Insulating Your Loft Joists: a Step-by-Step Guide

As we mentioned in our Introduction, there are two main ways to insulate your loft: one is via the rafters and the other is by insulating your joists. Insulating the joists is often the easier of the options and especially makes sense if you have no real plans to use the loft space.

Key points before you start

If your loft is easy to access, the insulating process should be pretty straightforward and can be undertaken as a DIY job. However, if you have any doubt in your own ability to carry out the work, we recommend getting a professional to do it.

Normally, people use mineral wool (either glass fibre or Rockwool) to do the job and if you intend to do the same it is imperative you wear protective clothing, goggles and a face mask, since the wool is an irritant.

You can use sheep wool insulation, which is much nicer to handle, 100% sustainable and actually is more breathable than the other types of wool insulation. If you are happy to pay a little extra we really recommend using this.

As a final warning, never stand between joists otherwise you will more than likely come through the ceiling which is never ideal. Using a board supported by several joists is the best way of working in the loft area regardless of whether you are insulating the joists or the rafters.

Preparing the loft space ready to insulate

Before you get to the business end of installing loft insulation up in your roof, you need to make sure you have prepared the loft space and also bought the right quantity of materials to do the job. Below is a quick step-by-step guide on what you should do before you begin any swork – but before this a quick word of warning!

Please remember not to step in between the joists – otherwise you will end up falling through the loft! Make sure you only stand of the joists themselves, ideally using a plank as a kneeling board.

A kneeling board should straddle several joists, thereby spreading the weight more evenly and reducing the load on the joist structure. The board should go across at least 3 joists to ensure it spreads the weight sufficiently. We also recommend covering your skin with suitable clothing and using a face mask, as stray fibreglass can act as an irritant if it comes into contact with skin.

Before installing the loft insulation:

    • Clean the space between the joists by vacuuming between them, removing any dust that may have settled there over time.
    • Make sure you pay attention to any cracks, dry rot or damage to the joists. If the are looks unsafe to work in, stop what you are doing and immediately seek professional help to repair these areas.
    • Ensure good lighting in the loft to provide decent visibility to help you do the work.
    • If you have boards fixed to your joists you need to remove these before you begin insulating the loft space.

Once you have the loft space ready to insulation we recommend doing a quick equipment check – below is a list of all the equipment you will need:

  • Tough pair of scissors to cut the loft insulationSafety Glasses
  • Measuring tape
  • Protective overalls and gloves when handling mineral wool (otherwise it is itchy!)
  • Safety goggles
  • A disposable mask

The protective overalls, gloves, goggles and mask are really worth using – the mineral wool insulation most people tend to use to insulate the loft is incredibly itchy, so making sure your skin is covered is a good way of avoiding this.

Using sheep wool insulation is another way to get around the itch issue.

Measuring the size of your loft

After you have cleared bulky objects you can see the area more clearly on what you can insulate. When measuring your loft space you need to start with the following:

    • Take a measuring device and measure the whole area of your loft (this is simply the width multiplied by the length).
    • Take into consideration the thickness you want to insulate to and if there is any pre-existing insulation in place, we recommend using 300mm if using wool insulation and 150mm if using the rigid insulating boards like Celotex or Kingspan.
    • Measure the width of your joists – for example our wool insulation comes in two widths 380mm and 570mm – the idea is you go for the one that is closest to width of your joists so it minimises cutting.
    • Measure the height of your joists – normally this is about 100mm. This means that if you are looking to insulate to a depth of 300mm in total, you will first lay down a layer of 100mm thick wool between the joists, then use 200mm thick wool laid across the joists at a 90 degree angle.

Write down these measurements on a piece of paper and then take them with you to a DIY shop to buy the insulation or visit an online retailer. Just a word of warning: insulating wool is really bulky and so unless you have a huge car, you might be better off having it delivered directly to your home.

Installing loft insulation between the joists

Insulating the loft at joist level is actually pretty easy to do as a DIY job, but if you feel uncomfortable doing the work then please call in a professional. Since different insulation products have slightly different insulating properties, building regulations use a U-value that needs to be attained to conform. This allows you to calculate the thickness of your chosen insulation product required to conform with building regulations, for example if you opt to use mineral wool, you will need to use 270mm-thick insulation to reach the 0.16 U-value specified in building regs.

