How much loft insulation do you need?

If you’re thinking of insulating your loft, you will need to purchase enough insulation for the job. But how much is enough and how do you work this out?

How to calculate how much insulation you need for a loft

Calculating how much insulation you need is an important first step. You don’t want to over or under-purchase and find that you’ve spent more money than you needed to or have to return to buy more. But first, you need to decide how thick you want the insulation to be.

How thick should loft insulation be?

The minimum recommended thickness of loft insulation has differed greatly over the years. In the 1980s, it was quite standard to install any thickness from 25 to 50 mm. Then, building regulations stated that homes should have as much as 100 mm of loft insulation. This gradually increased to 200 mm and now stands at 270 mm for new builds and is recommended for other properties (but not essential). This is why you must check how much insulation you currently have in your loft before purchasing more. You may find that you have anywhere from 25 to 200 mm up there, and knowing this and the insulation values will help to determine how much more you need to purchase. Some modern materials can provide the same thermal insulation whilst not being as thick.

When checking how much insulation you currently have, you must check whether the insulation fills the gap between joists or whether it comes above the joist. Additional insulation may even be laid over the top of the joists (this is more common in newer homes) so be sure to conduct a thorough check of what you have up there. The depth of the joists in new builds is usually 100 mm as an example.

Increasing the thickness of the insulation in your home could save you money each year on your heating bills. By taking your insulation from 50 mm up to 270 mm, for example, you could save nearly £50. A home that is sufficiently insulated will reduce the heat loss in your home,, meaning your gas boiler doesn’t need to work as hard to bring it up to the set temperature and keep it there. This is why it’s always worth investing in insulation, as the overall cost of heating your home will come down.

Now that you know how thick your insulation should be, it’s time to take some measurements to work out how many rolls you need to buy if you wanted to take this up to 270mm (rolls can vary in size and thickness). First, you should measure the thickness of your current insulation and work out how much you will need to take the depth to 270 mm. For example, if the loft insulation is currently 100 mm, you’ll need an additional 170 mm to bring it to the total. It may be that you need more than one layer of insulation to get the correct depth. For instance, if the depth is 70 mm, you would require either one layer of insulation at 200 mm or two layers at 100 mm.

Next, you should measure the total area of your loft. This can be done by measuring both the length and width of the space in metres and multiply the length by the width, which will give you the square metre area (m2) of your loft. It may be easier for two people to complete this task.

Now you have your measurements, you can work out how much insulation to buy. Use our handy table below to do this.

*Please note, the below table is based on a standard roll size of 1.14 metres wide by 5.68 metres long and with a thickness of 170 mm. Rolls can vary in length, so do check before buying.

Rolls of insulation are quite large and bulky, and so it’s much easier to have them delivered instead of going to pick them up, unless you have a very large car or van.

How much is loft insulation?

The cost of insulation will depend on how much you need, whether you’re installing it yourself and whether you’ve got a  grant for loft insulation.

Typically, one roll of insulation costs around £20* estimated. Use our table below to see how much your insulation could cost you depending on how many rolls you need.

*Prices at 22/10/2020

Remember, if the insulation is being installed by someone else, you should add around £300 to the total above. When it comes to getting a quote, you should shop around and contact a few different people to make sure you’re not paying over the odds.

Can you over insulate a house?

Above, we’ve provided the minimum thickness that your insulation should be to ensure that your home stays nice and toasty in winter. But is it possible to over insulate?

We’d recommend everything in moderation. While some people argue that an over-insulated house could cause damp, mould and other issues. Typically, upping your loft insulation from 270 mm to 400 mm isn’t really going to make a big difference, and so it’s probably worth sticking with the lesser amount. Once you’ve insulated your loft, you should have a look at other areas where your house could be losing heat, including the windows, doors, walls and floors. Rather than over insulating one part of the house, it’s best to spread a moderate amount all over. If you have worries about this then you should contact an insulation expert.