

Turn off the power at the mains consumer unit before touching any cables
Always move slowly and carefully in the loft and work from ‘crawling boards’, moving them along as you go. Never stand between joists and avoid standing directly on joists as much as possible
It’s generally recommended to leave a 75mm gap between the insulation and any hot chimneys, flues, or recessed light fittings, but be sure to check the manufacturer’s instructions
Wear safety goggles, a suitable face mask and gloves when fitting your insulation

Aftercare

Avoid applying weight to, or storing items on the insulation, as this will reduce its efficiency

Step by step

Start by lining your cold-water tank with the jacket, following the manufacturer's instructions.

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Next, insulate pipes, using a snap off knife to cut the insulation to size.

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Secure the pipe insulation with duct tape.

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Insulation shouldn't be laid over any electric cables. Where possible, clip them to the roof joists or gently lift them out of the way whilst you install, before laying them over the insulation when it’s in position.

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If you have recessed lights, now’s the time to install down light protectors, being sure to follow the manufacturer’s instructions.

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Once the insulation roll is in position, measure the width of roll you need to fit between the joists; this tends to be either 400mm or 600mm. Then, cut the roll to size using a panel saw or large bladed kitchen knife.

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Position the crawling boards in the furthest corner of the loft. You’ll need to start laying with a minimum gap of 25mm between the eaves and the insulation to ensure a suitable air gap. Maintain this gap along two opposite sides of the loft.

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Make sure that the wall plate is covered, then roll out the insulation, moving backwards so you are working towards the centre of the loft to avoid putting your weight directly onto the insulation.

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Gently push the insulation so it fits snuggly between the joists, but don't compress it as that will reduce its efficiency.

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When you reach a cross beam, use a straight edge or metal ruler and a snap off knife to cut the insulation to fit.

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To join two lengths of insulation, simply butt them up tightly.

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Continue to lay until the first layer is in position, cutting the insulation as required to fit any awkward spaces.

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To reach the recommended loft insulation depth of between 270-300mm, a second layer can be added. Simply unwrap the insulation and lay at 90-degrees to the first layer and ceiling joists.