To insulate your loft at joist level, please follow the steps below:

STEP 1: The first thing to do is to check whether there are light fittings that protrude between the joists (e.g. a GU10 spotlight will always have the fitting protruding into the loft), if there are, these will require capping prior to laying any insulation. These caps, sometimes referred to as downlight fire hoods or insulation guards, allow sufficient space around the light fitting for the heat to dissipate to stop the light getting too warm. Each light fitting will require a cap to prevent this overheating – they are simply placed over the light fitting and then you are good to go with the insulation.

STEP 2: The first layer of wool insulation needs to be laid between the joists, so you need to measure the distance between the joists to ensure you can get the right width of product. Typically the gap between the joists is either 380mm or 570mm so you will need a width of insulation similar to this, so it can fit snugly between the joists. Most insulation you buy will be partially perforated, allowing you to cut the roll of insulation easily to produce either 2 rolls that are 570mm wide or 3 rolls that our 380mm wide. If the insulation isn’t partially perforated, you will need to use scissors to cut it to the necessary thickness.

STEP 3: Once the insulation is the correct width, you need to roll it out between the joists; lightly press the insulation material to fit between the joists, but be careful not to overdo it and compress the material. Joists tend to be only about 100mm high (although this can vary), so match the insulation thickness you buy with the joist height – once installed, the insulation should come up to the top of the joists. You will need to work the insulation around and over any downlight caps that may now be present.

STEP 4: You now need to spread an additional 170mm – 200mm thick insulation at 90 degrees to the joists. Starting at the furthest point from the loft hatch slowly unroll the insulation over the top of the joists – make sure you use kneeling boards to spread the weight load and reduce the risk of ceiling collapsing as you move around in the loft space. This second thicker layer of insulation should have no gaps between the strips that you lay out – it should produce a continuous layer of insulation – you will no longer be able to see the joists. This will take the total thickness of the insulation in the loft to 270mm – 300mm as specified by building regulations. You may like to install even thicker than this, but the energy savings of doing so will be negligible.

STEP 5: As a final step you will need to insulate the loft hatch, by strapping on some material to the top of the hatch. This can be stapled to the top of the loft hatch, helping to preserve a consistent thermal barrier. You can also fit draught proof strips on the outside of the hatch to stop gusts of cold air in the winter. A really easy way to do this is to fill a black bag with insulation and then tape this down to the top of the hatch using thick tape.

Although 270mm – 300mm is the optimum depth recommended for mineral wool, but if you are planning on using one of the other insulating materials such as loose fill, it is worth reading the guidelines provided by the manufacturer to ensure you use a sufficient volume to give the required depth. Putting more than 270mm on insulation within your loft (e.g. 350mm) will help you heat your house for less, but the savings equal the cost of laying the additional insulation so you may deem this unnecessary.

Storing items in the loft but still insulating to building regulations

As we have said, the more insulation the better and to adhere to building regs you need 270mm of wool insulation as a minimum. One of the issues with putting this much insulation in the loft space is that you lose sight of the joists. Many people like to use the loft space as storage, however, so will insulate only to the top of the joists and then attach chipboard directly to the joists making a solid, walkable surface. Obviously if you install 300mm this is not possible, so prior to STEP 4 above, you will need to install loft stilts (or loft lifters).

These are relatively simple to install, and are essentially strong plastic stilts that raise the height of the joists, allowing you to install the extra insulation and then attach chipboard on top of the them. This allows you to still produce a useable surface while also maximising the thickness of insulation you install.

Using rigid insulation board as loft insulation

Some people may prefer to use rigid insulation board to insulate the loft space – for example Celotex or Kingspan – the process is pretty similar to above, although you will need less depth to achieve the same insulation levels (than mineral wool). You will also need a hacksaw to cut the insulation to size. If you do decide to use rigid insulation board, then you will still need to use chipboard if you want to walk on it.

Insulating water tanks in the loft

One issue that arises from insulating the loft space just above the ceiling (i.e. in the joists) is that the loft space itself will become incredibly cold. During very cold weather the temperature could even become sub-zero, which if you have water tanks in the loft, could be a big issue!

If you do have cold water tanks in the loft, first of all, never insulate underneath them; always allow heat to travel up through the roof into the bottom of the cold water tank. Also, it is worth insulating the cold water tanks themselves and lagging any pipework you can see in the loft that sits above the insulation you have installed in the joists. The kit required to insulate the cold water tanks is known as the Byelaw 30 and can be purchased from any good DIY store.